WO2012089847A2

WO2012089847A2 - Stability and visibility of a display device comprising an at least transparent sensor used for real-time measurements

Info

Publication number: WO2012089847A2
Application number: PCT/EP2012/050026
Authority: WO
Inventors: Arnout Robert Leontine VETSUYPENS; Wouter M. F. WOESTENBORGHS; Saso MLADENOVSKI
Original assignee: Barco N.V.
Priority date: 2010-12-31
Filing date: 2012-01-02
Publication date: 2012-07-05
Also published as: WO2012089847A3; GB201022141D0

Abstract

The invention relates to a display device having at least one sensor for detecting a property such as the intensity, colour and/or colour point of light emitted from at least one display area of a display device into the viewing angle of said display device.

Description

Stability and visibility of a display device comprising an at least transparent sensor used for real-time measurements.

FIELD OF THE INVENTION

The invention relates to a display device having at least one sensor for detecting a property such as the intensity, colour and/or chromaticity of light emitted from at least one display area of a display device into the viewing angle of said display device.

The invention also relates to the use of such a display device. BACKGROUND OF THE INVENTION

In modern medical facilities, high-quality medical imaging using display devices like liquid crystal display devices (LCD devices) is more important than ever before. In addition, other display technologies from which crucial data is needed to be retrieved by the human eye typically are provided with a sensor and a controller device coupled thereto. One type of sensor is coupled to a backlight device, for instance comprising light emitting diodes (LEDs), of the LCD device. It aims at stabilizing the output of the backlight device, which inherently varies as a consequence of the use of LEDs therein.

WO2008/050262 discloses one example of such sensor for an LED-based backlight. The backlight device is herein provided with a transparent outcoupling plate overlying its surface from which light is emitted. Structures, such as prismatic grooves, are defined in the outcoupling plate, so as to guide light to a side face, where the sensor is located. Particularly, the outcoupling plate is designed so as to achieve light spreading in addition to the light guiding to a side face. This provides an improved uniformity of the light output of the backlight device. However, a stabilization of merely the backlight is insufficient for obtaining a high-quality display system, such can be for instance applied for medical imaging applications. Moreover, when considering such outcoupling plate in front of a display, light spreading is not desired. EP1274066B1 discloses a display device wherein the sensing is applied in front of the display. Use is made herein of a light guide, f.i. a waveguide or fibre, to guide a portion of the light output to a sensor outside the viewing angle of the display. Light from a display area comprising a plurality of pixels is inserted into the light guide, for instance at one end of the fibre or into a continuous waveguide. Therewith, the area on the display blocked for light transmission is limited. Particularly, as disclosed in EP1274066, light rays traveling under a large angle to the axis of the light guide can be made to exit the structure, while ambient light cannot enter the light guide. By means of this small acceptance angle, it is avoided that ambient light enters the photodiode sensor without a need for shielding.

However, it is desired to further improve such a sensor system, i.e. sensor and light guide. One implementation shown in EP1274066 is that an end of a fiber is parallel to the output surface of the display and the fiber is bent. This is however not a most practical implementation.

Another such solution with a waveguide in front of a display is disclosed in WO2004/023443. The waveguide particularly includes a material of relatively higher refractive index surrounded by a material of relatively lower refractive index. A sensor is present at one edge of the waveguide. Alternatively, the waveguide may extend in four directions and the sensors may be present on four edges. This solution is intended (see example 3) for calibration measurements of an 10x10 passive matrix OLED display, wherein each pixel is turned on sequentially.

Another example of display devices having at least one sensor for detecting properties such as the intensity, has been disclosed in WO2010/081814. Surprisingly good results have been obtained with partially transparent sensors located in front of the display area and within the viewing angle. Preferably, the sensor comprises an organic photoconductive sensor. The device further can comprise at least partially transparent electrical conductors for conducting a measurement signal from the sensor within the viewing angle for transmission to a controller.

However, it is an object of the present invention to provide a sensor system that can be used for real-time measurements, which has a better image quality and that cannot be affected by previous measurements done (i.e. previous measurements create a 'sensor history'), which is an improvement over the solution of WO2010/081814. An advantage of the referred sensor system is that it can be used for real-time measurements, e. g. while the display is in use, and off-line, e. g. when the normal display functionality is interrupted, with a high signal to noise ratio and simultaneously can isolate the contributions of the signals from the backlight of the display and the signals from the ambient light.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, a display device is provided that comprises at least one display area provided with a plurality of pixels. For each display area an at least partially transparent sensor for detecting a property of light emitted from the said display area into a viewing angle of the display device is present. The sensor is located in a front section of said display device in front of said display area.

The display defined in the at least one display area of the display device may be of conventional technology, such as an liquid crystal device (LCD) with a backlight, for instance based on light emitting diodes (LEDs), or an electroluminescent device such as an organic light emitting (OLED) display. The display device suitably further comprises an electronic driving system and a controller receiving optical measurement signals generated in the at least one sensor and controlling the electronic driving system on the basis of the received optical measurement signals.

Surprisingly good results have been obtained with at least partially transparent sensors located in front of the display area and within the viewing angle. An expected disturbance of the display image tends to be at least substantially absent. Due to the direct incoupling of the light into the sensor, a proper transmission to the sensor is achieved without a coupling member. Such transparent sensor is suitably applied to an inner face of a cover member. Indeed, the transparent cover member may be used as a substrate in the manufacturing of the sensor. Particularly an organic or inorganic substrate can be used that has sufficient thermal stability to withstand operating temperature of vapour deposition and the high vacuum conditions, which is a preferred way of deposition of the layers constituting the sensor. Flexible substrates such as flexible polymeric substrates can also be used. Specific examples include chemical vapour deposition (CVD) and any type thereof for depositing inorganic semiconductors such as metal organic chemical vapour deposition (MOCVD), thermal vapour deposition. In addition, one can also apply low temperature deposition techniques such as printing and coating for depositing organic materials for instance. Another method, which can be used, is organic vapor phase deposition. When depositing organic materials, the temperatures at the substrate level are not much lower than any of the vapor deposition. Assembly is not excluded as a manufacturing technique. In addition, coating techniques can also be used on inorganic substrates such as glass substrates, however for polymers one must keep in mind that the solvent can dissolve the substrate in some cases.

In a suitable embodiment hereof, the device further comprises at least partially transparent electrical conductors for conducting a measurement signal from said sensor within said viewing angle for transmission to a controller. Substantially transparent conductor materials such as indium tin oxide (ITO) and the polymeric Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), typically referred to as PEDOT:PSS, are known well known partially transparent electrical conductors. Preferably, a thin oxide layer or transparent conductive oxide is used, for instance zinc oxide can also be used which is known to be a good transparent conductor. . In one most suitable embodiment, the sensor is provided with transparent electrodes that are defined in one layer with the said conductors (also called a lateral configuration). This reduces the number of layers that inherently lead to additional absorption and to interfaces that might slightly disturb the display image.

In a preferred embodiment, the sensor comprises an organic photoconductive sensor. Such organic materials have been a subject of advanced research over the past decades. Organic photoconductive sensors may be embodied as single layers, as bilayers and as general multilayer structures. They may be advantageously applied within the present display device. Particularly, the presence on the inner face of the cover member allows that the organic materials are present in a closed and controllable atmosphere, e.g. in a space between the cover member and the display which will provide protection from any potential external damaging. A getter may for instance be present to reduce negative impact of humidity or oxygen. . An example of a getter material is CaO Furthermore, vacuum conditions or a predefined atmosphere (for instance pure nitrogen, an inert gas) may be applied in said space upon assembly of the cover member to the display i.e. an encapsulation of the sensor.

A sensor comprising an organic photoconductive sensor suitably further comprises a first and a second electrode that advantageously are located adjacent to each other. The location adjacent to each other, preferably defined within one layer, allows a design with finger-shaped electrodes that are mutually interdigitated. Herewith, charges generated in the photoconductor are suitably collected by to the electrodes. Preferably the number of fingers per electrode is larger than 50, more preferably larger than 100, for instance in the range of 250- 2000. However the present invention is not limited to this amount.

Furthermore an organic photoconductive sensor can be a mono layer, a bi-layer or in general a multiple (>2) layer structure one preferred type of photoconductive sensensor is one wherein the organic photoconductvie sensor is a bilayer structure with a exciton generation layer and a charge transport layer, said charge transport layer being in contact with a first and a second electrode. Such a bilayer structure using nontransparent metal electrodes is for instance known from Applied Physics Letters 93 "Lateral organic bilayer heterojunction photoconductors" by John C. Ho, Alexi Arango and Vladimir Bulovic. The sensor described by J.C. Ho et al relates to a non-transparent sensor as it refers to gold electrodes, which will absorb the impinging light entirely. The bilayer comprises an EGL (PTCBI) or Exciton Generation Layer and a HTL (TPD) or Hole Transport Layer (HTL) (in contact with the electrodes).

In embodiments of the present invention, the sensor, preferably a semi- transparent sensor is intended to be used for measuring properties of light (luminance and/or color) on an active display area. Preferably it is used in front of the display active area, and advantageously it provides as high transmission of light as possible. In addition, the sensor does not introduce any visible artifacts such as visibility of the sensors' active areas, overall coloring, and advantageously the sensor according to embodiments of the present invention advantageously does not require spacers, getters etc. which introduce visible artifacts.

Furthermore the human eye is sensitive to contrast and color changes in bar or finger shaped patterns. When designing the transparent electrical conductors, preferably some specific geometry that are less visible for the human eye are used, which can result in less visible non-uniformities and thus a better image quality. For instance using transparent electrical conductor patterns, which consist of curved finger, patterns or finger patterns under an angle, instead of straight finger-shaped transparent electrical conductor patterns will be less easy to detect by the human eye. For instance, these techniques can be used to avoid moire effects that can occur due to the superposition of the finger pattern grid on top of the display pixel grid.

According to embodiments of the invention, parts of a floating electrical conductor system, with no specific signal, can be applied on regions outside the finger-shaped partially transparent electrical conductor patterns and ITO tracks that guide the electrical signal from the finger pattern towards the edge of the display, to have a more uniform global luminance and color output. These parts of a floating electrical conductor system have no function aside from improving the visibility. They are separated from the active, finger-shaped partially transparent electrical conductor pattern and ITO tracks that guide the electrical signal from the finger pattern towards the edge of the display, by for instance a gap to ensure there is no electrical contact between them. Furthermore, the width of this gap is small enough so that the gap itself is not noticeable by the human eye. Moreover, the floating electrical conductor system is typically made of the same material as the active, finger-shaped electrical conductors.

In addition, to reduce the visibility of the finger patterns themselves, a floating electrical conductor is applied next to the end of a finger of the pattern (this can be interpreted as a "nail" of the finger) which also has no other function except reducing the visibility of the finger pattern itself. They are separated from all active parts of the transparent conductor and there is no signal applied on them. In embodiments of the present invention, the gaps between the fingers of a partially transparent electrical conductor pattern are chosen such that the human eye is not able to distinguish any individual fingers. This is preferably enabled by appropriately choosing the gap between the fingers such that the angle subtended by the gap at a given distance will be smaller than the smallest details any (or a typical) human observer is able to discriminate. Evidently, there are limits in the possible choice of the gap sizes, as they can impact the performance of the sensor.

Furthermore, by widening the fingers and keeping the gap width constant, we increase the percentage of the finger pattern area, which is covered by the transparent conductor. In this way, the average transmission over the finger pattern is very close (almost the same) to the transmission of the areas with a floating electrical transparent conductor. This significantly reduces the visibility of the finger pattern area in reference to the floating electrical conductor system and makes the whole substrate uniform. In other words, by suitably selecting the finger width/gap ratio for a suitable gap, the human eye is unable to distinguish the sensor's finger pattern from the neighboring ITO tracks and floating ITO conductors, wherein the patterns and tracks conduct the signal towards the edge of the display, and parts of the floating electrical conductor system with no specific signal. As the number of fingers has to be maintained to reach a desired signal amplitude, the sensor's size will consequentially increase, when selecting a higher finger width to gap ratio for a given fixed gap. Moreover, in embodiments of the invention semi-random fingers are created by randomly choosing several points on the two edges of the fingers, where the fingers should go through, the different points then are for instance connected using a cubic spline interpolation. The adjacent finger preferably is limited in distance, in the sense that the gap in between the fingers should remain constant to ensure that the device's properties remain unaltered.

In addition, in other embodiments, the points can be chosen in a semi random way in the sense that they are limited to a specific area to avoid too high spatial frequencies in the fingers. Preferably, the ratio between the finger width and the gap between the fingers is non-constant in the created finger pattern, as a consequence of maintaining the gap size and choosing the points of the edge semi-randomly. In addition, when maintaining a specific gap size, optimized for the sensor's performance as described above, the average finger width can be design such as to reduce the visibility relative to the areas with floating ITO. .

In embodiments of the present invention, the encapsulation of the sensor based on organic photoconductive sensors is preferably done with a method different than the conventional approach (based on encapsulation plate, spacers and getters done in inert gas atmosphere). In particularly a space between the substrate with sensors and the encapsulation plate is filled, by preferably using an encapsulation glue which has a refractive index close to the refractive index of both the substrate and the encapsulation plate (typically made of glass), high transmission and whereby the glue has no coloring effect. This helps to remove a few unwanted visual artifacts such as the spacers and getters. Furthermore it improves the transmission of the sensor by reducing the reflection of internal optical interfaces. In addition it reduces the visibility of the finger pattern in reflection. This is because the glue is filling in all the gaps, which contain the organic photoconductor, and reduces the diffractive effects caused by the finger pattern.

Additionally the transmission of the sensor is improved by using a transparent conductor with a lower absorption coefficient or lower thickness, thus by for instance using ITO with a lower absorption coefficient or a lower thickness.

Furthermore, the transmission can be improved by using antireflection coatings on the external side of the substrate and the encapsulation. The antireflection coating can be tuned in such a way that it reduces the reflection over all wavelengths. For some wavelength regions the reflection is reduce more than for other wavelength regions which is helpful to remove any potential coloring of the sensor.

From experiments performed, it has been learned that sensors of the type described above based on organic photoconductive sensors can be affected by previous measurements done (i.e. previous measurements create a 'sensor history') which then as a result has an impact on the measured signal.

Therefore, it is proposed to perform a 'reset' of the sensor and thus erase the history, before the start of a new measurement. Part of the implementation of the reset can be for example by short-circuiting the sensor in such way that all the accumulated charges can leave the device. So the latter concept comprises applying a specific signal to the sensor to reset.

In addition to this resetting, one can furthermore apply a specific voltage or current signal to the sensor in order to obtain a more stable read-out. In order to measure the resistance of the sensor a signal is applied, e.g. a DC signal. This specific signal can be structured in such a way to overcome a specific problem, for example to avoid an overshoot. Instead of abruptly applying the required signal, a steadily increasing signal can be applied, for example by using the following formula:

Specific Signal (t) = RequiredSignal - a^_t where a > 1 . The type of voltage applied to the sensor can for instance be a block wave, a sinusoidal wave, ore more exotic shapes known by the skilled person. Preferably symmetrical waves going from a positive voltage to the same negative voltage are used. For example, good results were obtained using a block wave that switches between from +1 V and -1 V.

Preferably, the applied wave is repeated a multiple times, for instance repeated during 10 seconds, and then one preferably uses the measured data which is retrieved at a certain point during the wave propagation in time. Preferably, a specific point on the upper and lower flanks is focused during the consecutive cycles, and its output is plot over time. The points that are focused are typically the points on the flank right before the voltage switches from high to low or from low to high. For example, when applying a 1 Hz block wave, the final value at the end of the positive flank or at the end of the negative flank is used. Preferably, the optimal frequency depends on the layer thickness of the sensor and typically is chosen between 0.2-2.5 Hz. As a result, the organic layer stack of the sensor has a direct impact on its fundamental stability and the used layer thickness, and the stack composition has an impact on the stability. For instance when using following stack:

Sample Organic stack

LB0448 TMPB (40nm) / PTCBi (10nm)

LB0464 TMPB (40nm) / PTCBi (5nm)

LB0465 TMPB (100nm) / PTCBi (10nm)

LB0466 TMPB (40nm) / PTCBi (1 Onm) / TMPB (40nm)

The HTL thickness and the number of HTL layers impact the stability. In a case of a dual layer stack a thicker HTL in the range of 80-100 nm is preferred over a thinner HTL in the range of 40 nm for improving the stability.

In addition, the amplitude of the applied signal can have an impact on the resulting stability of the measured signal and typically is chosen between 0.5- 2V. An applied voltage signal with a lower amplitude (for instance 1 V) generally renders a more stable result than a signal with a higher amplitude (for instance 8V). The obtained measurement results can be asymmetrical, meaning that the positive flank renders a different result than the negative flank, and in addition they can converge differently. In addition, the measured value preferably is chosen as a value near the end of the flank of the outcoming block wave, just before the applied signal is inverted (from the positive to the negative value or vice versa). In some embodiments the measurement results are averaged or only one part (the positive of negative flank) is used. It is preferred, but not limited to have gaps between the fingers with a width of 6μιη to 30μιη, because the gaps have a proven impact on the signal amplitude and visibility.

When it comes to encapsulating the sensor by using a glue over its entire surface, as indicated in an earlier embodiment, the sensor performs as good as a sensor with standard encapsulation (using spacers, getters and inert gas atmosphere)

Alternatively, sensors comprising composite materials could be constructed. With composite materials nano/micro particles are proposed, either organic or inorganic dissolved in the organic layers, or an organic layer consisting of a combination of different organic materials (dopants). Since the organic photosensitive particles often exhibit a strongly wavelength sensitive absorption coefficient, this configuration can result in a less colored transmission spectrum, when suitable materials are selected and suitably applied, or can be used to improve the detection over the whole visible spectrum, or can improve the detection of a specific wavelength region. However, a disadvantage could be that the sensor only provides one output current per measurement for the entire spectrum. In other words, it is not evident to measure color online while using the display. This could be avoided by using three independent photoconductive sensors that measure red, green and blue independently. They could be conceived similarly to the previous descriptions, and stacked on top of each other, or adjacent to each other on the substrate, to obtain an online color measurement.

Alternatively, instead of using organic layers to generate charges and guide them to the electrodes, hybrid structures using a mix of organic and inorganic materials can be used. A bilayer device that uses a quantum-dot exciton generation layer and an organic charge transport layer can be used. For instance colloidal Cadmium Selende quantum dots and an organic charge transport layer comprising of Spiro-TPD. Although in a preferred embodiment, where photoconductive sensors are used, a disadvantage could be that the sensor only provides one output current per measurement for the entire spectrum. In other words, it is not evident to measure color online while using the display. This could be avoided by using three independent photoconductive sensors that measure red, green and blue independently, and provide a suitable calibration for the three independent photoconductive sensors. They could be conceived similarly to the previous descriptions, and stacked on top of each other, or adjacent to each other on the substrate, to obtain an online color measurement. Offline color measurements can be made without the three independent photoconductive sensors, by calibrating the sensor to an external sensor, which is able to measure tristimulus values (X, Y & Z), for a given spectrum. It is important to note that uniform patches should be displayed here, as will become clear from the later description of the methodology to measure online. This can be understood as follows. A human observer is unable to distinguish the brightness or chromaticity of light with a specific wavelength impinging on his retina. Instead, he possesses three distinct types of photoreceptors, sensitive to three distinct wavelength bands that define his chromatic response. This chromatic response can be expressed mathematically by color matching functions. Consequentially, three color matching functions, and have been defined by the CIE in 1931 . They can be considered physically as three independent spectral sensitivity curves of three independent optical detectors positioned at our retinas. These color-matching functions can be used to determine the CIE1931 XYZ tristimulus values, using the following formulae:

dX

JO

roc

J I(X) z(X) dX

Where I (λ) is the spectral power distribution of the captured light. The luminance corresponds to the Y component of the CIE XYZ tristimulus values. Since a sensor, according to embodiments of the present invention, has a characteristic spectral sensitivity curve that differs from the three color matching functions depicted above, it cannot be used as such to obtain any of the three tristimulus values. Although the first sensor according to embodiments of the present invention is typically sensitive in the entire visible spectrum with respect to the absorption spectrum of the sensor or alternatively, they are at least sensitive to the spectral power distributions of a (typical) display's primaries, XYZ values can be obtained after calibration for a specific type of spectral light distribution emitted by the display. Displays are typically either monochrome or color displays. In the case of monochrome (e.g. grayscale) displays, they only have a single primary (e.g. white), and hence emit light with a single spectral power distribution. Color displays have typically three primaries - red (R), green (G) and blue (B)- which have three distinct specific spectral power distributions. A calibration step preferably is applied to match the XYZ tristimulus values corresponding to the spectral power distributions of the display's primaries to the measurements made by the sensor according to embodiments of the present invention. In this calibration step, the basic idea is to match the XYZ tristimulus values of the specific spectral power distribution of the primaries to the values measured by the sensor, by capturing them both with the sensor and an external reference sensor. Since the sensor response according to embodiments of the present invention is non-linear, and the spectral power distribution associated with the primary may alter slightly depending on the digital driving level of the primary, it is insufficient to match them at a single level. Instead, they need to be matched ideally at every digital driving level. This will provide a relation between the actual tristimulus values and sensor measurements in the entire range of possible values. To obtain a conversion between any measured value, as measured by the sensor according to the preferred embodiment, and the desired tristimulus value, an interpolation is needed to obtain a continuous conversion curve. This results in three conversion curves per display primary that convert the measured value in the XYZ tristimulus values. In the case of a monochrome display, three conversion curves are obtained when using this calibration methodology. Obtaining the XYZ tristimulus values is now evident when using a monochrome display. The light to be measured can simply be generated on the display (in the form of uniform patches), and measured by the sensor according to embodiment of the present invention, when using the different conversion curves. In the case of a color display, this calibration needs to be done for each of the display's primaries. This results in 9 conversion curves, in the typical case when the display has 3 primaries. Note that a specific coloured patch with a specific driving of the red, green and blue primaries will have a specific spectrum, which is a superposition of the scaled spectra of the red, green and blue primaries, and hence every possible combination of the driving levels needs to be calibrated individually. Therefore, an alternative methodology can suitably be used: the red, green and blue primaries need to be calibrated individually for each digital driving level. During such a calibration a single primary patch is displayed while the other 2 channels (primaries) remain at the lowest possible driving level (emitting the least possible light, ideally no light at all). This suitable methodology implies that the red, green and blue driving of the patch needs to be done sequentially. The correct three conversion curves corresponding to the specific primary will need to be applied to obtain the XYZ tristimulus values from the measured values. This results in three sets of tristimulus values: (XRYRZR), (XGYGZG) and (XBYBZB). Since the XYZ tristimulus values are additive, the XYZ tristimulus values of the patch can be obtained using the following formulae: Xpatch=XR+XG+XB

Ypatch=YR+YG+YB

Zpatch=ZR+ZG+ZB

Note that we assume the display has no crosstalk in these formulae. Two parts can be distinguished in the XYZ tristimulus values. Y is directly a measure of brightness (luminance) of a color. The chromaticity, on the other hand, can be specified by two derived parameters, x and y. These parameters can be obtained from the XYZ tristimulus values using the following formulae:

Y

X + Y + Z

This offline color measurement which is enabled by calibrating the sensor to an external sensor which is able to measure tristimulus values (X, Y & Z) Thus allows measuring brightness as well as chromaticity.

According to a second aspect of the invention, a display device is provided that comprises at least one display areas with a plurality of pixels. For each display area, at least one sensor and an at least partially transparent optical coupling device are provided. The at least one sensor is designed for detecting a property of light emitted from the said display area into a viewing angle of the display device. The sensor is located outside or at least partially outside the viewing angle. The at least partially transparent optical coupling device is located in a front section of said display device. It comprises a light guide member for guiding at least one part of the light emitted from the said display area to the corresponding sensor. The coupling device further comprises an incoupling member for coupling the light into the light guide member. It is an advantage of the present invention to detect a property such as the brightness or the chromaticity of light emitted by at least one display area of a display device into the viewing angle of said display device without constraining the view on said display device. The use of the incoupling member solves the apparent contradiction of a waveguide parallel to the front surface that does not disturb a display image, and a signal-to-noise ratio sufficiently high for allowing real-time measurements. An additional advantage is that any scattering eventually occurring at or in the incoupling member is limited to a small number of locations over the front surface of the display image. However, when using waveguides a moire pattern can be observed at the edge of the waveguides, which can be considered to be a high risk, to lower this risk the described embodiments using organic photoconductive sensors can be applied.

Preferably, the light guide member is running in a plane which is parallel to a front surface of the display device. The incoupling member is suitably an incoupling member for laterally coupling the light into the light guide member of the coupling device. The result is a substantially planar incoupling member. This has the advantage of minimum disturbance of displayed images. Furthermore, the coupling device may be embedded in a layer or plate. It may be assembled to a cover member, i.e. front glass plate, of the display after its manufacturing, for instance by insert or transfer moulding. Alternatively, the cover member is used as a substrate for definition of the coupling device.

In one implementation, a plurality of light guide members is arranged as individual light guide members or part of a light guide member bundle. It is suitable that the light guide member is provided with a circular or rectangular cross-sectional shape when viewed perpendicular to the global propagation direction of light in a light guide member. A light guide with such a cross-section may be made adequately and moreover limits scattering of radiation. The cover member is typically a transparent substrate, for instance of glass or polymer material.

In any of the above embodiments the sensor or the sensors of the sensor system is/are located at a front edge of the display device. The incoupling member of this embodiment may be present on top of the light guide member or effectively inside the light guide member. One example of such location inside the light guide is that the incoupling member and the light guide member have a co-planar ground plane. The incoupling member may then extend above the light guide member or remain below a top face of the light guide member or be coplanar with such top face. Furthermore, the incoupling member may have an interface with the light guide member or may be integral with such light guide member.

In one particular embodiment, the or each incoupling member is cone-shaped. The incoupling member herein has a tip and a ground plane. The ground plane preferably has circular or oval shape. The tip is preferably facing towards the display area.

The incoupling member may be formed as a laterally prominent incoupling member. Most preferably, it is delimited by two laterally coaxially aligned cones, said cones having a mutual apex and different apex angles. The difference between the apex angles Δα=α1 - α2 is smaller than the double value of the critical angle (6_C) for total internal reflection (TIR) Δα < 26_c. Especially, the or each incoupling member fades seamlessly to the guide member of the coupling device. The or each incoupling member and the or each guide member are suitably formed integrally. In an alternative embodiment, the or each incoupling member is a diffraction grating. The diffraction grating allows that radiation of a limited set of wavelengths is transmitted through the light guide member. Different wavelengths (e.g. different colours) may be incoupled with gratings having mutually different grating periods. The range of wavelengths is preferably chosen so as to represent the intensity of the light most adequately.

In a further embodiment hereof, both the cone-shaped incoupling member and diffraction grating are present as incoupling members. These two different incoupling members may be coupled to one common light guide member or to separate light guide members, one for each, and typically leading to different sensors. By using a first and a second incoupling members of different type on one common light guide member, light extraction, at least of certain wavelengths, may be increased, thus further enhancing the signal to noise ratio. Additionally, because of the different operation of the incoupling members, the sensor may detect more specific variations. By using a first and a second incoupling member of different type in combination with a first and a second light guide member respectively, the different type of incoupling members may be applied for different type of measurements. For instance, one type, such as the cone-shaped incoupling member, may be applied for luminance measurements, whereas the diffraction grating or the phosphor discussed below may be applied for color measurements. Alternatively, one type, such as the cone-shaped incoupling member, may be used for a relative measurement, whereas an other type, such as the diffraction grating, is used for an absolute measurement. In this embodiment, the one incoupling member (plus light guide member and sensor) may be coupled to a larger set of pixels than the other one. One is for instance coupled to a display area comprising a set of pixels, the other one is coupled to a group of display areas.

In a further embodiment, the incoupling member comprises a transformer for transforming a wavelength of light emitted from the display area into a sensing wavelength. The transformer is for instance based on a phosphor. Such phosphor is suitably locally applied on top of the light guiding member. The phosphor may alternatively be incorporated into a material of the light guiding member. It could furthermore be applied on top of another incoupling member (e.g. on top of or in a diffraction grating or a cone-shaped member or another incoupling member).

The sensing wavelength is suitably a wavelength in the infrared range. This range has the advantage the light of the sensing wavelength is not visible anymore. Incoupling into and transport through the light guide member is thus not visible. In other words, any scattering of light is made invisible, and therewith disturbance of the emitted image of the display is prevented. Such scattering typically occurs simultaneously with the transformation of the wavelength, i.e. upon reemission of the light from the phosphor. The sensing wavelength is most suitably a wavelength in the near infrared range, for instance between 0.7 and 1 .0 micrometers, and particularly between 0.75 and 0.9 micrometers. Such a wavelength can be suitably detected with a commercially available photodetectors, for instance based on silicon.

A suitable phosphor for such transformation is for instance a Manganese Activated Zinc Sulphide Phosphor. Preferably, the phosphor is dissolved in a waveguide material, which is then spin coated on top of the substrate. The substrate is typically a glass substrate, for example BK7 glass with a refractive index of 1 ,51 . Using lithography, the parts are removed from which are undesired. Preferably, a rectangle is constructed which corresponds to the photosensitive area, in addition the remainder of the waveguide, used to transport the generated optical signal towards the edges, is created in a second iteration of this lithographic process. Another layer can be spin coated (without the dissolved phosphors) on the substrate, and the undesired parts are removed again using lithography. Waveguide materials from Rohm&Haas can be used or PMMA. Such a phosphor may emit in the desired wavelength micron region, where the manganese concentration is greater than 2%. Also other rare earth doped zinc sulfide phosphors can be used for infrared (IR) emission. Examples are ZnS:ErF3 and ZnS:NdF3 thin film phosphors, such as disclosed in J.Appl.Phys. 94(2003), 3147, which is incorporated herein by reference. Another example is ZnS:Tim_xAg_y, with x between 100 and 1000 ppm and y between 10 and 100 ppm, as disclosed in US4499005.

Instead of being an alternative to the before mentioned transparent sensor solution, the present sensor solution of coupling member and sensor may be applied in addition to such sensor solution. The combination enhances sensing solutions and the different type of sensor solutions have each their benefits. The one sensor solution may herein for instance be coupled to a larger set of pixels than another sensor solution. The display device suitably further comprises an electronic driving system and a controller receiving optical measurement signals generated in the at least one sensor and controlling the electronic driving system on the basis of the received optical measurement signals.

The display defined in the at least one display area of the display device may be of conventional technology, such as an liquid crystal device (LCD) with a backlight, for instance based on light emitting diodes (LEDs), or an electroluminescent device such as an organic light emitting (OLED) diodes.

While the foregoing description refers to the presence of at least one display area with a corresponding sensor solution, the number of display areas with a sensor is preferably larger than one, for instance two, four, eight or any plurality. It is preferable that each display area of the display is provided with a sensor solution, but that is not essential. For instance, merely one display area within a group of display areas could be provided with a sensor solution.

Furthermore, a display sensor constructed using multiple sensors such as described above, can be a separate device that can be fixed to a display screen. In such case there is a priori no guaranteed positional relationship between the display sensor and the screen. It is essential to know the position of the sensors relative to the active area of the screen. This can be done for example by applying specific video patterns (for instance squares) to the display and detecting which of the sensors responds. From the response the relative position between each sensor and the pixels of the display can be determined. In alternative embodiments, the display sensor comprises at least one sub sensor, whereby said display sensor can be a clip-on sensor, like a front glass, which can be appropriately designed to make electrical contact when connected to the rest of the display by for instance wires that conduct the measured electronic signals to and from the sensor. However, when mounting the clip-on sensor onto the display, the individual sub-sensors of the clip-on sensor can end up at a slightly different position on the display's active area. Therefore, an alignment procedure preferably is applied to match the position of the sensor to the position of a certain zone of pixels on the display's active area. The alignment algorithm is used to appropriately drive the display and to use the clip-on sensor to measure the required property of the emitted light. Afterwards, the results are processed to triangulate the sensors' location. For example a square (black and/or white) can be shown at a maximum driving level, at a background of a minimum driving level. When the square is translated over the screen, it will be detected by a single sensor, or by multiple sensors at a certain position, or in a certain range of potential positions on the screen, depending on the size of the square. Depending on the size of the square, the detection can occur faster but less accurate or slower but more accurate. Therefore, an optimization can be done by using for instance the following algorithm: starting with a large square to roughly determine the position, and in a second iteration, the size of the square is decreased and simultaneously the translation area is restricted to the roughly determined position from the first iteration.

In a further aspect according to the invention, use of the said display devices for sensing a light property while displaying an image is provided. Most suitably, the real-time detection is carried out for the signal generated by the sensor according to the preferred embodiment of this invention, this signal is generated according to the sensors' physical characteristics as a consequence of the light emitted by the display, according to its light emission characteristics for any displayed pattern. The detection of luminance and color (chromaticity) aspects may be carried out in a calibration mode, e.g. when the display is not in a display mode.

However, it is not excluded that luminance and chromaticity detection may also be carried out real-time, in the display mode. In some embodiments it can be suitable to do the measurements relative to a reference value.

In a preferred embodiment of this invention, the sensor does not exhibit an ideal spectral sensitivity according to the V (λ) curve, nor does it have suitable color filters to measure the tristimulus values. Therefore, real-time measurements are difficult as the sensor will not be calibrated for every possible spectrum that results from the driving of the R, G & B subpixels which generate light impinging on the sensor. A V(A) sensor following a ν(λ) curve describes the spectral response function of the human eye in the wavelength range from 380 nm to 780 nm and is used to establish the relation between the radiometric quantity that is a function of wavelength λ, and the corresponding photometric quantity. As an example, the photometric value luminous flux is obtained by integrating radiant power <t>e (λ) as follows:

780nm

Φ_ν = _rn |Φ_β(λ) ν(λ^λ

The unit of luminous flux Φν is lumen [Im] , the unit of Oe is Watt [W] and for V(A) is [1 /nm]. The factor Km = 683 Im/W establishes the relationship between the (physical) radiometric unit watt and the (physiological) photometric unit lumen. All other photometric quantities are also obtained from the integral of their corresponding radiometric quantities weighted with the V(A) curve.

It is clear from the explanation above, that measurements of luminance and illuminance require a spectral response that matches the V(A) curve as closely as possible. In general, a sensor according to embodiments of the present invention, is sensitive to the entire visible spectrum and doesn't have a spectral sensitivity over the visible spectrum that matches the V(A) curve. Therefore, an additional spectral filter is needed to obtain the correct spectral response.

On top of this non-ideal spectral sensitivity, the sensor as described in a preferred embodiment also does not operate as an ideal luminance sensor.

As the sensor used is not a perfect luminance sensor, as it does not only capture light in a very small opening angle, preferably the angular sensitivity is taken into account, as described in the following part.

For a given point on an ideal luminance sensor, the measured luminance corresponds to the light emitted by the pixel located directly under it (assuming that the sensor's sensitive area is parallel to the display's active area). On the contrary, the sensor according to embodiments of the present invention captures the pixel under the point together with some light emitted by surrounding pixels. More specifically, the values captured by the sensor cover a larger area than the size of the sensor itself. Because of this, the patterns used do not correspond to the actual patterns and therefore a correction has to be done in order to simulate the measurements of the sensor. To enable the latter preferably the luminance emission pattern of a pixel is measured as a function of the angles of its spherical coordinates, represented in Figure a. The range of the angles preferably are changed from -80 to 80 degrees with a step of 2 degrees for the inclination angle Θ and from 0 to 180 with a step of 5 degrees for the angle Φ. The distance preferably is kept constant over the measurements. When a luminance sensor is positioned parallel to the display's active area, the latter corresponds to an inclination angle of 0, meaning that only an orthogonal light ray is considered. In addition, the exact light sensitivity of the sensor can be characterized. These measurements can then be used in the optical simulation software to obtain the corrected pattern for the actual light the sensors will detect. Using this actual light output will provide an additional improvement and advantageous effect of the algorithm that will render more reliable results. As a result, for an appropriate real-time sensing while display of images in ongoing, further processing on sensed values is suitably carried out. Therein, an image displayed in a display area is used for treatment of the corresponding sensed value or sensed values, as well as the sensor's properties. Aspects of the image that are taken into account, are particularly its light properties, and more preferably light properties emitted by the individual pixels or an average thereof. Light properties of light emitted by individual pixels include their emission spectrum at every angle. An algorithm be used to calculate the expected response of the sensor, based on digital driving levels provided to the display and the physical behaviour of the sensor (this includes its spectral sensitivity over angle, its non-linearity and so on). When comparing the result of this algorithm to the actually measured light of a pixel or a group of pixels, it is possible to improve the display's performance by implementing a precorrection on the display's driving levels to obtain the desired light output. This precorrection may be an additional precorrection which can be added onto a precorrection that for example corrects the driving of the display such that a uniform light output over the display's active area is obtained.

In one embodiment, the difference between the sensing result and the theoretically calculated value is compared by a controller to a lower and/or an upper threshold value taking into account the reference. If the result is outside the accepted range of values, it is to be reviewed or corrected. One possibility for review is that one or more subsequent sensing results for the display area are calculated and compared by the controller. If more than a critical number of sensing values for one display area are outside the accepted range, then the setting for the display area is to be corrected so as to bring it within the accepted range. A critical number is for instance 2 out of 10. E.g. if 3 to 10 of sensing values are outside the accepted range, the controller takes action. Else, if the number of sensing values outside the accepted range is above a monitoring value but not higher than the critical value, then the controller may decide to continue monitoring. In order to balance processing effort, the controller may decide not to review all sensing results continuously, but to restrict the number of reviews to infrequent reviews with a specific time interval in between. Furthermore, this comparison process may be scheduled with a relatively low priority, such that it is only carried out when the processor is idle.

In another embodiment, such sensing result is stored in a memory. At the end of a monitoring period, such set of sensing results may be evaluated. One suitable evaluation is to find out whether the sensed values of the difference in light are systematically above or below the threshold value that, according to the settings specified by the driving of the display, should be emitted. If such systematic difference exists, the driving of the display may be adapted accordingly. In order to increase the robustness of the set of sensing results, certain sensing results may be left out of the set, such as for instance an upper and a lower value. Additionally, it may be that values corresponding to a certain display setting are looked at. For instance, sensing values corresponding to a high (RGB) driving levels are looked at only. This may be suitable to verify if the display behaves at high (RGB) driving levels similar to its behaviour at other settings, for instance low (RGB) driving levels. Alternatively, the sensed values of certain (RGB) driving level settings may be evaluated as these values are most reliable for reviewing driving level settings. Instead of high and low values, one may think of light measurements when emitting a predominantly green image versus the light measurements when emitting a predominantly yellow image. Additional calculations can be based on said set of sensed values. For instance, instead of merely determining a difference between sensed value and theoretically calculated value of the light output, which is the originally calibrated value, the derivative may be reviewed. This can then be used to see whether the difference increases or decreases. Again, the timescale of determining such derivative may be smaller or larger, preferably larger, than that of the absolute difference. It is not excluded that average values are used for determining the derivative over time. In another embodiments, use is made of sets of sensed values at a uniform driving of the display (or when applying another precorrection dedicated to achieve a uniform luminance output) for different display areas are compared to each other. In this manner, homogeneity of the display emittance (e.g. luminance) can be calculated.

The skilled reader will understand that use is made of storage of displays theoretically calculated values and sensed values for the said processing and calculations. The skilled person may further implement an efficient storage protocol. In the embodiment where the display is used in a room with ambient light; the sensed value is suitably compared to a reference value for calibration purposes. The calibration will be typically carried out per display area. In the case of using a display with a backlight, the calibration typically involves switching the backlight on and off to determine potential ambient light influences that might be measured during normal use of the display, for a display area and suitably one or more surrounding display areas. The difference between these measured values corresponds to the influence of the ambient light. This value needs to be determined because otherwise the calculated ideal value and the measured value will never match when the display is put in an environment that is not pitch black.

In case of using a display without backlight, the calibration typically involves switching the display off, within a display area and suitably surrounding display areas. The calibration is for instance carried out for a first time upon start up of the display. It may subsequently be repeated for display areas. Moments for such calibration during real-time use, which do not disturb a viewer, include for instance short transition periods between a first block and a second block of images. In case of consumer displays, such transition period is for instance an announcement of a new and regular program, such as the daily news. In case of professional displays, such as displays for medical use, such transition periods are for instance periods between reviewing a first medical image (X-ray, MRI and the like) and a second medical image. The controller will know or may determine such transition period. While the above method has been expressed in the claims as a use of the above mentioned sensor solutions, it is to be understood that the method is also applicable to any other sensor to be used with other display types. It is more generally a method of using a matrix of sensors in combination with a display. In the preferred embodiment, the matrix of sensors is designed such that it is permanently integrated into the display's design. Therefore, a matrix of transparent organic photoconductive sensors is used preferably, suitably designed to preserve the display's visual quality to the highest possible degree.

The goal can be either to assess the luminance or color uniformity of the spatial light emission of a display, based on at least two zones.

The present invention includes providing a sensing result by:

-Comparing the sensor value which is actually measured in the zone to the ideally measured value which should have been measured by the sensor for a specified display area with the applied display settings for said display area corresponding to the moment in time on which the sensor determination is based. This can either be based on a mathematical algorithm or on an additional calibration step, depending on whether a real-time measurements or offline measurements are made using uniform patches, and

-Evaluating the sensing result and/or evaluating a set of sensing results for defining a display evaluation parameter;

-If the display evaluation parameter is outside an accepted range, modify the display settings, or notify the user the display is out of the desired operating range, and/or continue monitoring said display area.

The average display settings as used herein are more preferably the ideally emitted luminance as discussed above.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic illustration of a display device with a sensor system according to a first embodiment of the invention;

Fig. 2 shows the coupling device of the sensor system illustrated in Fig. 1 ;

Fig. 3 shows a vertical sectional of a sensor system for use in the display device according to a third embodiment of the invention;

Fig. 4 shows a horizontal sectional view of a display device with a sensor system according to a fourth embodiment of the invention;

Fig. 5 shows a side view of a display device with a sensor system according to a second embodiment of the invention;

Fig 6a shows the first stage of amplification used for a display device with a sensor system; and

Fig 6b shows the second stage of amplification used for a display device with a sensor system; and

Fig 6c shows the first stage of amplification used for a display device with a sensor system; and

Fig. 7 illustrates the overview of the data path from the sensor to the processor; and

Fig. 8 is a schematic illustration of a display device with a sensor system and floating electrical conductor system according to an alternative embodiment of the invention;

Fig. 9 shows a schematic view of a network of sensors with a single layer of electrodes used in the display device.

Fig. 10 is a schematic illustration of a cross-section of a display device with a sensor system according to an embodiment of the invention.

Fig. 1 1 is a schematic illustration of a top view of an electrode comprising finger patterns according to an embodiment of the invention.

Fig. 12 shows transmission measurements of ITO coated glass substrates with different thickness of ITO layer.

Fig. 13 shows transmission measurement of a sensor with standard encapsulation (and ITO 65nm, HTL 40nm, EGL 10nm) and with glue encapsulation. Fig. 14 shows a schematic illustration of a top view of an electrode comprising finger patterns with a nail according to an embodiment of the invention.

Fig. 15 is a schematic illustration an electrode comprising a finger pattern whereby a floating electrical conductor system comprises parts with no electronic signal is applied in the regions outside the finger patterns according to an embodiment of the invention.

Figs. 16a and 16b schematically illustrate an electrode comprising a pattern whereby the pattern comprises semi-random fingers according to embodiments of the invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non- limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Similarly, it is to be noticed that the term "coupled", also used in the claims, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression "a device A coupled to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.

It is furthermore observed that the term "at least partially transparent" as used throughout the present application refers to an object that may be partially transparent for all wavelengths, fully transparent for all wavelengths, fully transparent for a range of wavelengths and partially transparent for the rest of the wavelengths. Typically, it refers to optical transparency, e.g. transparency for visible light. Partially transparent is herein understood as the property that the intensity of an image shown through the partially transparent member is reduced due to the said partially transparent member or its color is altered. Partially transparent refers particularly to a reduction of impinging light intensity of at most 40% at every wavelength of the visible spectrum, more preferably at most 25%, more preferably at most 10%, or even at most 5%. Typically the sensor design is such so as to be substantially transparent, i.e. with a reduction of impinging light intensity of at most 20% for every visible wavelength.

The term 'light guide' is used herein for reference to any structure that may guide light in a predefined direction. One preferred embodiment hereof is a waveguide, e.g. a light guide with a structure optimized for guiding light. Typically, such a structure is provided with surfaces that adequately reflect the light without substantial diffraction and/or scattering. Such surfaces may include angles of substantially 90 to 180 degrees with respect to each other. Another embodiment is for instance an optical fiber.

Moreover, the term 'display' is used herein for reference to the functional display. In case of a liquid crystal display, as an example, this is the layer stack provided with active matrix or passive matrix addressing. The functional display is subdivided in display areas. An image may be displayed in one or more of the display areas. The term 'display device' is used herein to refer to the complete apparatus, including sensors, light guide members and incoupling members. Suitably, the display device further comprises a controller, driving system and any other electronic circuitry needed for appropriate operation of the display device.

Fig. 1 shows a display device 1 formed as a liquid crystal display device (LCD device) 2. Alternatively the display device is formed as plasma display devices or any other kind of display device emitting light. The display's active area 3 of the display device 1 is divided into a number of groups 4 of display areas 5, wherein each display area 5 comprises a plurality of pixels. The display device 3 of this example comprises eight groups 4 of display areas 5; each group 4 comprises in this example ten display areas 5. Each of the display areas 5 is adapted for emitting light into a viewing angle of the display device to display an image to a viewer in front of the display device 1 .

Fig. 1 further shows a sensor system 6 with a sensor array 7 comprising, e.g. eight groups 8 of sensors which corresponds to the embodiment where the actual sensing is made outside the visual are of the display, and hence the light needs to be guided towards the edge of the display. This embodiment thus corresponds to a waveguide solution and not to the preferred organic photoconductive sensor embodiment, where the light is captured on top of (part of) the display area 5, and the generated electronic signal is guided towards the edge. In addition, in the preferred embodiment which uses organic photoconductive sensors to detect light, the actual sensor is created directly in front of the (part of) the sub area that needs to be sensed, and the consequentially generated electronic signal is guided towards the edge of the display, using semitransparent conductors. Each of said groups 8 comprises, e.g. ten sensors (individual sensors 9 are shown in Figs. 3, 4 and 5) and corresponds to one of the groups 4 of display areas 5. Each of the sensors 9 corresponds to one corresponding display area 5. The sensor system 6 further comprises coupling devices 10 for a display area 5 with the corresponding sensors 9. Each coupling device 10 comprises a light guide member 12 and an incoupling member 13 for coupling the light into the light guide member 12, as shown in Fig. 2. A specific incoupling member 13 shown in Fig. 2 which is cone- shaped, with a tip and a ground plane. It is to be understood that the tip of the incoupling member 13 is facing the display area 5. Light emitted from the display area 5 and arriving at the incoupling member 13, is then refracted at the surface of the incoupling member 13. The incoupling member 13 is formed, in one embodiment, as a laterally prominent incoupling member 14, which is delimited by two laterally coaxially aligned cones 15, 16, said cones 15, 16 having a mutual apex 17 and different apex angles a1 , a2. The diameter d of the cones 15, 16 delimiting the incoupling member 13 can for instance be equal or almost equal to the width of the light guide member 12. Said light was originally emitted (arrow 18) from the display area 5 into the viewing angle of the display device 1 , note that only light emitted in perpendicular direction is depicted, while a display typically emits in a broader opening angle. The direction of this originally emitted light is perpendicular to the alignment of a longitudinal axis 19 of the light guide member 12. All light guide members 12 run parallel in a common plane 20 to the sensor array 7 at one edge 21 of the display device 1 . Said edge 21 and the sensor array 7 are outside the viewing angle of the display device 1 .

Alternatively, use may be made of a diffraction grating as an incoupling member 13. Herein, the grating is provided with a spacing, also known as the distance between he laterally prominent parts. The spacing is in the order of the wavelength of the coupled light, particularly between 500nm and 2μιη. In a further embodiment, a phosphor is used. The size of the phosphor could be smaller than the wavelength of the light to detect. The light guide members 12 alternatively can be connected to one single sensor 9. All individual display areas 5 can be detected by a time sequential detection mode, e.g. by sequentially displaying a patch to be measured on the display areas 5. The light guide members 12 are for instance formed as transparent or almost transparent optical fibres 22 (or microscopic light conductors) absorbing just a small part of the light emitted by the specific display areas 5 of the display device 1 . The optical fibres 22 should be so small that a viewer does not notice them but large enough to carry a measurable amount of light. The light reduction due to the light guide members and the incoupling structures is for instance about 5% for any display area 5. More generally, optical waveguides may be applied instead of optical fibres, as discussed hereinafter.

Most of the display devices 1 are constructed with a front transparent plate such as a glass plate 23 serving as a transparent medium 24 in a front section 25 of the display device 1 . Other display devices 1 can be made rugged with other transparent media 24 in the front section 25. Suitably, the light guide member 12 is formed as a layer onto a transparent substrate such as glass. A material suitable for forming the light guide member 12 is for instance PMMA (polymethylmethacrylate). Another suitable material is for instance commercially available from Rohm&Haas under the tradename Lightlink™, with product numbers XP-5202A Waveguide Clad and XP-6701 A Waveguide Core. Suitably, a waveguide has a thickness in the order of 2-10 micrometer and a width in the order of micrometers to millimeters, or even centimeters. Typically, the waveguide comprises a core layer that is defined between one or more cladding layers. The core layer is for instance sandwiched between a first and a second cladding layer. The core layer is effectively carrying the light to the sensors. The interfaces between the core layer and the cladding layers define surfaces of the waveguide at which reflection takes place so as to guide the light in the desired direction. The incoupling member 13 is suitably defined so as to redirect light into the core layer of the waveguide.

Alternatively, parallel coupling devices 10 formed as fibres 22 with a higher refractive index are buried into the medium 24, especially the front glass plate 23. Above each area 5 the coupling device 10 is constructed on a predefined guide member 12 so light from that area 5 can be transported to the edge 21 of the display device. At the edge 21 the sensor array 7 captures light of each display area 5 on the display device 1 . This array 7 would of course require the same pitch as the fibres 22 in the plane 20 if the fibers run straight to the edge, without being tightened or bent. While fibres are mentioned herein as an example, another light guide member such as a waveguide, could be applied alternatively.

In Fig. 1 the coupling devices 10 are displayed with different lengths. In reality, full length coupling devices 10 may be present. The incoupling member 13 is therein present at the destination area 5 for coupling in the light (originally emitted from the corresponding display area 5 into the viewing angle of the display device 1 ) into the light guide member 12 of the coupling device 10. The light is afterwards coupled from an end section of the light guide member 12 into the corresponding sensor 9 of the sensor array at the edge 21 of the display device 1 . The sensors 9 preferably only measure light coming from the coupling devices 10.

In addition, the difference between a property of light in the coupling device 10 and that in the surrounding front glass plate 23 is measured. This combination of measuring methods leads to the highest accuracy. The property can be intensity or colour for example.

In one method, each coupling device 10 carries light that is representative for light coming out of a pre-determined area 5 of the display device 1 . Setting the display 3 full white or using a white dot jumping from one area to another area 5 gives exact measurements of the light output in each area 5. However, by this method it is not possible to perform continuous measurements without the viewer noticing it. In this case the relevant output light property, e.g. colour or luminance, should be calculated depending on the image information, radiation pattern of a pixel and position of a pixel with respect to the coupling device 1 1 . Image information determines the value of the relevant property of light, e.g. how much light is coming out of a specific area 5 (for example a pixel of the display 3) or its colour.

Consider the example of optical fibers 22 shaped like a beam, i.e. with a rectangular cross-section, in the plane parallel front glass plate 23, for instance a plate 23 made of fused silica. To guide the light through the fibers 22, the light must be travelling in one of the conductive modes. For light coming from outside the fibers 22 or from outside the plate 23, it is difficult to be coupled into one of the conductive modes. To get into a conductive mode a local alteration of the fiber 22 is needed. Such local alteration may be obtained in different manners, but in this case there are important requirements than just getting light inside the fiber 22. For accurate measuring it is important that only light from a specific direction (directed from the corresponding display area 5 into the viewing angle of the display device) enters into the corresponding coupling device 10 (fiber 22). Hence, light from outside the display device 1 ('noisy' light) will not interfere with the measurement.

Additionally, it is important that upon insertion into the light guide member, f.i. fiber or waveguide, the image displayed is hardly, not substantially or not at all disturbed. According to the invention, use is made of an incoupling member 13 for coupling light into the light guiding member. The incoupling member 13 is a structure with limited dimensions applied locally at a location corresponding to a display area. The incoupling member 13 has a surface area that typically much smaller than that of the display area, for instance at most 1 % of the display area, more preferably at most 0.1 % of the display area. Suitably, the incoupling member is designed such that it leads light to a lateral direction.

Additionally, the incoupling member may be designed to be optically transparent in at least a portion of its surface area for at least a portion of light falling upon it. In this manner the portion of the image corresponding to the location of the incoupling member is still transmitted to a viewer. As a result, it will not be visible. It is observed for clarity that such partial transparency of the incoupling member is highly preferred, but not deemed essential. Such minor portion is for instance in an edge region of the display area, or in an area between a first and a second adjacent pixel. This is particularly feasible if the incoupling member is relatively small, e.g. for instance at most 0.1 % of the display area.

In a further embodiment, the incoupling member is provided with a ground plane that is circular, oval or is provided with rounded edges. The ground plane of the incoupling member is typically the portion located at the side of the viewer. Hence, it is most essential for visibility. By using a ground plane without sharp edges or corners, this visibility is reduced and any scattering on such sharp edges are prevented. A perfect separation may be difficult to achieve, but with the sensor system 6 comprising the coupling device 10 shown in Fig. 2 a very good signal-to-noise-ratio (SNR) can be achieved.

In another preferred embodiment a coupling device such as an incoupling member is not required. For example, organic photoconductive sensors photoconductive sensors can be used as the sensors. The organic photoconductive sensors serve as sensors themselves (their resistivity alters depending on the impinging light) and because of that they can be placed directly on top of the location where they should measure. (For instance, a voltage is put over its electrodes, and a impinging-light dependent current consequentially flows through the sensor, which is measured by external electronics) Light collected for a particular display area 5 does not need to be guided towards a sensor 9 at the periphery of the display (i.e. contrary to what is exemplified by Fig. 3). In a preferred embodiment, light is collected by a transparent or semi-transparent sensor 101 placed on each display area 5. The conversion of photons into charge carriers is done at the display area 5 and not at the periphery of the display and therefore the sensor, although transparent, will not be visible but will be within / inside the viewing angle. Just as for the sensor system 6 of Fig. 1 , this embodiment may also have a sensor array 7 comprising, e.g. a plurality of groups, such as eight groups 8 of sensors 9, 101 . Each of said groups 8 comprises a plurality of sensors, e.g. ten sensors 9 and correspond to one of the groups 4 of display areas 5. Each of the sensors 9 corresponds to one corresponding display area 5, as illustrated in figure 8.

Fig. 5 shows a side view of a sensor system 9 according to a second embodiment of the invention. The sensor system of this embodiment comprises transparent sensors 33, which are arranged in a matrix with rows and columns. The sensors can for instance be photoconductive sensors, hybrid structures, composite sensors, etc. The sensor 33 is realized as a stack comprising two groups 34, 35 of parallel bands 36 in two different layers 37, 38 on a substrate 39, preferably the front glass plate 23. An interlayer 40 is placed between the bands 36 of the different groups 35, 36. This interlayer is the photosensitive layer of this embodiment. The bands (columns) of the first group 34 are running perpendicular to the bands (rows) of the second group 35, in a parallel plane. The sensor system 6 divides the display area 1 into different zones by design, which is clear for anyone skilled in the art, each with its own optical sensor 9 connected by transparent electrodes.

The addressing of the sensors may be accomplished by any known array addressing method and/or devices. For example, a multiplexer (not shown) can be used to enable addressing of all sensors. In addition a microcontroller is also present (not shown). The display can be adapted, e.g. by suitable software executed on a processing engine, to send a signal to the microcontroller (e.g. via a serial cable: RS232). This signal determines which sensor's output signal is transferred. For example, a 16 channel analogue multiplexer ADG1606 (of Analog Devices) is used, which allows connection of a maximum of 16 sensors to one drain terminal (using a 4 bit input on 4 selection pins).

The multiplexer is a carefully selected low-noise multiplexer. This is essential, because the signal measured is a low-current analogue signal, therefore very sensitive to noise. The very low (4.5 Ω) on-resistance makes this multiplexer ideal for this application where low distortion is needed. This on-resistance is negligible in comparison to the resistance range of the photoconductor itself (order of magnitude MQ-100GQ). Moreover, the power consumption for this CMOS multiplexer is low.

To control the multiplexer switching, a simple microcontroller was used (Basic Stamp 2) that can be programmed with Basic code: i.e. its input is a selection between 1 and 16; its output goes to the 4 selection pins of the multiplexer. To communicate with the sensor, a layered structure is foreseen. The layered structure begins from the high-level implementation in QAWeb, which can access BarcoMFD, a Barco in-house software program, which can eventually communicate with the firmware of the display, which handles the low-level communication with the sensor. The complete path to access to the functionality of the BarcoMFD is very long and complex. In fact, by communicating with an object from upper levels, the functionality can be accessed quite easily. The communication with the sensor is a two-way communication. For example, the command to "measure" can be sent from the software layer this will eventually be converted in a signal activating the sensor (serial communication to the ADC to ask for a conversion), which puts the desired voltage signal over the sensor's electrodes. The sensor (selected by the multiplexer at that moment in time) will respond with a signal depending on the incoming light, which will eventually result in a signal in the high-level software layer.

In order to reach the eventual high-level software layer, the analogue signal generated by the sensor and selected by the multiplexer will need to be filtered, amplified and digitized. The types of amplifiers used are LT2054 and LT2055: zero drift, low noise amplifiers. Different stages of amplification are used. Stages 1 to 3 are illustrated in Fig. 6a to 6c respectively. In a first stage the current to voltage amplification with factor 2.2x10⁶Ω. In a second stage closed loop amplification is adjustable between about 1 and 140 (using a digital potentiometer). And finally in a third stage low band pass filtering is enabled (first order, with fO at about 50Hz (cfr RC constant of 22ms)).

Digitization can be by an analog to digital, converter (ADC) such as an LTC 2420 - a 20bit ADC which allows to differentiate more than 10⁶ levels between a minimum and maximum value. For a typical maximum of 1000Cd/m² (white display, backlight driven at high current), it is possible to discriminate 0.001 Cd/m² if no noise is present. In addition the current timing in the circuit is mainly determined by setting of a ΔΣ-ADC such as LTC2420. Firstly, the most important time is the conversion time from analogue to digital (about 160ms, internal clock is used with 50Hz signal rejection). Secondly, the output time of the 24 clock cycles needs to read the 20bit digital raw value out of the serial register of LTC2420 which is of secondary importance (e.g. over a serial 3-wire interface).The choice of the ADC (and its setting) corresponds to the target of stable high resolution light signals (20bit digital value, averaged over a time of 160ms, using 50Hz filtering).

Additionally Fig. 7 illustrates the overview of data path from the sensor to the ADC. The ADC output can be provided to a processor, e.g. in a separate controller or in the display. Fig. 8 shows a schematic view of the sensor system 9 according to an alternative embodiment of the invention. The sensor system of this embodiment comprises transparent sensors 33, which are arranged in a matrix with rows and columns. Transparent sensors that are not connected to a sensor, a power supply or ground, the floating transparent sensors 41 , are positioned in a small distance between the regions that do conduct a specific region, for instance transparent sensors 33 or the sensor system 9. Suitable materials for the transparent electrodes are for instance ITO (Indium Tin Oxide) and poly-3,4- ethylenedioxythiophene polystyrene acid (known in the art are PEDOT-PSS). This sensor array 7 can be attached to the front glass or laminated on the front glass plate 23 of the display device 2, for instance a LCD.

The difference of using a structure comprising ITO or for instance a structure proposed in the article of J.H. Co et al in Applied Physics Letters 93 is the use of ITO instead of gold electrodes. The work function of the electrode material influences the efficiency of the sensor. In the bilayer photoconductor created in the previously mentioned article, a material with a higher work function is most likely more efficient. Therefore, Au is used which has a work function of around 5.1 eV, while ITO has a work function of typically 4.3-4.7 eV, this would result in a worse performance.. The art seems to teach away from ITO (at least when one expects a good enough sensor). The article cited above uses gold as electrode, US 6348290 suggests the use of a number of metals including Indium or an alloy of Indium (see also column 7 In 25-35, of US'290). Tin Oxide is not named, furthermore US 6348290 suggests using an alloy because of its superiority in e.g. electrical properties. However, when ITO is used in stead of gold, the inventors did not expect that the structure would work so well as to be usable for the monitoring of luminance in a high-end display. Also, the inventors' goal was not to create a transparent sensor, since gold electrodes are used, which are highly light absorbing.

The interlayer 40 is preferably an organic photoconductor, and may be a monolayer, a bilayer, or a multiple layer structure. Most suitably, the interlayer 40 comprises a exciton generation layer (EGL) and a charge transport layer (CTL). The charge transport layer (CTL) is in contact with a first and a second transparent electrode, between which electrodes a voltage difference may be applied. The thickness of the CTL can be is for instance in the range of 25 to 100 nm, f.i. 50 nm. The EGL layer may have a thickness in the order of 5 to 50 nm, for instance 20nm. The material for the EGL is a suitably a material known for use as an optically absorptive material in solar cells. It is for instance a perylene derivative. One specific example is 3,4,9,10-perylenetetracarboxylic bisbenzimidazole (PTCBI). The material for the CTL is typically an p-type organic semiconductor material. Various examples are known in the art of organic transistors and hole transport materials for use in organic light emitting diodes. Examples include pentacene, poly-3-hexylthiophene (P3HT), 2- methoxy, 5-(2'-ethyl-hexyloxy)-1 ,4-phenylene vinylene (MEH-PPV), A/,A/'-bis(3- methylphenyl)-A/,A/'-diphenyl-1 ,1 '-biphenyl-4,4'-diamine (TPD). Mixtures of small molecules and polymeric semiconductors in different blends could be used alternatively. The materials for the CTL and the EGL are preferably chosen such that the energy levels of the orbitals (HOMO, LUMO) are appropriately matched, so that excitons dissociate at the interface of both layers. A charge storage layer (CSL) may be present between the CTL and the EGL in one embodiment. Various materials may be used as charge storage layer, for instance based on low molecular organic material and a binder. Such materials are for instance known from US6617604, the contents of which are included herein by reference. In accordance with the invention, use is made of an at least partially transparent electrode materials. This is for instance ITO. Alternatively, a transparent conductor such as ITO or PEDOT:PSS may be combined with a metal layer sufficiently thin to be at least partially transparent. Suitable metals are for instance Au, Mo, Cr. Suitable thickness of such thin metal layer is particularly in the order of nanometers, for instance of less than 2 nm thickness). When ITO is used instead of gold, the inventors did not expect that the structure would work so well so as to be usable for the monitoring of luminance in a high-end display.

Instead of using a bilayer structure, a monolayer structure can also be used. This configuration is also tested in the referenced paper, with only a CTL. Again, in the paper, the electrodes are Au, whereas we made an embodiment with ITO electrodes, such that a (semi) transparent sensor can be created. Also, we created embodiments with other organic layers, such as PTCDA, with ITO electrodes. The organic photoconductor may be a patterned layer or may be a single sheet covering the entire display. In the latter case, each of the display area 5 will have its own set of electrodes but they will share a common organic photosensitive layer (simple or multiple). The added advantage of a single sheet covering the entire display is that the possible color specific absorption by the organic layer will be uniform across the display. In the case where several islands of organic material are separated on the display, non uniformity in luminance and or color is more difficult to compensate.

These organic photoconductive sensors are the preferred embodiment of the sensor 2 in this patent. Unless indicated otherwise, the detailed descriptions of this patent are elaborated with this specific embodiment in mind.

These organic photoconductive sensors serve as sensors and because of that, they can be placed directly on top of the location where they should measure. Consequentially, light collected for a particular display area does not need to be guided towards a sensor at the periphery of the display. In the most preferred embodiment, light is collected by a transparent or semi-transparent second sensor placed on each display area. The conversion of photons into charge carriers is done at the display area and not at the periphery of the display and therefore the sensor will be within / inside the viewing angle

Alternatively, second sensors comprising composite materials could be constructed. With composite materials nano/micro particles are proposed, either organic or inorganic dissolved in the organic layers, or an organic layer consisting of a combination of different organic materials (dopants). Since the organic photosensitive particles often exhibit a strongly wavelength sensitive absorption coefficient, this configuration can result in a less colored transmission spectrum, or can be used to improve the detection over the whole visible spectrum, or can improve the detection of a specific wavelength region. However, a disadvantage could be that the sensor only provides one output current per measurement for the entire spectrum for all these embodiments. In other words, the X, Y and Z tristimulus values of a given spectrum emitted by the display have to be measured sequentially, the latter can be enabled by measuring and calibrating the X, Y and Z components of light emitted with a certain spectrum as described earlier, in case the sensor is sensitive to the entire visible spectrum. This could be avoided by using three independent photoconductors that measure red, green and blue independently, which can each be calibrated to measure to measure X, Y and Z for a given spectrum. They could be conceived similarly to the previous descriptions, and stacked on top of each other, or adjacent to each other on the substrate, to obtain an online color measurement.

In addition, instead of using organic layers to generate charges, hybrid structures using a mix of organic and inorganic materials can be used. A bilayer device that uses a quantum-dot exciton generation layer and an organic charge transport layer can be used. For instance colloidal Cadmium Selende quantum dots and an organic charge transport layer comprising of Spiro-TPD. Furthermore an organic photoconductor can be a mono layer, a bi-layer or in general a multiple (>2) layer structure. An example of an organic bilayer photoconductor is known from Applied Physics Letters 93 "Lateral organic bilayer heterojunction photoconductors" by John C. Ho, Alexi Arango and Vladimir Bulovic. However, the bilayer disclosed by J.C. Ho uses gold electrodes, which are non-transparent, and therefore this sensor is not usable as a transparent sensor. The bilayer comprises an EGL (PTCBI) or Exciton Generation Layer and a CTL (TPD) or Charge Transport Layer. In another preferred embodiment, an alternative sensor, like the organic sensor described above, can be used. First of all, the sensor can be panchromatic, meaning that it is sensitive to the entire visual spectrum. This implies that the sensor can be sensitive to the red, green and blue spectra emitted by the display. The first downside of such a sensor is the lack of colour filters which are typically used for measuring the CIE XYZ components. The sensor can also be used to measure the brightness of light with a certain spectrum after calibrating it with the first sensor that includes the required filters. This calibration step is crucial, as the measured brightness will be relative to the source due to the lack of a V (λ) filter. The absorption spectrum of the exciton generation layer (organic material) is linked directly to the spectral sensitivity of the sensor. Therefore, the luminance versus the digital driving level (DDL) curve can be calibrated for all three primaries, and will differ for displays having different spectra. After the calibration, the luminance of the different primaries can be measured using a matrix of sensors in order to obtain the luminance of the color components.

The sensor has a design fundamentally based on a compromise between transparency and efficiency: light needs to be sensed, which implies that photons are to be absorbed, while we still desire that the sensor remains (almost) transparent. This effect adds up to the lack of a V(A) filter, such that (minor) errors can occur when the emitted spectrum is non-constant over the active area of the display.

However, the major advantage of the sensor is its ability for measuring over the entire active area of the display, which allows obtaining a global measurement result, instead of a local measurement near the border of the screen.

However, when using transparent conductors such as ITO conductors the transition area, on the display area, from regions where transparent conductors such as ITO is present to where the transparent conductors such as ITO is absent results in visible non-uniformities. In order to avoid this, in a preferred embodiment the regions with no transparent conductive material such as ITO are filled up with the same conductive material, e.g. ITO as well. These regions are not connected to a sensor, a power supply or ground, i.e. they are electrically "floating". Using floating transparent conductive regions such as ITO regions, with no specific signal, that retain a small distance between the regions that do conduct a specific region, advantageously results in a better image quality as the eye does not detect the difference between regions with and without the conductive material.

In addition, the distance between the first and second pattern of conductive materials is limited by two effects: the first is the limitation as a result of the potential occurrence of crosstalk between the floating and signal carrying parts of the transparent conductors such as ITO. The second is the technical limitation of the transparent conductor lithography such as ITO lithography. H. J. Kin et al, Fabrication of Alignment Layer Coated Indium-tin-oxide Prepared by Ultraviolet Nano-imprinting Lithography, Molecular Crystals and Liquid Crystals 530, p. 7-12 (2010) discloses how close the first and second pattern can be placed with respect to each other. One should note that if the resolution is insufficient, the floating parts could become connected to the signal carrying parts which may result in the failure of the sensor.

In one further implementation, the electrodes are provided with fingered shaped extensions. The extensions of the first and second electrode preferably form an interdigitated pattern. The number of fingers may be anything between 2 and 5000, more preferably between 250 and 2500, suitably between 500 and 1000. The surface area of a single transparent sensor may be in the order of square micrometers but is preferable in the order of square millimeters, for instance between 1 and 1000 square millimeters. One suitable shape is for instance a 1500 x 10 micrometers size, but a size of for instance 4 x 6 micrometers is not excluded either.

In the case of sensor based on organic photoconductive sensors with finger pattern electrodes, the semi-transparency of the sensor (i.e. not 100% transparency) can be the result of the reflection and absorption of light on its building blocks. In Figure 10, a cross-sectional view is given of a sensor, according to embodiments of the invention, which contains an ITO coated glass substrate. The active area of the sensor contains finger-shaped ITO electrodes that are mutually interdigitated. On top of the finger-patterned electrodes, organic layers are positioned, which are photosensitive layers of the photoconductive sensors used. If light is incident from the side of the glass substrate it is subjected to partial reflection and transmission at the air/glass interface due to the difference in the refractive indices (n_ai_r = 1 , n_gi_aSs = 1 .52). Afterwards it reaches the interface of glass and the thin film layer stack (which is preferably made of ITO and organic materials as described earlier) where it also propagates through a partial reflection, absorption and transmission. The reflection is due to the thin-film stack made of ITO and organic materials in which the light can experience interference effects or additionally diffraction effects at the position of the finger patterns. The organic materials on top of the ITO are the hole transporting layer (HTL) made for instance of TMPB material and the exciton generation layer (EGL) made for instance of PTCBi. The absorption is mainly caused by the ITO and EGL made of PTCBi material. These effects will be present also if we use multilayer stacks as mentioned in a previous embodiment. The next effect is the partial reflection and transmission on both interfaces of the encapsulation glass. Unfortunately, as a result of all of these mentioned losses the transmission of the sensor will be reduced. The absorption in the exciton generation layer is necessary for light detection and can only be reduced by reducing the exciton generation layer thickness which is not preferred because this leads to a reduced photo-current. Therefore the improvement in transmission needs to be done by minimizing losses at the other layers and interfaces.

In other embodiments, an antireflection coating (ARC) is applied on the external surfaces of the substrate glass and/or encapsulation glass to further improve the transmission and reducing the coloring. In addition, the transmission over all wavelength regions and also reduce the coloring of the sensor can be improved. The ARC can reduce the reflection (i.e. improving the transmission) over all visible wavelengths, but it can suitably be designed to reduce reflection relatively more in the green wavelength region and less in the blue and red wavelength region. Such an ARC can for instance be obtained from LASEROPTIK (item number B-00003) Furthermore LASEROPTIC also offers the possibility to make a customized ARC which can be tuned in such a way that coloring of another wavelength which is visible is reduced. In some embodiments when using conventional encapsulation techniques, the sensor comprises a finger width of 80μιη and gap width of 20μιη, a ITO thickness of 65 nm, a hole transporting layer thickness of 40nm and EGL thickness of 10nm, was visible in reflection which was due to the fact that the finger pattern works as a diffraction grating. When a diffraction grating is illuminated by white light, in reflection one can see dispersion of light which comes from the fact that different wavelengths are diffracted at different angles. In the case of our finger pattern, the diffractive effects are defined by the period of the grating (which is a combination of the finger width and gap width), in addition the depth of the grating and the contrast in the refractive indices of the organic materials, the ITO and the inert gas. A higher contrast of refractive indices leads to stronger diffraction effects. The depth of the grating will depend on the thickness of the ITO and the organic materials.

In other implementations, spacers (not shown) can be used which enable separation between the encapsulation glass and the organics. Furthermore it can also contain getters which result in absorption of any leftover humidity in the inert gas atmosphere and therefore for minimizing the degradation of the organic materials. In addition, by altering two of the loss mechanisms, namely absorption by the ITO (defined by its layer thickness and complex refractive index as function of wavelength) and reflection at the inert gas/encapsulation glass interface and the organic stack/inert gas interface, the transmission can be improved. In the first case one can apply ITO coated glass substrates with a higher transmission i.e. lower absorption in the ITO, by for instance changing the layer thickness of the ITO or choosing an ITO with a lower absorption coefficient. The effect of the ITO thickness on the percentage light transmitted is shown in the figure 12. Smaller ITO thickness of about 22 or 25nm significantly improve the percentage of transmitted light. For the described second loss mechanism, instead of encapsulating the sensor in an inert gas atmosphere and placing the glue only for instance at the edges of the encapsulation glass, an alternative embodiment can be used where the encapsulation glue is applied over the whole area of the sensor between the glass substrate and the encapsulation glass. The latter is enabled by using a drop of glue between the two glass plates, applying mechanical pressure on them, pushing out the inert gas by capillary forces and then curing the glue by UV exposure. In this way there is no gas left between the encapsulation glass and the organic materials. With this technique several problems of the prior art can be overcome, for instance one those not need the implementation of getters and spacers, and the transmission is improved whereas the visibility in reflection is reduced. The performance of the sensor with the glue is as good as the sensor with standard encapsulation. The latter is also illustrated in Fig. 13, which illustrates the transmission of a sensor with standard encapsulation (and ITO 65nm, HTL 40nm, EGL 10nm) and a sensor with glue encapsulation (and ITO =45nm, HTL 40nm, EGL 10nm) with improved ITO. When glue is used the reflection is reduced at the gas/encapsulation glass interface and the organic/gas interface because the refractive index of the glue is n_giue = 1 .54 i.e. it matches the refractive index of glass and is very close to the refractive index of the organic materials of 1 .8. Preferably, Norland Optical Adhesive 68 glue can be used, which enable very high transmission percentages and does not introduce any coloring. Fig. 14 illustrates how a floating conductive material can be used at the end of the fingers 140 in the form of nails (in this case made of ITO). There is no voltage applied on the nails and they are separated from the electrodes. The nails help for the edges of the finger pattern to become invisible for an observer. Fig. 15 schematically illustrates a single layer of electrodes 136. The electrodes 136 are preferably made of a transparent conducting material like e.g. ITO (Indium Tin Oxide). The part of the electrode corresponding to the photosensitive area 90, according to some embodiments, has a finger-shaped pattern,, as presented in figure 14, which is connected to the ITO track 81 that conducts the electric signal towards the edge of the active area. The width of the ITO track 151 , and the distance between two consecutive ITO tracks 150 is also shown. On the right hand side of figure 14 a floating electrical conductor system 141 , is applied in all the regions, where no ITO was present (as illustrated in on the left hand side of figure 14), outside the finger patterns of the electrode 136 according to an embodiment of the invention, to improve visibility. Fig. 16a schematically illustrates an electrode comprising a pattern whereby the pattern comprises semi-random fingers according to embodiments of the invention. In Fig. 16a the semi-random finger pattern is constructed by semirandomly choosing several points on the two edges of the finger where the fingers should go through. The different points are then connected using a cubic spline interpolation. The adjacent finger is limited in the sense that the gap in between the fingers should remain constant to ensure the device's properties remain unaltered. Also, the points can be chosen in a semi random way in the sense that they are limited to a specific area to avoid too high frequencies in the fingers. Fig. 16a illustrates a simulation of such a pattern comprising fingers. Each finger is determined by a set of control points, chosen at random through which for instance a cubic interpolation is run, resulting in a curve shape. To make sure the finger goes in the correct general direction, the control points are all put in an individual limited rectangle. This rectangle limits its position in horizontal and vertical direction. Rectangles for one of the fingers are positioned sequentially next to one another, and the curve is interpolated through the sequence of defined points. The oscillation of the curve can be increased by adding more control points in the interpolation, which boils down to altering the horizontal and vertical dimensions of the rectangle in which the control points are defined. If the fingers are oriented horizontally, like in Fig 16a, a rectangle with a smaller horizontal dimension and a larger vertical dimension allows bigger oscillations. In this example, the gap between the fingers is 20 microns wide and the sensor corresponds approximately to a 1 cm x 1 cm square. The width of the fingers is at least 2.5 times the gap between the fingers, and on average the fingers width is 1 1 .5 times the gap. The fingers are depicted in black, while the gaps are depicted in white. By choosing the control points at random, no specific, repetitive pattern should be visible.

Fig. 16b illustrates an alternative embodiment were the finger pattern is shaped like Euclidean spirals. Other patterns which result in reduced artifacts and a higher transmission of light, hereby said patterns are simulated as described above, can also be applied, for instance a sine wave pattern.

In connection with said further implementation, it is most suitable to build up the sensor on a substrate with said electrodes. The interlayer 40 therein overlies or underlies said electrodes. In other words, while Fig. 5 shows a design comprising a first and a second electrode layer (columns and bands), a single electrode layer may be sufficient. The latter is shown schematically in Fig. 9 where a network of sensors 9 with a single layer of electrodes 36 is illustrated. Electrodes 36 are made of a transparent conducting material like e.g. ITO (Indium Tin Oxide) and are covered by organic layer(s) 101 . In addition, the organic photoconductor needs not be limited laterally. The organic layer may be a single sheet covering the entire display (not shown). Each of the display areas 5 will have its own set of electrodes 36 but they will share a common organic photosensitive layer (simple or multiple). The added advantage of a single sheet covering the entire display is that the possible color specific absorption by the organic layer will be uniform across the display. In the case where several islands of organic material are separated on the display, non-uniformity in luminance and or color is more difficult to compensate.

In a further implementation, using the glue as described before can also significantly reduce the visibility of a finger pattern in reflection. The main reason for this is that the glue fills in the depth of the diffraction grating. This leads to a reduced contrast in the refractive indices and therefore smaller diffraction effects. Additionally when using a finger pattern with a suitable gap, and an increased finger width to finger gap ratio helps to reduce the visibility of the sensor. When applying the glue implementation a very high finger/gap ratio is not used, such that the finger pattern is made invisible in reflection . However, the larger finger/gap ratio is needed in order the finger pattern not to be noticeable in transmission when reference to the parts containing floating ITO and organic materials.

A suitable ratio between the finger width and the gap between the fingers in transmission can be estimated using simulations. More specifically, when maintaining a specific gap, optimized for the sensor's performance, the finger width has been increased, to reduce the percentage of the area with only ITO patterns. The metrics used for the simulations and to evaluate if there is a visible difference between the finger pattern region and the neighboring floating ITO region, are the number of JNDs between them, to evaluate if there is a difference in brightness, and the ΔΕ2000 metric to evaluate if there is a perceived difference in chromaticity. Note that the average value of the tristimulus X, Y and Z values calculated and used in the region of the finger pattern; in other words, we assume that the gap between the fingers is suitably chosen such that only an average tristimulus X,Y and Z value is perceived.. Of course, this also depends on the sensor design: the type of ITO and the thicknesses of the PTCBI Exciton Generation Layer and TMPB Hole Transporting Layer, and the encapsulation method.

The gaps between the fingers are preferably chosen such that the human eye is not able to distinguish any high-spatial frequency pattern where the fingers are located. To select appropriate values, a simulation can be performed, which is later confirmed by human observer tests. This is an optical simulation model built in a ray tracing optical simulation software program. The simulation includes a light source, a pattern according to embodiments of the present invention and an optical model of the human eye. This human eye model has the appropriate optical imperfections, introduced amongst others by the limited cone density on the retina, the cornea, and the lens in our eye. The human observer tests when using a bar finger shaped pattern comprising wide bars (e.g 4mm wide) with varying distances in between, to make sure one cannot distinguish the gaps in between the fingers. The distances were varied e.g. from 500μιτι down to 5 μιη. The minimal distance depends on the type of material used for the pattern, the thickness of the exciton generation layer and the methodology used to deposit the latter. A typical range for width to gap ratio is in the range 30 to 2 to 10 to 2 e.g. 20 to 2.

Preferably the latter selection is done in combination with the previous point, as they both require altering the finger pattern dimensions. Of course, there are limitations on the gap size because it can impact the performance. This resulted in a range of finger pattern dimensions that will be physically created and tested. Note that this depends on the type of ITO and the thickness of the PTCBI Exciton generation layer.

Keeping this in mind, typical gaps are in the range of 10-20 μιη. In a further implementation, when a finger pattern is used with a gap of 15 μιη and finger widths of 80 μιη for the embodiment comprising glue. For stability reasons one can apply a thicker HTL of about 80-1 OOnm which advantageously can lead to improved stability when the sensor is driven with an AC block wave voltage signal.

In the specific embodiment of the glue solution, very good results have been obtained using ITO with a thickness of 45nm by using a gap of 15 μιη and a finger width 151 of 80 μιη. the finger width/gap ratio is increased which results in a less visible finger pattern on the overall substrate when compared to the parts that don't have a pattern and only comprise a uniform ITO and organic layers. The range of suitable finger width to gap ratios depends on the exact design of the sensor, which includes the type of ITO used, the thickness of the organic layers, the type of encapsulation methodology used, the finger width to gap ratio and so on.

A specific embodiment of the conventional encapsulation methodology that does not contain the index-matching realized by the glue solution, a gap of 15μιη between each finger, and a finger width of 173μιη. However, as discussed earlier the small gaps of 15 μιη can be difficult to physically create using standard photolithography processes on large area ITO coated substrates. To overcome this problem one can use laser ablation technology which can make gaps as low as 10 μιη on large area substrates. This technology is available for example from the company Laserod. It however is observed that a sensor of a first and a second electrode with the interlayer may, on a higher level, be arranged in a matrix for appropriate addressing and read out, as known to the skilled person. Most suitably, the interlayer is deposited after provision of the electrodes. The substrate may be provided with a planarization layer.

Optionally, a transistor may be provided at the output of the photoconductive sensor, particularly for amplification of the signal for transmission over the conductors to a controller. Most suitably, use is made of an organic transistor. Electrodes may be defined in the same electrode material as those of the photodetector. Alternatively, particularly with a suitable, hidden location of the transistor, use may be made of gold electrodes. A organic field effect transistor device structure with a bottom gate structure, a pentacene semiconductor and parylene dielectric is suitably applied. Vias cut into the parylene allow the photoconductor access to the interdigitated electrode structure.

The interlayer 40 may be patterned to be limited to one display area 5, a group of display areas 5, or alternatively certain pixels within the display area 5. Alternatively, the interlayer is substantially unpatterned. Any color specific absorption by the transparent sensor will then be uniform across the display.

Alternatively, the organic layer(s), as illustrated in figure 8, may comprise nanoparticles or microparticles, either organic or inorganic and dissolved or dispersed in an organic layer. A further alternative are organic layer(s) 101 comprising a combination of different organic materials. As the organic photosensitive particles often exhibit a strongly wavelength dependent sensitive absorption coefficient, such a configuration can result in a less colored transmission spectrum. It may further be used to improve detection over the whole visible spectrum, or to improve the detection of a specific wavelength range.

Suitably, more than one transparent sensor may be present in a display area 5, as illustrated in figure 8. Additional sensors may be used for improvement of the measurement, but also to provide different colour-specific measurements. Additionally, by covering substantially the full front surface with transparent sensors, any reduction in intensity of the emitted light due to absorption and/or reflection in the at least partially transparent sensor will be less visible or even invisible, because position-dependant variations over the active area can be avoided this way.

Returning to figure 5, we note that by constructing the sensor 9 as shown in Fig. 5, the sensor surface of the transparent sensor 30R is automatically divided in different zones. A specific zone corresponds to a specific display area 5, preferably a zone consisting of a plurality of pixels, and can be addressed by placing the electric field across its columns and rows. The current that flows in the circuit at that given time is representative for the photonic current going through that zone.

This sensor system 6 cannot distinguish the direction of the light. Therefore the photocurrent going through the transparent sensor 30 can be either a pixel of the display area 5 or external (ambient) light. Therefore reference measurements with an inactive backlight device are suitably performed. Suitably, the transparent sensor is present in a front section between the front glass and the display. The front glass provides protection from external humidity (e.g. water spilled on front glass, the use of cleaning materials, etc.). Also, it provides protection form potential external damaging of the sensor. In order to minimize negative impact of any humidity present in said cavity between the front glass and the display, encapsulation of the sensor is preferred.

Fig. 3 shows another embodiment of the invention relating to a sensor system 6 for rear detection. Fig. 3 is a simplified representation of an optical stack of the display 3 comprising (from left to right) a diffuser, several collimator foils, a dual brightness enhancement film (DBEF) and a LED display element in the front section 25 of a display device 1 . At the backside 26 of the display 3 (left side) the sensor 9 of the sensor system 6 is added to measure all the light in the display area 5. A backlight device 27 is located between the sensor 9 and the stack of the display 3. The sensor 9 is counter sunken in a housing element (not shown) so only light close to the normal, perpendicular to the front surface 28, is detected.

The sensor system 6 shown in Fig. 3 can be used for performing an advantageous method for detecting a property of the light, e.g. the intensity or colour of the light emitted from at least one display area 5 of a liquid crystal display device 2 (LCD device) into the viewing angle of said display device 2, wherein said LCD device 2 comprises a backlight device 27 for lighting the display 3 formed as a liquid crystal display member of the display device 2, the method comprising the steps:

- Switching off the backlight device 27,

- Detecting the light emitted by at least one chosen display area 5 and

- Switching on the backlight device 27. There are three possible ways to do the detection of the light emitted by the at least one chosen display area 5: A very ambitious method is the use of the time of flight principle and measuring only the photons that react at the interface polarizer-air (not shown) at the front section 25 of the display device 1 . A second method uses an optical device 10 formed as a mirror 28 in front of the display 3 to achieve the same result but with higher luminance to measure. A third method consists of estimating the escaped energy out of the backlight cavity of the backlight device 27. Fig. 4 shows a horizontal sectional view of a display device 1 with a sensor system 6 according to a fourth embodiment of the invention. The present embodiment is a scanning sensor system. The sensor system 6 is realized as a solid state scanning sensor system localized the front section 25 of the display device 1 . The display device 1 is in this example a liquid crystalline display, but that is not essential. This embodiment provides effectively an incoupling member. The substrate or structures created therein (waveguide, fibers) may be used as light guide members. In accordance with this embodiment of the invention, the solid state scanning sensor system is a switchable mirror. Therewith, light may be redirected into a direction towards a sensor. The solid state scanning system in this manner integrates both the incoupling member and the light guide member. In one suitable embodiment, the solid state scanning sensor system is based on a perovskite crystalline or polycrystalline material, and particularly the electro- optical materials. Typical examples of such materials include lead zirconate titanate (PZT), lanthane doped lead zirconate titanate (PLZT), lead titanate (PT), bariumtitanate (BaTiO3), bariumstrontiumtitantate (BaSrTiO3). Such materials may be further doped with rare earth materials and may be provided by chemical vapour deposition, by sol-gel technology and as particles to be sintered. Many variations hereof are known from the fields of capacitors, actuators and microactuators (MEMS).

In one example, use was made of PLZT. An additional layer 29 can be added to the front glass plate 23 and may be an optical device 10 of the sensor system 6. This layer is a conductive transparent layer such as a tin oxide, e.g. preferably an ITO layer 29 (ITO: Indium Tin Oxide) that is divided in line electrodes by at least one transparent isolating layer 30. The isolating layer 30 is only a few microns (μιη) thick and placed under an angle β. The isolating layer 30 is any suitable transparent insulating layer of which a PLZT layer (PLZT: lanthanum- doped lead zirconate titanate) is one example. The insulating layer preferably has a similar refractive index to that of the conductive layer or at least an area of the conductive layer surrounding the insulating layer, e.g. 5% or less difference in refractive index. However, when using ITO and PLZT, this difference can be larger as a PLZT layer an have a refractive index of 2.48, whereas ITO has a refractive index of 1 .7. The isolating layer 31 is an electro- optical switchable mirror 31 for deflecting at least one part of the light emitted from the display area 5 to the corresponding sensor 9 and is driven by a voltage. The insulating layer can be an assembly of at least one ITO sub-layer and at least one glass or IPMRA sub-layer.

In one further example, a four layered structure was manufactured. Starting from a substrate, f.i. a corning glass substrate, a first transparent electrode layer was provided. This was for instance ITO in a thickness of 30 nm. Thereon, a PZT layer was grown, in this example by CVD technology. The layer thickness was approximately 1 micrometer. The deposition of the PZT layer may be optimized with nucleation layers as well as the deposition of several subsequent layers, that do not need to have the same composition. A further electrode layer was provided on top of the PZT layer, for instance in a thickness of 100 nm. In one suitable example, this electrode layer was patterned in fingered shapes. More than one electrode may be defined in this electrode layer. Subsequently, a polymer was deposited. The polymer was added to mask the ITO finger pattern. When to this structure a voltage is applied between the bottom electrode and the fingers on top of the PZT the refractive index of the PZT under each of the fingers will change. This change in refractive index will result in the appearance of a diffraction pattern. The finger pattern of the top electrode is preferably chosen so that a diffraction pattern with the same period would diffract light into direction that would undergo total internal reflection at the next interface of the glass with air. The light is thereafter guided into the glass, which directs the light to the sensors positioned at the edge. Therewith, all it is achieved that diffraction orders higher than zero are coupled into the glass and remain in the glass. Optionally, specific light guiding structures, e.g. waveguides may be applied in or directly on the substrate.

While it will be appreciated that the use of ITO is here highly advantageous, it is observed that this embodiment of the invention is not limited to the use of ITO electrodes. Other partially transparent materials may be used as well. Furthermore, it is not excluded that an alternative electrode pattern is designed with which the PZT layer may be switched so as to enable diffraction into the substrate or another light guide member.

The solid state scanning sensor system has no moving parts and is advantageous when it comes to durability. Another benefit is that the solid state scanning sensor system can be made quite thin and doesn't create dust when functioning.

An alternative solution can be the use a reflecting surface or mirror 28 that scans (passes over) the display 3, thereby reflecting light in the direction of the sensor array 7. Other optical devices may be used that are able to deflect, reflect, bend, scatter, or diffract the light towards the sensor or sensors.

The sensor array 7 can be a photodiode array 32 without or with filters to measure intensity or colour of the light. Capturing and optionally storing measured light in function of the mirror position results in accurate light property map, e.g. colour or luminance map of the output emitted by the display 3. A comparable result can be achieved by passing the detector array 9 itself over the different display areas 5.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1 . A display device comprising a transparent substrate and a layer of partially transparent electrical conductors on the substrate having a first pattern, some of the partially transparent electrical conductors being connected to electronic devices on the substrate further comprising means for reducing the visibility of electrical conductors, said means comprising any of the following:

partially transparent electrical conductors in a second pattern which are at a floating potential (not connected to ground, an electronic or a power supply) the second pattern being interposed with the first pattern,

an encapsulating material of the partially transparent electrical conductors in the first or second pattern,

an antireflection coating

electrical conductors in the first or second pattern having a width and separated by a gap, the width to a ratio being in the range 30 to 2 to 10 to 2.

2. A display device comprising at least one display area provided with a plurality of pixels, with for each display area an at least partially transparent sensor for detecting a property of light emitted from the said display area into a viewing angle of the display device, which sensor is located in a front section of said display device in front of said display area and

a controller for applying a specific signal to the at least partially transparent sensor to obtain an optimized measurement independent of any previous measurement, before a measurement is made.

3. A display device comprising at least one display area provided with a plurality of pixels, with for each display area an at least partially transparent sensor for detecting a property of light emitted from the said display area into a viewing angle of the display device, which sensor is located in a front section of said display device in front of said display area and means for displaying a test pattern on the display area, and means for detecting the property of light by the sensor and determining a positional relationship between the sensor and the display area.

4. A display device comprising at least one display area provided with a plurality of pixels, with for each display area an at least partially transparent sensor for detecting a property of light emitted from the said display area into a viewing angle of the display device, which sensor is located in a front section of said display device in front of said display area, wherein at least partially transparent sensor comprises partially transparent electrical conductors in the form of fingers having a width and whereby in between the fingers a gap is present, whereby said gap is such that the viewing angle subtended by the gap is smaller than the resolving power of the human eye within the viewing angle of the display device at a viewing distance of 500mm.

5. The display according to claim 4, whereby the ratio between the finger width and finger gap is in the range 30 to 2 to 10 to 2.

6. The display according to claim 4, whereby the electrical conductor in the form of fingers further comprises a floating electrical conductor adjacent at an end of a finger whereby said floating electrical conductor is separated from all electrically active parts of the electrical conductor and no signal applied.

7. The display according to any of claims 4 to 6, whereby the fingers of the partially transparent electrical conductor are curved.

8. A display device comprising at least one display area provided with a plurality of pixels, with for each display area an at least partially transparent sensor for detecting a property of light emitted from the said display area into a viewing angle of the display device, which sensor is located in a front section of said display device in front of said display area, wherein at least partially transparent sensor comprises partially transparent electrical conductors, whereby the partially transparent electrical conducts have a semi-random pattern.

9. The device of claim 8, whereby said semi-random pattern is created by choosing several points on two edges of the electrical conductors wherein the pattern goes through said several points and by connecting said several points by using an interpolation and whereby the resulting gap between the electrical conductors is constant.

10. Display device according to any previous claim 1 , and having at least one display area provided with a plurality of pixels, whereby for each display area the electronic devices comprise an at least partially transparent sensor for detecting a property of light emitted from the said display area into a viewing angle of the display device, which sensor is located in a front section of said display device in front of said display area.

1 1 . The display device as claimed in any of Claims 1 to 10, comprising first patterns of partially transparent electrical conductors are for conducting a measurement signal from said sensor within said viewing angle for transmission to a controller.

12. The display device as claimed in any of Claims 2 to 1 1 , wherein the sensor is transparent.

13. The display device as claimed in any of claims 2 to 12, wherein the sensor comprises an organic photoconductor.

14. The display device as claimed in Claim 13, wherein the organic photoconductor is a bilayer structure with an exciton generation layer and a charge transport layer, said charge transport layer being in contact with a first and a second electrode.

15. The display device as claimed in any of the claims 2 to 14, wherein the sensor comprises at least partially transparent electrodes, which have the form of fingers and of which the fingers of the partially transparent electrical conductor are curved.

16. The display device as claimed in claim 15, wherein the at least partially transparent electrodes comprise an electrically conductive oxide.

17. The display device according to any previous claims further comprising an at least partially transparent optical coupling device (10) located in a front section of said display device and comprising a light guide member (12) for guiding at least one part of the light emitted from the said display area (5) to the corresponding sensor (9),

wherein said coupling device (10) further comprises an incoupling member (13) for coupling the light into the light guide member (12).

18. The display device as claimed in claim 17, wherein the light guide member is running in a plane (20) which is parallel to a front surface (21 ) of the display device (1 ) and wherein the incoupling member (13) is an incoupling member (13) for laterally coupling the light into the light guide member (12) of the coupling device (1 1 ).

19. The display device as claimed in claim 17 or 18, wherein the light guide member is provided with a circular or rectangular cross-sectional shape when viewed in a plane normal to the front surface and normal to a main extension of the light guide member.

20. The display device as claimed in Claim 17, wherein the incoupling member is cone-shaped.

21 . The display device as claimed in Claim 18, wherein the incoupling member (13) is formed as a laterally prominent incoupling member (14), which is delimited by two laterally coaxially aligned cones (15,16), said cones (15,16) having a mutual apex (17) and different apex angles (a1 ,a2).

22. The display device as claimed in Claim 1 17, wherein the incoupling member (13) is a diffraction grating.

23. The display device as claimed in any of the Claims 17, 18 to 20, wherein the incoupling member (13) further transforms a wavelength of light emitted from the display area into a sensing wavelength.

24. The display device as claimed in Claim 23, wherein the sensing wavelength is in the infrared range, particularly between 0.7 and 3 micrometers.

25. The display device as claimed in Claim 23 or 24, wherein the incoupling member is provided with a phosphor for said transformation.

26. The display device as claimed in any of the claims 17 to 25, wherein the coupling device (10) is part of a cover member having an inner face and an outer face opposed to the inner face, said inner face facing the at least one display area, wherein the coupling device is present at the inner face.

27. Use of the display device as claimed in any of the previous claims for simultaneous display of an image and sensing a light property in at least one display area.

28. Use as claimed in claim 27, wherein the light property is the luminance and wherein color measurements are sensed by the at least one sensor of the display device in a calibration mode.