GB2542778A

GB2542778A - Front lit reflective display

Info

Publication number: GB2542778A
Application number: GB1517099.6A
Authority: GB
Inventors: Tydtgat Claude; Gerets Peter; Van Den Bossche Bart
Original assignee: Barco NV
Current assignee: Barco NV
Priority date: 2015-09-28
Filing date: 2015-09-28
Publication date: 2017-04-05
Also published as: GB201517099D0; WO2017055374A1

Abstract

A display comprises an array of reflective pixels 2, a transparent front lighting film 1 having a first surface 6 and a second surface 7; a plurality of solid state light sources such as light emitting diodes (LEDs) 5 provided in recesses 3 on the first surface 6 of the front lighting film 1; wherein the second surface 7 of the front lighting film is positioned parallel and adjacent to the array of reflective pixels. The plurality of LEDs on the first surface of the front lighting film are arranged to illuminate the array of reflective pixels and are substantially co-extensive therewith. The LEDs are arranged such that if a display pixel is hidden by an LED when viewed by one eye of a viewer 9 it will be visible by the other eye of the viewer. The LED array must be separated from the pixel array by a particular distance to ensure every pixel is visible by at least one eye. This distance is determined (see fig 8) by calculating the product of; the distance between the viewer and the display; and the ratio of the size of the LED and the intra-ocular distance. Front lighting film 1 having a thickness equal to the calculated distance is placed over the pixel array and the LEDs are formed thereon, ensuring the correct separation.

Description

Front Lit Reflective Display

The present invention relates to a front lit reflective display and components therefore.

Reflective displays require a front light system to facilitate viewing of images displayed. The front lighting for reflective displays can consist of an external light source as in e.g. 6,483,613 “Reflective Display Device and a Light Source for a Display Device” or US2013/0278900 “illumination Systems for Reflective Displays”. This type of front lighting has several disadvantages like e.g. uniformity of the illumination, additional space required etc...

The front lighting can also consist of a light guide attached to the front surface of the display as in e.g. US9,097,825 “White Tape in a Front Lighting Display Component Stack”, EP 1 544 657 “Broadband full white reflective display structure” or US 8,920,018 “Front Light Module”. This light guide has an illumination source placed at the edge of the light guide and the light is guided within the light guide by total internal reflection of the surface along the light guide, so that the light crosses the front of the display. Part of the surface of the light guide has a light scattering structure which causes light inside the light guide to be scattered out of the light guide to strike the display surface. Such a system usually has a slightly rough surface which may be the display itself that is index matched to the light guide, or has an array of microprisms specifically designed for this purpose. The display illuminated by the scattered light is viewed by looking through the light guide.

Such a front lighting system with a light guide film has many disadvantages.

First, since the light is strongest at the point of entry into the light guide film, the light tends to fall off in intensity rapidly at points which are further from the illuminating source. To compensate for this, a very complicated light prism structure may be utilized. This light prism structure may have different amounts of scattering near the source and at the points away from the source. Although such a system provides some degree of improvement in lighting uniformity, the uniformity, however, is still unsatisfactory, and the system must be customized for every application. Secondly, since this lighting system is based on scattering phenomenon, any scratch or piece of dirt in the light guide film also scatters the light which shows up as defects. As a result, it is very difficult for such a system to function properly. In addition, if a flexible front light is required, the front light system must be very thin; but a thin front light guide film exacerbates nonuniformity of the light along the length of the light guide film as described above.

In US 8,743,077, a front lit keymat assembly for use in a device such as e.g. a cell phone is described. The keymat has keys or buttons on its top surface. As seen on figure 1, a reflective display is overlaid on the keys and it can display visually viewable information, such as alphabets, numbers or operational commands. The areas between the keys may also be covered by a reflective display. OLED light sources are embedded in a front light film positioned above the keymat. There are no OLED in transparent areas of the top film. Those transparent areas correspond to the keys on the keymat.

The front light film is then placed over (e.g., laminated over) the keymat. The reflective display produces images to be shown on the keys and the images are visible through the transparent areas of the front light film. The light generated by the OLED light sources shines sideways onto the keys and illuminates them when required.

In this case, since the active display is only over the keys, the LED material may be one uniform layer filling all of the areas on the front light film, except the transparent areas. A problem with the proposed front light film is that if it is used to light a display (e.g. for display of detailed images), the OLED will hide up to 1 % of the pixels which may be unacceptable in certain applications There is a need for improvement in the art.

Summary of the invention.

In a first aspect of the invention, a display comprises an array of reflective pixels, a front lighting film made in a transparent material and having a first surface and a second surface; a plurality of solid state light sources such as light emitting diodes (5) on the first surface of the front lighting film; wherein the second surface of the front lighting film is positioned parallel and adjacent to the array of reflective pixels.

The plurality of solid state light sources such as light emitting diodes on the first surface of the front lighting film are arranged to illuminate the array of reflective pixels and are substantially co-extensive therewith; wherein pixels of the array of reflective pixels have a first spacing with a first pitch X along a row and a second spacing with a second pitch Y along a column, at least some of the plurality of light emitting diodes comprising diodes with a width A aligned in a direction of the rows of the array of reflective pixels and a length B aligned along a direction of the columns of the array of reflective, wherein the width A is larger than X and/or the length B is larger than Y.

The direction of the rows can be perpendicular to the symmetry plane of the viewer when in normal use (OR parallel to the local horizontal); the thickness of the front lighting film being greater or equal to the product of a (chosen) distance between viewer and display and the ratio of A and the inter-ocular distance (T > = L4 x d/D)

The front lighting film preferably has a thickness that it is at least the product of the minimum distance at which a viewer will look at the display and the ratio between a lateral dimension of the LED and the inter-ocular distance It is an advantage of that aspect of the invention that at least one eye of the viewer will see all the pixels of the display.

The distance between two adjacent LEDs of the plurality of LEDS is for instance 5 mm or alternatively preferably more than 10 mm or alternatively more than 15 mm.

It is another advantage of this aspect of the present invention that solid state light sources such as LED’s positioned on the first surface of the front lighting film will act therewith as a diffuser for the light travelling through the front lighting film and will contribute to a better uniformity of the ligh distribution across the array of reflective pixels.

Hence in accordance with this embodiment of the present invention, the pixels of the arrays of reflective pixels will be smaller than the solid state light sources such as LED’s or a mask (if a mask is used) and the distance from the display (L3, beyond which the pixels will not be resolved by a normal human eye) might be smaller than L1. In those cases, if L4 is the distance at which a viewer is expected to look at the display (like e.g. 40 to 76 cm for the screen), T will be chosen such that T > L4 x d/D.

In another aspect of the invention, a display comprises an array of reflective pixels, a front lighting film made in a transparent material and having a first surface and a second surface; a plurality of solid state light sources such as light emitting diodes (5) on the first surface of the front lighting film; wherein the second surface of the front lighting film is positioned parallel and adjacent to the array of reflective pixels, the plurality of solid state light sources such as light emitting diodes on the first surface of the front lighting film are arranged to illuminate the array of reflective pixels and are substantially co-extensive therewith; and the front lighting film preferably has a thickness that it is at least the product of the minimum distance at which a viewer will look at the display and the ratio between a lateral dimension of the LED and the inter-ocular distance.

In yet another aspect of the invention, a display comprises an array of reflective pixels, a front lighting film made in a transparent material and having a first surface and a second surface; a plurality of solid state light sources such as light emitting diodes (5) on the first surface of the front lighting film; wherein the second surface of the front lighting film is positioned parallel and adjacent to the array of reflective pixels, the plurality of solid state light sources such as light emitting diodes on the first surface of the front lighting film are arranged to illuminate the array of reflective pixels and are substantially co-extensive therewith, the direction of the rows can be perpendicular to the symmetry plane of the viewer when in normal use (OR parallel to the local horizontal); the thickness of the front lighting film being greater or equal to the product of a (chosen) distance between viewer and display and the ratio of A and the interocular distance (T > = L4 x d/D)

In any of the embodiments of the present invention, each of the solid state light sources such as LEDs can be positioned in a recess in the first side of the front lighting film. A phosphor can be positioned between a solid state light source such as a LED and the front lighting film. The phosphor can for instance be deposited at the bottom of the recess in which a solid state light source such as a LED is positioned. Positioning the phosphor on the bottom of the recess prior to positioning the solid state light source such as a LED can have a positive impact on the diffusion of light in the front lighting film and thereby improve the uniformity of the lighting on the reflective display.

In another aspect of the invention, the the solid state light sources such as LEDs are positioned on a second transparent substrate on which conductive electrodes have been formed.

The second substrate is positioned above the front lighting film, the recess in the front lighting film for receiving the solid state light sources such as LEDs being aligned with the solid state light sources such as LEDs on the second transparent substrate Alternatively, the recesses and the phosphor are on the second transparent substrate.

The LEDs can be bare die LEDs (i.e. the semi-conducting device is left unpackaged). The LEDs may have dimensions smaller than 1000 pm X 1000 pm.

The phosphor in the recesses has an advantage of improving diffusion of the light throughout the front lighting film. It also offers the possibility to match color tuning to the reflective display. Indeed, by illuminating with a certain color, it is possible to shift the color gamut of the reflective display. A black (light absorbing) guard or mask can be arranged on top of each LED. The out periphery of the black guard may be larger than the LED The side of the black guard directly closest to the LED can be covered with a reflector. Alternatively, a white diffusor replaces the reflector.

When a black guard or mask is used, the thickness of the front lighting film is at least the product of the minimum distance at which a viewer will look at the display and the ratio between a lateral dimension of the black guard or mask and the inter-ocular distance.

In a further aspect of the invention, at least one of the the solid state light sources such as LEDs is used as a sensor. The parasitic capacitance, and more generally the impedance of the at least one of the solid state light sources such as LEDs can be used as touch sensor and turn the front lighting film into a touch sensing film. The impedance of the solid state light source such as a LED is measured while it emits light or while it does not emit light.

In a further aspect of the invention, at least one of the solid state light sources such as the LEDs is used as a photo-sensor while it does not emit light. The solid state light source such as a LED can be used to evaluate the ambient light illuminating the reflective display and the light emitted by the solid state light sources such as the LEDs is tuned accordingly. A least one of the LED can be used as a temperature sensor to measure the temperature of the reflective display and adjust power dissipation of the display if necessary.

In a further aspect of the invention the plurality of solid state light sources such as LEDs are arranged as a first array on the second transparent substrate to illuminate the reflective display. In a further aspect of the invention, the second transparent substrate comprises a second array of solid state light sources such as LED’s which are used as sensors and in particular photo-sensors. It is an advantage of that aspect of the invention that the second array of the solid state light sources such as the LED’s will increase the number of elements diffusing light within the front lighting film. It is another advantage of this aspect of the present invention that monitoring of the light across the front lighting film with the second array of solid state light sources such as LED’s is easier.

Brief Description of the figures.

Figure 1. A front lit reflective display of the prior art.

Figure 2. Front lit display according to an embodiment the present invention.

Figure 3. Array of LED on transparent carrier and front lighting film according to an embodiment the present invention.

Figure 4. Array of LED on transparent carrier and electrodes connecting LED in series according to an embodiment the present invention.

Figure 5. First array of LED and second array of LED on transparent carrier according to an embodiment the present invention.

Figure 6. Mask over LED and relation to total internal reflection angle according to an embodiment the present invention.

Figure 7. Alternative embodiment of a front lit reflective display according to the invention.

Figure 8. Use of the parallax to guarantee visibility of all pixels by at least one eye of the viewer according to an embodiment the present invention.

Description of preferred 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 nonlimiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. 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, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions unless it is specifically stated as such. 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. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. 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.

The term “light emitting diode” includes any solid state light source such as including OLED’s.

The present invention proposes a front lighting for a reflective display as well as the display itself. Examples of reflective displays contemplated by the inventors for use in any of the embodiments of the present invention include ones with digital mirror displays (DMD), interferometric modulators (iMoD), reflective liquid crystal displays (LCD), electro-optic displays in general and electro-phoretic displays in particular.

The term "electro-optic", as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material or device (i.e. pixel element) having first and second display states differing in at least one optical property, the material or device being changed from its first to its second display state by application of an electric field to the material. Although the optical property is typically color perceptible to the human eye, it may be another optical property, such as optical transmission, reflectance, and luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.

The “interocular distance” is the distance between the centers of rotation of the eyeballs of an individual and is defined by the mean interpupillary distance (IPD). Mean IPD has been quoted in the stereoscopic literature as being anything from 58 mm to 70 mm. It is known to vary with respect to age, gender and race. The mean adult IPD is around 63 mm, whereby the vast majority of adults have IPDs in the range 50-75 mm, the wider range of 45-80 mm is likely to include (almost) all adults, and the minimum IPD for children (down to five years old) is around 40 mm see .

As shown on figure 2, a transparent film 1 is positioned on an array of reflective pixels 2. The transparent film 1 can be for instance PMMA. The transparent film 1 has a first surface 6 and a second surface 7; the second surface 7 being closest to the reflective display 6. Surfaces 6 and 7 are major surfaces. The transparent film 1 is used as front lighting film for the reflective display. The array of reflexive pixels 2 can be any of the pixels mentioned above, for instance can be liquid crystal based.

Recesses 3 are preferably formed on the first surface 6 of the transparent film 1. A phosphor material 4 is preferably deposited at the bottom of the recesses 3 by any suitable means, e.g. by dispensing or screen printing. A Light Emitting Diode 5 is positioned in each of the recesses 3, the phosphor 4 being between the LED 5 and the bottom of the recess in transparent film 1 in which LED 5 is positioned. The phosphor is for instance a white phosphor like YAG: Ce and the LED is a blue LED with a peak around 450 nm or an UV LED with a peak around 375 nm or lower. Instead of using a white phosphor, the LED can be a multicolor LED (which can emit Red, Green and Blue light) associated to a diffusor (which will mix the R, G and B component to form e.g. white light or any shade in the color triangle allowed by the R, G, B components).

The LEDs can be positioned individually in the recesses or the LEDs can be preassembled as an array on a transparent carrier film 12 with the corresponding conductive electrodes in a material like a conducting metaloxide e.g. Indium Tin Oxide (ITO). The carrier film 12 with the conductive electrodes 13 and the LEDs 5 forming an array 14 are then positioned over the transparent substrate 1. To each LED 5 on the carrier film 12 corresponds a recess 3 in the first surface 6 of the transparent substrate 1 for receiving the LED. Figure 3 shows how LEDs 5 on the carrier 12 are aligned with the recesses 3 in the front lighting film 1.

The light emitted by the LEDs 5 illuminates the array of reflective pixels 2. Some of the light reflected by the reflective pixels of the array 2 will undergo one or more TIR (Total Internal Reflections) on either the first surface 6 of the transparent substrate 1 or at the interface between carrier 12 and air. The LED’s 5 can be in an array which is co-extensive with the front lighting film.

Uniformity of the illumination is improved by reflection of the light on the LED 5 and/or the associated phosphor 4, thereby making the addition of diffusers at the first surface 6 of the front lighting film optionally redundant. A mask 8 can be positioned above each of the LEDs 5. The mask 8 has a first surface 8a facing away from a viewer 9 looking at the reflective display through the transparent film 1. The first surface 8a is preferably black, i.e. matt and light absorbing. The mask 8 has a second surface 8b facing away from the viewer 9. The second surface 8b can be reflecting and reflect light emitted by the LED 5 towards the transparent film 1 and the array of reflective pixels 2. The mask 8 is larger than the LED 5 it covers. Alternatively, the mask 8 has the same properties on both surfaces 8a and 8b. Mask 8 can be Titanium Oxide formed by e.g. silk screening on transparent film 12. Mask 8 can be black ink dispensed on the carrier substrate 12.

The size of the mask 8 is such that all light rays coming from the LED or phosphor towards the reflector are not fulfilling the TIR condition. Indeed, for light rays hitting just aside the reflector, there is no need to place a reflector as due to TIR they will be reflected anyway. As seen on figure 6, with y the depth of a recess 3 and x a lateral dimension of mask 8 that extends from the border of the recess 3 to the border of the mask 8 it is easy to determine x in function of y and the TIR angle Θ.

The LED 5 can be a bare die (i.e. the semi-conducting material is not protected from its environment by e.g. a package in ceramic or plastic). The shape of the bare die can usually be approximated by a parallelepiped as on figure 3. The typical dimensions of such LEDs 5 are 150 pm X 250 pm and emits blue or UV light that will excite the phosphor material 4. The LED 5 can be as large as or larger than 1000 pm X 1000 pm and as small as 50 pm X 50 pm or smaller.

The LED 5 being between the array of reflective pixels and the viewer, it is expected that some of the pixels of the array will be at least partially hidden from the viewer. This is certainly the case with the front lighting device described in US 8,743,077.

Surprisingly, all pixels of the array can be visible to the viewer even if the LEDs used to light the array of reflexive displays have lateral dimensions larger than the pixel pitch of the array of reflective pixels as long as the front lighting film is thick enough. The lateral dimensions of the LED are the dimensions of the LED footprint on the first surface of the front lighting film. Bare die LED can usually be considered as parallelepiped with a thickness e, a length It and a width w as seen on figure 3. The same thing can be said for the mask 8. In some cases (e.g. deposition of a drop of ink), the mask will better be approximated by a disk of radius r. In that case, the lateral dimension will be the radius r of the mask.

Typically, pixels of the array of reflective pixels will have a first spacing with a first pitch X along a row and a second spacing with a second pitch Y along a column, at least some of the plurality of light emitting diodes comprising diodes with a width A aligned in a direction of the rows of the array of reflective pixels and a length B aligned along a direction of the columns of the array of reflective, wherein the width A is larger than X and/or the length B is larger than Y.

Optionally, the front lighting film has a thickness at least half as large as the distance between two adjacent LEDs.

The inventors realized that light emitting diodes as large as 500 pm X 500 pm (or even larger) can be used with reflective pixels with a pixel pitch of 400 urn or less. With parallax, at least one eye of the viewer will see the pixel or pixels “hidden” by the light emitting diodes.

As seen on figure 8 there is a distance Lmax at which e.g. the right eye will still see a point P, the point P being hidden from the left eye by the LED 5. Beyond Lmax, neither the right eye nor the left eye will see the point P behind the LED.

If T is the thickness of the front lighting film 1, d a lateral dimension of the LED 5 or the mask 8 masking the LED (when a mask is used), D the inter-ocular distance and Lmax the distance between the eyes and the reflective pixels array 2 beyond which the point P will not be visible anymore by the right eye; then the following applies: Lmax = T x D/d (1).

For d = 0.5 mm, T = 10 mm and D = 65 mm, we have Lmax = 1300 m.

In other words, with light emitting diodes with a footprint as large as 500 pm x 500 pm and a front lighting film 10 mm thick, all pixels of an array of reflective pixels can still be seen to a distance of as much as 1300 mm from the reflective pixels even though the pixel pitch is smaller than 500 pm.

It is as if LEDs positioned between the viewer and the array of reflective pixels were not there at all.

The possibility to use LEDs larger than the pixels is advantageous because manipulation of larger LED is easier and reduce the cost of assembly. Larger LED can thus be used without impact on performance even with high definition reflective liquid crystal arrays (a liquid crystal panel where the backlight is replaced by a mirror) for which the pixel pitch is smaller than the lateral dimensions of the light emitting diodes positioned between the viewer and the array of reflective pixels. For distance smaller than Lmax, if the line of sight of an eye is blocked by the LED (and the one or more pixels in the shadow of that LED are not visible to that eye), the other eye will see the one or more pixels.

For distances larger than Lmax, there are circumstances were the LED will not be resolved anymore by the human eye and will thus be virtually invisible.

The inventors found thus a way to light an array of reflective pixels with sources of light positioned directly between the viewer and some of the pixels of the array of reflective pixels while preserving the visibility of all the pixels of the array of reflective pixels.

Example. Relation (1) can be used to design a front lit reflective displays when at least two parameters are known.

Let us take a 32 inches liquid crystal panel with 1920 x 1080 pixels. For such a display, a pixel is as small as 350 pm X 370 pm. If the available LEDs are 500 pm X 500 pm (for this example, we will assume that the mask 8 has the same footprint as the LED) and if the distance between the viewer and the screen (in normal use) is less than 1 meter, the thickness of the front lighting film must be at least 5,5 mm (five millimeter and a half).

Relation (1) can be used as a design rule to design a front lit display according to the invention. For a given diode size d and an average inter-ocular distance D, there is a first distance L1 between display and viewer beyond which the diode will not be resolved anymore (humans with normal eyesight being able to resolve objects seen under an angle of T). For a given T, there is a second distance L2 that corresponds to the maximum distance beyond which some pixels will not be visible anymore. Ideally, one should have L2 > L1. A first design rule would be to have T > L1 x d/D.

In some circumstances, the pixels of the arrays of reflective pixels will be smaller than the LED 5 or the mask 8 (if a mask is used) and the distance L3 (beyond which the pixels will not be resolved by a normal human eye) might be smaller than L1. In those cases, if L4 is the distance at which a viewer is expected to look at the display (like e.g. 40 to 76 cm for the screen), T will be chosen such that T > L4 x d/D.

As explained earlier, even though L1 might be larger than L4, all pixels of the display will be visible to at least one eye of the viewer even when the LEDs 5 are larger than the pixels of the array of reflective pixels.

Figure 4 represent an array 14 of LEDs 5 on a transparent carrier 12, each diode 5 being connected to electrically conducting electrodes 13. The LEDs can be connected e.g. in parallel or in series. On figure 4, the LEDs aligned along direction AA’ are connected in series.

Figure 5 represents an array 14 of LEDs 5 complemented by an array 16 of LEDs 17 (that can be identical to LEDs 5) that are used as sensors. While LEDs 5 can still be connected in parallel or series, LEDs 17 can be addressed individually (LEDs 16 form a passive or active matrix). It is an advantage if the array 16 of LEDs 17 has the same periodicity as the array 14. In the case of a rectangular array (the LEDs forms a regular network of perpendicular lines and columns), the LEDs 16 are ideally placed in the center of the rectangle formed by four adjacent LEDs 5 as seen on figure 5. Arrays of LED can be easier to assemble with the rest of the structure in particular if recesses are used in the front lighting film) than haphazardly distributed LEDs or if more complex distribution of the LEDs is used. The second array 16 of LED 17, can be used as photo-sensors to detect touch. In this case there will be modulation of light measured by a photo-sensor in function of position of finger(s) on the surface of the display hence enabling a touchscreen display.

The LEDs used in both arrays can be identical and assembled on the same carrier. The recesses made in the transparent film 1 can be different for LED 17 and LED 5 or can be identical in order to simplify manufacturing. The LED 17 can be used to evaluate uniformity of the lighting, detect failure of the LED’s and allow corrective measures to be taken (e.g. the LED surrounding a defect LED would emit more light to compensate for the defect LED). A mask 8 can also cover the LED 17 of the second array 16 to shield them from ambient light (i.e. light not generated by LED 5). At least one LED 17 can be left without mask 8 in order not to be shielded from ambient light and allow a more straightforward measurement of the ambient light. For that at least one LED 17, a masking material (e.g. a drop of ink can be dispensed in the recess where there would normally be a phosphor material) can be used to shield that LED from light in the transparent film 1 and allow an easier measurement of ambient light.

If LEDs 5 are also to be used as sensors to sense local conditions, it may be advantageous to use an active matrix (with thin film transistors on the carrier substrate 12) as well to connect each LED 5 individually to a power driver and/or a sensor interface.

The array of LED 16 can for instance be used as an array of photo-sensors to evaluate the uniformity of the light lighting the array of reflective pixels 2.

If the LEDs 5 are turned off, the LEDs 16 can also be used to evaluate the ambient light (i.e. light not emitted by LEDs 5 but reaching the array of reflective pixels 2.

The light emitted by LEDs 5 can be isolated from ambient light by synchronous detection.

Any of the LEDs 17 can also be used as temperature sensor as well. If necessary, the LED of array 16 used as temperature sensor can be shielded to make temperature measurement easier (by avoiding signaling fluctuations caused by changes in ambient light). A LED 5 or 17 that can be addressed individually can also be used to detect a touch event (i.e. the front lighting device of the present invention can double as touchscreen). The impedance of a LED being influenced by its environment, change in the LED’s impedance can be used to evaluate whether or not a finger or stylus is touching the carrier film 12 in the vicinity of a LED 5 or a LED 17.

Powering a LED 5 and measuring its impedance can be achieved at the same time if e.g. measuring the impedance is done by injecting a low amplitude high frequency signal through the diode 5 ot 17 and measuring the high frequency voltage drop across the LED 5.

The array 16 can be composed of other components than diodes.

Figure 7 shows an alternative embodiment of the invention. A transparent carrier substrate 20 is used to assemble an array of LEDs 5. A reflector 21 can be formed on the substrate 20 below each of the LED 5. The reflector 21 can be larger than LED 5 as discussed for the previous embodiments. Electrodes 13 are formed to contact the LEDs 5. All LEDs 5 or group of LEDs 5 can be connected in parallel or in series.

An optical bonding layer 18 is formed on the LEDs 5. The optical bonding layer 18 has a first surface and a second surface. The first surface of the bonding layer 18 is in contact with the array of LEDs 15, the electrodes 13 and the transparent carrier substrate 20.

Recesses 19 can be formed in the second surface of the optical bonding layer 18 at the same level as the LED 5 for receiving a phosphor material. A phosphor material 4 is deposited (e.g. by dispensing or screen printing) in the recesses 19. The blue and/or UV light emitted by LED 5 excites the phosphor material 4 to produce white light.

Another optical bonding layer 22 can be positioned between the lighting film 1 and the array of reflective pixels 2.

The bonding layer 18 is important as for normal conditions (when the front illumination is off; daylight condition) the reflectivity of the structure must be reduced as much as possible, otherwise contrast would be poor. So the whole structure should have no reflections at either boundary. So it would be best if all materials used for the layers have as equal as possible refractive indexes which can be bonded by indexed matching glue. The bonding layer 18 can be silicone based. The bonding layer 22 is special as it must attach to the array 2, it needs to have the refractive index of the PMMA on top. Without this, we introduce again a reflection at the bottom of the PMMA. If air would be between 1 and 2 we need to have a good AR coating on the bottom of 1. A uniform lighting of the array of reflective pixels can be a moot point if not all pixels of the array of pixels are visible (for instance, one expects that the LEDs 5 and associated mask 8 will hide the pixels right under them or a part of the pixels under them if the pixels are larger than the mask 8 and the LED 5).

Claims

Claims.

1. A display comprising an array of reflective pixels having rows and columns, a front lighting film made in a transparent material and having a first surface and a second surface; a plurality of light emitting diodes on the first surface of the front lighting film arranged to illuminate the array of reflective pixels and being co-extensive therewith; wherein the second surface of the front lighting film is positioned adjacent to the array of reflective pixels; wherein pixels of the array of reflective pixels have a first spacing with a first pitch X along a row and a second spacing with a second pitch Y along a column, at least some of the plurality of light emitting diodes comprising diodes with a width A aligned in a direction of the rows of the array of reflective pixels and a length B aligned along a direction of the columns of the array of reflective, wherein the width A is larger than X and/or the length B is larger than Y.

2. A display according to claim 1 wherein the front lighting film has a thickness at least half as large as the distance between two adjacent LEDs.

3. A display according to claim 1 or 2 wherein the direction of the rows is perpendicular to the symmetry plane of the viewer when in normal use or parallel to the local horizontal; the thickness of the front lighting film being greater or equal to the product of a distance between viewer and display and the ratio of A and the inter-ocular distance (T > = L4 x d/D).

4. A display according to any of the preceding claims further characterized in that a phosphor material is positioned between the front lighting film and the plurality of light emitting diodes.

5. A display according to any of the preceding claims further characterized in that the plurality of light emitting diodes are assembled on a transparent carrier substrate and the transparent carrier substrate is aligned.

6. A display according to any of the preceding claims further characterized in that there are recesses in the front lighting film corresponding to the plurality of light emitting diodes on the transparent carrier substrate.

7. A display according to claim 2 characterized in that an optical bonding layer is positioned between the transparent carrier substrate and the front lighting film, the plurality of) light emitting diodes and a phosphor material being positioned on opposite surfaces of the optical bonding layer.

8. A display according to any of the preceding claims further characterized in that the plurality of light emitting diodes are connected to an active matrix

9. A display according to claim 6 further characterized in that at least one of the plurality of light emitting diodes is used as photo-sensor, temperature sensor or touch sensor.

10. A display according to claim 9 further characterized in that the light emitting diodes are in a second array positioned on the transparent carrier substrate and is used as an array of photo-sensor and/or temperature sensor and/or touch sensor.

11 .A display according to any of the preceding claims further characterized in that the array of reflective pixels is a liquid crystal panel.

12. A display according to any preceding claim further characterized in that lateral dimensions of each of the plurality of light emitting diodes are larger than the pixel pitch of the pixels in the array of reflective pixels.

13. A display according to any preceding claim wherein the light emitting diodes are OLEd’s.

14. A display comprising an array of reflective pixels, a front lighting film made in a transparent material and having a first surface and a second surface; a plurality of solid state light sources such as light emitting diodes (5) on the first surface of the front lighting film; wherein the second surface of the front lighting film is positioned parallel and adjacent to the array of reflective pixels, the plurality of solid state light sources such as light emitting diodes on the first surface of the front lighting film are arranged to illuminate the array of reflective pixels and are substantially co-extensive therewith; and the front lighting film preferably has a thickness that it is at least the product of the minimum distance at which a viewer will look at the display and the ratio between a lateral dimension of the LED and the inter-ocular distance.

15. A display comprises an array of reflective pixels, a front lighting film made in a transparent material and having a first surface and a second surface; a plurality of solid state light sources such as light emitting diodes on the first surface of the front lighting film; wherein the second surface of the front lighting film is positioned parallel and adjacent to the array of reflective pixels, the plurality of solid state light sources such as light emitting diodes on the first surface of the front lighting film are arranged to illuminate the array of reflective pixels and are substantially co-extensive therewith, the direction of the rows can be perpendicular to the symmetry plane of the viewer when in normal use or parallel to the local horizontal; the thickness of the front lighting film being greater or equal to the product of a (chosen) distance between viewer and display and the ratio of A and the inter-ocular distance (T > = L4 x d/D)