WO2005104075A2

WO2005104075A2 - Display with optically coupled light sensor

Info

Publication number: WO2005104075A2
Application number: PCT/GB2005/001397
Authority: WO
Inventors: Paul Routley
Original assignee: Cambridge Display Technology Limited
Priority date: 2004-04-22
Filing date: 2005-04-11
Publication date: 2005-11-03
Also published as: GB2429107A; GB0408960D0; WO2005104075A3; GB0620559D0; GB2429107B

Abstract

This invention generally relates to electronic displays, in particular organic light emitting diode (OLED) -based displays, and to techniques for compensating for variations in display element brightness such as those caused by ageing. A method of compensating for variations in brightness of an OLED display element of a passive OLED display device, the display device having a plurality of said OLED display elements mounted on a substrate, the method comprising: driving a said display element to emit light; detecting a brightness said emitted light after waveguiding by said substrate; and compensating for a variation in said display element brightness responsive to said detecting.

Description

Displays, Drivers and Related Methods

This invention generally relates to electronic displays, in particular organic light emitting diode (OLED) -based displays, and to techniques for compensating for variations in display element brightness such as those caused by ageing.

Displays fabricated using OLEDs provide a number of advantages over other flat panel technologies. They are bright, colourful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in US 4,539,507.

A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.

Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image. Other passive displays include segmented displays in which a plurality of segments share a common electrode and a segment may be lit up by applying a voltage to its other electrode. A simple segmented display need not be scanned but in a display comprising a plurality of segmented regions the electrodes may be multiplexed (to reduce their number) and then scanned.

Figure la shows a vertical cross section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in Figure la). The structure of the device is somewhat simplified for the purposes of illustration.

The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic or some other substantially transparent material. An anode layer 104 is deposited on the substrate, typically comprising around 150 nm thickness of ITO (indium tin oxide), over part of which is provided a metal contact layer. Typically the contact layer comprises around 500nm of aluminium, or a layer of aluminium sandwiched between layers of chrome, and this is sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal are available from Corning, USA. The contact metal over the ITO helps provide reduced resistance pathways where the anode connections do not need to be transparent, in particular for external contacts to the device. The contact metal is removed from the ITO where it is not wanted, in particular where it would otherwise obscure the display, by a standard process of photolithography followed by etching.

A substantially transparent hole transport layer 106 is deposited over the anode layer, followed by an electroluminescent layer 108, and a cathode 110. The electroluminescent layer 108 may comprise, for example, a PPN (poly(p- phenylenevinylene)) and the hole transport layer 106, which helps match the hole energy levels of the anode layer 104 and electroluminescent layer 108, may comprise a conductive transparent polymer, for example PEDOT:PSS (polystyrene-sulphonate- doped polyethylene-dioxythiophene) from H C Starck of Germany. In a typical polymer-based device the hole transport layer 106 may comprise around 200 nm of PEDOT; a light emitting polymer layer 108 is typically around 70 nm in thickness. These organic layers may be deposited by spin coating (afterwards removing material from unwanted areas by plasma etching or laser ablation) or by inkjet printing. In this latter case banks 112 may be formed on the substrate, for example using photoresist, to define wells into which the organic layers may be deposited. Such wells define light emitting areas or pixels of the display.

Cathode layer 110 typically comprises a low work function metal such as calcium or barium (for example deposited by physical vapour deposition) covered with a thicker, capping layer of aluminium. Optionally an additional layer may be provided immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved or enhanced through the use of cathode separators (not shown in Figure la).

The same basic structure may also be employed for small molecule devices.

Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.

To illuminate the OLED power is applied between the anode and cathode, represented in Figure 1 a by battery 118. In the example shown in Figure 1 a light is emitted through transparent anode 104 and substrate 102 and the cathode is generally reflective; such devices are referred to as "bottom emitters". Devices which emit through the cathode ("top emitters") may also be constructed, for example by keeping the thickness of cathode layer 110 less than around 50-100 nm so that the cathode is substantially transparent.

Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image. Referring now to Figure lb, this shows a simplified cross-section through a passive matrix OLED display device 150, in which like elements to those of figure la are indicated by like reference numerals. As shown the hole transport 106 and electroluminescent 108 layers are subdivided into a plurality of pixels 152 at the intersection of mutually perpendicular anode and cathode lines defined in the anode metal 104 and cathode layer 110 respectively. In the figure conductive lines 154 defined in the cathode layer 110 run into the page and a cross-section through one of a plurality of anode lines 158 running at right angles to the cathode lines is shown. An electroluminescent pixel 152 at the intersection of a cathode and anode line may be addressed by applying a voltage between the relevant lines. The anode metal layer 104 provides external contacts to the display 150 and may be used for both anode and cathode connections to the OLEDs (by running the cathode layer pattern over anode metal lead-outs).

The above mentioned OLED materials, and in particular the light emitting polymer material and the cathode, are susceptible to oxidation and to moisture. The device is therefore encapsulated in a metal can 111, attached by UN-curable epoxy glue 113 onto anode metal layer 104, small glass beads within the glue preventing the metal can touching and shorting out the contacts. Preferably the anode metal contacts are thinned where they pass under the lip of the metal can 111 to facilitate exposure of glue 113 to UN light for curing.

Although, as previously mentioned, OLEDs offer a number of significant advantages over other types of display in some areas, particularly for blue devices, device lifetimes would benefit from improvement. The present inventors have recognised that as well as improvements in the chemistry, electronic techniques may also be employed to, in effect, compensate for variations in OLED brightness due, in particular to ageing, but also arising from other effects such as manufacture intolerances.

According to a first aspect of the present invention there is therefore provided a method of compensating for variations in brightness of an OLED display element of a passive OLED display device, the display device having a plurality of said OLED display elements mounted on a substrate, the method comprising: driving a said display element to emit light; detecting a brightness said emitted light after waveguiding by said substrate; and compensating for a variation in said display element brightness responsive to said detecting.

In embodiments waveguiding some of the light emitted by the display element within the substrate before detection facilitates monitoring a plurality of display elements over the area of the display and, furthermore, enables a photodetector such as aphotodiode to be positioned towards an edge of the display so that it does not interfere with or obscure a part of the display area. Waveguiding light within the substrate before detection also facilitates practical fabrication of an OLED display incorporating means for brightness compensation.

The waveguided light may be detected by being partially coupled out of the substrate through the front surface of the substrate (that is facing an observer) or, in embodiments, through the back surface of the substrate. Preferably however, the waveguided light is detected at the edge of the substrate, that is at a side edge of the display because it is within this thickness that the light is propagating. It has been found in practice that this, what may be termed direct, detection of the waveguided light can detect light from display elements at a greater range from the detector than with other light detecting positions, in embodiments from all the display elements of a display.

In preferred embodiments the method includes modulating the emitted light and then selectively detecting modulated light in order to reduce the sensitivity of the light detector to ambient illumination. The brightness compensating may determine a compensation with reference to a reference level of brightness expected for the display or for the type or class of display, for example determine at manufacture. Such a reference brightness level may be stored within the display or within a driver for the display or may be based upon a knowledge of the display type. Preferably the method determines and stores a brightness compensation value for a display element, for example as part of a calibration procedure performed at intervals such as weekly, daily or whenever the display is switched on. If ambient light is a particular problem calibration may only be performed when ambient light is below a threshold level, for example at night. In preferred embodiments of the method the calibration procedure determines a brightness compensation value for each display element of the display and stores these values for later use, in particular so that when the display is driven with a particular drive level this drive level can be adjusted using the brightness compensation value before being applied to the display so that a user always perceives substantially the same brightness for a given drive level.

The above techniques can be employed to compensate for variations in brightness related to manufacture intolerances, but they are particularly useful for compensating for age-related brightness reductions. The techniques are applicable to both passive matrix displays and to segmented displays, whether compensation is preferably applied on a per segment-basis. For example for displaying lettering it has been noted that segments for descenders after a while tend to be brighter than the main body of the text and embodiments of the above-described method may be employed to adjust the brightness levels of display elements such that all appear at substantially the same brightness.

The invention further provides processor control code, in particular on a carrier, to implement embodiments of the above described methods.

In another aspect the invention provides a passive OLED display device comprising a plurality of OLED display elements mounted on a substrate, the device including at least one photosensitive detector configured to detect light emitted by a said display element and waveguided to said detector by said substrate.

The display device may comprise a passive matrix display or a multi-segment display; in general the display will be a bottom-emitter although top-emitting embodiments may also be provided where emitted light couples into the substrate. Preferably the detector, in embodiments a photo diode, is located adjacent an edge of the display, and is preferably configured to detect light leaving a side edge of the substrate. Thus preferably the detector arrangement is such that light is waveguided into the detector. In embodiments the detector is mounted on the substrate.

The invention also provides a driver for a passive OLED display device, the driver having an input to receive drive level data for a said display element, and further including: means for driving a said OLED display element to emit light; means for detecting said emitted light using a signal from said photosensitive detector; and means for adjusting said drive level for a display element responsive to said detecting.

In embodiments the driver is configured to operate in accordance with embodiments of the above described methods, preferably driving a display element such that it emits modulator light, selectively detecting this in order to separate from the display element from background light.

The invention further provides processor control code for a passive OLED display driver, in particular on a carrier, the code comprising code to: drive an OLED to emit light; input hght level data relating to a level of hght emitted by said OLED; compare said light level data with reference data; and determine drive correction data for said OLED responsive to said comparison.

The processor control code may be provided on a data carrier such as a disk, programmed memory such a read-only memory (firmware) or on a data carrier such as an optical or electrical signal carrier. The code may comprise code in a conventional programming language such as C or lower level code or code for setting up or controlling an ASIC or FPGA or code for a hardware description language such as Verilog (trademark). The skilled person will appreciate that such code may be distributed between a plurality of coupled components.

The applicants have further recognised that the above described techniques for detecting and compensating for variations in brightness of an element of a display may also be employed to provide a touch-sensitive display screen.

Thus in a further aspect the invention provides a method of detecting touching of a display device, the device having a plurality of light emitting elements emitting light through a substrate, the method comprising: driving said display device to illuminate one or more of said display elements; detecting a level of light emitted by said one or more display elements after waveguiding of said light within said substrate to a detector spaced apart from said one or more display elements; and detecting said touch by detecting a change in said light level caused by touching said substrate between said one or more display elements and said detector, said touching attenuating said waveguided light within said substrate.

Thus in embodiments of the method the display is operated normally and when the screen (substrate) is touched light from one or more illuminated display elements of the display waveguiding within the substrate is partially coupled out of the substrate, thus resulting in a reduction in detected light level, indicating that the display screen has been touched. Li other embodiments a particular display element or pixels can be illuminated in order to define touch sensitive regions of the screen or substrate.

Thus another aspect of the invention comprises use of a display device as described above in order to sense touching of the substrate, in particular to determine when, and optionally where the substrate is touched.

These and other aspects of the present invention will now be further described by way of example only, with reference to the accompanying figures in which:

Figures la and lb show, respectively, a vertical cross section through an example of an OLED device, and a cross section through a passive matrix OLED display;

Figures 2a and 2b show, respectively, light waveguiding within an OLED display substrate, and an example of an OLED display including a photodiode according to an embodiment of the present invention;

Figures 3 a and 3b show, respectively, a schematic diagram of an OLED display driver, and details of the driver of Figure 3 a, both embodying aspects of the present invention; and

Figure 4 shows a flow diagram illustrating operation of the display driver of Figure 3.

Referring first to Figure 2a, this shows an OLED display device 200 similar to that shown in Figure lb, like elements being indicated by like reference numerals, illustrating optical waveguiding within the device. In Figure 2 display element 201 is illuminated and light 202 from this display element passes through transparent anode layer 104 and substrate 102 towards an observer. However some of the light 202 emitted by element 201 , depending upon the angle at which it is emitted, is waveguided either within organic layers 106, 108 (light 204) or within anode layer 104 (light 206) or within substrate 102 (light 208). The light 208 waveguided within substrate 102 can travel large distances within the substrate and is therefore detectable at, among other positions, the edge of the display.

Display device 200 incorporates one or more photodiodes 210a, b configured to detect the light 208 waveguided in substrate 102. There is a number of possible positions for such photodiodes, in particular a photodiode may be placed on a front 102a or back 102b face of the substrate 102 and/or anode layer 104 to detect light internally reflecting from one or other of these display faces. Photodiode 210a is an example of a photodiode in such a position; in other arrangements a photodiode or other photodetector may be placed within can 111 and fabricated, for example, from polymer material. A photodiode on a front or back face of the substrate and/or anode layer receives light because a portion of the waveguided light 208 is coupled into the photodiode at points where the light internally reflects adjacent the photodiode - in other words photodiode 210a attenuates the internal reflection and couples some light out of substrate 102. However a preferable location for a photodiode is that of photodiode 210b in Figure 2a which, as can be understood from the diagram, receives more of the waveguided light (not just that totally internally reflecting adjacent the photodiode). Thus a photodiode at this position is expected to have greater sensitivity and/or to be able to detect light from pixels at a greater range from the photodiode.

Referring now to Figure 2b, this shows an actually fabricated OLED display device 250 incorporating photodiodes at a range of positions. The display illustrated in Figure 2b is a passive matrix display of 7 pixels by 5 pixels with 2mm by 2mm planar unfocussed photodiodes attached, in this example, at positions comprising a position 252 on an active pixel, a position 254 at the edge of the display, towards a comer, and outside can 111 , a position 256 at the edge of the display and inside can 111 , a position 258 at the edge of the display, towards a corner and inside can 111, and a position 260 at the edge of the display, chosen to detect light leaving a side edge of the display (substrate). Pixels of the display were driven with a current of 5 mA and the resultant current in the various photodiodes, in microamps, was recorded. For each photodiode the current when the pixel nearest the photodiode was illuminated was recorded, together with the current when pixels 1, 2, 3 and 4 pixels away from the nearest pixel were illuminated, together with current when the furthest pixel from the photodiode was illuminated, and the results are given in Table 1 below.

Table 1

By comparison with the figures given in Table 1 the signal for room illumination from a photodiode was approximately 4 microamps and, for a display in direct sunlight, approximately 900 microamps.

Although in more developed embodiments the photodiode may be reduced in size, photodiode positions at the edge of the display are preferable to those such as position 252 on an active pixel, so that the display is not partially obscured. Reviewing the results in Table 1 in more detail, it can be seen that only the photodiode attached at position 260 on the edge of the glass substrate receive light from all the pixels, and in particular from the furthest pixel from the photodiode, albeit that the absolute level of signal from a pixel in this position is less than at some other positions for closer pixels (which may shine more directly into a photodiode). Thus these results show that it is preferable to detect the waveguided light in a direction substantially perpendicular to the emitting surface; this also shows the least variation in signal with respect to relative pixel-photodiode position. Although the signal is small it is relatively straightforward to detect electronically but difficulties may arise in extracting a signal where a display device incorporating a photodiode is exposed to direct sunlight. To reliably detect the photodiode signal in such a case a display element may be driven using a pulse drive scheme to modulate the illuminated pixel, and a lock-in amplifier arrangement may then be employed to detect this modulated signal against the much larger background. One embodiment of an OLED display driver configured to compensate for pixel ageing effects using measurements from photodiodes as described above is shown in Figures 3 a and 3b, described below.

Figure 3a, shows a schematic diagram of a passive matrix OLED display and driver 300 embodying aspects of the present invention. In Figure 3a a passive matrix OLED display 302 comprises a plurality n of row lines 304 each with a corresponding row electrode contact 306 and a plurality m of column lines 308 with a corresponding plurality of column electrode contacts 310. An OLED 312 is connected between each pair of row and column lines with, in the illustrated arrangement, its anode connected to the column line. The row electrodes 306 are driven by row driver circuits 312 and the column electrodes 310 are driven by column drivers 314.

The driver for each row typically comprises a MOS transistor to selectively connect a row electrode to ground; the driver for each column generally comprises a controllable current source. Row driver circuits 312 have a control input 311 for selecting one or more row electrodes for connection to ground. Column drivers 314 have a control input 313 for setting the current drive to one or more of the column electrodes. Preferably control inputs 313 and 311 are digital inputs for ease of interfacing and preferably control input 313 sets the current drives for all the m columns of display 302.

A two-dimensional image may be presented on display 302 by selecting each row in turn and driving all the pixels in the selected row using column drivers 314, then selecting the next row and repeating the process to build up an image using a conventional raster scan pattern. Where a greyscale or colour display is to be provided a variable current drive is provided or the duration of the emitted light varied for each column according to the desired pixel brightness. In some embodiments of row driver circuitry 312 the raster scan function may be provided automatically by the row drivers under control of the control input 311. In a driver for a simple segmented display there may, in effect, just be a single "row" and thus no associated row driver, although in more complex segmented displays the drive to sets of segments may be scanned similarly to rows of a matrix-format display.

Data for display on display 302 is provided on data and control bus 320 which may be either a parallel or a serial bus. Bus 320 provides an input to a frame store or memory 322 which stores display data for each pixel of display 302, in effect forming in the memory an image of the data for display. Thus, for example, one or more bits of memory may be associated with each pixel, defining a greyscale pixel brightness level or a pixel colour. The data in frame store 322 is stored in such a way that the brightness values of pixels in a row may be read out and, in the illustrated embodiment, frame store 322 is dual ported, outputting data read from the frame store on a second, read data bus 324; alternatively the functions of data buses 320 and 324 may be combined in a single bus.

The passive matrix OLED driver 300 also incorporates a display drive controller 316, for providing display data to control input 313 of column drivers 314 and for providing a row select or scan control output to control input 311 of row drivers 312 for controlling the raster scanning of the display. Display drive controller 316 operates in a conventional manner to read data from frame memory 322 and to provide control data signals to control inputs 313 and 311 to display this data on passive matrix display 302. The timing of processing performed by display drive controller 316 is controlled by a clock signal from clock generator 318. The display drive controller 316 is coupled to data bus 324 for reading data from frame memory 322 although the frame memory 322 may be integrated with the display drive controller.

Turning now to the display analysis/brightness compensation circuitry, the display 302 incorporates a photodiode 303, as described above. An output from this photodiode provides an input to a display analyser 330 coupled to display drive controller 316 and to a brightness compensation module 332. The brightness compensation module, conceptually, modifies the data on read data bus 324 output from frame memory 322 to adjust data in frame memory 322 specifying a desired brightness level so that the actual brightness of display 302 corresponds more closely to the desired brightness. Typically this is achieved by increasing the actual drive to the display for a given desired brightness level as the display ages, to compensate for a reduction in display efficiency. The brightness compensation module 332 may thus additionally or alternatively be inserted in data bus 320, to modify input data before it is written into frame memory 322.

The display analyser 330 receives data and control signals from display driver controller 316, typically a signal to indicate that the display is in a calibration mode, and data to indicate that a particular pixel is illuminated, together with the drive level for that pixel (which may comprise a pulse with modulated signal to facilitate implementation of a lock-in amplifier within display analyser module 330. In turn in embodiments display analyser 330 may provide drive adjust data to controller 316, as described further below. Display analyser module 330 also provides brightness compensation data to brightness compensation module 332.

Referring now to Figure 3b, this illustrates an example implementation of a brightness compensation module 332, here comprising a frame compensation memory 332a and a multiplier 332b. Display analyser module 330 provides data and address information to a dual ported frame compensation memory 332a to write brightness compensation data into the memory at a first port. A second port outputs this data to multiply 332b which multiplies brightness level data on bus 324 output from frame memory 322 by a brightness compensation factor from memory 332a, providing adjusted brightness level data to controller 316. Both memories 322 and 332a are addressed by controller 316 in tandem to allow the controller to reputedly refresh the display in a raster scan.

Figure 4 illustrates operation of the display and driver 300 of Figures 3 a and 3b in a brightness compensation calibration mode. Thus at step S400 the calibration mode is entered and, at step S402 controller 316 drives display 302 to illuminate a selected segment or pixel, optionally under control of display analyser 330. Display analyser 330 then measures a segment/pixel brightness signal level from photodiode 303 and, optionally, may perform further processing to substantially remove the effects of ambient illumination and/or convert the signal level output of photodiode 303 into a brightness level. Then at step S406 the display analyser 330 reads reference brightness data from a data store such as a local read-only memory (ROM) and compares the measured brightness level with the expected or reference level. The reference level data preferably incorporates distance compensation data as the expected signal from photodiode 303 in general depends upon the distance of the illuminated segment or pixel from the photodiode. The reference data for a particular type of display may be determined from measurements of a known good display, in which case the ROM may incorporate reference data for a number of display types, selectable according to the driven display. Storing data for a known good display facilitates compensation for manufacturing variations as well as display ageing. Alternatively a set of reference values may be read from the display and stored, for example in Flash RAM, during an initialisation procedure run when, say, the display is new or first used.

The comparison of step S406 may directly determine a correction value for the segment/pixel or alternatively a feedback loop may be employed, as shown by steps S408, S409, in which drive to segment/pixel is repetitively adjusted until brightness is at a desired level, to determine a brightness correction value. Once the correction value has been determined this is stored, at step S410, in frame compensation memory 332a at an address corresponding to that of the illuminated segments/pixel. Preferably the correction value is expressed as a correction factor, for example 10% to increase the drive by 10% from that specified. Generally the correction value will increase the drive to a display. There may be instances in which the drive is decreased - for example if a display is driven hard for a long period its efficiency can reduce and recover overnight, and some display types exhibit an increase in efficiency during the first part of their life.

Preferably calibration is performed for each segment/pixel of a display so, at step S412, the procedure checks whether there are any more segments/pixels to calibrate, if so looping back to step S402, if not the procedure ending at step S414.

The calibration procedure of Figure 4 may be implemented by processor control code or microcode or in hardware, for example using a state machine implemented on a PLA (programmable logic array), in which case processor control code may employed to define such hardware in terms of its function. The calibration procedure may be performed at intervals, for example every time the display is turned on, or at periodic intervals such as weekly. Additionally or alternatively the calibration procedure may be operated in the background when the display is working normally, in which case step S402 is performed by display drive controller 316 as part of the normal display process, display analyser 330 receiving data specifying which segment pixel is illuminated and its drive level, so that this drive level can be taken into account in the comparison with the expected brightness level at step S406.

The above described display and display driver have applications other than brightness compensation for ageing manufacturing variations. For example, referring back to Figure 2a, it can be seen that touching the front face 102a of substrate 102 will tend to reduce the efficiency of total internal reflection within the substrate where the display is touched, coupling some light out of the display and attenuating the light level reaching a photodiode such as photodiode 210a, b. This effect can be used to provide a touch sensitive display since by measuring the brightness level signal from, say, photodiode 210b touching of the display between illuminated display element 201 and the photodiode can be detected as a reduction in this signal level.

No doubt many other effective alternatives will occur to the skilled person, and it will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

1. A method of compensating for variations in brightness of an OLED display element of a passive OLED display device, the display device having a plurality of said OLED display elements mounted on a substrate, the method comprising: driving a said display element to emit light; detecting a brightness said emitted light after waveguiding by said substrate; and compensating for a variation in said display element brightness responsive to said detecting.

2. A method as claimed in claim 1 wherein said detecting of said emitted light comprises detecting said light adjacent at an edge of said display device.

3. A method as claimed in claim 2 wherein said detecting comprises detecting light emitted by a front or back face of said substrate.

4. A method as claimed in claim 2 wherein said detecting comprises detecting light emitted by a side edge of said substrate.

5. A method as claimed in any one of claims 1 to 4 further comprising modulating said driving to modulate said emitted light, and wherein said detecting comprises selectively detecting said modulated emitted light.

6. A method as claimed in any one of claims 1 to 5 further comprising comparing said detected brightness with a reference level, and performing said compensating responsive to a result of said comparing.

7. A method as claimed in any one of claims 1 to 6 further comprising determining and storing a brightness compensation value for said display element, for said brightness variation compensating, and adjusting a brightness level of said display element using said compensation value.

8. A method as claimed in any one of claims 1 to 6 wherein said compensating comprises: determining a brightness compensation value for said display element; receiving a display element drive level signal; adjusting said drive level signal using said brightness compensation value for said display element; and driving said display element using said adjusted drive level signal.

9. A method as claimed in any one of claims 1 to 8 comprising driving each of said plurality of display elements in turn and storing for each a respective said brightness compensation value.

10. A method as claimed in any one of claims 1 to 9 wherein said brightness variation comprises an OLED display element age-related brightness variation and/or a manufacturing-related brightness variation.

11. Processor control code, in particular on a carrier, to implement the method of any one of claims 1 to 10.

12. A passive OLED display device comprising a plurality of OLED display elements mounted on a substrate, the device including at least one photosensitive detector configured to detect light emitted by a said display element and waveguided to said detector by said substrate.

13. A passive OLED display device as claimed in claim 12 wherein said detector is located adjacent an edge of said display device.

14. A passive OLED display device as claimed in claim 12 or 13 wherein said detector is configured to detect light coupled out of a front or back face of said substrate.

15. A passive OLED display device as claimed in claim 12 or 13 wherein said detector is configured to detect light leaving a side edge of said substrate.

16. A driver for the passive OLED display device of any one of claims 12 to 15, the driver having an input to receive drive level data for a said display element, and further including: means for driving a said OLED display element to emit light; means for detecting said emitted light using a signal from said photosensitive detector; and means for adjusting said drive level for a display element responsive to said detecting.

17. A driver as claimed in claim 16, further comprising means for determining and storing a said drive level adjustment.

18. A driver as claimed in claim 17 configured for determining and storing a said drive level adjustment for each said display element of a display.

19. A driver as claimed in any one of claims 16 to 18 wherein said means for driving includes means for modulating said emitted light, and wherein said means for detecting includes means for selectively detecting said modulation.

20. A combination of the driver of any one of claims 15 to 19 and the display device of any one of claims 12 to 15.

21. Processor control code for a passive OLED display driver, in particular on a carrier, the code comprising code to: drive an OLED to emit light; input light level data relating to a level of light emitted by said OLED; compare said light level data with reference data; and determine drive correction data for said OLED responsive to said comparison.

22. A method of detecting touching of a display device, the device having a plurality of light emitting elements emitting light through a substrate, the method comprising: driving said display device to illuminate one or more of said display elements; detecting a level of light emitted by said one or more display elements after waveguiding of said light within said substrate to a detector spaced apart from said one or more display elements; and detecting said touch by detecting a change in said light level caused by touching said substrate between said one or more display elements and said detector, said touching attenuating said waveguided light within said substrate.

23. Use of a display device as claimed in any one of claims 12 to 15 to sense touching of said substrate.