WO2005083663A2

WO2005083663A2 - Display operating method and calibration method for e.g. a video wall

Info

Publication number: WO2005083663A2
Application number: PCT/GB2005/000689
Authority: WO
Inventors: Timothy Martin Coker; Paul Alastair Silsby; Simon Paul Smith
Original assignee: Inputpearl Limited
Priority date: 2004-02-24
Filing date: 2005-02-24
Publication date: 2005-09-09
Also published as: GB0404091D0; WO2005083663A3

Abstract

A display for displaying a moving image is disclosed. The display has a plurality of modules or sub-modules, each having at least one respective light source at least partially enclosed by a reflector. Each module or sub-module has an array of pixels for modulating and transmitting light from the respective light source to form an image. Each light source has an associated detection means for detecting the brightness of said at least one or more of said light sources, the detection means being arranged to sample light from a sampling location within the space enclosed by the reflector. Signals from the detection means are collated to provide a measurement of the uniformity of light from the respective light sources. A processor controls the light sources to adjust for temperature and aging variations of modules in the display. Also disclosed are methods for calibration and operation of the display.

Description

DISPLAY AND METHOD OF OPERATION

The present invention relates to a display. Particularly, but not necessarily exclusively, it relates to a large display such as may be used in public places for displaying a moving or fixed image. Such large screens are used for a variety of applications. There are known screens of comparatively low addressability, that is they have relatively few individually addressable pixels, and they display- messages mainly in the form of text (e.g. as disclosed in US-A-5394167) . Such message indicators are known for traffic and public transport systems applications and are used to provide concise information to travellers, for example train times, advance warning of delays etc. In other cases screens are of a much higher addressability and can display moving or fixed images. Such screens, for example, may be installed in a sports stadium to show video or television pictures or images, or to display appropriate information such as match or event statistics Such screens may also be employed at outdoor events such as rock concerts or festivals to display moving images to large audiences. Screens may be temporarily rigged on a stage or mounted on a truck, for example. Large screens may also be used as advertising hoardings or billboards. Such displays may be implemented using a number of different technologies. In some circumstances, it is possible to use projection to display images onto a large screen, e.g. in the same way as in a cinema or movie theatre. However such technology is generally only of use where the ambient light levels are low. It is also possible to construct large screen displays using arrays of conventional displays or television monitors, e.g. cathode ray tubes, which screens are sometimes referred to as "video walls" . Such screens are usually not very bright and so such technology is also tends only to be of use where the ambient light levels are low. Furthermore, there are inevitably gaps between the monitors or displays, and this significantly detracts from the overall appearance and picture quality of the screen. A large liquid crystal display (LCD) is disclosed in WO 03/16989. This display is made up of an arrangement of modules . Each module provides a number of liquid crystal pixels. Each pixel modulates and transmits light from a light source disposed behind the pixel. By appropriate control of the state of each pixel, an image can be displayed. Typically, each pixel is made up of red, green and blue sub-pixels- to provide a full colour image. The state of each sub-pixel is controlled by a pixel driver such that each sub-pixel controls the amount of light from the light source that it transmits. Also known are large display screens that use light-emitting diode (LED) pixels. Such displays differ fundamentally from LCDs in that the light is emitted by and not transmitted through the pixels. Thus, LED displays do not have light sources that are independent of the pixels. Again, typically, each pixel is made up of red, green and blue sub-pixels to provide a full colour image. The brightness and colour of each sub- pixel is modulated by altering the electrical drive to each sub-pixel. The present- inventors have realised that a significant problem with displays can be, over time, a deterioration of the uniformity of the light emitted across the entire display. For example, in the case of an LED display, the individual pixels may age at different rates so that some are dimmer than others for a given electrical drive signal. In the case of a modular liquid crystal display, different light sources (e.g. lamps in different modules) may have different brightness at the same input power. These factors can lead to a loss in spatial uniformity of brightness of the light given out by the display. The present invention has been made preferably in order to detect, address, ameliorate, reduce or even eliminate this problem. In a general aspect, the invention provides detection means for detecting the brightness of light emitted by light sources of the display. In a first preferred aspect, the invention provides a display for displaying an image, the display having a plurality of modules or sub-modules, each having at least one respective lamp, each lamp at least partially enclosed by a reflector; and an array of pixels in each module or sub-module for modulating and transmitting light from the respective lamp or lamps to form an image, wherein each lamp has an associated detection means for detecting the brightness of said lamp, the detection means being arranged to sample light from a sampling location within the space enclosed by the reflector. In a second preferred aspect, the invention provides a method of operation of a display according to the first aspect, the method including the step of detecting the brightness of one or more of the lamps using the detection means . In a third preferred aspect, the present invention provides a method of calibrating a display according to the first aspect, the method including the steps of: driving each lamp to provide observably or measurably uniform illumination levels across the display; and setting each detection means so that each detection means outputs equal brightness values for said equal illumination levels. In a fourth preferred aspect, the present invention provides a method of operating a display according to the first aspect, each module being calibrated for brightness and having a module controller for receiving a brightness value from each detection means and at least one lamp driver for independently controlling the brightness of each lamp, the display further having a screen controller, the method including the steps of : the screen controller providing a single calibrated control input (for example 1500Cd/m²) to said module controllers; and each module controller controlling said at least one lamp driver within the module according to the single calibrated control input in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display. In a fifth preferred aspect, the present invention provides a method of operating a display according to the first aspect, each module having a module controller for receiving a brightness value from each detection means and at least one lamp driver for independently controlling the brightness of each lamp, the display further having a screen controller for controlling the module controllers, the method including the steps of : the module controllers sending said brightness values to the screen controller; and the screen controller sending respective control inputs to said module controllers on the basis of the brightness values received from said module controllers, in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display. Preferred and/or optional features will now be set out. These are applicable in any combination or independently with any aspect of the invention, unless ^' the context demands otherwise. Using the invention, it is possible to detect or measure the brightness of one or more of the lamps. It is to be understood that the term lamp is intended to mean a light source. This brightness information may, for example, be collated centrally to give an overall measurement of the uniformity of brightness of the light sources. This can provide the display operator with a useful indication of the performance of the display. Preferably, therefore, the display includes processing means for processing signals from the detection means. The processing means may collate the signals from the detection means to provide a measurement of the uniformity of brightness of the light sources. The processing means may, for example, be a computer suitably configured for this task. In order to provide a high quality image, the detection means is preferably calibrated. In general, calibration can be performed in two basic ways: relative and absolute. Relative calibration allows the detection means to report the same value when all the lamps are the same brightness, but that this value may be arbitrary. In the case of absolute calibration, this value is not arbitrary but is a measurement of brightness in accepted units (e.g. candelas per square metre - Cd/m² - in the SI system) . In principle, each detection means can be individually calibrated. However, it is preferred to carry out a relative calibration of all the detectors and then a single measurement with a calibrated instrument in order effectively to calibrate all the detectors at once. Thus even an absolute calibration of all detection means within a screen would typically include the step of carrying out a relative calibration first. Most preferably, this operation is carried out at several brightness settings such that intermediate brightness values can be interpolated rather than needing to be measured explicitly. The autonomous mode described below relies on the detection means being at least relatively calibrated, preferably absolutely calibrated. However, to achieve the aim of a uniform screen image, relative calibration will suffice. Preferably, each module or sub-module includes at least one lamp driver. The function of the lamp driver is typically to provide independent control (i.e. independent from the other lamps) of the brightness of each lamp in said module or sub-module. Typically, the lamp driver is a ballast such as a dimmable electronic ballast that allows control of the lamp brightness. Preferably, each module includes a module controller. One of the functions of the module controller is typically to provide control input to the lamp driver or drivers in order to control the brightness of each lamp. In use, control input may be provided to the^' lamp driver from the- odule controller on the basis of the brightness detected by said detection means. The display may have a screen controller. The function of the screen controller is typically to provide control over aspects of the display as a whole, including the overall brightness level of the display, for example. Preferably, each module is calibrated for brightness. In this case, the screen controller may provide a single calibrated control input to the module controllers. Due to the calibration of the modules, the intended result is that a substantially equal light output is provided from each lamp, thus providing uniform illumination across the display. In an alternative embodiment, it is possible for 1 the screen controller to provide respective control inputs to each module controller on the basis of the brightness detected by the detection means (in each module) . Thus, the control input sent to a module is tailored to that particular module by the brightness detected in that module. In this way it is also possible to provide uniform illumination across the display. Preferably, the determination of observably equal illumination levels is carried out using a remote detection means arranged in use to detect the brightness of light transmitted through some or all of said pixels. The method may include the additional step of measuring said illumination level to provide absolute calibration for the display. The remote detection means is preferably external to said modules or sub-modules . For example-, the remote detection means may be an imaging device arranged to image said pixel arrays . The imaging device may be a camera such as a video camera. Preferably, the display allows dynamic control of the drive signal supplied to the lamp from the lamp driver in order to compensate for variations in lamp performance over time, e.g. due to temperature variations or ageing effects. Preferably, each module or sub-module has an associated respective light-conductive member arranged to carry light from the sampling location within the space enclosed by the reflector to said detection means . Each light-conductive member may be arranged so that the sampling location is substantially the same within each reflector. Preferably, this location in one at which there is a relatively low spatial variation in brightness, in use, so that minor differences in location of each light-conductive member in different reflectors do not lead to artificial differences in detected brightness values . The light-conductive member may be one or more flexible optical fibres. However, preferably it is a light pipe. The light pipe may be relatively rigid so that it is disposed in a fixed location with respect to the light source. Typically, the light-conductive member samples light from its free end. The detection means may be, for example, a photodiode . In some circumstances, such as where the photodiode is very sensitive, the detection means may have a filter disposed in the light-conductive path between itself and the light-conductive member, in order to reduce the light intensity to a level within a range most suited to that detection means. Typically, the control of each lamp is carried out by the respective lamp driver. Preferably, such control is carried out on the basis of the brightness detected by said detection means, in order to provide a substantially uniform light output from each lamp. In this way, the lamp driver may regulate the power supplied to the respective lamp. For example, the lamp driver may itself be controlled on the basis of feedback from the detection means . In any of the aspects of the invention, preferably each pixel modulates and transmits light from a respective light source, e.g. a lamp. The pixels are preferably capable of greyscale transmission. Preferably, each pixel is capable of transmitting light at a number of levels between the pixel's minimum and maximum transmittance . This number of levels is preferably 1 or more. Preferably, the number of levels corresponds to 2, 3, 4, 5, 6, 7, 8 or higher bit brightness resolution. Most preferably, the transmission of light through each pixel is observably continuously variable. Most preferably, the pixels are liquid crystal pixels . Each pixel may include two or more sub-pixels, typically 3 sub-pixels, each sub-pixel being for . transmission of a different colour, e.g. red, green or blue . Preferably, the display is a large screen display. The screen .may have an area of 3 m² or more, 5 m² or more, or 10 m² or more. Preferably, the screen has an area of at least 15 m². Preferred embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a schematic side view of an arrangement for use with an embodiment of . the invention. Fig. 2 shows diagrammatically a flow of signals for the arrangement of Fig . 1. Fig. 3 shows a flow of signals between a screen controller and display modules in an embodiment of the invention. Fig/ 4 shows a flow of signals similar to Fig. 2 but with more than one lamp per module. Fig. 5 shows a schematic view of an alternative embodiment of the invention, having external detection means . In what follows, similar features in the different drawings are referred to using the same reference numbers, when appropriate. Fig. 1 shows a schematic side view of part of a display module for use in the present invention. The housing of the module is not shown but a lamp 13 is held^' on lamp support 18 within a reflector assembly 12. Lamp 13 is energised to produce light. The light is reflected in a generally forward direction towards front face 10 of the module. Front face 10 has an array of controllable pixels (not shown) for variably modulating and transmitting the light from the lamp and reflector, so as to produce a viewable image. Fig. 1 also shows the arrangement by which a light pipe 11 samples light from within the reflector assembly 12 due to lamp 13 and transfers it to detection means 14. In this figure, and purely by way of example, the detection means is a surface mounted photo-diode, mounted onto a PCB 15. Additional circuitry on the PCB (not shown, but typically a trans-impedance amplifier) generates a brightness signal 16 that is transferred to the module control means or the screen controller-. In this way, the brightness of the lamp 13 can be monitored. Fig. 2 shows the arrangement within a module whereby lamp 13 is controlled to a certain brightness. The lamp is controlled and powered by a lamp driver, typically a dimmable electronic ballast 22 that will be known to the skilled person. The ballast is itself controlled by suitable known means within module controller 23. The arrow 24 shows conceptually the feedback between lamp 13 and detection means 14 (a photodiode is depicted here as an example) . The module controller 23 takes signal (brightness value 16) from the detection means, which is representative of the brightness of the lamp, and uses it to control the ballast 22. In this way, known control techniques can be used to ensure that the brightness of the lamp remains substantially steady at a set value. In an additional feature, this set value can be fed to the module controller by the screen controller (not shown) as a control input, or ^demand' 27. This feature is of particular utility where the detection means (and thus the module) is calibrated, thus the screen controller can issue an explicit demand value (single calibrated value) to each module within the screen, the module controller then being able to control the lamp (via the lamp driver) to this value. In this way, lamps across the whole screen can be made to be at the same brightness . This is an example of the autonomous mode described further below. In an alternate embodiment, the module controller returns the brightness value from the detection means to the screen controller (not shown) as a signal 28. The screen controller will then know the brightness of all lamps within the screen and can then, by passing the appropriate respective control inputs (demand values) to each module controller, control all lamps within the screen to the same value. Fig. 3 shows a screen controller 31 controlling an array of modules 32, although only three modules are shown here by way of example. Each module returns a brightness value 33 to the controller 31 which, in turn, sends a brightness demand value 34 to each module (it being understood that the actual values, for each module, of signals 33 and 34 are likely to be different) . In one embodiment, the non-autonomous mode, the screen controller controls all the lamps to the same value by analysing all the returned values 33 and then issuing demand values 34 to each module such that all the module lamps are controlled to the same value. One strategy for this is to control all the lamps down to the brightness of the dimmest of the lamps in the screen. An alternate embodiment does not require the return brightness values, but relies on each detection means being appropriately calibrated, the screen controller can then issue a single demand value which the modules autonomously use to control their lamps to the required value. This is referred to as the autonomous mode . In the autonomous mode, each detection means is calibrated so that equal brightness values from different detection means are set to indicate observably equal illumination levels from the lamps. The determination of observably equal illumination levels may be carried out using known instruments (e.g. light meters) or by a skilled observer. In the non-autonomous mode such calibration is not necessary. Instead, the lamps are driven to provide observably equal illumination levels across the display, as in the autonomous mode above. However, in this case, the brightness values from the detections means are taken as they are by the screen controller (via the module controllers) . These are collated, typically in a look-up table. The screen controller then knows that those brightness values from those detection means will correspond to a uniform illumination level across the screen. So, to provide that uniform illumination level, the screen controller sends respective control inputs to the module controllers, differences between those respective control inputs accounting for any differences between the different brightness values from the detection means. This process may be repeated for different uniform illumination levels, the resulting look-up table then providing the screen controller with the capability of demanding those different uniform illumination levels from the screen (by sending the appropriate control inputs to the module controllers) . Illumination levels intermediate those stored in the look up table may be provided by interpolation of the relevant brightness values for each detection means. In Fig. 4 the module controller is shown controlling more than one lamp, two. are shown here purely by way of example. Lamps 401 and 402 are each monitored by detection means 403 and 404. The module controller 405 takes a signal 406, 407 from each detection means and uses it to control the lamps via ballasts 408, 409. Again the arrows 410 and 411 are indicative of the feedback from lamp to detection means. Signal 412 is the brightness value returned to the screen controller (equivalent to signal 33 in Fig. 3) and signal 413 is the control input (demand) from the screen controller (cf item 34 above) . In this case, where modules have more than one lamp, there are a number of alternate embodiments of the invention. If the modules can operate autonomously then the autonomous mode can be used, it being left to the module to control all of its lamps. However if the module cannot autonomously control its lamps then the module controller returns brightness values for each of its lamps in signal 412, whereupon the screen controller can use the non-autonomous method described above, given that the demand values 413 will include individual values for each lamp within a module. A further embodiment, for a multi-lamp module, which might be described as semi-autonomous, is wherein the module only controls its lamps such that they are all the same brightness within that module; the module would then report this single value to the screen controller as signal 412 and then drive this single value up or down according to control input (demand value) 413 from the screen controller. Fig. 5 shows a display according to a further embodiment, wherein an external detection means 51 is used to measure the brightness of the lamps within the module array 54. This brightness information is passed to the screen controller 55 as signal 52. Whilst the autonomous or semi-autonomous modes described above work for this further embodiment, these would entail passing the brightness information to each module, which is inefficient. The best mode of use for this further embodiment is the non-autonomous mode described above, wherein the screen controller sends brightness demand values 53 to each module (or each lamp within each module) . In this example, the detection means 51 is a digital camera suitably aligned to capture images of the whole screen. These images are passed as signal 52, for the screen controller to process them and extract the specific brightness information for each module. Alternatively, the processing can be done at the detection means, in which case the signal 52 now represents explicit brightness information. In a still further embodiment, it is possible to combine aspects of the previous embodiments. In particular, this further embodiment uses the external detection means 51 to calibrate the internal detection means 14, 403, 404 (within each module) . In order to calibrate the modules of the display, the lamps in the display are driven to emit light in the normal way. Then, each lamp driver is adjusted (by its module controller) in order to provide an observably uniform level of illumination across the display. The assessment of "observably uniform" may be carried out by a skilled observer or by a remote detection means, as outlined above. In the autonomous mode, once the uniform illumination state has been achieved, each detector is set so that each outputs equal brightness values. In this way, the calibration of the display or modules is ' relative in that each detector is calibrated with respect to the others. Thus, a control input signal from the screen controller, for example, will give rise to a uniform illumination across the display. In the relative calibration state set out above, the actual brightness of the display is not measured. In order to do so, to provide absolute calibration to the display, it is possible to measure the brightness of the uniform illumination level (e.g. using a light meter or digital imaging device) to provide absolute calibration for the display. It is possible also to measure the brightness of the display in a similar way for different uniform illumination levels. By selecting appropriate illumination levels and measuring them, it is possible to provide a basis for subsequent 'interpolation of intermediate illumination levels. During operation of the display, the brightness of each lamp may vary over time for a given drive signal . There are various causes for this, e.g. temperature variation and ageing effects. Thus, there is a need for dynamic control of the drive signal in order to avoid this variation, that is to provide a uniform level of illumination over the screen that is also stable over time. This dynamic control is provided in different ways depending on whether the display is operating in the autonomous or non-autonomous mode. In both modes, the detected brightness value is compared with the expected detected brightness value and the drive signal provided by the lamp driver is adjusted accordingly. In the autonomous mode this control is carried out at the module, In the non-autonomous mode this control is carried out at the screen controller. The above embodiments have been described by way of example. Modifications of these embodiments, further embodiments and modifications thereof will be apparent to the skilled person on reading this disclosure and are within the scope of the invention.

Claims

CLAIMS :

1. A display for displaying an image, the display having: a plurality of modules or sub-modules, each having at least one respective lamp, each lamp at least partially enclosed by a reflector; and an array of pixels in each module or sub-module for modulating and transmitting light from the respective lamp or -lamps to form an image, wherein each lamp, has an associated detection means for detecting the brightness of said lamp, the detection means being arranged to sample light from a sampling location within the space enclosed by the reflector.

2. A display according to claim 1 having processing means for collating signals from the detection means to provide a measurement of the uniformity of brightness of the lamps across the display.

3. A display according to claim 1 or claim 2 wherein each lamp has an associated light-conductive member arranged to carry light from the sampling location within the space enclosed by the reflector to the said detection means .

4. A display according to any one of claims 1 to 3 wherein each module or sub-module includes at least .one lamp driver for independently controlling the brightness of each lamp in said module or sub-module.

5. A display according to claim 4 wherein each module includes a module controller- for providing control input to said at least one lamp driver to control the brightness of each lamp.

6. A display according to claim 5 wherein said control input is provided to the lamp driver from the module controller on the basis of the brightness detected by said detection means.

7. A display according to claim 6 wherein each module is calibrated for brightness, the display having a screen controller arranged to provide a single calibrated control input to said module controllers in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display.

8. A display according to claim 5 having a screen controller arranged to provide respective control inputs to said module controllers on the basis of the brightness detected by said detection means, in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display.

9. A display according to any one of claims 1 to 8 further having a remote detection means arranged in use to detect the brightness of light transmitted through some or all of said pixels.

10. A display according to claim 9 wherein the remote detection means is an imaging device arranged to image said pixels.

11. A method of calibrating a display, the display having : a plurality of modules or sub-modules, each having at least one respective lamp at least partially enclosed by a reflector; and an array of pixels in each module or sub-module for modulating and transmitting light from the respective lamp to form an image, wherein each lamp has an associated detection means for detecting the brightness of said lamp, the detection means being arranged to sample light from a sampling location within the space enclosed by the reflector, and the detection means outputting a brightness value as a result of the operation of its respective lamp, the method including the step: driving each lamp to provide observably or measurably uniform illumination levels across the display.

12. A method according to- claim 11 further including the step of setting each detection means so that each detection means outputs equal brightness values for said equal illumination levels.

13. A method according to claim 11, the display having a screen controller, the method further including the step of receiving brightness values for each detection means and collating said brightness values and relating said brightness values to the corresponding display uniform illumination, level .

14. A method according to any one of claims 11 to 13 wherein the determination of uniform illumination levels is carried out using a remote detection means arranged in use to detect the brightness of light transmitted through some or all of said pixels.

15. A method according to any one of claims 12-14^' including at least one further additional step of providing at least one different uniform illumination level to provide further calibration values for subsequent interpolation of intermediate uniform illumination levels.

16. A method according to any one of claims 11 to 15 including the additional step of measuring said uniform illumination level or levels to provide absolute calibration for the display.

17. ^• A method of operating a display, the, display having: a plurality of modules or sub-modules, each having at least one respective lamp at least partially enclosed by a reflector; and an array of pixels in each module or sub-module for modulating and transmitting light from the respective lamp to form an image, wherein each lamp has an associated detection means for detecting the brightness of said lamp, the detection' means being arranged to sample light from a sampling location within the space enclosed by the reflector, and the detection means outputting a brightness value as a result of the operation of its respective lamp, each module being calibrated for brightness and having a module controller for receiving a brightness value from each detection means and at least one lamp driver for independently controlling the brightness of each lamp, the display further having a screen controller the method including the steps of : the screen controller providing a single calibrated control input to said module controllers; and each module controller controlling said at least one lamp driver within the module according to the single calibrated control input in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display.

18. A method of operating a display, the display having: a plurality of modules or sub-modules, each having at least one respective- lamp at least partially enclosed by a reflector; and an array of pixels in each module or sub-module for modulating and transmitting light from the respective lamp to form an image, wherein each lamp has an associated detection means for detecting the brightness of said lamp, the detection means being arranged to sample light from a sampling location within the space enclosed by the reflector, and the detection means outputting a brightness value as a result of the operation of its respective lamp, each module having a module controller for receiving a brightness value from each detection means and at least one lamp driver for independently controlling the brightness of each lamp, the display further having a screen controller for controlling the module controllers, the method including the steps of : the module controllers sending said brightness values to the screen controller; and the screen controller sending respective control inputs to said module controllers on the basis of the brightness values received from said module controllers, in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display.

19. A method of operating a display, the display having: a plurality of modules, each having at least two lamps each at least partially enclosed by a respective reflector; and an array of pixels in each module for modulating and transmitting light from said lamps to form an image, wherein each lamp has an associated detection means for detecting the brightness of said lamp, the detection means being arranged to sample light from a sampling location within the space enclosed by the lamp's reflector, and the detection means outputting a brightness value as a result of the operation of its respective lamp, each module having: a module controller, and a lamp driver for each lamp to independently control the lamp brightness, the display further having a screen controller for controlling the module controllers, the method including the step of: the module controller receiving said brightness values from said detection means and sending respective driver control inputs to said lamp drivers on the basis of the brightness values, in order to provide a^" substantially equal light output from each lamp in the module that is also stable over time.

20. A method according to claim 19 further including the steps : each module controller sending an output signal to the screen controller, the output signal indicating the brightness of the lamps in the module; and the screen controller sending respective control inputs to said module controllers on the basis of the output signals received from said module controllers, in order to provide a substantially equal light output from each lamp in the display, thereby providing uniform illumination across the display.

21. A method according to claim 19 wherein each module is calibrated for brightness, the method further including the steps : the screen controller providing a single calibrated control input to said module controllers; and each module controller controlling said lamp drivers within the module according to the single calibrated control input in order to provide a substantially equal light output from each lamp, thereby providing uniform illumination across the display.