EP1394763A1

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Publication number: EP1394763A1
Application number: EP02017703A
Authority: EP
Inventors: Christian Henn; Olivier Delbeke
Original assignee: Conrac GmbH
Current assignee: Conrac GmbH
Priority date: 2002-08-07
Filing date: 2002-08-07
Publication date: 2004-03-03

Abstract

A device for compensating a degradation of a light output of a cell of a plasmadisplay which occurs during the operation of the plasma display. The device forcompensating comprises means for providing and storing data concerning brightnessand duration of said brightness of at least one cell of the plasma display over aspecified time period. Furthermore the device for compensating comprises means forproviding a compensation value for the at least one cell of the plasma display forcompensating the degradation of the light output of the at least one cell based on thestored data of the at least one cell to ensure that the brightness of the cellcorresponds approximately to a brightness of a cell with a non-degraded light output.In accordance with the present invention there is provided a device and a methodthat increases the operating lifetime of a plasma display and reduces the occurrenceof the burn-in effects in an flexible way.

Description

BACKGROUND OF THEINVENTION1. Field of the Invention

The invention generally relates to computer displays and public displays, and inparticular to a device and method for compensating the degradation of a light outputof a plasma display panel.

2. Description of the Related Art

Displays are an important but expensive part of the interface between man andmachine. Therefore, one of the most significant expressions of the digital age are theomnipresent displays showing data, information and pictures. These displays areused for wrist watches, calculators and laptops but also more and more as publicdisplays. This new generation of flat panel displays can be found at many places likepresentation rooms, show rooms, mini theatres and in the public. They are used atwork or at school for video conferencing or presentations, in public or companyfacilities as information displays or at airports, train and bus stations for departureand arrival information. Many other applications emerge now as the displaysbecome more reliable and stable in quality.

All modern displays are based on a conversion of electrical signals in optical signals.For a business application the electro-optical properties for display have to fit to theapplication conditions. According to their physical properties displays are divided intoactive and passive displays. Active displays, e.g. cathode ray tube, light emittingdiode displays, vacuum fluorescence displays and plasma displays are emitting lightto create the image. Passive displays, e.g., liquid crystal displays do not create theemitted light but use the ambient light.

Active displays use phosphors for generating visible light, either by exiting withelectrones (CRT, VFD, FED), or UV light (plasma display) or fast charged carriers(electro luminescence display).

As shown in FIG. 1 colored plasma display panels have a simple construction,basically consisting of two thin sheets of glass separated by a few hundred microns,thefront glass substrate 110 and therear glass substrate 120. The space betweenthe sheets of glass is filled withcells 130 containing a rare gas (xenon, neon etc.).Each cell is coated on the bottom and at the surface of thebarrier rib 140 with adifferent color of phosphor in red, green or blue. Electrodes (150, 160 and 170) canbe found at the top and bottom of each sheet of glass or substrate. In ordinaryfluorescent slides, voltage is supplied to the electrodes, resulting in anelectricaldischarge 180. This creates an ultraviolet emission, which emits fluorescentmaterials that coat the inside of the bulb.

In the color plasma display panel (PDP) voltage is supplied to selected cellsdepending on the image and the red, green and blue phosphor combine to create thecolors in the images displayed. As shown in FIG. 2 therefore three cells, one red,one green and one blue are combined to form one pixel.

Referring back to FIG. 1 to create the red, green or blue light in the cells first avoltage is supplied to thescan electrode 170 and thesustain electrode 160, effectinga preparatory, or "priming" electrical discharge. Then voltage is supplied to thescanelectrode 170 and thedata electrode 150 of the cell to which data is to be written,resulting in a discharge. This in turn creates a wall charge on the dielectric layer ofthe addressed cell. In a third step, voltage is applied again to thescan electrode 170and thesustain electrode 160, generating a discharge which is emitting ultravioletrays. If an AC voltage is continuously applied, the process occurs repeatedly,resulting in the emission of light for displaying an image. Finally, in order toneutralize the wall charge in the cell in preparation for the next screen update, alower voltage is supplied to the scan electrode and the sustain electrode, resulting ina low level discharge.

One important aspect for public or information displays is the size as especially largecolor displays are ideally suited for information display systems in airports, railwaystations and convention centers. The currently available 42 inches to 61 inches colorplasma displays combine the advantage of low depth and weight with a viewingangle of more than 160°. Although CRT and TFT may be used as public displays,presently, the only flat display technology for business applications with a largediagonal measurement is the plasma technology. Its basic principle uses the plasmaeffect for light generation as described above.

In general, an information display is a display device with an integrated PC. Thecontroller may be a computer with network and graphics abilities. An embeddeddisplay controller allows the integration into already existing FIDS networks. Specialversions with integrated PC are already available. They are offering an integratedPC platform based on an Intel Pentium 3 processor. Additionally several free PCI orISA slots allow the integration of network or ISDN cards,hardware MPEG 2 decodersand any other cards that are suited for the application. Furthermore, these moderndisplays include a processor, a Random Access Memory, a graphic card or a graphiccontroller, interfaces like RS232, RS485, USP or parallel interfaces and interfaces formouse and keyboards, hard disks or other system components. Furthermore, theyinclude a power supply, climate control circuit, a TV cable tuner, audio amplifiers andintegrate loud speakers or have interfaces for external loudspeakers.

In this way, such a high performance PC platform provides all necessary functions forstate of the art advertising or multimedia presentations including live videosequences and high resolution graphics. All PC based operating systems andapplication programs can be implemented and make the unit into an intelligentdisplay terminal.

There are also offered information displays based on a controller system. Thecontroller system includes an integrated graphic engine board which provides notonly the local intelligence but also the control of key display functions from a centralserver in the network. The graphic engine board controls the picture performance tominimize a jitter or a phase or scaling error. A gamma correction circuitry ensurestrue color display of graphics or videos. An integrated hardware watchdog monitors the controller function and automatically reboots the unit in case of system shutdownor fault. Theoptional hardware MPEG 2 video audio decoder allows programmablepropping and positioning of full motion video and graphics overlay.

The embedded controller types provide a signal and a control interface to the specificdisplay subsystem used. The created pages are saved in a video memory. It is readout with the pixel clock and transferred to the display. The control interface can bean l²C or a control bit interface. The displayed image on the screen can be seen as acopy of the video memory content.

The contrast control function on an embedded controller is realized inside thegraphics controller chip. Contrast control varies the end level of the gammacorrection color look-up table.

On internal direct digital links between the video memory output and the displaymodule input the video memory is organized with the physical resolution of theconnected display. The range of resolutions range from VGA to UXGA or 16:9formats like the popular WVGA with 848X 480 picture elements. FIG. 2 showsschematically such a display. Another resolution method is ALIS (alternate lighting ofsurfaces method) that alternately displays odd and even lines at high speeds. Thistechnology makes it possible to create high resolution images using about the samenumber of electrodes as used in conventional VGA technology. ALIS technologyprovides a pixel resolution of 1024 X1024 compared with WXGA displays providing768 X 1365 .

Furthermore, an optional touch screen system offers an excellent large screen man-machineinterface for any point of information application. All these terminals arebuilt for 24 hour operation in professional applications and uses high grade industrialcomponents.

Due to their network capabilities a system integrator can configure the displaysaccording to special requirements. In most of the applications the informationdisplays are connected as remote displays with a network connection to a server.The operating system of the application's specific software installed on the embedded controller, communicates with the server and builds pages with thecommands coming via the network. Specific software allows the complete control ofmost of the display alignment and control functions.

Their technology makes the color plasma displays immune against magneticinterference and hence suitable for installation near railways, industrial applicationswith high electric energy and even for mobile use without the need of additionalshielding. The normal linearity, geometry, conversion and focussing problems wellknown to users of conventional CRTs monitors simply do not exist, there is a uniformclarity to the digital operation of the color plasma displays where each cell isaddressed individually.

The color depth of video memories varies depending on quality requirements anddisplayable numbers of colors between 4 and 10 bits per primary color red (R), green(G) and blue (B). For example, 8 bits per color is equivalent to 256 shades of eachcolor resulting in 16.7 million colors.

The contrast control of a display varies the gain of the video input signal. It can varyfrom 0% to 100%. The full set of contrast control uses the maximum dynamic rangeof the connected display.

As mentioned above, the walls of the cells are coated with phosphors for convertingthe UV rays invisible light. On the one hand phosphors must have a high quantumefficiency, that means that the absorbed UV light is transformed in maximum possibleamount of visible light. And on the other hand, the phosphors may not luminescencelonger than 10 milliseconds, otherwise a picture refresh rate of 100 Hz would beinfluenced by the luminescence. As mentioned above, the cells are filled with raregases or a mixture of rare gases where generally a mixture of neon and xenon with a3 to 5% ratio of xenon is used. This neon xenon plasma emits radiation in the rangeof 140 to 190 nm. As suitable phosphors for PDPs are used Y(V,P)O₄:Eu or(Y,Gd)BO₃:Eu for red color, Zn₂SiO₄:Mn for the color green and BaMgAl₁₀O₁₇:Eu forthe color blue. The blue phosphor BaMgAl₁₀O₁₇:Eu has a lower stability under VUVlight as the red and green phosphors. Therefore, during the operation time the bluecolors of the plasma display will fade out and the original white of the display will shift to a yellow. This degradation of the blue phosphor is caused by oxidation of theactivators Eu²⁺ to Eu³⁺. The use of different phosphors may improve the stability buton the other hand the luminescence will be prolonged, which influences the refreshrate of the display.

The phosphor elements in a plasma display do not radiate their colored light at aconstant amplitude during their life span. The intensity degrade is high during thefirst few of hours. After that period of time the curve is more horizontal, but stilllowering during the course of the remainder of its lifetime. Due to the rapid changesin brightness at the very beginning it is necessary to let the display undergo "pre-burn-in"period. This is usually a couple of days the usual lifetime of a commercialplasma display is about 10 000 hours. After that period the brightness of the displaywill be much lower and, in the case of static images, certain areas will be less ormore bright. These are so-called "burned-in-mark areas".

Therefore it is necessary to develop a special PDP driving mechanism to increasethe operating lifetime and reduce occurrence of so-called burn-in affects. Asmentioned above, PDPs are based on phosphors like cathode ray tube basemonitors. Red, green and blue phosphors are exited by UV light to generate coloredlight. The light output of the phosphors degrades over time resulting in lowerbrightness over the lifetime of the product. For moving pictures and typical videosequences, all colors and all areas of the screen are stressed and worn out evenly soit is almost impossible to notice the reduction in brightness. In airport applicationshowever, where the image displayed is often static (e.g. airline logo) the screen is notstressed equally. This leads to an uneven brightness degradation, which results inthe burn-in of the image on the screen. For example, if a blue circle is displayedcontinuously in the center of the screen, only the blue phosphor would be worn out inthis area. If after some time it tries to display a full white picture, the area of the bluecircle would appear quite yellow. This is because the red and green, which are notworn at all, will make a greater contribution than that of the worn out blue with theresult been yellowish. This phenomenon is well known and can of course bereduced by careful use of the display and using simple tricks such as screen savers,inverse colors and moving the images where possible. However, a lot of applications do not allow temporary removal of the usable information or to change the standardcolors of a logo.

Another way of burn-in reduction is an orbiter function that slowly, but imperceptibly,moves the image over a number of pixels. This amount of pixels must be small(smaller than 10 pixels) to maintain as much usable display area as possible. As aconsequence, this system will not be effective when large areas of the display (e.g.,big airline logo) are being used. It will only result in smoother transitions from theburned-in area to the non burned-in area.

Moreover, the intensity degrade is high during the first 10^th of hours. After that periodof time, the curve is more horizontal, but still lowering during the course of theremainder of its lifetime. Due to the rapid changes in brightness at the verybeginning it is necessary to let the display undergo a "pre-burn-in" period. This isusually a couple of days.

The useful lifetime of a commercial plasma display is about 10 000 hours. After thatperiod the brightness of the display will be much lower and, in the case of staticimages, certain areas will be less or more bright.

US 5821917 discloses a system and method to compensate for the effects of agingof the phosphors and faceplate upon color accuracy in a cathode ray tube whereinbeam current measurements are made upon individual cathodes of a cathode raytube to sample the individual beam currents at periodic intervals. The sum totals ofthe individual beam current measurements are then stored in a non-volatile memoryand correction factors are calculated for both luminous efficiency degradation anddeviations in hue, based on the stored sum total beam current measurement.

Due to rationalization and the need for information there is a big demand for publicdisplays providing the necessary quality in color stability and offering the possibilitiesfor a flexible administration. Moreover, due to this claimed flexibility there is a needto reduce the occurrence of burn-in effects in a way that also allows themultifunctional use of these displays.

SUMMARY OF THE INVENTION

An improved device and method for compensating a degradation of a light output of acell of a plasma display panel which occurs during the operation of the plasmadisplay, is provided that may allow to compensate the effects of aging of thephosphors upon color accuracy in plasma display panels.

In one embodiment, a device for compensating a degradation of a light output of acell of a plasma display which occurs during the operation of the plasma display isprovided. The device comprises means for providing and storing data concerningbrightness and duration of that brightness of at least one cell of the plasma displayover a specified time period. Furthermore, the device comprises means for providinga compensation value for the at least one cell of the plasma display for compensatingthe degradation of the light output of the at least one cell based on the stored data ofthe at least one cell to ensure that the brightness of the cell correspondsapproximately to a brightness of a cell with a non-degraded light output.

In a preferred embodiment, a device for compensating a degradation of a light outputof a cell of a plasma display which occurs during the operation of the plasma displayis provided. The device comprises means for providing and storing data concerningbrightness and duration of that brightness of cells of the plasma display over aspecified time period. Furthermore, the device comprises means for providing acompensation value for the cells of the plasma display for compensating thedegradation of the light output of the cells based on the stored data of the cells toensure that the brightness of the cell corresponds approximately to a brightness ofcells with a non-degraded light output. The device further comprises a graphicscontroller adapted for generating a video signal for displaying pictures on the plasmadisplay and wherein the means for providing a compensation value further comprisesa programmable signal processing device comprising a compensation frame bufferfor storing the compensation value for the at least one cell of the plasma display.The means for providing a compensation value further comprises a high speedmultiplication unit for correcting video signals using the compensation value of the atleast one cell and a buffer for storing the results of the high speed multiplication. The high speed multiplication unit is connected to the graphics controller for receivingvideo signals generated by the graphics controller and to the programmable signalprocessing device for receiving the compensation values. Furthermore, the meansfor providing and storing data are providing and storing data for each single cell ofthe plasma display. Additionally, the programmable signal processing deviceprovides compensation values for each single cell of the plasma display based on thestored data of each single cell regarding the degradation of different phosphors usedin the different cell types and stores the compensation values for each single cell inthe compensation frame buffer at every refresh cycle the plasma displays operatedwith. Initial values for the compensation values of the cells are stored in thecompensation frame buffer so that an initial maximum adjustable brightness of thecells is limited to a specific percentage of a maximum allowable brightness of thecells. Additionally, the programmable signal processing device further comprises aconfiguration unit with an interface for inputting data for changing the compensationvalue of the at least one cell in the compensation frame buffer.

In second preferred embodiment, a device for compensating a degradation of a lightoutput of a cell of a plasma display which occurs during the operation of the plasmadisplay is provided. The device comprises means for providing and storing dataconcerning brightness and duration of that brightness of at least one cell of theplasma display over a specified time period. Furthermore, the device comprisesmeans for providing a compensation value for the cells of the plasma display forcompensating the degradation of the light output of the cells based on the storeddata of the cells to ensure that the brightness of the cells corresponds approximatelyto a brightness of cells with a non-degraded light output. The device forcompensating a degradation of a light output further comprises a graphics controlleradapted for generating a video signal for displaying pictures on the plasma displayand wherein the means for providing a compensation value further comprises aprogrammable signal processing device with a compensation frame buffer for storingthe compensation values for the cells of the plasma display. The programmablesignal processing device is connected to the graphics controller for receiving thevideo signals and is correcting the video signals using the compensation values ofthe cells. The compensation frame buffer is a Flash Memory, a Random AccessMemory or a Hard Disk. The device for compensating a degradation of the light output includes the interface which is connected to the programmable signalprocessing device and the programmable signal processing device is configurablewhile the interface over a computer network or a telecommunication network.Additionally, the programmable signal processing device further comprises aconfiguration unit with an interface for inputting data for changing the compensationvalue of the at least one cell in the compensation frame buffer. The programmablesignal processing device provides compensation values for each single cell of theplasma display based on the stored data regarding the degradation of differentphosphors used in different cell types and stores the compensation values for eachsingle cell in the compensation frame buffer. The means for providing and storingdata providing and storing data for each single cell of the plasma detail.Furthermore, the means for providing and storing data are providing and storing dataat every refresh cycle the plasma display is operated with. Additionally theprogrammable signal processing device provides compensation values at everyrefresh cycle the plasma display is operated with.

In a third preferred embodiment there is provided a calibration system for calibratinga compensation value for the cells of the plasma display to compensate thedegradation of the light output of the cells, which comprises the device forcompensating a degradation of a light output according to the first and secondembodiment. The calibration system further comprises an electronic camera fortaking screen images of the plasma display for measuring brightness of at least onecell of the plasma display and a PC or a laptop connected to the electronic cameraand the configuration unit of the device for compensating a degradation of a lightoutput. The programmable signal processing device further comprises theconfiguration unit with an interface for inputting data for changing the compensationvalue of the at least one cell of the compensation frame buffer. The electroniccamera is transmitting data of the taken screen images to the electronic device. Theelectronic device is transforming the transmitted data to the compensation values forthe at least one cell of the plasma display and is transmitting the compensationvalues to the configuration unit to change the compensation value of the at least onecell in the compensation frame buffer.

Furthermore, another embodiment is provided a method for compensating adegradation of a light output of a cell of a plasma display which occurs during theoperation of the plasma display, comprising the steps of providing and storing dataconcerning brightness and duration of that brightness of at least one cell of theplasma display over a specified time period and providing a compensation value forthe at least one cell of the plasma display for compensating the degradation of thelight output of the at least one cell based on the stored data of the at least one cell toensure that the brightness of the cell corresponds approximately to a brightness of acell with a non-degraded light output.

In accordance with the present invention there is provided a device and a methodthat increases the operating lifetime of a plasma display and reduces the occurrenceof the burn-in effects in an flexible way. It is suitable for both kinds of displays eithershowing static or dynamic pictures. The device and method is usable for differentkinds of displays as it is configurable and offers to be administrated via computernetwork. Furthermore, it offers the advantage that it can be integrated in alreadyexisting displays. Using the calibration system in accordance with the presentinvention a time and cost saving device and method is offered for adjusting thebrightness of each cell of a display whenever it is need.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are incorporated into and form a part of thespecification for the purpose of explaining the principles of the invention. Thedrawings are not to be construed as limiting the invention to only the illustrated anddescribed examples of how the invention can be made and used. Further featuresand advantages will become apparent from the following and more particulardescription of the invention, as illustrated in the accompanying drawings, wherein:

FIG. 1: is showing a schematical diagram illustrating the structure of a cell of aplasma display;
FIG. 2: is showing a schematical diagram illustrating the set-up of a plasmadisplay;
FIG. 3: is showing a schematical diagram of a first preferred embodiment of thepresent invention;
FIG. 4: is showing a diagram displaying the brightness over time of plasmadisplays without a device for compensating a degradation of a light outputof the cells of the plasma display comparing with plasma displaysincluding a preferred embodiment of the present invention.
FIG. 5: is showing a schematical diagram illustrating the application of anotherpreferred embodiment of the present invention;
FIG. 6: is showing a schematical diagram illustrating a preferred embodiment ofthe calibration system in accordance with the present invention;
FIG. 7: is showing another schematical diagram illustrating a preferredembodiment of the calibration system in accordance with the presentinvention;

DETAILED DESCRIPTION OF THE INVENTION

The illustrative embodiment of the present invention will be described withinreference to the figure drawings.

FIG. 3 illustrates in a schematical diagram the device for compensating adegradation of a light output of a cell for plasma display according to a first preferredembodiment of the present invention. The compensating device has aunit 310 forproviding and storing data concerning brightness and duration of the brightness ofcells of a plasma display. Theunit 310 for providing and storing data is capable ofproviding the brightness and duration data in accordance to a specific time period.For this the unit collects data for each cell of the plasma display how long and howintensively the cell was activated. Thedata unit 310 is connected to acompensationunit 320, which is providing a compensation value for the cells of the plasma display.

Thecompensation unit 320 is providing compensation values for compensating thedegradation of the light output of the cells of the plasma display based on the storeddata of the cells to ensure that the brightness of the cell corresponds approximatelyto a brightness of a cell of the plasma display with a non-degraded light output.Preferably, thecompensation unit 320 comprises a programmable signal processingdevice with acompensation frame buffer 340 for storing the compensation values.Furthermore, the compensatingdevice 300 includes a graphic controller which isgenerating video signals for displaying pictures on a display preferably on a plasmadisplay. Additionally, the compensatingdevice 300 includes a highspeedmultiplication unit 350 for correcting video signals using the compensation valueprovided by thecompensation unit 320. For this the highspeed multiplication unit350 is connected to the graphics controller and to thecompensation unit 320.Results of the high speed multiplication are transmitted to abuffer 360 for providingthe compensated video signals to a display preferably plasma display. Forsynchronizing the video signals generated in the graphics controller andcompensation values provided by thecompensation unit 320 thedata unit 310 andunit compensation 320 are connected to the graphics controller for synchronizing theprocessing cycles. For receiving digital signals of pictures to be displayed on thedisplay thegraphics controller 330 and thedata unit 310 and thecompensation unit320 have an interface to be connected to a computer bus system or a computernetwork or a telecommunication network. Preferably, these units comprise a PCIslave interface.

As shown in FIG. 3 in an alternative of the preferred embodiment of the presentinvention thedata unit 310 and thecompensation unit 320 are integrated in a fieldprogrammable gate array (FPGA). This is a programmable hardware block whichincludes also the PCI slave interface and additional registers for horizontal andvertical resolution programming. It further comprises a glue logic for reading andwriting the compensation values in thecompensation frame buffer 340. The read outof the compensation values of the compensation buffer and the serialization of thecompensation values is synchronized with a high speed pixel clock also included inthe FPGA. The high speed clock is synchronized with the clock of the graphicscontroller. Preferably the FPGA circuit is clocked with 100 MHz. Alternatively, theclock is operated with a lower or higher frequency in accordance with the application.

For the design of the FPGA circuit the color resolution, the display resolution and therefresh rate of the display has to be considered.

For a preferred high speed alternative all components like graphics controller, videoframe buffer, data unit, compensation unit, compensation frame buffer, high speedmultiplication unit and buffers for the compensated video signal for the display paneletc. are integrated on one printed circuit board (PCB). As output devices foroutputting the compensated video signal are preferably used LVTTL or LVDSdevices. This increases the performance of the compensatingdevice 300 andreduces the number of piece parts.

Alternatively, the programmable signal processing device on the FPGA or the FPGAitself comprises a serial or parallel interface or an interface to a computer network ortelecommunication network. Preferably the FPGA includes a RS232 or a USB andan Ethernet interface. Via these interfaces the FPGA or the programmable signalprocessing device is configured or updated. This interface is used to select specificoperation modes of the programmable signal processing device and the FPGAfunctions. Via these interfaces it is possible to adjust the clock frequency inaccordance with the application and to store or update programs in a memory of theFPGA.

Therefore it is possible to configure the programmable signal processing device inaccordance with the application either that it determines compensation values of thecells of the display using a look-up table or calculating the compensation valuesusing a specific formula. This specific formula is changeable in accordance with thedisplay which is operated by the compensatingdevice 300.

Furthermore, the programmable signal processing device is either configurable thatonly cells of a specific area of the displays are compensated or that all cells of thedisplays are compensated. Furthermore, the three different cell types (red, greenand blue or cyan, magenta and yellow), forming one pixel are compensatedregarding the degradation of the different phosphors used in the different cell types.

Moreover, the time cycles for providing and storing the data concerning brightnessand duration of the brightness are adjustable to a specific time period and also thetime cycle for providing the compensation values for all cells or only for a part of thecells of the display. It is also possible to adjust the time cycle for providing andstoring the data and providing the compensation values to the refresh cycle of thedisplay, e.g. 100 Hz.

Via the above-mentioned network interface or an additional configuration interfacethe compensation values in the compensation frame buffer are thereby changeable.It is configurable whether all compensation values in the compensation frame bufferare changed or only a specific compensation value in the compensation frame buffer.

In accordance with a preferred embodiment of the present invention the initial valuefor the compensation values in the compensation frame buffer are defined so that aninitial maximum adjustable brightness of the cells is limited to a specific percentageof a maximum allowable brightness of the cells. Preferably, this compensation valueis set to a value that the initial maximum brightness of the cells is 80% of a maximumallowable brightness of the cells. This might be seen as a commercial disadvantage,but it will certainly drastically lengthen the display's lifetime not only concerning thecolor quality. The remaining 20% can be used to compensate the degradation inbrightness on a cell per cell or pixel or pixel basis. Without this margin acompensation of the degradation of the brightness is not possible.

As mentioned above thedata unit 310 or alternatively the FPGA is continuouslygathering information of the RGB brightness and the duration of the brightness ofevery cell or every pixel formed by the three different cell types. That information isstored in a memory included in the FPGA. A formula will calculate a compensationvalue for each RGB cell of each pixel and hold this in the compensation frame buffer.Alternatively the compensation values for each RGB cell of each pixel are providedby a look-up table. Before an update of the image or part of the image is done thevideo signal will be multiplied by the compensation values in the highspeedmultiplication unit 350 for each pixel. The high speed multiplication unit is capable todo this for a 24 bit or higher video mode. Furthermore the preferred embodiment is capable of compensating MPEG video for digitized analogue video coming from areal time analogue source.

As mentioned above, in the case that a blue circle is displayed continuously in thecenter of a screen, only the blue phosphor would be worn out in this area. One triesto display a full white picture and after some time the area of the blue circle wouldappear quite yellow without compensating the degradation of the light output of theblue cells. This is because the red and green cells which are not as worn out at all,will make a greater contribution to the brightness than the worn out blue. The resultis a yellowish color instead of a white color.

Using the preferred embodiment of the present invention all blue cells previouslyshowing that circle will be compensated with a slightly higher brightness value.Therefore it is possible to prevent a yellowish circle in the white picture. Thiscompensation procedure is done as long as there is a margin in the brightness valuethat can be programmed. Once 100% of the brightness signal is reached the cellsare addressed with, the degradation of the brightness will start to become visible.Therefore the preferred embodiment of the present invention lengthens the lifetime ofdisplays by displaying a perfect picture for a much longer period than in a non-compensateddisplay.

As already mentioned in the preferred embodiment, thedata unit 310 is collecting thedata concerning the brightness and the duration of the brightness for specified cellswithin a specific time period. The data is either accumulated by the data unit and thenprovided to thecompensation unit 320 or provided to thecompensation unit 320without accumulating. For the second alternative the compensation unit isaccumulating the data and alternatively storing the history of the data.

In a second preferred embodiment there is provided a device for compensating adegradation of a light output of cells of a plasma display, which comprises adata unit310 for providing and storing data concerning brightness and duration of thebrightness of cells of a plasma display and acompensation unit 320 for providing acompensation value for cells of a plasma display for compensating the degradation ofthe light output of the cells based on the stored data of the cells. The compensatingdevice 300 further comprises a graphics controller, which is generating video signalsfor displaying pictures on a display. Thecompensation unit 320 for providingcompensation values further comprises a programmable signal processing devicewith acompensation frame buffer 340 for storing the compensation values. Thesignal processing device is either connected to the graphics controller for receivingsignal or a computer bus system to correct the video signals of the graphicscontroller using the compensation values of the cells stored in the compensationframe buffer. The data concerning the brightness and the duration of the brightnessof the cells is preferably stored in a Flash Memory, Random Access Memory or on aHard Drive. The programmable signal processing device calculates thecompensation values for cells either of a specific part of the display or the wholedisplay. This can be done by a specific formula or compensation values are providedby using a look-up table. After calculation of the compensation values of a part of thecells or all cells of a display the compensation values will be stored in thecompensation frame buffer 340. But preferably the compensation values for all cellsare calculated and updated for every refresh cycle.

Preferably the graphics controller and the programmable signal processing devicehave an interface to a computer bus system, a computer or a telecommunicationnetwork or/and a serial and parallel interface. Alternatively the graphics controller,the programmable signal processing device, the compensation frame buffer and anadditional video frame buffer for the graphics controller are arranged on one PCB. Ina preferred alternative the PCB further includes processor like Pentium 3, RandomAccess Memory, Serial RS232, Interface, Parallel Interface and an Interface for amouse and a keyboard and a USB interface. Furthermore, this preferred alternativeprovides a hard disk and an Ethernet interface and is capable of running anoperations system. Furthermore it provides a power supply for the electroniccomponents. FIG. 6 shows in a schematic diagram also a block diagram of apreferred alternative 610 of this embodiment, with the integrated components likeadditional climate control etc.

In the preferred alternative the calculation of the compensation values of the cells ofthe display is done by a software module which is executed by the processor incombination with the Random Access Memory and the Hard Disk. It continuously gathers information of the brightness and the duration of the brightness of every cellor pixel, which is built by the three different cell types. Before an update of the imageor a part of the image is done, the compensation software will correct the RGBvalues of the video signal with the compensation values for each cell respective pixel.Furthermore, the second preferred embodiment of the present invention includes aconfiguration unit with an interface for inputting data for changing the compensationvalues in the compensation frame buffer. This ensures that initial compensationvalues can be manually or automatically configured in accordance with a specificapplication or a specific display type.

By storing and accumulating the data for the intensity and the duration of thebrightness of each cell separately this data can be reused for calculating an improvedset of compensation values in the case that an improved formula or improved look-uptable is provided for calculating the compensation values.

Preferably an initial value for the compensation values is stored in the compensationframe buffer, so that an initial maximum adjustable brightness of the cells is limited toa specific percentage of a maximum allowable brightness of the cells. The remainingmargin can be used to compensate the degradation of the brightness of the displayon a pixel per pixel basis. Alternatively different formulas or a look-up table are usedfor calculating the compensation values of the different cell types (red, green andblue or cyan magenta and yellow). As mentioned above, also this preferredembodiment is configurable so that the degradation of the brightness of cells of a partof the display or of the whole display is compensated. Furthermore, it is adjustablevia the computer network interface in which time period the data concerningbrightness and duration of the brightness are stored and in which time period thecompensation values for the cells are calculated and updated in the compensationframe buffer. Preferably this is done with every refresh rate of the display, e.g. 100Hz.

FIG. 4 illustrates a degradation of brightness of two different information displayscompared with the information displays using a preferred embodiment of the presentinvention. As it can be seen from the illustration, the brightness level of the displayusing e.g. ALIS technology or WVGA technology degrades with time. Although the uncompensated screens show a higher brightness during the beginning of theiroperation time the compensated displays offer a constant brightness level of the cellsover a long operation time of several thousand hours. This is necessary for offeringan adequate color quality.

FIG. 5 illustrates an alternative of the first and second preferred embodiment in aschematic diagram, wherein the compensatingdevice 300 is a stand alone devicewhich is addressed, configured and programmed via a computer network or atelecommunication network. For this the compensatingdevice 300 includes aninterface to be connected to a computer network or to a wireless telecommunicationnetwork or an ISDN- or a DSL interface.

Furthermore, the compensatingdevices 300 are connected to plasma monitors 550,CRTs 560 orLCDs 570 and are configured according to the application via thenetwork 520 with acomputer 510 also connected to the computer network or to atelecommunication network. The pictures to be displayed on the plasma display,CRT or LCD are either provided by thecomputer 510, which is configuring thecompensating device or other computers (530, 540) connected to the computer ortelecommunication network which are allowed to address the compensatingdevice300 via an lP address. In that way there are displayed images, logos, photos,animations, tables imported from a database or videos in different formats.

As the compensatingdevices 300 are configurable via network an alternative versionof the first and second preferred embodiment offers a function that the look-up tableor formula for calculating the compensation values for the cells are updated via thenetwork continuously in accordance with the newest data concerning the phosphorsused in the cells or other properties of the displays. Additionally the compensationvalues are updated or refreshed via this function by asystem administrator 510.

This embodiment also offers the possibility to update the programs, formulas andlook-up tables by replacing an EPROM or flash memory of the programmable signalprocessing device.

Another alternative of the first and second preferred embodiments of the presentinvention, is that the compensatingdevice 300 is integrated into a plasma display orin an information display. Integrating the compensatingdevice 300 into aninformation display offers the opportunity that redundant parts like processor, harddisk, random access memory, PCI bus, etc. have to be provided only once.

In a further alternative to the first and second preferred embodiments of the presentinvention the compensatingdevice 300 may be provided as a PCI slot card which isconnectable to the PCI bus of an information display. In that way the compensatingdevice 300 is integrated in already existing information displays in a very economicand effective way.

The relationship between life-times, stress levels, ambient temperatures, averagebrightness levels is very complex. It requires a high level a data sets, which describethe degradation of the primary phosphors red, green, and blue under the variousoperating conditions.

The proposed software concept, which traces the stress levels of display load, thenumber of address ability of single picture elements, sustain frequency control andambient temperature will provide the data for future automatic software and hardwarecompensation algorithm.

In the following a calibration system is proposed for analysing and compensating theactual brightness of the plasma display cells. The gathered data are processed,stored, and distributed to the information display for a pixel wise compensation. Thecalibration system is a complete solutions for on-side compensation on installedinformation displays.

FIG. 6 illustrates a schematic diagram of a third preferred embodiment of the presentinvention. A calibration system is shown for calibrating a compensation value of cellsof information display to compensate the degradation of the light output of the cells.The calibration system comprises a compensatingdevice 300 in accordance with thefirst and second preferred embodiments of the present invention. Furthermore thecalibration system comprises anelectronic camera 620 for taking screen images of the plasma display for measuring the brightness of the cells of the information displayand alaptop 630 or computer, which is connected to the electronic camera and to thecompensatingdevice 300. Theelectronic camera 620 is transmitting data of thetaken screen image to the computer or laptop and the laptop or computer istransforming the transmitted data of the electronic camera to compensation values ofthe cells of the plasma display. Furthermore the PC or laptop is transmitting thecalculated compensation values via a serial, parallel or Ethernet interface to thecompensatingdevice 300 where the values are stored in the compensation framebuffer.

Alternatively the PC or laptop provides only the date concerning brightness and theduration of the brightness of the cells and transmits this data to the compensatingdevice 300 where the compensation values are calculated.

Preferably a progressive high resolution CCD camera is used with a 12 bit accuracyand a digital output to ensure signal fidelity. The data of the CCD camera isoutputted via a RS422 to the measurement PC or a laptop. Furthermore a highperformance lens is used for the CCD camera to provide a distortion free image ofthe screen. This is necessary in the case that the compensation values for eachsingle cell of the plasma or information display has to be calibrated.

FIG. 7 illustrates a schematic diagram of thecamera 620 in accordance with a thirdpreferred embodiment of the present invention. The camera is assembled to a blackchamber head. The head is a black ended box with one open side. The camera islocated opposite to the open side of the center of that area. The area of the openside is equivalent to the display screen size.

For collecting data the black box with the camera is moved against the surface of thescreen. The black box eliminates effects resulting from ambient light sources andreflections. The measuring box uses soft and flexible edges at the open side to closethe junction between box and display surface.

The camera takes the screen images with its digital sensor. Best suited is a sensorresolution which corresponds to the physical resolution of the display device under test. The camera transmits an image data via a serial link to the measurement PC orlaptop. The software running on the computer processes the data and generates thecompensation values pixel by pixel or cell by cell for the three primary colors red,green and blue.

Referring back to Fig. 6 in addition to the above mention alternative the tables ormatrices with the compensation values are transferred to aserver 640 in a computernetwork to be stored. Each of the displays within the network has its own networkaddress. This clear identification tool allows the storage of compensation matricesand remote distribution from the server to the displays.

The server or the local direct links save the data to the embedded compensatingdevice. The compensation values are added as positive or negative values to acompensation correction software tool or saved into a dedicated second videomemory or in the compensation frame buffer.

For taking the measurement data full screen images of a black screen (R 0,G 0, B0), of a 80% white screen (R 208, G 208, B 208), of a red screen (R 208,G 0, B 0),of a green (R 0, G 208, B 0), and a blue screen (R 0,G 0, B 208) are taken.

For fixed patterns the compensation software adds the compensation data to theimage data before writing the data into the video memory or buffers. For movingpictures a computer processor speed is not sufficient enough. An additionalembedded signal processor adds the data from the image video memory or videoframe buffer with the data from the compensation memory or compensation framebuffer and transfers the final results to the buffers or directly to the connected display.

The compensation can be done by multiplying a positive or negative factor to theimage data included in the video signal. As long as the display offers dynamic rangefor neutral compensation the positive factor will be the preferable method in order tokeep the image performance and luminance (light output) on the level at thebeginning of the product life. Multiplication increases the data in the video memoryand keeps the brightness offset unchanged. The higher the data value the brighterthe image on the screen.

The analysis software on the measurement PC checks the image array for themaximum, the minimum and the relative values. The maximum value corresponds tothe picture element with the lowest stress level. The minimum value corresponds tothe picture element with the strongest stress level.

By collecting data regarding the light output of each cell of the display, all effects fordegradation of the phosphor efficiency are taken into account. The reasons fordifferences in light output are the operating time, average and peak brightness duringoperating time, cell temperature, basic light output variations (about 10%, notlocalised) and production process variations.

The maximum value equals to a compensation factor of 1. The minimum valuerequires a compensation factor to generate a light output which equals the luminanceof the cell with the maximum measurement values. All other values are equallydistributed between the maximum and the minimum values. Depending on the colordepths used a limited and discrete number of compensation values can be offered.This limitation might cause small variations in luminance output from picture elementto element.

Nevertheless, the dynamic range of an information display comes to its end, also withcompensation, but much later. At that moment compensation of an equallydistributed luminance can still be maintained. However the constant distributionrequires a lower total luminance of the display. From that time on the minimumluminance value has to be taken as the reference to compensate all other sub-pixelsagainst the sub-pixel with the lowest light output.

The measurement cycles are directly related to the offered degradation of theperformance of the installed display devices and the kind of information displayed onthe screen.

The visibility of the phosphor decay depends, besides the stress levels for the picturecells, on the used color selection. When using character with changing mixed colorson a dark or grey background the visibility of the degradation occurs later.

When using characters with strong primary colors with maximum intensity on a whitebackground the degradation becomes obvious much earlier. In such applicationsdifferences of less than 5% in luminance can be detected on the screen.

Additional screen savers during the non-operating hours, orbiting systems andperiodic change of the page layouts or the line structure and position of theinformation automatically equal the stress levels of the single cells and prolong thetime interval between active phosphor compensation additionally to compensationdevice.

In relation to the above characteristics of an information display application, serviceintervals between 6 months and 24 months seem to be realistic for the nowadaysavailable quality of plasma display devices. This offers the possibility to use aneffective calculation method to calculate the compensation values for compensatingthe degradation between two service periods without the need to consider allphysical stress levels in the formula. Nevertheless, a good color quality is maintainedwithin the service interval. Updating the matrices of the compensation values forevery pixel with the calibration system brings back the full color quality to the display.

The measurement time and phosphor compensation can be a manual and anautomated process. The duration of an automated process is less than half an hour.

The above stated scenarios are not technology dependent. So this compensationmethod can also be used in the case of a worn out CRT.