Patent Application
20080018570
The present invention relates to display boards comprising light emitting elements and methods of constructing and operating these. More particularly, the present invention relates to aging of the light emitting elements of such display boards and methods of operating these taking into account aging.
Electronic displays can use transmissive or emissive materials to generate pictures or light. Emissive materials are usually phosphorescent or electroluminescent materials. Examples are inorganic electroluminescent materials such as applied in thin film and thick film electroluminescent displays (EL-displays, for example thin film TFEL displays as manufactured by Sharp, Planar, LiteArray or iFire/Westaim) or light emitting diodes (LEDs),. Another group is organic electroluminescent materials (such as Organic Light Emitting Diode or OLED materials) deposited in layers comprising small molecule or polymer technology or phosphorescent OLED, where the electroluminescent materials are doped with a phosphorescent material. Yet another group of materials are phosphors, commonly used in the well-established cathode ray tubes (CRT) or plasma displays (PDP) and even in emerging technologies like laser diode projection displays where the laser beam is used to excite a phosphor imbedded in a projection screen.
Two basic types of displays exist: fixed format displays which comprise a matrix or array of “cells” or “pixels” that are individually addressable, each producing or controlling light over a small area, and displays without such a fixed format, such as scanning electron beam displays, e.g. a CRT display. Fixed format relates to pixelation of the display as well as to the fact that individual parts of the image signal are assigned to specific pixels in the display.
Tiled displays may comprise modules made up of tiled arrays which are themselves tiled into supermodules. Modular or tiled emissive displays, such as e.g. tiled LED or OLED displays, are made from smaller modules or display boards that are then combined into larger tiles. These tiled emissive displays or display tiles are manufactured as a complete unit that can be further combined with other display tiles to create displays of any size and shape.
All light emitting elements on display boards and display tiles can be formed from different batches, can have different production dates, different run times, etc, i.e. they can have different properties. In the factory, i.e. before real use, all light emitting element products are calibrated under controlled circumstances. However, there is one parameter which can only be compensated based on statistical data and not on actual data, and that is the aging or degradation of the light emitting elements when they are being used. Age differences occur, for example, due to the varying ON times of the individual light emitting elements (i.e., the amount of time that the light emitting elements have been active) and due to temperature variations within a given display area.
For large-screen applications, where the display may consist of a set of tiled display boards, there is the possibility that one display will age at a faster rate than another, because of varying ON times of its light emitting elements and/or because of temperature differences. Typically, when a tiled display is manufactured, it is calibrated for a uniform image. The challenge in a display comprising light emitting elements is to make its light output uniform, i.e. to make all light emitting elements on the display board to have the same brightness, even after having been used.
In EP 1 158 483 a system 10 is described which corrects for the aging of the pixels in a display. The system 10 comprises a solid-state display device 12. The system 10 uses reference pixels 14 to enable the measurement of pixel performance and a feedback mechanism responsive to the measured pixel performance to modify the operating characteristics of the display device 10 (see
According to EP 1 158 483, the measurement circuit 18 monitors the performance of the reference pixel 14. The measured performance values are compared to the expected or desired performance by the analysis circuit 20. These comparisons can be based on a priori knowledge of the characteristics of the device 12 or simply compared to some arbitrary value empirically shown to give good performance. In either case, once a determination is made that the performance of the device 12 needs to be modified, the analysis circuitry 20 signals the feedback and control mechanism which then initiates the change.
In the system 10 according to EP 1 158 483, however, errors in the measurement circuit 18 can lead to errors in the correction or change. Furthermore, the value the measured performance values are compared to is not exactly measured under the same circumstances as the measured performance values and thus can include small deviations from a reference value which would be measured under the same circumstances as the performance value. This could lead to errors in the correction or change.
It is an object of the present invention to provide good display boards and a good method for determining aging of light emitting elements in such a display board.
The above objective is accomplished by a method and device according to the present invention.
In a first aspect, the present invention provides a display board comprising an array of addressable light emitting elements and driving means for driving the light emitting elements with image data. The display board furthermore comprises aging determination means comprising:
at least one reference light emitting element, the driving means being adapted for driving the at least one reference light emitting element with calibration data,
light measurement means for measuring light emitted by the reference light emitting element, and for measuring light representative of the light emitted by the light emitting elements, and
comparison means for comparing measured light emitted by the reference light emitting element with measured light representative of the light emitted by the light emitting elements and for, based on the comparison result, deciding on aging of the light emitting elements in the array.
Light representative of the light emitted by the light emitting elements may be light emitted by the light emitting elements themselves. Alternatively, this may be light emitted by a reference light emitting element.
In embodiments of the present invention, the light emitting elements and the at least one reference light emitting element may be of different types, i.e. light emitting elements having different performance properties. For example the light emitting elements of the display board may be power LEDs and the at least one reference light emitting element may be a cheaper type of LEDs such as SMD LEDs.
In alternative embodiments of the present invention, the light emitting elements of the display board and the at least one reference light emitting element may be of the same type, i.e. have same performance properties. They may for example be both power LEDs, or they may both be SMD LEDs.
In an embodiment, the present invention provides a display board comprising an array of addressable light emitting elements and driving means for driving the light emitting elements with image data. The display board furthermore comprises aging determination means comprising:
With first moments in time is meant the moments at which the display is running, in other words, when the light emitting elements of the array are driven with image data. With second moments in time is meant the moments at which intermediate calibrations are performed.
An advantage of the display board according to embodiments of the invention is that both the reference and the aged value are determined on a same display board. This leads to more reliable and more correct determination of aging with respect to prior art devices where the aged value is compared to pre-determined values.
According to embodiments of the invention, the value derived from the image data may be an average value of the image data.
The display board may furthermore comprise compensation means for compensating the light emitting elements in the array for aging based on the decision on aging. However, according to other embodiments, the compensation means may also be located outside the display board.
An advantage hereof is that at every moment in time, compensation for aging differences between the light emitting elements of the array can be performed.
The display board may furthermore comprise a controller for controlling the driving means.
According to embodiments of the invention, the array of light emitting elements may be provided at a first side of the display board and the first and second reference light emitting elements may be provided at a second side of the display board, the second side being opposite to the first side.
An advantage hereof is that adding the first and second reference light emitting elements does not alter the size of the display board and does not disturb the image as it is not part of the array of display elements.
According to other embodiments of the invention, the array of light emitting elements may be provided at a first side of the display board and the first reference light emitting element may be provided at the first side of the display board and the second reference light emitting element may be provided at a second side of the display board, the second side being opposite to the first side.
According to still other embodiments of the invention, the first and second reference light emitting elements may be provided at a same side of the display board as the array of light emitting elements.
According to some embodiments, the first reference light emitting device may be part of the array of light emitting devices.
In particular embodiments, the first and second reference light emitting elements may be coupled to a same light measurement means.
An advantage hereof is that there is not only compensated for aging of the display light emitting elements, but that there is also compensated for aging drift of the light measurement means, e.g. photodiode or phototransistor. This is because if the difference is made between the measurements both performed by a same light measurement means, possible errors emanating from the light measurement means can be exclude excluded.
The light measurement means may comprise at least one photodetector or phototransistor.
According to embodiments of the invention, the display board may comprise light emitting elements of different colours and a first and a second reference light emitting element may be provided for each colour.
According to other embodiments of the invention, the display board may comprise multi-coloured light emitting elements and the aging determination means may comprise one first and one second reference light emitting element, the first and second light emitting elements being multi-coloured light emitting elements.
The light emitting elements of the array may be LEDs.
The display board according to embodiments of the invention may be incorporated in a display tile.
A plurality of display tiles may form a display.
In a second aspect, the present invention provides a method for determining aging of a display board, the display board comprising an array of light emitting elements, driving means for driving the light emitting elements with image data, and at least one reference light emitting element. The method comprises:
measuring light representative of the light emitted by the light emitting elements,
driving the reference light emitting element with calibration data and measuring light emitted by the reference light emitting element, and
comparing the light representative of the light emitted by the light emitting elements with the light emitted by the reference light emitting element and,
based on the comparison result, deciding on aging of the light emitting elements in the array.
It is an advantage of embodiments of the present invention that a reference light emitting element essentially not driven, so not showing ageing effect, is on-board of the display board. Such reference light emitting element may be of the same type or of different type compared to the light emitting elements of the display board.
Measuring light representative of the light emitted by the light emitting elements may comprise measuring light emitted by the light emitting elements themselves.
In an alternative embodiment measuring light representative of the light emitted by the light emitting elements may comprise measuring light emitted by a reference light emitting element. In this embodiment, a method is provided for determining aging of a display board, the display board comprising an array of light emitting elements, driving means for driving the light emitting elements with image data and at least a first and second reference light emitting elements. The method comprises:
An advantage of the method according to embodiments of the invention is that both the reference and the aged value are determined on a same display board. This leads to more reliable and more correct determination of aging with respect to prior art devices where the aged value is compared to pre-determined values.
The first calibration data may be equal to or may be different from the second calibration data.
The method may comprise before driving the first and second reference light emitting elements with first and second reference data respectively, driving the light emitting elements of the display board with image data and driving the first reference light emitting element with a value derived from the image data.
According to embodiments of the invention, the value derived from the image data may be an average value of image data.
In a further aspect of the invention, a method is provided for calibrating a display board, the display board comprising an array of light emitting elements. The method comprises:
Compensating the light emitting element in the array for aging may be performed by adapting a driving parameter of the light emitting elements of the array.
According to embodiments of the invention, the driving parameter may be a voltage.
According to other embodiments of the invention, the driving parameter may be a current.
Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.
Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.
The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention as defined by the claims. The reference figures quoted below refer to the attached drawings.
In the different figures, the same reference signs refer to the same or analogous elements.
The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.
Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.
It is to be noticed that the term “comprising”, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression “a device comprising means A and B” should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.
With “light” in the present invention is meant electromagnetic radiation with a wavelength between 375 and 1000 nm, i.e. visible light, IR radiation, near IR and UV radiation.
The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of persons skilled in the art without departing from the true spirit or technical teaching of the invention, the invention being limited only by the terms of the appended claims.
The present invention, in embodiments thereof, provides a display board comprising an array of light emitting elements, e.g. LEDs, and age determination means, as well as a method for detecting aging of a display board. The method according to embodiments of the present invention yields data which can then be used for adapting the driving of the light emitting elements of the array, e.g. LEDs, so as to correct for decreasing light intensity because of aging of the light emitting elements, e.g. LEDs, of the array.
Embodiments of the present invention may be applied to passive or active matrix displays and to monochrome or colour displays. Furthermore, the displays may be flat or curved displays. The boards and/or tiles optionally used in such displays may be flat or curved themselves as well.
When in the description and claims is referred to an array of light emitting elements, e.g. LEDs, a structure is meant in which the light emitting elements, e.g. LEDs, are logically organised in rows and columns. The terms “column” and “row” are used to describe sets of the array of light emitting elements, e.g. LEDs, which are linked together. The linking can be in the form of a Cartesian array of rows and columns however the present invention is not limited thereto. As will be understood by those skilled in the art, columns and rows can be easily interchanged and it is intended in this disclosure that these terms be interchangeable. Also, non-Cartesian arrays may be constructed and are included within the scope of the invention. Accordingly the terms “row” and “column” should be interpreted widely. Each display element, e.g. LED, may be individually addressable. According to embodiments of the invention, display boards may comprise current addressed or voltage addressed light emitting elements, e.g. LEDs.
Hereinafter, the present invention will be described by means of LEDs as light emitting elements. This is not limiting the invention in any way; any suitable light emitting element known by a person skilled in the art may be used with the present invention.
When in the description and the claims the term “light emitting element” is used, it is meant to cover an active light emitting element that can be addressed electronically and includes the following possibilities: ELs (electroluminescent devices) in general, TFELs (thin films ELs), LEDs (light emitting diodes), OLEDs (organic light emitting diodes) and PLEDs (polymeric light emitting diodes).
The present invention will mainly be described with reference to LED's but the present invention is not limited thereto.
An embodiment of the present invention, as illustrated in
The first reference LED 34 is, during functioning of the display board 30, driven with reference data equal to a value derived from the image data for driving the LEDs 31 of the array e.g. by means of an algorithm on the display board 30. According to embodiments of the invention, the algorithm may comprise deriving an average value of the image data. According to other embodiments, the algorithm may comprise deriving a peak value of the image data. According to still other embodiments, the algorithm may comprise deriving a combination of a peak value and an average value, or in other words off-setting an average value of the image data with a peak value of the image data. In the following description, reference data for driving the first reference LED 34 will be referred to as being equal to an average of the image data for driving the LEDs 31 of the array. It has to be understood that this is not limiting the invention in any away and that other algorithms as described above can also be used to determine the value of the reference data in accordance with embodiments of the present invention. This means that the first reference LED 34 has substantially the same usage, and thus shows substantially the same aging, as the LEDs 31 of the array on the display board 30. The first reference LED 34 may also be called an average LED. This first reference LED 34 corresponds, throughout the useful life of the display board 30, with the average actual history of the LEDs 31. At the time of an intermediate calibration of the display board 30, i.e. when the display board 30 is calibrated during use after a particular period thereof, the first reference LED 34 is driven with first calibration data.
The second reference LED 35 is normally not used. This LED 35 corresponds with a LED with the initial state of the LEDs 31 of the display board 30. This means that, during functioning of the display board 30, when the LEDs 31 of the array on the display board 30 are in use and thus when the first reference LED 34 is driven with reference data equal to a value derived from the image data for driving the LEDs 31 of the array by means of an algorithm, the second reference LED 35 is not driven. The second reference LED 35 is only used at intermediate calibration time and is then driven with second calibration data. The second reference LED 35 is a LED which corresponds with the “new state” of the LEDs 31 of the array on the display board 30 at the time of factory calibration. According to embodiments of the present invention, the first and second calibration data may be the same or may be different. When the first and second calibration data are the same, a same output would be expected for the first and second reference LED 34, 35. However, in some cases, the outputs of the first and second reference LED 34, 35 can be different. This difference is a calibration difference and is not attributed to aging, but should be corrected for when determining aging of the LEDs 31 of the array. Correction for the calibration difference can be done by means of specific software.
The aging determination means 33 furthermore comprises light measurement means 36 for, during intermediate calibration, measuring light emitted by the first and second reference LED 34, 35 and comparison means 37 for comparing light emitted by the first reference LED 34 with light emitted by the second reference LED 35 and for, based on the comparison result, deciding on aging of the LEDs 31 of the array on the display board 30. The light measurement means 36 are adapted for measuring the brightness levels of the first and second reference LEDs 34, 35. The light measurement means 36 may be photodiodes. The light measurement means preferably have an optical transfert curve which is as flat as possible over the spectrum to be measured. The measurement resolution is preferably high enough to measure small enough differences between radiated light of the first and second reference LEDs 34, 35.
In a next step, the light emitted by the first reference LED 34 is compared to the light emitted by the second reference LED 35 by comparison means 37. The difference between the light emitted by the first reference LED 34 and the light emitted by the second reference LED 35 is an indication for the aging status of the LEDs 31 of the array on the display board 30.
The difference between the light emitted by the first reference LED 34 and the light emitted by the second reference LED 35 obtained as described above may then be used to correct overall calibration values for adapting the driving parameter of the LEDs 31, bringing the actual LED aging into account. This can be done by changing the driving parameter of the driving means 32 by means of controller 38. For example, when the LEDs 31 of the array on the display board 30 are voltage-driven, correction for aging may be done by adapting the voltage the LEDs 31 are driven with based on the calibration values, such that no loss of brightness occurs because of aging of the LEDs 31. When the LEDs 31 of the array on the display board 30 are current-driven, the current the LEDs 31 are driven with may be adapted based on the calibration values, such that no loss of brightness occurs because of aging of the LEDs 31.
An advantage of the display board 30 and method according to embodiments of the present invention is that both the light emitted by the first reference LED 34 and the light emitted by the second reference LED 35 are determined on a same display board 30 or in other words, are both measured under the same circumstances. Therefore, compared to the prior art where the light emitted by a reference LED is compared with an a priori knowledge of the characteristics of the device or simply compared to some arbitrary value empirically shown to give good performance, embodiments of the present invention may lead to more reliable and up to date determination and thus of subsequent compensation for the aging problem of the LEDs 31.
Furthermore, when using a single light measuring means 36 for determining the light emitted by the first reference LED 34 and the second reference LED 35, in case a difference is made between the light emitted by the first reference LED 34 and the light emitted by the second reference LED 35, possible errors emanating from the light measurement means 36 may be minimised or even excluded.
An additional advantage of particular embodiments, i.e. the embodiments where the light emitted by the first reference LED 34 is measured by the same light measurement means 36 as the light emitted by the second reference LED 35 is that also can be compensated for aging drift of the light measurement means 36, e.g. photodetector, phototransistor, photoelectric cell, photodiode, . . . , because the drift on this component is always re-normalized by making the difference between the light emitted by the first reference LED 34 and measured by the light measurement means 36 and light emitted by the second reference LED 35 and measured by the same light measurement means 36.
An extended version of a process in accordance with embodiments of the present invention is illustrated hereinafter, referring to
Phase 1 is the initial phase, illustrated in the flow chart of
Phase 2 is the normal life of the display board 30. The first reference LED 34 is driven with reference data equal to a value derived from the image data for driving the LEDs 31 of the array e.g. is driven at the average value of the display LEDs 31. The second reference LED 35 is not driven. This second reference LED will only be used when field recalibration is executed.
Phase 3 is the in-field recalibration phase, illustrated in the flow chart of
A concept of embodiments of the method is that the actual initial reference is on board of the display board 30 (by means of the second reference LED 35) and by re-measuring the second reference LED 35 during an in-field recalibration, the electrical drift of the measurement means 36 is eliminated. The only difference between measurements is then the optical difference caused by the difference in light coupling between first reference LED 34/optical measurement means 36 and second reference LED 35/optical measurement means 36. Since the latter is constant, the difference in aging between first and second reference LEDs 34, 35 remains. The adjustment results in a level 1 adjustment on the display board 30 LEDs 31 and the drive levels of the first and second reference LEDs 34, 35.
Hereinafter some examples will be discussed of possible implementations of the display board 30 according to embodiments of the present invention.
According to particular embodiments, the at least first and second reference LED 34, 35 may be provided at a side of the display board 30 opposite to the side of the display board 30 where the image is shown intended to be looked at. This is illustrated in
An advantage of the example illustrated in
Another advantage of the embodiments illustrated in
According to other embodiments, and as illustrated in
According to other embodiments, illustrated in
A disadvantage of the embodiments illustrated in
According to still other embodiments of the invention, both the first and second reference LED 34, 35 may be provided at the front side of the display board 30 next to the array of LEDs 31, as illustrated in
The display board 30 according to the embodiment illustrated in
The edges of the display board 30 may be covered by a cover 39, as illustrated in
The above-described embodiments all relate to display boards 30 comprising one kind of LEDs, i.e. all the LEDs on the display board 30 are of a same colour and thus the above-described embodiments relate to monochrome display boards and thus only require one first and one second reference LED 34, 35.
However, according to other embodiments of the present invention, the display board 30 may comprise LEDs 31 of different colours. It is known that LEDs 31 with different colours age in a different way. Therefore, the aging determination means 33 may comprise a first reference LED 34 and a second reference LED 35 for each colour. For example, if the display board 30 comprises red, green and blue LEDs the aging determination means 33 may comprise a red first and second reference LED, a green first and second reference LED and a blue first and second reference LED.
According to other embodiments of the present invention, the display board 30 may comprise multi-colour LEDs, each LED comprising e.g. three colours. In this case, only one first reference LED 34 and one second reference LED 35 may be provided, the first and second reference LEDs 34, 35 being the same multi-colour LEDs as the multi-colour LEDs 31 of the array on the display board 30.
According to still other embodiments, not all light emitting elements, e.g. LEDs, are of the same type. For example, the display LEDs 31 may be power LEDs, as typically applied in display applications, e.g. outdoor display applications, where LEDs are used to form the pixels of the display board 30. Most often it is too expensive to provide, on top of the display LEDs 31 also first and second reference LEDs 34, 35 as power LEDs, as such power LEDs are much more expensive than other LEDs. Of course, if there is no objection to extend the display board 30 to carry first and second reference “power LEDs” 34, 35, the basic principle of aging compensation as in embodiments of the present invention set out above can be applied. In the case of a power LED based application, however, the first and second reference LEDs 34, 35 under the form of a power LED would add a significant cost to the display board 30. Therefore, first and second reference LEDs 34, 35, according to embodiments of the present invention, can be replaced by a cheaper alternative. Only one cheap reference LED needs to be provided; however, a plurality of reference LEDs may be provided. The one or more reference LEDs should show the same ageing characteristics as the display LEDs 31. This embodiment requires that the measurement means 36 can sample the light of the power LEDs 31 and also the light of the cheaper reference LED 35. The process again comprises 3 phases:
Phase 1 is the Initial phase and is illustrated in the flow chart of
Phase 2 is the normal life of the display board 30. The power LEDs 31 are driven as normal. The second reference LED 35 is not driven at all.
Phase 3 is the in-field recalibration phase, illustrated in the flow chart of
The measurement result of the second reference LED 35 is compared with the original value, step 118, which may have been stored in a memory. The difference between these values determines the drift of the measurement means 36, which can also be used for the power LED measurements.
The difference between brightness levels of the second reference LED 35 and power LEDs 31 is determined, step 117.
The brightness level of the second reference LED 35 is compared to the brightness level of the power LEDs 31, step 119. A compensation for optical coupling differences may be made. Measuring both values of the power LEDs 31 and the second reference LEDs 35 with the same measurement means 36 fully eliminates drifts of the measurement means 36. The optical coupling difference is known and constant over life, and is taken into account in the compensation calculation, yielding a correction of the drive of the power LEDs 31 in order to compensate for their aging (and thus run-time), step 120.
Again it is a concept of this embodiment that the actual initial reference is on board of the display board 30 (by means of the reference LED 35) and by re-measuring the reference LED 35 during an in-field recalibration, the electrical drift of the measurement means 36 is eliminated. It is to be understood that although preferred embodiments, specific constructions and configurations, as well as materials, have been discussed herein for devices according to embodiments of the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06014750.1 | Jul 2006 | EP | regional |
