The invention relates to an active matrix display and a method of displaying an image on an active matrix display.
JP-A-11-015437 discloses a LED display device which corrects differences of luminance characteristics between the LED elements by performing luminance corrections to the display data of the red, green, and blue LED elements. A luminance correction factor has to be stored for each LED element.
It is an object of the invention to provide an active matrix display in which the pixel to pixel non-uniformity at low luminance levels is improved without requiring storing a correction factor for each LED element.
A first aspect of the invention provides an active matrix display as claimed in claim 1. A second aspect of the invention provides a method of displaying an image on an active matrix display as claimed in claim 10. Advantageous embodiments are defined in the dependent claims.
The active matrix display comprises a pixel comprising sub-pixels. The sub-pixels each are driven to generate a desired amount of light which contributes to the luminance of the pixel. Usually, different sub-pixels of a pixel have different colors. For example, in a full color display, the pixel may comprise three sub-pixels which generate blue, red and green light, respectively. Alternatively, the pixel may comprise four sub-pixels which generate blue, red, green and white light. It is also possible to replace the red, green, blue sub-pixels by yellow, cyan and magenta pixels or to add the yellow, cyan and magenta pixels.
A drive circuit receives an input signal which determines a desired luminance and a desired color of the pixel. More in detail, the drive circuit drives the sub-pixels of the pixel such that the desired luminance and color of the pixel is obtained by the combination of the light emitted by the sub-pixels. The drive of the sub-pixels depends on the number and type of sub-pixels used.
It is determined whether a desired luminance of the pixel is below a predetermined level. Usually, the luminance of the pixel can be calculated from the video input signal which has to be displayed. This video input signal may be a composite signal, a YUV signal or a RGB signal. If the video input signal is a YUV signal (Y=luminance, U and V represent the color information), the luminance signal may be used. If the video input signal is an RGB signal (Red, Green, Blue), the R, G and B components may be summed using appropriate weighting factors to obtain the corresponding luminance value. It is also possible to use the drive signals of the sub-pixels to determine the luminance of the pixel. If the desired luminance of a pixel is below the predetermined level, the drive circuit is controlled to drive only a subset of the sub-pixels required to obtain the desired color of this pixel. Or said differently, the number of sub-pixels which contribute to the luminance of the pixel is lower than the number of sub-pixels which have to contribute to obtain the desired color of the pixel. The desired color of the pixel is determined by the image to be displayed. Thus, less sub-pixels are driven if the luminance of the pixel is below a predetermined level. The use of less sub-pixels to generate the same luminance or luminance increases the current density in the sub-pixel used and thus decreases the non-uniformity. Although the correct luminance is obtained, the color of the pixel deviates from the desired color. However, at low luminance, the human eye is less sensitive to the actual color displayed but is still very sensitive to the luminance. It usually is less noticeable if a color error is produced at low luminance. The predetermined level of the luminance below which less sub-pixels are driven than required to obtain the desired color, depends on the image content and the construction of the pixels. In practical implementations of a particular construction of the pixels, this predetermined level is optimally selected between 0.5 and 6% of the maximum luminance. If the color of the pixels of which the luminance is below the predetermined level (further referred to as threshold pixels) is replaced by white light (only the white sub-pixel is driven) the particular level can be selected higher than if only one of the saturated colors (only the red, green, or blue sub-pixel is driven) is used. In the latter case, the predetermined level can be selected higher if the desired color is nearer to one of the saturated colors.
For multi-primary displays the number of optimal required colors to obtain the desired pixel color e.g. white can be higher (e.g. RGBCMY) than the number of minimal required colors e.g. RGB or CMY or only GM etc.
In an embodiment as claimed in claim 2, the pixel comprises three sub-pixels generating light having different colors. Preferably the colors are the primary colors red, green, and blue, respectively. If it is detected that the luminance of the pixel is below the predetermined level, only one or two of the three sub-pixels are driven. The sub-pixels are driven to obtain the correct desired luminance. This will give rise to a deviation from the desired color, if more sub-pixels are required to obtain the desired color. For example, if all three sub-pixels have to be driven to obtain the correct desired luminance and color, if the luminance of the pixel is below the predetermined level, only one or two sub-pixels are driven such that the desired luminance is displayed at the wrong color. Alternatively, if two sub-pixels have to be driven to obtain the correct desired luminance and color, if the luminance of the pixel is below the predetermined level, only one sub-pixel is driven such that the desired luminance is displayed at the wrong color. If only one sub-pixel is to be driven to obtain the correct desired luminance and color, no improvement of the luminance uniformity is possible. Also at a luminance of the pixel below the predetermined level still one sub-color is driven.
In an embodiment as claimed in claim 4, the means for controlling are arranged to control the drive circuit to drive only a single one of the sub-pixels if the desired luminance is below the predetermined level. If only a single sub-pixel is driven, the maximum current is obtained in this sub-pixel, and the luminance uniformity will be improved.
In an embodiment as claimed in claim 5, the number of sub-pixels selected to contribute to the desired luminance gradually decrease dependent on the level of the luminance of the pixel.
In an embodiment as claimed in claim 6, the means for controlling comprises means for determining the sub-pixels to be driven out of the available sub-pixel colors to obtain a color of the at least one pixel nearest to the desired color. For example, the color coordinates of the desired color are determined, and the primary color is selected of which the color coordinates have the smallest difference with the color coordinates of the desired color.
In an embodiment as claimed in claim 7, the pixel comprises sub-pixels of which one generates white light. Preferably, the other sub-pixels generate light being red, green, blue, respectively. In such a matrix display, the extra white pixel allows to boost the luminance level of white.
In an embodiment as claimed in claim 8, the means for controlling are arranged to control the drive circuit to drive only the sub-pixel generating the white light. This provides less noticeable disturbance because the eye sensitivity shifts to black/white for low luminance. At low luminance levels, it is therefore possible to generate white light instead of light which has a primary color.
In an embodiment as claimed in claim 9, the active matrix display further comprises a further pixel including further sub-pixels. The further pixel is arranged adjacent to the first mentioned pixel. The drive circuit is controlled to drive only a subset of the first mentioned sub-pixels and only a subset of the further sub-pixels. If the desired luminance of at least one of the first mentioned pixel or the further pixel is below the predetermined level, the subset of the first mentioned sub-pixels and the subset of the further sub-pixels is selected to obtain a color being substantially an average of the desired color of the first mentioned pixel and a desired color of the further pixel. This approach has the advantage that it is possible to generate the correct color, but at a lower resolution.
In an embodiment as claimed in claim 11, the active matrix display comprises three adjacent pixels. Each one of the three pixels comprises a red, green and blue sub-pixel. If the desired luminance of the pixel or the sub-pixels is below the predetermined level, the controller controls the driver to drive only: the red sub-pixel of the first one of the three pixels, the green sub-pixel of the second one of the three pixels, and the blue sub-pixel of the third one of the three pixels. Again, besides the correct luminance, the desired color can be obtained at a higher current of the driven sub-pixels. The pixels which in combination produce the correct desired color and the correct desired luminance may comprise more than three sub-pixels. This combination of pixels may comprise more than three pixels.
In an embodiment as claimed in claim 13, the pixel comprises a red, green, blue, magenta, yellow, and cyan sub-pixel. The controller controls the driver to only drive a sub-pixel of the pixel if the luminance of this sub-pixel is above an associated predetermined level.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
In the drawings:
Each sub-pixel 10 comprises a LED L1, L2, L3, L4 (further collectively referred to as L) and a pixel driving circuit PD. The LED's L may be, for example, inorganic electroluminescence (EL) devices, organic EL devices, cold cathodes, or organic LED's like polymer or small molecule LED's. Especially, Polymer and Small Molecule OLED's have opened a new path to make high quality displays. The advantages of these displays are the self-emissive technology, a high luminance, a near-perfect viewing angle and a fast response time. These advantages indicate that the OLED technology holds the promise of providing a better front of screen performance than LCD displays. The different LED's may generate different colors or the LED's may generate, for example, white light and suitable color filters are implemented. In
It is possible to use passive matrix and active matrix addressing. For relatively large displays (>7″), which are considered in the now following, active matrix addressing is required to reduce the power consumption.
By way of example, in
Each pixel driving circuit PD receives a select signal from its associated select electrode SE, a data signal D from its associated data electrode DE, a power supply voltage VB from its associated power supply electrode PE, and supplies a voltage Vd and a current Id to its associated LED L. Although for each pixel 10 the same references are used to indicate the same elements, the value of signals, voltages and data may be different.
The current Id is driven through the LED L via the pixel driving circuit PD and the power supply electrode PE. The luminance of the LED is determined by the level of the current Id which is flowing through the LED. The current Id is determined by the data signal level D on the data electrode DE. The select electrodes (also commonly referred to as address lines) SE are used to select (or address) the rows of pixels 10 one by one. In practice, more address lines per display line may be used, for example to control the duty cycle of the current Id supplied to the LED's L. It is possible to select more than one row of pixels 10 at a time.
The operation of the circuit is elucidated in the now following. When a row of pixels is selected by an appropriate voltage on the select electrode SE with which this row of pixels is associated, the transistor T1 is conductive. The data signal D which has a level indicating the required luminance of the LED L is fed to the control electrode of the transistor T2. The data signal D defines the gate-to-source voltage, Vgs, of the transistor T2, and thus determines the desired current Id flowing from the power supply electrode PE to the LED L. After the select period of the row of pixels, the voltage on the select electrode SE is changed such that the transistor T1 becomes a high resistance. The data voltage D which is stored on the capacitor C still drives the transistor T2 to obtain the desired current Id through the LED L. The current Id will change when the select electrode SE is selected again and the data voltage D is changed.
The current Id is supplied by the power supply electrode PE which receives the power supply voltage VB via a resistor Rt. The resistor Rt represents the resistance of the power supply electrode towards the pixel 10 shown. It has to be noted that other pixels 10 associated with the same power supply electrode PE may carry current too; this current is denoted by Io. Both the currents Id and Io flow through the resistor Rt and thus cause a voltage drop in the power supply electrode PE. The pixel driving circuit PD will only function correctly if the voltage Vp across the series arrangement of the main current path of the transistor T2 and the LED L is sufficiently high to obtain the current Id. The resistor Rt and its influence is not relevant to the present invention.
It is commonly known how to drive the sub-pixels 10 of the pixels to display an image on the display 1. In short, an input signal IV (see
The construction of the pixel driving circuits PD is not essential to the invention. For example, some alternative pixel driving circuits PD are disclosed in the publication “A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays”, D. Fish et al, SID 02 Digest, pages 968-971.
As will be elucidated with respect to the now following Figs., the present invention differs from the known drive of the pixels P in that is determined for each pixel P whether the luminance of a pixel P is below a predetermined threshold. If this is true, less sub-pixels 10 of this pixel P are selected to contribute to the luminance of the pixel P than required to obtain the desired color of this pixel P. Preferably, with the sub-set of sub-pixels 10 driven, still the desired luminance of the pixel P is obtained. Thus, the luminance of at least one of the sub-pixel(s) 10 used to contribute has to increase to still be able to substantially produce the desired luminance. The color of the pixel P will deviate from the desired color. A deviation from the desired color at low luminance levels is less noticeable. However, a deviation from the desired luminance would be more visible. The higher luminance of the sub-pixel(s) 10 used is realized by a higher current Id through the sub-pixel(s) 10 and consequently, as will be elucidated in the following, the non-uniformity of the luminance of the pixels P decreases. Thus, the luminance uniformity is improved at the cost of color deviations, which, however, are not very visible at the low luminance levels involved. It is more important to keep the luminance level substantially equal to the desired luminance.
Thus, the non-uniformity of the voltage programmed current driven pixels P is caused by variations in the threshold voltage and the mobility of the transistor T2. The usually used Low Temperature Poly-Silicon TFT inherently suffer from point to point variations in their threshold voltage and mobility due to the random variations in the silicon grains formed when annealing. The variations in these parameters cause different currents Id in different sub-pixels 10 at a same given gate-source voltage of the transistors T2.
The current Id of a sub-pixel 10 depends on the TFT mobility μ and the TFT threshold Vt according to equation 1.
Id˜μ(Vgs−Vt)2 equation 1
Consequently, the luminance of the sub-pixels show random deviations with respect to each other although the same gate source voltages are applied. These random luminance deviations, or luminance non-uniformities, are visible in the image displayed as random noise. The percentage variation of the current Id through the TFT T2 with respect to its threshold voltage and mobility must be below about 2% to be invisible.
Several advanced pixel designs were proposed to mitigate these luminance non-uniformities. Among these designs are solutions called digital display, threshold voltage shift display, current mirror display, and on optical feedback circuit, which all add circuitry to compensate for the non-uniformity. In contrast, the present invention may use any drive circuit, also the simple drive circuit shown in
It is known that eyes of human beings are less color sensitive at low luminance levels. From perception investigations we found that, depending on the image content, color errors are acceptable below a luminance level of 0.5 to 6% of the maximum luminance. Thus below this threshold, the pixel color information (the color coordinates in the x-y plane, see
In the usual, precise, color reproduction mode, all the sub-pixels 10 required to obtain the desired color are driven to contribute to the luminance of the pixel P. In the low-luminance mode, only a subset of these sub-pixels 10 is driven to contribute to the luminance of the pixel P. The number of sub-pixels 10 activated during the low-luminance mode may depend on the luminance of the pixel P. The transition from the precise color reproduction mode to the low-luminance mode can be realized in a single step or, alternatively, in a number of consecutive steps wherein with decreasing luminance fewer sub-pixels 10 contribute to the luminance of the pixel P.
The three luminance level transitions shown in
During each frame period of the input signal IV, the transition has to be calculated for each pixel P for which the luminance is below the highest or single threshold level VT1 or VT10. In general, and especially for OLED displays, the aperture of the various sub-pixels 10, and the dimensions of the TFT T2 are optimized with respect to the efficiencies and lifetime of the light emitting materials of the different colors of the sub-pixels 10. The most suitable threshold(s) and transition step strategies can be determined experimentally by looking to the effect reached on the display, taking all these parameters into account.
This effect is illustrated in
Every color is completely determined by its x and y color coordinates because these coordinates determine the tint and the saturation of the color. At a particular ratio of the primary colors R, G, B (dependent on the color coordinates of the primary colors R, G, B, this ratio may, for example, in the NTSC standard, be: 30:59:11) white W is obtained. The tint of the color of a point AC in the color triangle CT is found as the junction SC of a line through this point AC and the white point W and the locus VC. The saturation of the color of this point AC is determined by the ratio of the distance between on the one hand the points AC and W and on the other hand between the points AC and SC.
Instead of using the three primary colors R, G, B shown it is possible to use more primaries, for example to obtain a polygon covering a larger area of the locus VC than the triangle CT does. It is also possible to add a primary color white W. In the matrix display discussed, the sub-pixels 10 have the different colors determining the polygon indicating which colors can be displayed.
The adaptation circuit 4 further receives the Y, U, V signals and supplies the adapted Y′, U′, V′ signals which depend on the received Y, U, V signals and the control signal CA. The adapted Y′ signal is substantially equal to the received Y signal such that the luminance is substantially independent on the number of sub-pixels 10 which contribute to the luminance of the pixel P. If the control signal CA indicates that the luminance Y of the pixel P is below the threshold level VT, the adapted U′, V′ signals are determined from the received U, V signals preferably such that even now less sub-pixels 10 contribute to the luminance of the pixel P, the resultant color is as near as possible to the desired color. For example, the adaptation circuit 4 may comprise a look up table comprising U′ and V′ values for the primary colors R, G and B of the display, and a decision circuit which determines which one of the primary colors R, G, B has U′ and V′ values nearest to the U′ and V′ values of the desired color. For pixels P for which the luminance Y is above the threshold level VT, the adaptation circuit 4 does not adapt the Y, U, V signals received and supplies the adapted Y′, U′, V′ signals which are identical to the Y, U, V signals. The determination of the U′ and V′ values may be performed, for example, with a processor which, for example, calculates gain factors which are used to control the gain of the U and V signals. The adaptation of the level of the Y, U, V signals may then be performed with gain controlled amplifiers. The conversion circuit 5 converts the Y′, U′, V′ signals into R′, G′, B′ signals which are stored in the frame memory FB and which are processed in a know way to be displayed on the display 1.
Alternatively, after correction from the NTSC standard to the color coordinates of the RGB primary colors of the display, the R, G, B signals may be processed directly without converting them to the Y, U, V signals. Usually, the RGB colors of the display differ from the NTSC RGB, i.e. a color correction is required anyhow. The luminance of the R, G, B signals can be calculated as a weighted sum. If the weighted sum is above a threshold level, the R, G, B signals are not adapted. If the weighted sum is below the threshold level, level of the R, G, B signals is adapted such that at least one of the signals gets a zero level while at least one of the others gets an increased level such that the luminance is substantially kept the same. The increased level of the non-zero signal(s) is selected to obtain a color which is nearest to the desired color. The conversion from R, G, B to Y, U, V and the other way around is now not required, but extra calculation power is required to calculate the luminance Y and the color coordinates from the R, G, and B levels. A processor may be used to determine the weighted sum, to detect whether the weighted sum is below the threshold level, to calculate or to find in a look up table adapted levels R′, G′, B′ or correction factors to be applied to the R, G, B signals. Thus, the processor may calculate the adapted levels R′, G′, B′ directly or may calculate the correction factors which are supplied to gain controlled amplifiers. The gain controlled amplifiers receive the R, G, B signals and supply the R′, G′, B′ signals, respectively, dependent on the correction factors.
The controller CO receives the line synchronization signal Hs and the frame synchronization signal Vs of the input video IV to supply a control signal CPR to the input processor, a control signal CR to the select driver SD, a control signal CC to the data driver DD, and a control signal CP to the power supply PS.
The input processor comprises the conversion circuit 2, the threshold circuit 3, the adaptation circuit 4, and the conversion circuit 5. The complete driver circuit 6 comprises the input processor, the frame memory FB, the select driver SD, the data driver DD, the power supply PS and the controller CO. The control signal CPR controls the conversion circuit 2 to retrieve, process and store the R, B, G signals or values of the input signal IV in synchronization with the horizontal synchronization signal Hs and the vertical synchronization signal Vs. The control signals CR, CC and CP synchronize the selection of the rows of pixels 10, the supply of data D to the selected row of pixels 10, and the supply of the power supply voltages VB. The power supply voltages VB may be fixed making the control signal CP superfluous.
It has been found that an acceptable threshold VT depends on the image content and the algorithm used. The acceptable threshold VT varies between 0.5% and 6% of the maximum luminance. When the color of the so-called below threshold pixels P is replaced by white, the threshold value VT can be selected higher than when the color is replaced by red R, green G, or blue B. In the latter case, the threshold VT may be dependent on the saturation of the color of the pixel P: the threshold VT is selected higher at a more saturated color.
An embodiment in accordance with the invention is directed to the situation wherein a number of neighboring pixels Pi have a luminance below the threshold value VT. This often occurs in dark areas of the image. Now, the average luminance and color of a group of neighboring pixels Pi is determined. For example, such groups comprise three neighboring pixels Pi. The average luminance and color is represented by using of each one of the neighboring pixels Pi of the group only one of sub-pixels Lj. The sub-pixels Lj used have different colors. For example, as shown in
Further improvements in uniformity can be obtained by implementing more complex drive circuits than shown in
In yet another embodiment in accordance with the invention, a multi-primary display comprises, pixels P100 which comprise seven sub-pixels R, G, B, C, M, Y, W, even a larger freedom exists to select a subset of the sub-pixels 10 at low luminance levels of the pixel P to improve the uniformity. For example, it may be prevented to drive any sub-pixels 10 below the threshold luminance to avoid the non-uniformities to become clearly visible. Thus, the sub-pixels 10 of which the luminance is above the threshold generate light, while the sub-pixels 10 of which the luminance is below the threshold are switched off.
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
Number | Date | Country | Kind |
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04251578.3 | Mar 2004 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB05/50842 | 3/8/2005 | WO | 9/14/2006 |