WO2005015533A1

WO2005015533A1 - Uniformity correction of black and white states in liquid crystal displays

Info

Publication number: WO2005015533A1
Application number: PCT/IB2004/051330
Authority: WO
Inventors: Jeffrey A. Shimizu; Peter J. Janssen; Sandeep Dalal
Original assignee: Koninklijke Philips Electronics, N.V.
Priority date: 2003-08-11
Filing date: 2004-07-29
Publication date: 2005-02-17

Abstract

In a LCD (Liquid Crystal Display), uniformity correction of black and white is employed to improve picture quality and contrast. Extending the drive range and allocating a few gray scale levels to this extended range can accomplish the correction without penalty to contrast. Such an arrangement can be particularly helpful for projection display systems such as those based on LCOS (liquid crystal on silicon).

Description

Uniformity Correction of Black and White States in Liquid Crystal Displays

This invention relates to liquid crystal displays (LCD's), and more particularly relates to the correction of brightness and color uniformity of LCD light modulator panels, for displays and the like. An LCD panel is comprised of a layer of liquid crystal material that is sandwiched between two plates. The thickness of the liquid crystal layer is determined by the distance between the plates, which is referred to as the 'cell gap'. The plates include electrodes for applying voltage across the panel to induce the liquid crystal material to undergo an electro-optic response, such as an orientation change, which in turn changes the light polarization vector through the panel. By varying the voltage, and by incorporating a polarization filter, the light from the panel can be controlled between bright and dark limits. Light intensities between the bright and dark limits are referred to as gray scales. By employing a matrix of line electrodes in rows and columns across the panel, an array of picture elements (pixels) is defined at the crossings of the rows and columns, at each of which the voltage can be separately controlled in accordance with image data, and images can thus be formed with resolution commensurate with the pitch between pixels. In one type of panel, the so-called active matrix panel, a switch is associated with each pixel to prevent stray voltages from influencing the state of the liquid crystal at the pixel during addressing of other pixels with image data. In a particular type of active matrix panel, the so-called LCOS (Liquid Crystal On Silicon) panel, one of the plates is a silicon chip into which the switches and associated electronic structures are integrated. In LCD's, particularly LCD projection systems, spatial uniformity of brightness and color can be difficult to achieve. This is a known problem and current projection displays often make use of uniformity correction. Uniformity correction samples the image brightness of all three colors at a limited number of gray scales. Errors in uniformity, especially those that lead to color variation, are corrected for by altering the pixel values of the image at different spatial positions on the display. A difficulty occurs in trying to correct for uniformity errors of the black and white states of the display. Black and white states refer to the condition when all the pixels are driven to the minimum gray value or the maximum gray value for the display. A gray value is represented by a drive voltage; this implies all pixels will have the same applied drive voltage for the black state. For example, to correct for the dark-state, all pixels would be increased in brightness to the brightest area when driven to black. Similarly for white-state correction, brightness would be reduced to the dimmest area under full white drive. The result is to reduce both brightness and contrast of the image. This penalty is high. Thus correction of the white and black states is typically not done. The electro-optic response of the LCD is described by the brightness versus voltage (B-V) curve, an example of which is shown in Figure 1. With an LCD, the B-V curve can be different at different spatial locations on the display. In particular the B-V curve shifts with changes in the cell gap thickness, which can be difficult to keep uniform. Other effects can also cause the B-V curve to shift, including non-uniform compensation elements, mechanical stress, or birefringence in optical components. As a result, if the entire panel is driven to a "black" state, a non-uniform intensity pattern is displayed. This effect is particularly noticeable in the case of large displays such as projection displays. An example of variation in B-V response near the "black" state, at three different pixel areas on an LCD panel, is shown in Figure 2. The three B-V curves represent the response at the Center, at Max V, where the greatest voltage is required to achieve a black state, and at Min V, where the smallest voltage is required to achieve a black state. Typically the system is adjusted for the center point, or an average over the central area of the display. For the example in Figure 2, the black voltage would be set at 6.2V. That is, gray level 0 would drive the panel at 6.2V at all locations on the panel. However, different locations on the panel would produce different brightness as compared to the center of the panel. Referring to Figure 2, in a black state, areas of the panel corresponding to Max V would produce more brightness than pixels in the center of the panel. Further, in areas corresponding to Min V, a contrast inversion is seen. That is, as the voltage increases beyond that needed to achieve a black state, the gray level (brightness) increases once again. Thus, at the black voltage of 6.2V, the Min V areas are also brighter than the pixels in the center of the panel. Because of this non-uniform behavior of the display, a uniform black field, with all pixels driven at gray level 0 and producing the minimum brightness at all locations on the panel, is not achievable. As the drive voltage decreases in accordance with the desired gamma-corrected brightness function, the gray level (brightness) is increased, until a maximum, typically 255, is reached, corresponding to a white state. The non-uniform behaviour of the display also prevents a uniform white state. Thus, significant brightness and color variations can occur at the screen for both the black and white states. In accordance with the invention, uniformity correction of black and white states of an LCD is employed to improve picture quality and contrast. Extending the drive range and allocating a few gray scale levels to this extended range can accomplish the correction without penalty to contrast. Such an arrangement can be particularly useful for projection display systems such as those based on LCOS (liquid crystal on silicon) display panels. The LCD panels described herein are 'drive to black' systems, in which minimum brightness is achieved at maximum voltage. However, it will be appreciated by those skilled in the art that an LCD may also be operated to achieve maximum brightness at maximum voltage, as in so-called 'drive to white' systems. The principles of the invention are equally applicable to both systems. Thus, the generic term 'end state' is sometimes employed herein to describe the states of maximum and minimum brightness at the ends of the grey scale, regardless of voltage. An 'end state' corresponds to a measure of brightness, not to the gray level or voltage drive needed to achieve it. In accordance with one embodiment of the invention, a method is provided for correction of an end state at different areas of an LCD panel, wherein the end states occur at opposite ends of a range of voltages (drive range) used to drive the LCD panel, and wherein the range of voltages is determined in accordance with a gray scale having a plurality of grey levels ranging from 0 (or a minimum) at one end of the gray scale, to 255 (or a maximum) at the other end of the gray scale. The method for correction of a first end state corresponding to the upper end of the drive range comprises the steps of: (a) determining the largest voltage required to achieve a first end state at any area of the LCD panel; (b) setting a first end of the gray scale to correspond to said largest voltage; (c) determining smaller voltages required to achieve the first end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the gray levels applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said first end of the gray scale to correspond to each of said smaller voltages; and (2) adjusting said first end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel. For correction of a second end state corresponding to the lower end of the drive range, the steps comprise: (a) determining the smallest voltage required to achieve the second end state at any area of the LCD panel; (b) setting a second end of the gray scale to correspond to said smallest voltage; (c) determining larger voltages required to achieve the second end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: ( 1 ) allocating gray levels near said second end of the gray scale to correspond to each of said larger voltages; and (2) adjusting said second end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel. In the case in which the end state corresponding to the largest voltage is the black state and the end of the gray scale corresponding to the black state is the 0 or minimum end of the gray scale, the 0 or minimum end of the gray scale is adjusted by adding zero or a positive number to said zero or minimum end of the gray scale. In the case in which the end state corresponding to the smallest voltage is the white state and the end of the gray scale corresponding to the white state is the maximum end of the gray scale, the maximum end of the gray scale is adjusted by subtracting zero or a positive number from said maximum end of the gray scale. According to a preferred embodiment of the invention, a method of correcting a data signal input for an LCD display is provided, the method comprising the steps of: (a) uniformity-correcting the data signal input for at least one of the end states of the LCD display in one of the manners described above; followed by (b) gamma-correcting the uniformity-corrected data signal in accordance with one of the known methods. In accordance with another aspect of the invention, a liquid crystal display (LCD) system is provided, the system comprising: (a) a light-modulating LCD panel; (b) an illumination system for providing unmodulated light to the LCD panel; and (c) electrical signal input source for providing a display signal to the LCD panel; wherein the electrical signal input source includes means for correcting the end state uniformity of the LCD panel, said means comprising means for: (a) determining the largest voltage required to achieve a first end state at any area of the LCD panel; (b) setting a first end of the gray scale to correspond to said largest voltage; (c) determining smaller voltages required to achieve the first end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said first end of the gray scale to correspond to each of said smaller voltages; and (2) adjusting said first end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel. In accordance with a preferred embodiment, the illumination system of the display system comprises: means for splitting the unmodulated light into red, green and blue beams, and means for continuously and sequentially scrolling the red, green and blue beams across the light modulating LCD panel. In accordance with another preferred embodiment, the LCD panel is an LCOS panel. Figure 1 is a typical B-V curve illustrating the brightness versus voltage response of a liquid crystal display; Figure 2 shows B-V curves at three different points on an LCD panel, illustrating the variation in the maximum voltage required for maximum black; Figure 3 is a schematic illustration of the steps involved in one embodiment of the uniformity correction of the invention; Figure 4 is a schematic illustration of one embodiment of a method for adjusting the drive voltage of an LCOS projection display system, including correction of the uniformity of the black and white states of the LCOS panel; Figure 5 is a schematic illustration of one embodiment of the illumination architecture for a light engine for a single panel scrolling color projector; and Figure 6 is a schematic illustration of one embodiment of a light valve projection display system incorporating an LCD panel corrected for uniformity of the black and white states in accordance with the method of the invention.

In accordance with the teachings of the invention, uniformity correction of the black and/or white states of an LCD panel is implemented by expansion of the voltage range used for drive the panel. This may be achieved by mapping most pixels in the black (or white) state to a gray value above the minimum grey value (typically zero) and below the maximum value (typically 255 for an 8 bit system). The voltage for black, gray level 0, (or white, gray level 255) is set at the maximum (minimum) required to achieve the lowest (highest) brightness, anywhere in the picture. Thus the best system brightness and contrast is maintained, with only a minor reduction in dynamic range due to the loss of a few gray levels. The invention will be described in terms of a preferred embodiment, in which uniformity correction is carried out for an LCOS panel used in a single panel scrolling color projection display and operating in the 'normally white' ('drive to black') mode. That is, with no drive voltage applied, the display is nearly white, and with high drive voltage the panel is nearly black. The single panel scrolling color projection display system is characterized by a single LCOS light modulating panel having a raster of individual picture elements or pixels, which panel is illuminated by horizontally elongated red, green and blue illumination bars or stripes. The stripes are continuously scrolled vertically across the panel while the illuminated rows of pixels are synchronously addressed with display information corresponding to the color of the then incident stripe. See, for example, United States Patent 5,410,370, "Single panel color projection video display improved scanning" issued to P. Janssen on March 25, 1994, and United States Patent 5,416,514, "Single panel color projection video display having control circuitry for synchronizing the color illumination system with reading/writing of the light valve" issued to P. Janssen et al. on May 16, 1995, the entire specifications of which are hereby incorporated by reference herein. Such single panel systems are to be distinguished from the more conventional three-panel systems, in which separate red, green and blue beams each fully illuminate and are modulated by a separate light modulator panel. The modulated beams are then superimposed on a display screen to produce a full color display. In accordance with the invention, an incoming display signal, such as a color video signal, in the form of a stream of digital data, having gray scale image information encoded in 0-255 gray levels, is corrected for uniformity of the black and white states by a series of operations or steps. First, the panel is sampled with drive voltages to determine the maximum drive voltage required to achieve the best black state, i.e., the state where the modulated light level is as small as possible. Next, the lower end of the gray scale, zero, is set to this voltage. In Figure 2, this is the curve MaxV, and the black voltage is 6.9V. Then, the panel is sampled with smaller drive voltages at a pre-set number of areas of the panel to determine where the best black can be obtained with less than the maximum drive voltage, and a number of gray levels, e.g., ten gray levels, are allocated to the voltage range between minimum and maximum best black voltage. Then in the example of Figure 2, gray level 10 would correspond to 5.6V and gray level 5 would correspond to 6.2 V. Uniformity correction of the black state, gray level 0, would then add 0 to the area at Max V, add 5 to the Center area, and add 10 to the area at Min V. When this black level uniformity correction is applied, then all areas are driven fully to the best black when a full field of the black state is called for. Thus, the black state uniformity is improved, maximum contrast is maintained or achieved in all areas, and contrast inversion is avoided. In the same manner uniformity correction can be applied to the white state. In an 8-bit system, white corresponds to gray level 255, which is mapped to the minimum voltage required to achieve highest brightness anywhere on the panel. Uniformity correction is then applied at the full field white state that would reduce the gray value below 255 in selected areas. The correction of black and white states in accordance with the invention results in a slight reduction of dynamic range due to some loss of gray levels. Consider the Center of the picture in the example of Figure 2. With correction, black is mapped to gray level 5 and white is mapped to gray level 250. Thus the full range encompasses 246 gray levels instead of the normal 256 levels. The penalty is small relative to the gain in uniformity, contrast, and overall picture quality. In the drive-to-black system of Fig.2, the first end state is a minimum level of brightness (black state) achieved at a maximum drive voltage. Because of non- uniformity, different 'maximum' voltages are required at different locations on the panel. The highest voltage needed to achieve this black state anywhere on the panel for this system is MaxV. The gray level corresponding to this highest voltage, MaxV, is the first end of the gray scale. Typically, this gray level is equal to 0, and is designated GO. In a drive-to-white system, the first end state is a maximum level of brightness (white state) achieved at a maximum drive voltage, and the gray level corresponding to this highest voltage is the first end of the gray scale. Typically, in an 8 bit system, this gray level is equal to 255. In either the drive-to-black or drive-to-white system, the other voltages that achieve the same end state are necessarily smaller than the highest voltage, designated herein Vh. These can be designated VI, V2, .. Vn, respectively., In the example of Fig. 2, V2 corresponds to Min V, VI corresponds to Center, and Vh corresponds to MaxV, where V2 < Vl < Vh. In order to obtain correction factors for the correction of non-uniformity in the black state, a gray level Gl is first allocated to the voltage VI and gray level G2 is allocated to the voltage V2, where G2 > Gl > GO. This allocation of gray levels to voltages is achieved by use of an appropriately defined gamma table. Once corresponding gray levels are defined for all sampled locations on the panel, correction factors can be defined for all of these locations. The correction factors are simply the differences in gray levels (G2 - GO), (Gl -GO), etc. Gray level G2 applied to the panel at specified location produces the same brightness level (the same 'first end state') as gray level GO applied to location identified as MaxV. Therefore we have achieved uniformity at those locations. If G2 is the highest gray level allocated for correcting the black state uniformity, then the number of gray levels that remain to be allocated for the other brightness levels is reduced from an original 256 gray levels to a number which is 256 - G2. Similarly we will allocate a certain number of gray levels for uniformity correction at the bright end of the brightness scale and the number of remaining gray levels will be reduced yet again. This is the tradeoff for obtaining brightness and color uniformity in the black and white states. In accordance with a preferred embodiment of the invention, as shown in Figure 4, after the data signal has been corrected for uniformity of the black and white states, it is then processed for gamma correction in the conventional manner, or in the manner described in co-pending United States patent application S.N. , filed (Attorney Docket No. PHID 703,346). As part of this gamma correction, the data signal is converted from digital to analog voltage signals within a predetermined range, which are then input into the light modulating LCOS panel for projection. A typical voltage range for such a system is 0.0-15 volts.

In a single panel scrolling color system, incoming white light is split into separate red, green and blue color components by a light engine using dichroic elements. The illumination architecture for a light engine 1 for such a scrolling color projector is shown schematically in Fig. 5. White light from source S is split into a blue component B and a green/red component G/R by dichroic element 2. The B component is directed by lens 3 and mirror 4 to prism scanner 5. The G/R component is passed by dichroic element 2 through lens 6 to dichroic element 7, which splits the G/R component into a green component G and a red component R. The G component is reflected by element 7 to prism scanner 8, while the red component is passed through dichroic element 7 to prism scanner 9. The scanned R, G, B components are then directed to recombination dichroic elements 10 and 11 by mirror 12 and relay lenses 13 through 17. Fig. 6 is a block diagram of a single panel color projection display system 20 of the invention. Illumination system 21 includes light collection system 22, which provides an illumination beam of stripe-shaped cross-section to beam splitting and scrolling engine 23. Illumination system 21 generates sequentially scrolling red, green and blue stripes, for sequentially scrolling across the surface of light-modulating LCD panel 24, preferably a LCOS panel, which modulates the scrolling light beams synchronously with the input of display information from electrical signal input source 25. Projection lens 26 projects the modulated light onto a display surface, not shown. In accordance with the invention, signal input source 25 includes means for correcting the end state uniformity of the LCD panel, by (a) determining the largest voltage required to achieve a first end state at any area of the LCD panel; (b) setting a first end of the gray scale to correspond to said largest voltage; (c) determining smaller voltages required to achieve the first end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said first end of the gray scale to correspond to each of said smaller voltages; and (2) adjusting said first end of the gray scale to correspond to the allocated gray scales at said at least two different areas of the LCD panel. The invention has necessarily been described in terms of a limited number of embodiments. However, other embodiments and variations of embodiments will be apparent to those skilled in the art, and these are intended to be encompassed within the scope of the appended claims.

Claims

CLAIMS:

1. A method for correction of an end state at different areas of an LCD panel, wherein the end states occur at opposite ends of a range of voltages (drive range) used to drive the LCD panel, and wherein the range of voltages is determined in accordance with a gray scale having a plurality of grey levels ranging from 0 or a minimum at one end of the gray scale and to a maximum at the other end of the gray scale, the method comprising the steps of: (a) determining the largest voltage required to achieve a first end state at any area of the LCD panel; (b) setting a first end of the gray scale to correspond to said largest voltage; (c) determining smaller voltages required to achieve the first end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said first end of the gray scale to correspond to each of said smaller voltages; and (2) adjusting said first end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel.

2. The method of claim 1 in which a second end state is corrected by the steps of: (a) determining the smallest voltage required to achieve the second end state at any area of the LCD panel; (b) setting a second end of the gray scale to correspond to said smallest voltage; (c) determining larger voltages required to achieve the second end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said second end of the gray scale to correspond to each of said larger voltages; and (2) adjusting said second end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel.

3. The method of claim 1 in which the end state corresponding to the largest voltage is the black state and the end of the gray scale corresponding to the black state is the 0 or minimum end of the gray scale.

4. The method of claim 2 in which the end state corresponding to the smallest voltage is the white state and the end of the gray scale corresponding to the white state is the maximum end of the gray scale.

5. The method of claim 3 in which the 0 or minimum end of the gray scale is adjusted by adding zero or a positive number to said zero or minimum end of the gray scale.

6. The method of claim 4 in which the maximum end of the gray scale is adjusted by subtracting zero or a positive number from said maximum end of the gray scale.

7. The method of claim 1 in which following correction of the end state, gamma correction of the LCD panel is carried out.

8. The method of claim 2 in which following correction of the end state, gamma correction of the LCD panel is carried out.

9. A method for correction of an end state at different areas of an LCD panel, wherein the end states occur at opposite ends of a range of voltages (drive range) used to drive the LCD panel, and wherein the range of voltages is determined in accordance with a gray scale having a plurality of grey levels ranging from 0 or a minimum at one end of the gray scale and to a maximum at the other end of the gray scale, the method comprising the steps of: (a) determining the smallest voltage required to achieve the second end state at any area of the LCD panel; (b) setting a second end of the gray scale to correspond to said smallest voltage; (c) determining larger voltages required to achieve the second end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said second end of the gray scale to correspond to each of said larger voltages; and (2) adjusting said second end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel.

10. The method of claim 9 in which a second end state is corrected by the steps of: (a) determining the largest voltage required to achieve a first end state at any area of the LCD panel; (b) setting a first end of the gray scale to correspond to said largest voltage; (c) determining smaller voltages required to achieve the first end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating a gray level near said first end of the gray scale to correspond to each of said smaller voltages; and (2) adjusting said first end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel.

11. The method of claim 9 in which the end state corresponding to the smallest voltage is the white state and the end of the gray scale corresponding to the white state is the maximum end of the gray scale.

12. The method of claim 10 in which the end state corresponding to the largest voltage is the black state and the end of the gray scale corresponding to the black state is the 0 or minimum end of the gray scale.

13. The method of claim 9 in which the maximum end of the gray scale is adjusted by subtracting zero or a positive number from said maximum end of the gray scale.

14. The method of claim 10 in which the 0 or minimum end of the gray scale is adjusted by adding zero or a positive number to said zero or minimum end of the gray scale.

15. The method of claim 9 in which following correction of the end state, gamma correction of the LCD panel is carried out.

16. The method of claim 10 in which following correction of the end state, gamma correction of the LCD panel is carried out.

17. A method of correcting a data signal input for an LCD display, the method comprising the steps of: (a) uniformity-correcting the data signal input for at least one of the end states of the LCD display in accordance with the steps of claim 1; followed by (b) gamma-correcting the uniformity-corrected data signal.

18. A method of correcting a data signal input for an LCD display, the method comprising the steps of: (a) uniformity-correcting the data signal input for at least one of the end states of the LCD display in accordance with the steps of claim 2; followed by (b) gamma-correcting the uniformity-corrected data signal.

19. A method of correcting a data signal input for an LCD display, the method comprising the steps of: (a) uniformity-correcting the data signal input for at least one of the end states of the LCD display in accordance with the steps of claim 9; followed by (b) gamma-correcting the uniformity-corrected data signal.

20. A method of correcting a data signal input for an LCD display, the method comprising the steps of: (a) uniformity-correcting the data signal input for at least one of the end states of the LCD display in accordance with the steps of claim 10; followed by (b) gamma-correcting the uniformity-corrected data signal.

21. A liquid crystal display (LCD) system comprising: (a) a light-modulating LCD panel; (b) an illumination system for providing unmodulated light to the LCD panel; and (c) electrical signal input source for providing a display signal to the LCD panel; wherein the electrical signal input source includes means for correcting the end state uniformity of the LCD panel, said means comprising means for: (a) determining the largest voltage required to achieve a first end state at any area of the LCD panel; (b) setting a first end of the gray scale to correspond to said largest voltage; (c) determining smaller voltages required to achieve the first end state for at least two areas of the LCD panel different from said any area; (d) applying correction factors for the voltages applied to said at least two different areas of the LCD panel, by: (1) allocating gray levels near said first end of the gray scale to correspond to each of said smaller voltages; and (2) adjusting said first end of the gray scale to correspond to the allocated gray levels at said at least two different areas of the LCD panel.

22. The system of claim 21 in which the illumination system comprises: means for splitting the unmodulated light into red, green and blue beams, and means for continuously and sequentially scrolling the red, green and blue beams across the light modulating LCD panel.

23. The system of claim 21 in which the LCD panel is an LCOS panel.

24. The system of claim 21 comprising a projection lens for projecting the light- modulated image onto a projection display surface.