WO2004056103A1

WO2004056103A1 - Color non-uniformity correction method and apparatus having optical and electronic lcd panel compensation

Info

Publication number: WO2004056103A1
Application number: PCT/IB2003/005913
Authority: WO
Inventors: Michael Bakhmutsky; Viktor Gornstein
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-12-18
Filing date: 2003-12-11
Publication date: 2004-07-01
Also published as: JP2006510931A; KR20050084308A; CN1726703A; AU2003303006A1; TW200501043A; EP1576811A1

Abstract

A method and apparatus for correcting video non-uniformity in a projection system includes correcting video non-uniformity of a liquid crystal display; and correcting optical non-uniformity of an optical system using optical compensation.

Description

COLOR NON-UNIFORMITY CORRECTION METHOD AND APPARATUS HAVING OPTICAL AND ELECTRONIC LCD PANEL COMPENSATION

The present invention relates generally to video processing for color displays, and in particular to a method and apparatus for providing color non-uniformity correction in color displays, including liquid crystal (LC) color displays.

Color displays are used in a variety of electronic devices. These include monitors for personal computers, televisions, and other video displays. These displays may be direct-view, cathode-ray tube devices, or projection devices.

One type of projection device is based on the optical properties of liquid crystals, such as nematic crystals. These projection devices may include a layer of liquid crystal disposed over a semiconductor transistor array. Often, the array is one of complementary metal-oxide-semiconductor (CMOS) transistors that are used to selectively produce electric fields across the layer of liquid crystal. These electric fields change the polarization angle of the liquid crystal material molecules enabling the modulation of light that traverses this material. The light may be reflected by reflective elements or may be transmitted to the screen. In either case, the modulated light is projected onto a screen by optical elements forming a video image. In case of reflection, projection devices are referred to as liquid crystal on semiconductor (LCOS) projection displays.

Some factors that impact image quality of displays are resolution, brightness, contrast, and color depth. Resolution is the number of pixels a screen displays. Often, the resolution is expressed in a particular pixel dimension (e.g. 800 x 600 for many computer monitors). In this example, the monitor has 800 pixels in the horizontal dimension, and 600 pixels in the vertical dimension. Of course, the greater the number of pixels for a given display area, the smaller area of each pixel, and the greater the resolution.

Color depth defines how many colors can be displayed on a screen. Generally, color depth is described in binary logic (bits). Each of the three primary colors used in color displays (red, blue, green) has a number of bits that describes its color depth, or the number of shades of a particular color that may be displayed. The number of colors is normally described via binary exponential notation (e.g., 2⁸ (referred to as 8 bit video) for 256 shades of each of the three primary colors). As can be readily appreciated, the greater the number of color bits, the greater the number of shades of the color, and the greater the color depth. Of course, the greater the color depth, the better is the display quality.

While the resolution, brightness, contrast, and color depth may be chosen for a particular desired image quality, certain factors may deteriorate the image quality. For example, differences in both the optical path and the imager (LC panel) characteristics in LCOS projection devices can have deleterious effects on the quality of the projected image.

What is needed is a correction method and apparatus, which overcomes certain drawbacks of known correction schemes, such as video clipping.

In accordance with an exemplary embodiment of the present invention, a method comprises correcting video non-uniformity of a liquid crystal display; and correcting optical non-uniformity of an optical system using optical compensation.

In accordance with another exemplary embodiment of the present invention, apparatus for correcting a video non-uniformity comprises an electronic correction device that corrects video non-uniformity of a liquid crystal display; and an optical device that corrects optical non-uniformity of an optical projection system.

The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Fig. 1 is a perspective view of a video correction apparatus in accordance with an exemplary embodiment of the present invention.

Fig. 2 is an illustration of bilinear interpolation of correction data in accordance with an exemplary embodiment of the present invention.

In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well-known devices, methods and materials may be omitted so as to not obscure the description of the present invention.

Briefly, the present invention relates to a method and apparatus for providing color non-uniformity correction in real-time in LCD projection systems by correcting the video non-uniformity of both the LCD and the optical system. The correction for the LCD panel non-uniformity is effected electronically by a bilinear interpolation technique that does not require the storage of all of the correction data for all the pixels of all the colors in a memory. The correction of optical video non-uniformity of the projection system includes an optical element that decreases the brightness level at regions that exceed the desired brightness. Beneficially, video signal clipping and its deleterious affects are substantially avoided by virtue of the exemplary embodiments.

Fig. 1 shows an LCD apparatus 100 in accordance with an exemplary embodiment. An LCD 101 is connected to an electronic correction device 102, which provides the electronic correction of video non-uniformity due to LCD panel non-uniformity. A video input 107 is coupled to the electronic correction device 102. The output of the electronic correction devicel02 is input to an LCD driver 108, which is connected to the LCD 101 as shown. The electronic correction is illustratively effected by a bilinear interpolation correction technique that is described in further detail herein. The electronic correction device 102 includes the required elements to effect the calculation of the correction data (e.g., an interpolator) and to control the LCD in order to alter the video level at each pixel as needed.

The output 105 from the LCD is incident on an optical element 103, which is illustratively the optical inverse of the projection system's non-uniformity distribution. For example, the optical element may be a photographic negative of the screen taken when the video levels applied are uniform gray with no presence of LCD 101 in the system. In this manner, the light 105 is transmitted through the optical element 103, and emerges as light 106 that is incident on the screen.

As will become clearer as the present description continues, by correcting for video non-uniformity with the electronic correction device 102, and correcting for video non- uniformity of the optical system with the optical element, the level of correction is increased, and the image quality at the image screen 104 is enhanced. However, clipping of the video signal is substantially avoided.

In general, the electronic device 102 provides electronic color non-uniformity correction by video modification. In order to achieve this correction, the brightness distributions for each pixel at a number of video levels are evaluated individually for each color path. As can be appreciated, in order to save memory, not all pixels are evaluated. Rather, a limited number of pixels located at a grid of points spaced with respect to each other by a predetermined number of pixels both vertically and horizontally are evaluated. Differences between the actual and the nominal brightness for the particular video level under evaluation are calculated and stored as correction coefficients. To provide a high-quality correction to the video signal, the color correction data is obtained for a variety of color levels. In an exemplary embodiment, four correction data are stored in a memory device in such a manner that they are readily available for computations. These data are used for the bi-linear (or other two-dimensional) interpolation at a particular video level in a grid (segment) of a particular area having a particular number of pixels. To effect color correction across the image screen, a plurality of color correction data representative of different locations of the display are stored for real-time interpolation.

Fig. 2 shows conceptual view of a bilinear interpolation scheme in accordance with an exemplary embodiment of the present invention. The interpolation block 201 includes four measured and stored correction coefficients (202, 203, 204 and 205) at points representative of points on an image screen. The correction coefficient for any interpolated point on a map of correction data 200 (e.g., interpolated point 205) is illustratively determined by the electronic correction device 102 by a technique described in U.S. Patent Application Serial Number 10/179,319, entitled "Color Non-Uniformity Correction Method and Apparatus," to Michael Bhatmustsky, and filed on June 24, 2002. The disclosure of this application is incorporated herein and for all purposes.

It is noted that the relationship between light and the electronic video signal from the LCD is not linear. As such, any error in the video level applied results in further error due to this non-linearity. In order to effect sufficient correction of the video signal, it is necessary to have multiple correction data sets (several maps of correction data obtained at preset video levels for each color individually). To correct the video signal, the level of the video data is added to the interpolated correction. Conceptually, there may be multiple maps (corresponding to other video levels at which the correction data were measured) above map 200, and an interpolated coefficient would be determined by further cross- interpolating between these maps in real-time depending on currently processed video level. While the technique described for correcting video accounting for the non-linear relationship between the video signal and the light transmitted to the screen is certainly useful, it requires a large number of data points for the various levels and colors of the image. Moreover, the process of correction may result in high margins of video modification in the LCD display systems in which these devices are disposed. Ultimately, this can lead to clipping of the video signal, particularly when the video level is high and the correction or compensation is high. As can be appreciated, when then sum of the video correction and the video level is greater than a particular video level, the video signal is clipped and suitable correction is not attained. This reduces the overall video quality. For example, consider 255-level video. If the video level of a particular pixel is at 190, and the correction coefficient is determined to be 100, applying this level of correction exceeds the maximum video level of 255. As a result the video signal will be clipped, and there will be harmonics in the signal that are manifest as undesirable artifacts on the image display. However, in accordance with exemplary embodiments of the present invention, because a portion of the correction is addressed by the optical element 103, the amount of electronic correction required from the electronic correction device 200 is reduced, which means lower level of electronic correction and less clipping in video processing.

As referenced above, in accordance with an exemplary embodiment of the present invention, in order to address the problems of clipping of the video signal, the amount of electronic correction relied upon is limited to avoid clipping the video signal. To this end, a portion of the compensation needed to achieve the higher levels of correction, which can cause clipping in an all-electronic correction method, is effected via the electronic correction device, while the remaining portion is exacted using the optical element 103. Thereby color non-uniformity is corrected by a combination of the electronic correction device, which controls the LCD panel, and the optical element, which provides optical correction. Accordingly, by not requiring the electronic correction device to correct for video non-uniformity from both the LCD panel and the optical system, video clipping is avoided, and the overall correction capability is increased.

The optical element 103 is usefully the inverse of the image screen 104, generally resulting from a gray level uniform video without the LCD in the system, although variations are certainly possible. Illustratively, three separate inverse images are used (i.e., one per video color). Alternatively, one inverse image may be used to compensate for overall brightness non-uniformity, with the remaining correction exacted electronically via the electronic correction device 102. In many applications of the exemplary embodiments, the optical element 103 may be a type of photographic negative. However, it is noted that the optical element 103 may be fabricated by image processing techniques, to include digital photography and other digital imaging techniques. Finally, it is noted that optical element 103 may be a color negative (inverse image) of the image screen; although three black-and-white negatives may be used in the paths of the primary colors.

Illustratively, the optical element 103 may be fabricated by installing mirrors instead of the LCD panel in the projection system in which the LCD is used. The optical element may be tailored to the particular projection system, or a plurality of negatives for a given type of projection system could be made from a production version thereof. In the former case, the image quality is better; and in the latter the mass production capabilities are advantageous. Regardless of the degree of correction achieved by the optical element 103, the remaining correction is effected using the electronic correction device 102 and the technique(s) described above.

The compensation for non-uniformities due to the optical system by the optical element 103 illustratively is effected as follows. The output from the LCD is projected light onto the image screen with the video correction being manifest in the intensity of the light at each pixel on the image screen. As referenced above, this may result in an increase in the compensation beyond the video level limits. Alternatively, the intensity of the light may be too low due to an under-correction in the interpolation method described above. However, by capturing the inverse (negative) of the image screen in the optical element 103, each pixel is tempered to a certain correction. To wit, pixels that are too bright due to over-correction are transmitted through a correspondingly 'darker' portion of the optical element; while pixels that are under-corrected so that the intensity level is too low on the image screen are transmitted through a region of the optical element that is the optical inverse of this low level, and the corresponding pixel has a greater intensity at the screen.

It is noted that in either of the above cases, the optical element 103 also corrects the color intensity of each pixel (e.g., the optical element 103 is a color negative or inverse of the screen image). It is further noted that the optical element alone cannot achieve high- quality correction because it is not able to provide the non-linear video compensation. To wit, the optical element 103 will correct all levels the same way. However, in combination with the electronic correction device 102, the optical element provides suitable video correction.

The correction technique and apparatus of the exemplary embodiments is advantageous for a variety of reasons. For example, because the electronic correction/compensation does not have to correct for errors in the optical system, the required range of video modification can be reduced. This eliminates the problem of clipping of the corrected video signal. Moreover, because the range of video modification is reduced, the computational accuracy of the correction is enhanced.

Another benefit of an exemplary embodiment is realized in manufacturing. Because of the combination of optical and electronic correction, the correction levels in such a system are lower, and the correction margins are subsequently higher than in known systems. This can solve more severe LCD problems and lead to a significant increase in the LCD panel yields.

Finally, it is noted that while the exemplary embodiments thus far described effect LCD induced video non-uniformity strictly electronically, and optical non-uniformity via the strictly via the optical element, this is not essential. To this end, some of the optical non-uniformity may in fact be compensated electronically by the method described above. For example, in a mass production setting where a mass-produced optical element is made for a type of projection system, calibration of each LCD device may be realized by providing some optical correction electronically.

The invention being thus described, it would be obvious that the same may be varied in many ways by one of ordinary skill in the art having had the benefit of the present disclosure. Such variations are not regarded as a departure from the spirit and scope of the invention, and such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims and their legal equivalents.

Claims

CLAIMS:

1. A method of correcting video non-uniformity in a projection system, the method comprising: correcting video non-uniformity of a liquid crystal display (101); and correcting optical non-uniformity of an optical system of the projection system using optical compensation.

2. A method as recited in claim 1, wherein said correcting optical non-uniformity includes providing an optical element (103) in a light path before an image screen (105).

3. A method as recited in claim 2, wherein said optical element is an inverse image of light projected on said image screen (105).

4. A method as recited in claim 3, wherein said inverse image is a photographic negative.

5. A method as recited in claim 1, wherein said correcting video non-uniformity includes electronic correction of a video signal.

6. A method as recited in claim 1, wherein said correcting optical non-uniformity includes optical correction of an optical signal.

7. A method as recited in claim 5, wherein clipping of said video signal is substantially avoided by the method.

8. A method as recited in claim 1, wherein said correcting video non-uniformity includes interpolating a plurality of correction data to determine correction coefficients.

9. A method as recited in claim 3, wherein said inverse image further comprises a plurality of photographic negatives.

10. A method as recited in claim 3, wherein said inverse image is a digital photographic negative.

11. An apparatus for correcting a video signal in a projection system comprising: an electronic correction device (102) that corrects video non-uniformity of the liquid crystal display (101); and an optical element (103) that corrects optical non-uniformity of the projection system.

12. An apparatus as recited in claim 11, wherein said electronic correction device includes an interpolator, which determines a plurality of correction coefficients.

13. An apparatus as recited in claim 12, wherein said optical element is an inverse image of an image screen of said projection system.

14. An apparatus as recited in claim 12 wherein said electronic correction device (102) corrects said video non-uniformity by correcting a video signal with a subset of said correction coefficients.

15. An apparatus as recited in claim 14, wherein clipping of the video signal is substantially avoided.

16. An apparatus as recited in claim 13, wherein said inverse image is at least one photographic negative of light projected on said screen.

17. An apparatus as recited in claim 11, wherein said optical element (103) comprises a negative image of light projected on an image screen (105) for each of the primary colors.

18. An apparatus as recited in claim 13, wherein said photographic negative is a digital negative.

19. An apparatus as recited in claim 12, wherein said interpolator is a bilinear interpolator.