Method and apparatus for determining and correcting for illumination variations in a digital projector

The present invention relates to display devices, and more particularly to methods and apparatus for correcting for variations in the display devices.

Digital projection systems can exhibit variations in gain and intensity (offset) from pixel to pixel. These variations can be due to variations in the driving electronics, the light modulator, the projection optics, the display screen, and the illumination optics. Electronic systems can be developed to correct for the variations by applying gain and offset correction values to each pixel. U.S. Pat. No. 6,061,102 issued May 9, 2000 to Sheppard et al. shows a target brightness map that is generated for each of a plurality of preselected shading application points and the target brightness map is used to correct the input signal to the display to remove variations in the display. The shading process is automatically performed during projector setup.

One problem with this approach is that the Sheppard et al. device uses a CCD sensor wherein each pixel in the sensor senses a large number of pixels in the display and is therefore unable to correct for variations in individual pixel intensity. There is a need therefore for an improved method and apparatus for correcting variations in display pixel intensity that avoids the problems noted above.

The need is met according to the present invention by providing a method of correcting pixel by pixel variations in a display, the method including the steps of creating a defect map of pixel intensity offsets for the display; correcting an input signal according to the defect map; and displaying the corrected input signal on the display, wherein the offset defect map is created by modulating a pixel in the display at a predetermined rate at a predetermined intensity; sensing the display with a photosensor to generate a sensed signal; demodulating the signal with a synchronous demodulator at the predetermined rate to produce a demodulated signal representing a sensed intensity; and employing the sensed intensity and the predetermined intensity to generate a correction offset.

The method and apparatus of the present invention has the advantage of correcting for each pixel of the display. The components required are low cost, and correction data is collected for the entire electro-optical projection system.

FIG. 1 is schematic diagram of a projection display system incorporating the correction method of the present invention;

FIG. 2 is a graph illustrating the computation of a gain correction value according to one embodiment of the present invention;

FIG. 3 is a flow chart illustrating the correction method according to the present invention; and

FIG. 4 is a flow chart illustrating the step of creating a defect map according to the present invention.

The present invention collects data on pixel gain and intensity using a simple sensor that collects light from the entire projected image. To obtain the correction data, all pixels are driven to black, or a predetermined level except for the pixel under observation. The pixel under observation is driven at a predetermined frequency that is preferably some fraction of the refresh rate of the display. A photosensor senses the entire display to produce a signal representing the light from the pixel. To increase sensor accuracy according to the present invention, the sensor is equipped with a synchronous demodulator that demodulates the signal at the predetermined frequency. The sensed intensity and the predetermined intensity are employed to generate an offset correction. The process is repeated for a plurality of intensities to generate a gain correction. The process is repeated for all pixels of the modulator and correction values created.

Referring to FIG. 1 , a projection display 10 according to the present invention includes a light source 12 , such as a xenon projection lamp. The light from the light source 12 is conditioned by light source optics 14 to improve uniformity and color balance, remove heat, and concentrate the light onto the surface of a light modulator 16 , such as the JVC DILA reflective liquid crystal light modulator. Light reflected from the light modulator 16 is directed to a display screen 20 by projection optics 18 . Drive electronics 22 applies a corrected input signal to the light modulator 16 . Correction electronics 24 applies a pixel by pixel correction to the input signal to correct for pixel by pixel variations in gain and offset. The corrections are stored in a defect map contained in a memory 26 such as a nonvolatile RAM.

According to the present invention, the corrections stored in the defect map are generated by driving one pixel at a time at a predetermined frequency. The signal for driving the pixel is generated, for example by a programmed microprocessor 28 . A photosensor 30 is directed at the display screen 20 , or alternatively at the projection optics 18 to generate a sensed signal. The sensed signal is detected by a synchronous detector 32 and converted to a digital signal by analog to digital converter 34 . The digital detected signal is supplied to the microprocessor 28 where an intensity offset correction O is generated by comparing the signal sent to drive the pixel with the sensed demodulated signal. By driving the pixel at a predetermined frequency and sensing the detected signal with a synchronous demodulator at the predetermined frequency, the relatively weak signal produced by a single pixel can be efficiently detected in the presence of flare light and other noise that may corrupt the signal. The offset correction value O is recorded in the defect map. When the display is operating, the offset correction value O is added to the input pixel values that correspond to the pixels position on the display.

Referring to FIG. 2 , if the pixels in the display have gain variations, a gain correction value can also be generated by driving a pixel at several different input intensity levels and sensing the signal at each intensity level to produce a corresponding plurality of output values 31 . A line is fit through the output values 31 to produce a gain curve 33 . The slope S 1 of the gain curve 33 is the gain of the pixel. The ratio of the slope S 2 of an ideal gain curve 35 to the slope S 1 of the sensed gain is computed as a gain correction value G for the pixel. The gain correction value G is recorded in the defect map. When the display is operating, the input pixel values are multiplied by the gain correction values G that correspond to the pixels position on the display.

Referring to FIGS. 3 and 4 , a method of using the apparatus of FIG. 1 according to the present invention will be described. First a defect map is created 36 that contains the corrections to be applied to all of the pixels of the display. The defect map is then used to correct 38 each pixel of the input signal to the display. Finally the corrected input signal is displayed 40 . As shown in FIG. 4 , the defect map is created by modulating 42 a single pixel at a predetermined rate and intensity while keeping all other pixels at zero or some predetermined level. The display is sensed 44 with a photosensor to generate a signal. The sensed signal is demodulated 46 with a synchronous demodulator, and a correction offset is generated 48 from the demodulated signal. The steps are then repeated 50 for every pixel in the display.

For simplicity, the method has been described with respect to one channel, however it will be understood that the method can be applied to each channel of a color display device. It will also be understood that a cluster of pixels can be modulated rather than a single pixel when typical defects in the display occur over regions that are greater than one pixel, thereby reducing the time to characterize the display.

The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

10 projection display

12 light source

14 light source optics

16 light modulator

18 projection optics

20 display screen

22 drive electronics

24 correction electronics

26 memory

28 programmed microprocessor

30 photosensor

31 output values

32 synchronous detector

33 gain curve

34 analog to digital converter

35 ideal gain curve

36 create defect map step

38 correct signal step

40 display corrected signal step

42 modulate single pixel step

44 sense display step

46 demodulate signal step

48 generated correction offset step

50 repeat for each pixel step