1. Field of the Invention
The invention pertains to the field of transmissive displays and more particularly to a method of determining input values for providing the primary color luminances required for a set of specified colors to be displayed thereon.
2. Description of the Prior Art
Transmissive display systems have been developed to provide flat-panel monitors for numerous applications, including aircraft instrumentation, personal computers, laptop and notebook computers, and the like. Such displays potentially offer greater luminance, higher contrast ratios, greater sharpness, and better spatial uniformity than CRT displays. These systems utilize a light source, termed a backlight, to illuminate the pixels on the flat panel. Light intensity from the backlight is normally maintained at constant level and color is provided by the relative luminosity of the light transmitted through three primary color filters, usually selected as red, green, and blue, associated with each pixel on the screen. The intensity of the light from each filter is controlled by analog signals which in turn are selected by digital signals representative of the desired pixel color. These analog signals are selected from a look-up table which is accessed by the color representative digital signals.
Due to the backlight leakage through the primary color filters, the black level in a transmissive display is not as dark as the black level in a CRT. Consequently, transmissive displays have lower contrast ratios than CRTs. Further, when the luminance of a primary color is reduced by decreasing the video level, the measured color coordinates shift. This is largely the result of mixing the intended level of the primary colors with a backlight leakage component, which is also represented in the black level.
Colors produced on the screen of an uncompensated transmissive display may vary from the desired luminance and chromaticity of the target colors. Such variances may be caused by factors such as primary filter color variations, external flare, nonlinearities, and backlight leakage. Since measured color coordinates of the primary colors are not constant with input signal levels, due to backlight leakage, proper addition of the primary colors may not always be achieved. This problem is most severe when very low signal levels are required for use in low light ambient conditions. Consequently, when applied to transmissive displays, the prior art calibration methods can fail to achieve specified accuracy of chromaticity and luminance. To provide required chromaticity and luminance, a transmissive display system must be calibrated by modifying the process used to generate the input signals and calculating compensated input values which may be stored in a look-up table.
In the prior art, monitors were color calibrated and adjustments were made, if needed, either manually or automatically. The manual system, which is time consuming, requires a color calibration for each of a multiplicity of specified colors followed by a manual adjustment of the analog signals to bring each of the specified colors within predetermined tolerances. The closed loop system, which is expensive to set-up and maintain, monitors a specified color while the control signals are varied until the color being calibrated is within the predetermined tolerance. This procedure is repeated for each of the specified colors. Besides their difficulty and expense, these processes can fail when the chromaticity of primary colors varies with input signal level.