1. Field of the Invention
The present invention relates to correlated color temperature detection of ambient or projected light, and, in particular, to a digital color temperature detector fabricated on an integrated chip.
2. Description of the Related Art
In color reproduction fields such as commercial printing and photography, it is known that the correlated color temperature of the viewing light affects the way in which an observer perceives a color image. More particularly, an observer will perceive the same color image differently when viewed under lights having different correlated color temperatures. For example, a color image which looks normal when viewed in early morning daylight will look bluish and washed out when viewed under overcast midday skies.
Correlated color temperature is characterized in color reproduction fields according to the temperature in degrees Kelvin (.degree.K.) of a black body radiator which radiates the same color light as the light in question. FIG. 1 is a chromaticity diagram in which Planckian locus (or hereinafter "white line") 1 gives the temperatures of whites from about 1500.degree. K. to about 10,000.degree. K. The white color temperature of viewing light depends on the color content of the viewing light as shown by line 1. Thus, the aforementioned early morning daylight has a white color temperature of about 3,000.degree. K. (hereinafter "D30") while overcast midday skies has a white color temperature of about 10,000.degree. K. (hereinafter "D100"). A color image viewed at D60 will have a relatively reddish tone, whereas the same color image viewed at D100 will have a relatively bluish tone.
Because of these perceptual differences, conventional color reproduction practice accepts 5,000.degree. K. (hereinafter "D50") as a standard white color temperature. In accordance with this convention, commercial color reproduction facilities ordinarily evaluate color images for color fidelity in a room whose light is controlled to a white color temperature of D50.
Recently, however, low-cost high-quality color reproduction equipment has become available to individual users. Such users are not ordinarily in a position to provide a room having ambient light controlled to D50. And, even if such rooms are available, the color image is not ordinarily displayed in a room whose ambient light is D50. Rather, such color images are more likely to be displayed in rooms not having a white color temperature of D50 and may, for example, be used in an office building as part of a business presentation where the viewing light is far different from D50.
Since the white color temperature affects the perception of color, it has been proposed to modify the colors in a color image based on a measurement of white color temperature of the viewing light. For example, "Color Equalization" by J. Schwartz, Journal Of Image Science And Technology, Vol. 36, No. 4, July/August, 1992, suggests to equalize a color image based on the white color temperature of viewing light by adjusting the amount of individual inks used during a printing process based on the color temperature of the viewing light.
Heretofore, however, it has not been possible to measure the white color temperature of viewing light simply and effectively. Instead, white color temperatures have been estimated, or complicated measuring devices have been used to measure red, green and blue tristimulus values of the viewing light in order to find the corresponding white color temperature. Such measuring devices, however, are not ordinarily designed to serve the single use of measuring the white color temperature. To the contrary, such devices are designed to serve many different purposes besides white color temperature calculation. Consequently, such conventional devices are large, expensive, and complicated, and typically require trained technical personnel for operation.