Illuminant discriminator with improved boundary conditions

Abstract

The present invention is an apparatus and an associated method for determining the dominant scene illuminant which apparatus is more reliable and accurate than previous illuminant detectors. Mixed illuminant detection has been added to the choice of illuminant categories, to take care of the cases where one illuminant is not dominant and color correction is best handled by printing algorithms. A new boundary condition has been devised to eliminate detection errors seen when fluorescent illumination mixes with certain quantities of daylight causes a tungsten reading. The apparatus incorporates a thresholding technique to improve over existing art. The apparatus is comprised of a means for converting illuminate light into corresponding electrical signals. The electrical signals are then directed to a log amplifier wherein they are compressed to approximately form a signal having one term equal to the log of the DC term plus a ratio of the dominant AC components to the DC components. A second portion of the apparatus receives the signals from the log amplifier and provides two filtered outputs which are multiples of the frequency of expected artificial illumination sources. Each of the output signals is compared against a plurality of threshold signals to identify which illuminant components are present. The combination of detected components are then compared against the components of known illumination sources with the closest match identifying the unknown source. Means are provided for combining the output signals from both of the filters to identify mixed sources.