Analysis and measurement of pattern noise in color-filter arrays for image sensors

Pattern-noise measurements on a color-filter array (CFA) for an image sensor are valuable for quantifying the pixel-to-pixel transmittance uniformity important for good image quality. The pattern noise, however, changes with test conditions and is not an intrinsic array property. We develop here a relationship between the pattern noise and the fluctuations in density parameters that produce it. Determining these fundamental parameters makes clear the source of the noise and permits calculation of the filter-array performance for any conditions of interest. The analysis shows how the filter-array spectral shapes influence the pattern noise and how it can be reduced. The pattern noise of CFA's of the dyed photopolymer type is reported. Monochromatic measurements, using microdensitometry and an image sensor with an RGBG striped array, confirm a predicted increase in pattern noise with dye density. Fitting the data to the model yields fluctuations in dye coverage of 0.2-0.4 percent, depending on the dye. For a frame-transfer CCD in white light (5500 K) this corresponds to pattern noise of 0.4 percent or less. Power spectra show that these variations in dye concentration occur largely over sensor dimensions, not pixel spacings.