Three ISO speeds for digital cameras, yielding the minimum, typical, and maximum exposures recommended for use, are defined in International Standard 12232, which is under revision. The typical and minimum acceptable exposures are based upon signal-to-noise criteria, described in ISO 12232, in which visual (perceptually relevant) noise is computed as a weighted sum of variances from a luminance (Y) and two chrominance (R-Y, B-Y) channels. The weights of the two chrominance variances, C1 (R-Y) and C2 (B-Y), are in need of reevaluation because of: (1) changes in linearization procedures being introduced in the revision of ISO 12232; (2) the limited nature of the original experiment to determine C1 and C2 and (3) suspicion that the initial C1 and C2 values were too high, overemphasizing the contributions of chrominance noise to perception. This paper describes the image simulations, psychophysical experiment, and analyses conducted to determine new values for the chrominance weights to be used in the revised standard. The values obtained, C1 = 0.279 (standard error = SE = 0.025) and C2 = 0.088 (SE = 0.017), are approximately one-half as large as those in the original version of ISO 12232. Systematic variation of the weights with the color of noise-sensitive uniform areas in the scenes is observed, but the effect is small and does not have a practical impact on the standard.
The new Kodak KAI-11000CM image sensor-a 35-mm format, 11-Megapixel interline CCD-has been characterized to evaluate its performance in photography applications. Traditional sensor performance parameters, including quantum efficiency, charge capacity, dark current, and read noise are summarized. The impact of these performance parameters on image quality is discussed. A photographic evaluation of the sensor, including measurements of signal-to-noise and color fidelity, is described.