The expanding use of CCD sensor based, digital cameras necessitates the development of a reliable method of determining an equivalent ISO speed for such cameras. This paper will briefly describe the physical differences between image capture, and the signal and noise properties of film and CCD based imaging systems. The model first addresses the ISO speed of a monochrome CCD sensor. The results of this model indicate that an equivalent ISO speed can be found when the exposure, Em, in Lux-seconds at a signal-to- noise ration of 30 is used to calculate the speed from the expression developed for film based systems, ISO speed equals 0.8/Em. The model shows that calibration exposures of 0.01 seconds or less must be used to eliminate the effect of dark current on determining the 'correct' ISO speed. The model is then sued to calculate the effective ISO speed for a series of CCD sensors for different pixel areas, nose characteristics and quantum efficiencies. The same model is used for color cameras by introducing a color filter. The model is also used to determine what set of color filters will provide a balanced exposure for a given light source and what electronic processing must be done to compensate for different equivalent ISO speeds between the color filter elements. The effect on ISO speed and color balance as a function of illuminant is considered. The model is also used to map the signal-to-noise characteristic of the CCD camera under usage equivalent to a film based camera.