PHOTOGRAPHIC SPEED OF SOLID-STATE SENSOR ARRAYS

There is an interest in expressing the sensitivity of solid-state image sensor arrays in conventional photographic speed terms. Photographic speed expressed in ASA units is defined for film, but can be adapted to other image sensors by relating ASA to signal and noise levels in the threshold response region. To do this requires defining a picture element (pixel) in film analogous to the pixel area of solid-state arrays. The ASA speed of solid-state arrays has been calculated using this approach, and its dependence on exposing intensity and time, pixel area, and dark-current noise levels evaluated. A speed-resolution relationship has also been calculated. Dark-current fluctuations over the sensor array are at present the dominant source of noise, and limit sensor imaging to rather low effective ASA values and moderately high scene illumination levels. Effective ASA values of several thousand can be achieved with short exposure times if dark-current fluctuations can be reduced below 0. 5 nA/cm**2, and signal processing noise kept low.