Modulation transfer function and number of equivalent elements for SPRITE detectors.

SPRITE (signal processing in the element) detectors are three-contact photoconductive structures made of HgCdTe, in which a time-delay-and-integration function is performed in the detector element itself without the need for external circuitry. Spatial frequency-dependent expressions are developed for the modulation transfer function and the number of equivalent elements N(eq) of the SPRITE. The development is based on a Green's function method, which accounts for carrier generation, recombination, and diffusion processes. The usual low-frequency approach of defining a square resolution element on the SPRITE is avoided. The resulting expressions are functions of spatial frequency and are also dependent on physical variables such as the length of the SPRITE element, carrier lifetime, carrier mobility, and operating voltage. These expressions are then applied to the design of a SPRITE element, optimized for operation over a particular range of spatial frequencies.