Investigation of Non-linearity in Focal Plane Array Detectors

A new measurement technique for focal plane linearity was investigated experimentally. The spatial harmonic distortion test for linearity consists of projecting sinusoidal (Young\u27s double slit) interference fringes onto a detector array. With a linear detector responsivity, a sinusoid input is mapped to a sinusoid output. If the detectors in the array exhibit a nonlinear response characteristic (i.e., saturation), then the output waveform will exhibit a distortion. By examining the Fourier transform of the output data sets, the amount of harmonic distortion present is visualized. Two representative focal planes were measured. A Vidicon tube camera and a solid-state Charge Injection Device (CID) camera were used in the experiments. First, for calibration, flat-field tests (zero spatial frequency) were performed, yielding output signal as a function of input irradiance. Characterization of the nonlinearity was then investigated via harmonic distortion tests. Test results showed appreciable harmonic distortion in the vidicon tube camera system. Minute amounts of nonlinearity were detected in the CID solid state camera system. Also a quantity was accessed which is completely unavailable from the flat-field tests, that is, the spatial frequency dependence of nonlinearity. This effect is analogous to adjacency effects seen in photographic media