With increased requirements for better performance being placed on thermal imaging systems, new characterization figures of merit are being developed to assess infrared focal plane array (IRFPA) attributes. Post correction uniformity (PCU) is a parameter that determines how successfully a thermal imaging system can eliminate spatial noise from scanning and staring focal plane arrays. Requirements on PCU, particularly for the more sensitive IRFPAs and applications, are quite rigorous. Test issues of l/f noise, drift, and repeatability become critical and require a rethinking of accepted methods. As infrared sensors have become more sensitive, the need to characterize these focal plane arrays under more controlled and realistic test conditions has emerged. The U.S. Army Night Vision and Electronic SEnsors Directorate (NVESD) has attempted to address these issues by developing a unique capability to measure the PCU of IR focal plane arrays using software algorithms and a specialized mechanical modulator. The modulator is a two foot diameter, two toothed (one reflective and one emissive) blade, which is used to facilitate the real-time collection of test, gain, and offset flux levels. This paper addresses (1) the significance of PCU from a system perspective, (2) discuss the limitations of various PCU measurement techniques, (3) present the NVESD approach for measuring PCU, and (4) report PCU data collected using these techniques.
A new measurement technique for focal plane linearity was investigated experimentally. The spatial harmonic distortion test consists of projecting spatial sine waves of irradiance onto a focal plane by means of a Young's fringe technique. If the detectors in the array have a linear responsivity, a sinusoidal input waveform is mapped to a sinusoidal output. However, if the detectors in the array have a nonlinear responsivity (i.e., saturation), then the output waveform will exhibit harmonic distortion. When the Fourier transform of the array data is taken, the content at the second and third harmonics of the original sine-wave spatial frequency indicates the amount of nonlinearity in the aggregate array response. Measurement results are included for two focal planes: a vidicon tube camera and a solid-state charge-injection device (CID) camera. The minimum harmonic distortion measured was 3%. The sensitivity of this test is limited ultimately by the amount of spatial nonuniformity. Numerical and analytical models are given that indicate the minimum detectable harmonic distortion is in the range of a few percent. This test also allows measurement of the spatial-frequency dependence of the nonlinearity, a quantity that is not accessible with the usual flat-field techniques for linearity assessment.
The design and performance of an IR FPA test system are described. The parameters, techniques, and equipment required for an IR test system are discussed. An analysis of focal planes is presented to extract the last dB of performance, emphasizing the minimization of required device testing time. The CECOM test system is presented, including the instrument controller, the five source types for characterizing IR FPAs, the electronics, and the protective plexiglass enclosure. Critical test parameters and bias and clocking supply specifications are set forth. Printers which display the results are described, and a buffer/spooler is suggested to increase output rate. Data presentation is discussed. Tests on FPAs and readouts, such as dc uniformity, rms noise, 1/f noise, detectivity, linearity, and responsivity, can be conducted in an automated mode. The possibility of automated testing of optical crosstalk, spectral response, and optical area is mentioned. Future improvements to the facility are listed. Improved yields and increased FPA testing throughput in IRFPA production programs can result from the IRFPA test system facilities.
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