Proc. SPIE. 4486, Infrared Spaceborne Remote Sensing IX
KEYWORDS: Staring arrays, Signal to noise ratio, Point spread functions, Modulation, Imaging systems, Spatial frequencies, Sensors, Fourier transforms, Modulation transfer functions, Contrast transfer function
A sensor's Modulation Transfer Function (MTF) characteristics determine the upper limits of the image quality, i.e. the image resolution or sharpness. The MTF describes the image quality in terms of contrast as a function of spatial frequency, normalized to unity at zero spatial frequency. Unfortunately, characterization of the MTF of semiconductor-based focal plane arrays (FPAs) has typically been one of the more difficult and error-prone performance testing procedures. Discussed in this paper are several commonly used techniques for measuring the MTF in the visible and IR wavelength regions. A brief description of the physical nature of FPAs is presented. This description will show that, because the MTF varies as a function of illumination wavelength and can vary as a function of illumination intensity, the conditions and techniques used for MTF measurement should be chosen based on the ultimate application of the FPA. Trade-offs between complexities in measurement and complexities in analysis as applied to an MTF characterization will be discussed as well as the importance of proper sensor calibration and the measurement of the optical test system's MTF. A discussion of analysis validation, including the effects of noise, using simulated image data is given. Following this, a sample comparison of a sensor's MTF curves as measured by a variety of techniques is presented. Opto-mechanical issues that can significantly influence the quality of an MTF measurement are also discussed.
The noise limitations of IR imaging systems are often described as arising from predominantly temporal sources, such as background, thermal, and readout noise. Actual systems can also be limited by structured pattern) noises and drift. This is especially true with the advent of highly integrated hybrid focal plane arrays, which consist of an infrared detector array, usually photovoltaic, mated to a silicon CMOS VLSI readout device. In these arrays the large detector count, high density, and complexity create new susceptibilities for image noise. Low natural backgrounds in some systems, and the need to save aperture area in others, often force the designer to work at low background levels and high quanturn efficiencies, eliminating options for simpler FPA architectures and making control of focal plane elements more challenging. The desire to dc couple the array output and use an infrequent detector equalization update further complicates the issue. We discuss several noise processes in low-background hybrid arrays, both observed and anticipated. These processes are described through example circuits, typical of proposed and assembled lR focal plane arrays.