Overview: In this chapter we discuss a fundamental area of application involving imaging systems. Of course, this application area is far more encompassing than our cursory treatment here. Imaging systems are typically classified as passive or active. A passive system is one that is based on receiving emitted radiation by the target (e.g., blackbody radiation), or reflected radiation by the target from natural sources (e.g., reflected sunlight or moonlight). An active system refers to one in which the target is intentionally illuminated by a source such as a laser. Our treatment here is primarily for active systems.
A coherent imaging system is one in which the illumination wave and reflected wave are both coherent radiation. Such imaging systems are linear with respect to the optical electromagnetic field and can therefore be analyzed by conventional linear shift-invariant (LSI) principles, which involve the notions of impulse response and transfer function. In the open atmosphere a coherent illumination wave will suffer irradiance and phase distortions, leading to a partially coherent beam. Even in the partially coherent case, the imaging system is called an incoherent imaging system. Incoherent systems are linear with respect to irradiance instead of optical field. The useful parameters in this case are the point spread function (PSF) and optical transfer function (OTF), which are related through two-dimensional Fourier transforms. The modulus of the OTF, known as the modulation transfer function (MTF), is used to describe image quality whereas the phase transfer function (PTF) determines image position and orientation.
Adaptive optics (AO) methods are commonly used to improve images. Imaging performance measures of an AO system, such as resolution and Strehl ratio, are defined in terms of the PSF or MTF. These particular metrics involve Fried's atmospheric coherence width, also known as the seeing parameter. In the use of a beacon or guide star, the isoplanatic angle arises as another important parameter that describes the useable field of view.
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