This chapter discusses several measures of the performance of infrared optical systems with emphasis on how properties of the window material affect performance. We conclude by considering the ability of infrared windows to serve as microwave windows.
When a point source of light is viewed through an optical system, diffraction of light at the system aperture broadens the image to a finite width and creates a series of faint concentric rings around the central spot. This pattern of rings shown at the right in Fig. 2.1 is called an Airy disk. In the optical system at the left of Fig. 2.1, the lens diameter is the aperture, D, and the focal length is L. The f number is f# = LâD.
In a diffraction limited optical system, the optical wavefront does not deviate from its ideal shape by more than one-fourth of the wavelength, Î». Suppose that we look at two equal intensity point sources of light through an ideal, diffraction limited optical system whose aperture has diameter, D. The resolution of the system is the minimum angular separation between the two sources such that it is barely possible to discern two separate sources. With less separation, the two sources appear to be one (Fig. 2.2). The Rayleigh criterion for resolution is that the central spot of one Airy disk overlaps the first dark ring of the adjacent Airy disk. This means that the trough between the two bright spots is 74% as bright as the peaks.
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