Consider a perfectly plane, monochromatic wavefront, incident at an angle normal to the face of a window. Deviation from perfect
surface flatness, as well as inhomogeneity of the bulk material refractive index of the window, will cause a deformation of
the transmitted wavefront away from the ideal plane wave. In a retroreflector, each of the faces plus the material will affect
the emergent wavefront. Consequently, any reflecting or refracting element can be characterized by the distortions imparted to
a perfect incident wavefront.
In the following sections we will look at two methods of measuring the performance of optics. Interferometer measurements actually
measure the mechanical deviations of the optic being tested from that of a reference plate or surface. The modulation transfer function (MTF)
measures the ability of a lens or system totransfer object contrast to the image and provides a quantitative measure of image quality
which is far superior to any classic resolution criteria.
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