ABSTRACT
Infrared lenses differ from lenses designed for the visual region in several important aspects:
There are a lot less materials to choose from. Fortunately, available materials have high index of refraction and low dispersion (germanium, zinc selenide, etc.).
Due to the high cost of these materials, and their relatively poor transmission, thickness should be kept to a minimum. Many of these materials are polycrystalline and exhibit some scattering; this is another reason to keep the lenses thin.
The long wavelength means a much lower resolution requirement.
The walls of the housing are emitting radiation and therefore contribute to the background.
Detectors are often linear arrays, in contrast to film or the eye. These detectors are usually cooled.
One must check that the detector is not being imaged back onto itself in a process known as narcissus; see Hudson (1969:275) and the discussion in Chapter 1.
