3.1 Optical Materials
3.1.1 Materials for the infrared
A large number of optical materials transmit in the infrared region of the spectrum. However, the list of materials is quite limited when one considers physical characteristics, workability, and cost. Table 3.1 indicates the materials most commonly used in infrared zoom-lens systems for the 3- to 5-Î¼m and 8- to 12-Î¼m regions. It is apparent that indices of refraction are higher than they are for optical materials in the visible spectrum. This is an advantage in the correction of third-order and higher-order aberrations. For example, with a lens shaped for minimum spherical aberration, the angular spherical aberration Î²SPH for an object at infinity can be expressed by Î²SPH=n(4nâ1)128(nâ1)2(n+2)(F3), where n = index of refraction and F=fd,orfocallengthdiameter. The variation with a refractive index can readily be seen by tabulating Î² for an fâ1 lens as an example in Table 3.1. The advantage of using a high-index material like silicon or germanium is quite apparent from these calculations.
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