TABLE OF CONTENTS To be useful as an optical material, a substance must meet certain basic requirements. It should be able to accept a smooth polish, be mechanically and chemically stable, have a homogeneous index of refraction, be free of undesirable artifacts, and of course transmit (or reflect) radiant energy in the wavelength region in which it is to be used.
The two characteristics of an optical material which are of primary interest to the optical engineer are its transmission and its index of refraction, both of which vary with wavelength. The transmission of an optical element must be considered as two separate effects. At the boundary surface between two optical media, a fraction of the incident light is reflected. For light normally incident on the boundary the fraction is given by
| (10.1) |  |
where n and n' are the indices of the two media (a more complete expression for Fresnel surface reflection is given in Chap. 11).
Within the optical element, some of the radiation may be absorbed by the material. Assume that a 1-mm thickness of a filter material transmits 25 percent of the incident radiation at a given wavelength (excluding surface reflections). Then 2 mm will transmit 25 percent of 25 percent and 3 mm will transmit 0.25 0.25 0.25 = 1.56 percent. Therefore, if t is the transmission of a unit thickness of material, the transmission through a thickness of x units will be given by
| (10.2) |  |
This...
