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Certain elementary concepts in optics are necessary to appreciate infrared system design both for radiometric and resolution calculations. Only those concepts necessary are discussed here, and they are not derived. Further reading is available in several texts.
A ray is defined as the normal to wavefront, and describes the path of the light in a geometric fashion. A beam is a collection of rays. Beams can be collimated, in which case all the rays are parallel. They can be divergent or convergent, in which case they emanate or converge to a point with a geometry that looks like the sharpened end of a pencil. These are called conical beams.
The refractive index of a medium is defined as the ratio of the velocity of light in vacuo to that in the medium. All media have a refractive index greater than one. The refractive index of a mirror is â1 by convention. The light goes at the same speed, but in the opposite direction.
When light is incident from a medium of a particular refractive index onto a medium that has a different refractive index, it is both reflected and refracted at the surface between them. This is illustrated in Fig. 3-1. The direction of the (specularly) reflected beam is determined by θ i and θ r , the angles of incidence and reflection. These are both measured with respect to the surface normal, as shown. The angle of reflection is equal to the angle of incidence.
The direction of the refracted beam, the one that is in the medium and is deviated to some extent from the incident beam, is determined by Snell's law: the sine of the angle of refraction times the refractive index of the medium is equal to the sine of the angle of incidence times the refractive index of the incident medium.
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