Radiometry deals with calculating or measuring a quantity of radiation, whether in optical systems, in the atmosphere, on the Earth, on planets, or elsewhere. This chapter reviews some of the principles of optics that are essential and even useful in radiometry.
8.1 Photons, Waves, and Rays
Radiation can be described in terms of rays, waves, photons, and even probability amplitudes. The latter is not necessary in radiometry, but an interesting use of it is described by Feynman: the derivation of the law of reflection.
Light is electromagnetic radiation of the same type as fm, am, and short-wave radio waves, television, and X-rays. They can be described mathematically by a wave function Î¨. A traveling wave will be a function with an argument of tâzâv , where t is time, z is the direction of travel, and v is the velocity of travel.
Light has also been shown to consist of photons, small clumps of energy, hc/Î». This model is particularly suitable in analyzing the detection of radiation.
One can visualize waves by analogy to water waves in a pool. When a rock is dropped in a placid pool, waves arise. The crests expand in circles away from the point of entry. Each crest represents a wavefront. Each trough also represents a wavefront. These wavefronts are circles, the perimeters of circles. The rays are normal to them, the radii of the circles. The direction of the travel of light can be represented by the rays, the normals to wavefronts.
The reader should realize that these are all just models of radiation. Each is appropriate for describing a certain application or situation.
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