Conservation of energy requires an accounting of all of the energy in the incident light beam. Generally, a consideration of the reflectance and transmission intensities and of the absorption accounts for most of the incident light. In some instances, scattering also plays a role; this is not considered here, but is covered in the next section. If there is no absorption in the spectral range of interest, then reflectance and transmittance must equal unity, or 100 percent if the values are given as percentages. In this text, we deal with the ratio form of these values in equations, unless otherwise noted, and the percentage form is used in the narrative because of its clearer meaning.
The mathematical derivations of the equations governing the behavior of thin film interference filters indicate that the transmission of a coating is independent of the direction of the arrival of the light (Heavens, 1965, p. 77). This is well established in practice, and it simplifies the measurement of transmission, as the side of incidence of the filter is unimportant.
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