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Absorption spectroscopy refers to a range of techniques employing the interaction of electromagnetic radiation with matter. (Spectroscopy is a word that has come to denote an even wider variety of techniques used in physics and chemistry.) In absorption spectroscopy, the intensity of a beam of light of measured before and after interaction with a sample is compared. When combined with the word spectroscopy, the words transmission and remission refer to the direction of travel of the beam measured after absorption to that before. The descriptions of the experimental arrangement usually assume that there is a unique direction of light incident upon the sample, and that a plane perpendicular to this direction passes through the sample. Light that is scattered from the sample toward a detector on the opposite side of the sample is said to be detected in transmission and treated according to the theory of transmission spectroscopy. Light that is scattered from the sample toward a detector on the same side of the sample is said to be detected in remission and it is this light that is the subject of remission spectroscopy. The remitted radiation may be composed of two kinds of radiation referred to as specular reflection (when the angle of reflection is equal to the angle of incidence) and diffuse reflection (at all other angles).