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transmission factor[tranz′mish·ən ‚fak·tər]
The transmission factor of a medium τ is the ratio of the radiation flux Φ passing through the medium to the flux Φ0 incident upon the surface of the medium: τ = Φ/Φ0. The concept of a transmission factor is used most frequently with regard to luminous fluxes. The value of the transmission factor of a body depends on the dimensions, shape, and surface condition of the body, as well as on the angle of incidence, spectral composition, and polarization of the incident radiation (seePOLARIZATION OF LIGHT). A distinction is made between transmission factors for direct transmission, in which the medium does not scatter the radiation passing through it; for diffuse transmission, in which the medium diffuses all radiation penetrating it; and for mixed transmission, in which there is partial diffusion. The value of the transmission factor for radiation of only one wavelength (monochromatic light) is called the monochromatic transmission factor. This coefficient is found from measurements of the illumination and the luminance. The determination of the transmission factor is a photometric measurement.