In a conventional transmission photometer, the photocell collects the undeviated transmitted light plus some scattered by the sample particles at small angles. The Mie equations directly predict optical efficiencies in terms of total scattering. Presently, these equations plus numerical methods are used to calculate the effective extinction efficiencies that govern transmittances given by conventional optical systems. The effects on observed transmittances of two instrument properties, the convergence of the incident beam, and the angle of acceptance of the photocell have been examined, both theoretically and experimentally. Increase of γ2, the planar half-angle of acceptance of the photocell, substantially increases observed transmission, especially for large particles. Increase of γ1, the angle of convergence of the incident beam, decreases transmittance when γ1 ≦ γ2. Increases of both the real and imaginary parts of the particle refractive index decrease the effects of a nonneglible γ2.
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