Mie theory is used to investigate the degree to which a response from an optical particle counter (OPC) can be made monotonic when sizing particles of a given refractive index. This investigation is restricted to OPC designs that use monochromatic illumination and have coaxial light-collection optics. By systematically exploring the angular limits of the collection optics, it is found that, for certain limits, a very nearly monotonic response can be achieved for a nonresonant cavity OPC for a range of refractive indices. In contrast, for a resonant cavity instrument, this investigation did not reveal any angular limits where the response is close to being monotonic, unless the refractive index of the particles being sized has a large imaginary component.
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