Angular Truncation Error of the Integrating Nephelometer in the Fog Droplet Size Range

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Abstract
Integrating nephelometers have an inherent error, known as the angular truncation error, which arises from the fact that they integrate scattered light intensity over a truncated angular range from ϕ1 > 0° to ϕ2 < 180°. Typically, ϕ1 is between 5° and 10°. Since the proportion of light scattered in the extreme forward direction increases as scatterer size increases, this error can become sizeable for measurements made in fogs. Correction factors ξ(α) for angular truncation are computed for spherical water droplets having values of the size parameter α (= πd/λ) in the range 3–200, for values of ϕ1 of 6°–9° and for ϕ2 = 170°. If monochromatic light of wavelength 0.55 μm is assumed, this range of α corresponds to droplet diameters between 0.53 and 35 μm. The magnitude of ξ(α) increases nonmonotonically from a value between 1.03 and 1.07 (depending on ϕ1) for α = 3 to a value between 2.0 and 2.14 for α = 100. In the range 100 < α ≤ 200, ξ(α) varies by less than 7%. Angular truncation correction factors applicable to the MRI model 1580 Fog Visinmeter are also computed.

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