The Relative Merit of White and Monochromatic Light for the Determination of Visibility by Backscattering Measurements

The response of a simulated single-ended transmissometer, based on the Mie theory of the scattering of light, has been computed for monochromatic light similar to that of a ruby laser (λ = 0.7 μ) and for white light (0.4 μ ≤ λ ≤ 0.7 μ) for many fog and haze models. The results for white light are compared with data obtained from actual field measurements. The strong dependence of backscatter on the size distribution of the scatterers and on the spectral energy distribution of the source is illustrated, and the limitations of the single-ended transmissometer as a device for determining the visibility in haze and fog are discussed. A table of Mie extinction efficiency (or scattering coefficient) and intensity function (for the backscatter direction) is presented for integral values of size parameter up to 100.