The obscuration ratio of a surface contaminated by particles is a useful characterization of the degree of cleanliness of the surface. The many effects proportional to the square of particle size are also proportional to the obscuration ratio. However, the calculation of the fraction of the total surface area covered by particles for the typical distributions encountered cannot be performed by elementary methods. A useful method for the calculation of the obscuration ratio for particle distributions assumed to follow a generalized MIL-STD-1246A form is presented. In addition, the validity of an estimation method for measured, discrete distributions is established by the derivation of upper and lower bounds on the desired quantity.

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