Fiber counting: a source of error corrected

Optical counting of fibers (from a filter sample of industrial air, for example) involves determining how many fibers are within counting fields which are squares of side lengths. It is correct to count all those fibers which are completely within the field and half of those which intersect only one side. It is shown here to be incorrect to count half those fibers which intersect two sides; the magnitude of this error is calculated, the calculations being verified through a Monte Carlo simulation. The fractional overcount for fibers of length L less than or equal to s is (L/s)2/2pi.