Area Function, also variously referred to in spaceflight contamination control parlance as 'percent obscuration' or 'percent area coverage' (PAC), is an important parameter in the evaluation of witness plates used to monitor cleanliness in hardware ground processing environments. Computed-based image analysis tools can provide a rapid, accurate, and reliable means by which to obtain area fraction information from such witness plates. We present an example of measuring area fractions using A Cambridge 360 scanning electron microscope (SEM) in combination with a PGT/IMIX image analysis system to examine silicon wafer witness plate specimens. The SEM/Image analysis system sued in this work was shown to measure area fractions within 1 to 3 percent of the true PAC, with a precision of +/- 7.7 percent. Image collection and processing operations such as background equalization, erosion and dilation, were performed on secondary electron emission SEM source images. Secondary emission was found to produce source images most amenable to image processing given the material composition of the fallout on the specimen witness pates examined most amenable to image processing given the material composition of the fallout on the specimen witness plates examined here, but the application of other signal types are also discussed. The results presented provide the basis for a generalized discussion of issues basic to the use of computer-based image analysis tools, i.e., accuracy, precision, background equalization, contrast, and magnification.