Abstract: A measurement tool connects to an automatic inspection machine for identifying and measuring microscopic dimensions such as area, diameter, height and line width of defects and lines of a photographic mask. An operator draws a rough region of interest around a feature and the tool automatically identifies the feature and calculates its dimensions. For features less than one micron in size, the size of light photons interferes with measurement, so a non-linear polynomial calibration curve is developed for each machine. Features of known sizes are measured on a production machine to produce a calibration curve for each type of defect or line. Features of unknown sizes are measured on the same machine and the measured size in pixels are calibrated using the calibration curve to return a more accurate reading in microns.