1. A method comprising: receiving a first bitmap image of a substrate and a reference bitmap image from an image scan system; binning values of the first bitmap image wherein the binning values includes: determining a number of occurrences of each of a plurality of values represented in the first bitmap image; binning values of the reference bitmap image wherein the binning values includes: determining a number of occurrences of each of the plurality of values represented in the reference bitmap image; comparing the determined number of occurrences of each of the plurality of values represented in the first bit image to the determined number of occurrences of each of the plurality of values represented in the reference bit image; and based on the comparing, determining whether a defect captured in the first bitmap image will image a photosensitive material in a photolithography process.