Traditionally, the common window method is used to quantify image quality in optical lithography. The common window method can take dose variation, focus error, mask critical dimension error and aberrations into account. However, the demerit of the common window method is its computation time. In this paper, a new metric called Normalized Process Latitude (NPL) is proposed. The NPL considers dose variation, focus error, mask critical dimension error and aberrations to output its final quantification value. Its processing time for quantifying one feature is usually within 10 seconds on a PC with 1 GHz CPU and 256 MB DRAM. We perform several comparisons between the total window value and the NPL. It is found that the NPL draws similar conclusion as the total window. We can conclude that NPL is a sensible figure of merit for image quantification.