Optical end of line metrology (OELM) is a new method to measure relative line shortening effects using conventional optical overlay instruments. in this technique, a frame that has two adjacent sides that are constructed of lines and spaces is imaged onto a wafer. Since gratings below 0.5 ?m cannot be resolved using conventional optics, the alignment tool sees the sides composed of lines and spaces as solid edges. The purpose of this paper is to characterize the errors implicit with this approach. OELM results are compared with scanning electron microscopy (SEM) line shortening measurements, which showed that optical measurements exaggerated the physical effect. A simple aerial image analogy predicts that optical line shortening measurements are pitch and duty cycle dependent. OELM measurements require calibration as a function of pitch and line width.