1. A device for measuring a critical dimension of a pattern formed on a display substrate, the device comprising: a region of interest (ROI) setting unit configured to: i) receive image data from an external source, ii) set a region of interest in the image data, iii) determine whether the region of interest is larger than a reference region, and iv) generate a pattern image based at least in part on the region of interest; a design file memory storing a plurality of design patterns; a matching unit configured to match the pattern image to one of the design patterns; and a measurement unit configured to measure the critical dimension of the pattern in the pattern image, wherein the device is further configured to select the image data as the pattern image when the region of interest is larger than the reference region.