We propose a method using a machine vision technique to detect the defects caused by spatially non-uniform brightness on LCD panels. The detection method is based on human vision, so subjective experiments were conducted to investigate the correlation between the parameters related to non-uniformity and the degree of easy observability. The visibility of the defects depends mainly on the spatial gradient of luminance variation. The proposed method uses the spatial gradient calculated using extracted contours to detect the defects caused by non-uniform brightness. The detection method is composed of four parts: the extraction of contours, the calculation of spatial gradient, the decision about defects, and the display of defects. Finally, the spatial gradient has a near-logarithmic relationship to the observability in judging the degree of how easily the detected defects are observed by the human inspector.