This paper describes techniques for detecting changes between two infrared (IR) images of an area, taken under different ambient conditions. Change detection requires accurate registration between the two images as well as photonormalization of one image with respect to the other. Because of the high contrast of IR imagery, registration between the two images can be accomplished accurately using cross-correlation. Diurnal temperature variations complicate the detection of valid changes between two IR images taken at different times of the day. In the first approach, targets are detected on each of the original images before differencing. A second technique is described for overcoming diurnal temperature variations by photonormalization.