Mach bands, a visual phenomenon resulting from lateral inhibitory impulses in the retina are recognized as lucent or dense lines at the borders of different radiographic densities. A number of clinical situations have been described in which mach bands may cause difficulty in radiographic diagnosis. Photodensitometric measurement of the film can differentiate the true change in film density from the Mach band which is an optical illusion, Authors present several examples of photodensitometric tracings of Mach bands, with the brief review of the mechanism of their production.