True about “MACH BAND” is/are:

Incipient occlusal lesions are usually not detected on the radiograph unless the lesion has reached the dentine. The only change which may sometimes be detected is a fine grey shadow just below the DT junction. However, a similar but usually less broad shadow is frequently apparent on the image of the unaffected teeth below of above the occlusal enamel, this line of increased density at the junction is an optical illusion called “MACH BAND” effect. This phenomenon occurs along boundaries of relatively sharp contrast and is more pronounced when the contrast between adjacent areas is greater. This may also occur between a metallic restoration and sound enamel, such that it may be perceived as recurrent caries.