With phosphor-based electronic displays (including cathode-ray and plasma displays), the prolonged display of a menu bar or other graphical elements can create a permanent ghost-like image of these objects. It happens because the phosphor compounds that emit the light lose their luminosity with use. As a result, when certain areas of the display are used more frequently than others, over time the lower luminosity areas become visible to the naked eye, and the result is called burn-in. Although a ghost image is the most noticeable effect, a more common result is that the image quality continuously and gradually declines as luminosity variations develop over time, resulting in a "muddy" image.