A scanner, also known as an image scanner or computer scanner, is a device that optically scans images, printed text, handwriting, or an object, and converts it to a digital image. A flatbed scanner is composed of a glass pane, under which there is a bright light (often xenon or cold cathode fluorescent) which illuminates the pane. Images to be scanned are placed face down on the glass, an opaque cover is lowered over it to exclude ambient light, and the sensor array and light source move across the pane, reading the entire area. An image is therefore visible to the detector only because of the light it reflects.