As long as there is light, the photosensitive section of a CCD will generate electrons. Since the pixels must be read out, if the readout process takes a long time, the image quality will degrade. This is the same principle as photographing moving objects with a slow shutter speed. Normally, signal generation is stopped during readout. However, stopping light exposure requires some form of mechanical shutter or electronic shutter. Some CCDs are designed so that signals are quickly transferred to a metal area of the CCD as soon as they are generated. Since light cannot penetrate metal, signal generation ceases, allowing the CCD to read out without interference. This rapid transfer to a protected region is known as an electronic shutter, as electronic control is used to regulate the duration of signal generation.