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An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating-gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in electronic circuits. Once programmed, an EPROM can be erased only by exposing it to strong ultraviolet light. That UV light has usually a wavelength of 235nm (for optimum erasure time) and belongs to the UVC range of UV light. EPROMs are easily recognizable by the transparent quartz window in the top of the package, through which the silicon chip can be seen, and which permits UV light during erasing.