Featured Definitions Lumens The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the power of light perceived by the human eye. Beam Spread The beam divergence... within a solid material to a fluid medium, such as air or a liquid. Heat sinks are primarily used to remove unwanted heat from a device to keep it from overheating. LED A light...-emitting diode (LED) is a semiconductor light source. UV Rating Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x...