Lumen

A unit of measurement of the amount of brightness that comes from a light source. The standard lumen rating of a data projector is the average of photometer readings at several points on a full white image on the screen.
Technically, lumens measure "luminous flux." A wax candle generates 13 lumens; a 100 watt bulb generates 1,200. The lumen rating is a critical specification when choosing a data projector. In a darkened room, 500 lumens may be ample; however, in a conference room with normal lighting, 1,000 lumens would be better. In a room with bright daylight, 2,000 lumens is preferred.