The lumen is a measurement of light defined by the International System of Units (SI). It describes the output of devices that emit light that is visible to the human eye. While lumens apply to all light-emitting objects, ANSI lumens are a slightly different way of measuring light, specifically for projectors.

Lumens measure the volume of light emitted from a given source. While the newest “official” definition of a lumen is highly technical, it’s easier to understand the slightly older meaning, circa 2018. It describes one lumen as the amount of light you would get unhindered from one standard tealight candle (this measurement is called a candela). It’s an oversimplification. For us non-scientists, it’s good enough for… well, non-scientific work.

An important distinction is that lumens measure light as it is emitted directly from the source. Lumens are therefore essential when selecting products such as light bulbs. While the common wisdom is to buy light bulbs based on wattage, it is better to look at lumens instead, or better yet, lumens compared to wattage. It tells you how efficient the bulb is at converting electricity into light. For example, to produce the same light output (lumens), LED bulbs use about one-tenth the wattage of older incandescent bulbs.

ANSI lumens are similar to SI lumens in that they are both measurements of light output. However, ANSI lumens are generally considered more accurate because the standard includes a number of specific conditions for measurement. When measuring ANSI lumens, it is assumed that the room is 25°C, that the source is emitting a specific white light, and that the result is based on an overall average of 9 measurements taken from different points.