Lumen

The lumen (lm) is the unit of luminous flux, the total amount of light leaving a source. It accounts not only for energy but for how bright the human eye perceives that light to be; because the eye is more sensitive to some colors than others, the measurement is weighted accordingly. The lumen figure printed on a lamp or fitting tells you how much light it produces.

Watts versus lumens

In the days of incandescent bulbs, people got used to judging light by wattage. But watts measure the energy consumed, not the light produced. Efficient sources such as LEDs produce the same light with far less energy, so lumens are the number to check when choosing a lamp. Lumens per watt describe how efficient a source is, a value known as efficacy.

Lumens and lux

Lumens describe the total light coming out of a source; lux describes how much of it lands on a surface. Two fittings with the same lumen output can behave very differently: one may focus its light into a narrow beam on a single spot, the other spread it over a wide area. That is why lumens alone are not enough in lighting design; the beam distribution and the light level reaching each surface are considered together.

Lumens in rendering

Many physically based render engines let you enter light intensity in real units such as lumens. A light set up with the value from a manufacturer’s catalogue gives a believable brightness in the scene and, together with correct exposure settings, offers a reliable sense of how bright the space will really be.