Lumens

Overview

Lumens (unit symbol lm) is the SI unit of luminous flux (formula symbol Φ). It indicates the total amount of visible light emitted by a source in all directions. Unlike electrical power in watts, the lumen value evaluates light based on the sensitivity of the human eye (luminosity function), thus weighting each wavelength according to how bright it appears to the eye.

On set, the distinction between lumens and lux is crucial: lumens describe the source (the total amount of light emitted), while lux describes the illuminance at the subject – how much light actually hits a surface. What arrives at the subject is relevant for exposure; therefore, in practice, cinematographers and gaffers measure in lux or footcandles, not lumens.

Definition and Relationships

A lumen is defined as candela times steradian (1 lm = 1 cd·sr). A light source with a luminous intensity of 1 candela, radiating uniformly into a solid angle of one steradian, emits exactly one lumen of luminous flux into that solid angle. An isotropic (radiating uniformly in all directions) point source with 1 candela produces a total luminous flux of approximately 12.57 lumens over the full sphere (4π sr).

The most important relationships at a glance:

From this, it follows that the same luminous flux is distributed with varying intensity depending on the area. 1,000 lumens spread over 1 m² result in 1,000 lux; the same 1,000 lumens spread over 10 m² result in only 100 lux. The illuminance unit commonly used in Anglo-American regions, the footcandle (fc), is approximately equal to 10.76 lux.

Luminous Efficacy of Different Lamp Types

The amount of lumens a lamp delivers per watt consumed is called luminous efficacy (lm/W). The theoretical maximum value is around 683 lm/W – this corresponds to monochromatic light at 555 nm, the wavelength of maximum eye sensitivity. Real light sources achieve very different values depending on the technology:

This explains why HMIs and LEDs emit significantly more light than tungsten fresnels for the same electrical power – relevant for power planning, heat development, and battery operation on set.

Usage on Set

• Equipment Selection: Manufacturers often specify the luminous flux of modern LED lights in lumens. This value helps to roughly compare lights by brightness but says nothing about the light distribution (beam angle) or the illuminance arriving at the subject.
• Exposure: For camera exposure, the illuminance at the subject (lux/footcandle), measured with a light meter, is what counts. Lumens alone do not allow for direct aperture determination, as distance and beam spread significantly alter illuminance (inverse square law).
• Caution with Comparisons: Lumen specifications say nothing about color rendering (CRI/TLCI), color temperature, or spectrum. Two lights with the same lumen value can appear very different on skin and in the image.