What’s the Difference: Luminance, Luminous Flux, Illuminance, Luminous Intensity, Lux, Lumens?

The American Heritage Dictionary tells us:

Light (noun) līt. 1. Physics. a. Electromagnetic radiation that is visible, perceivable by the normal human eye as colors between red and violet, having frequencies between 400 terahertz and 790 terahertz and wavelengths between 750 nanometers and 380 nanometers. Also called visible light. b. Electromagnetic radiation of any frequency or wavelength.

Beyond this basic definition, we have tools and language to precisely describe, quantify, and analyze the electromagnetic waves (visible and invisible) that are all around us, emanating from the sun, from flames, and from artificial sources such as light bulbs. The radiation of light is made up of sub-atomic particles (photons), which have no mass but are simply carriers of energy. They are described by their energy or, inversely, by their wavelength.

The amount of light emitted from or reflected off an object is called its luminance—commonly referred to as brightness. Luminous flux is a measurement of the total amount of light a light source emits, integrated over the entire angular span of the light. We quantify luminous flux in units of lumens (lm), a photometric unit of measurement. Luminous intensity is a measure of the light that shines from the source in a given direction. Illuminance refers to the amount of light that shines onto a surface, measured in lumens per square meter (lm/m²), also called lux. Lux is an essential consideration for interior lighting design for spaces where humans live and work.

The diagram below illustrates the meanings of these different terms: luminance (measured in nits, candela/m²), luminous intensity, luminous flux (measured in lumens), and illuminance (measured in lumens/m², aka lux).

Related Luminus Help Center Articles

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Useful References:

radiometry-and-photometry-faq

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