Visible Radiation - FAQ


2.1.	Luminance, illuminance, intensity, flux, etc.: what are the differences?

An optical source emits a luminous flux (F) of energy toward a sensor. This flux propagates from the source to the sensor.

Relationships between luminous parameters are summarized into the below table:

Luminous parameters

Luminance is a quantity representing the brightness of sources independently from its size and its emitting cone. This value is consequently often used to compare tow sources.

The USI unit of luminous flux is Lumen, abbreviation lm.

1 lumen = 1/683 Watt at 555 nm

555 nm is the wavelength to which the eye is the most sensitive.

The USI unit of luminous intensity is Candela, abbreviation cd.

1 cd = 1 lm/sr

1sr = 1 steradian is the cone of light spreading out from the source which would illuminate 1 m² at 1 m distance from the source.

The USI unit of luminance or Radiance is Candela per square meter, abbreviation cd/m².

Other units are available:

Luminance units

The USI unit of illuminance or irradiance is Lux, abbreviation lx.

1 lux = 1 lm/m²

Other units are available:

Illuminance units