Visible radiation

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

    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.

  • What is the unit of the luminous flux?

    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.

  • What is the unit of the luminous intensity?

    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.

  • What are the units of the luminance?

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

    Other units are available:

    Luminance units
  • What are the units of the illuminance?

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

    1 lux = 1 lm/m²

    Other units are available:

    Illuminance units