Basic Lighting Terminology

Basic parameters used in lighting

Luminous flux Φ

Luminous Intensity I

Illuminance E

Luminance L


Luminous flux

The most fundamental quantity used to measure light is luminous flux.

Luminous flux is radiant flux multiplied, wavelength by wavelength, by the relative spectral sensitivity of the human visual system, over the wavelength range 380 nm to 780 nm.

Lumens (lm)

The international (SI) unit of luminous flux. This is the amount of light produced from a light source.

Luminous intensity

This describes the quantity of light that is radiated in a particular direction. This is a useful measurement for directive lighting elements such as reflectors. It is represented by the luminous intensity distribution curve and its unit is the Candela (cd).

 Illuminance (E)

This is the quantity of luminous flux (lumens) falling on a surface and its unit is Lux.  The simple equation to calculate illuminance is:

Illuminance E (lux) = luminous flux (lumens)

Area (m²)

Luminance (L)

This is the only lighting unit perceived by the eye. It describes the light source’s impression of surface brightness and is measured in cd/m²

Maintained Illuminance (Em)

This is the value below which the illuminance level should not fall in the visual task area. The illuminance and its distribution on the task area and on the surrounding area have a great impact on how quickly, safely and comfortably a person perceives and carries out the visual task.

Light Output Ratio (LOR)

The LOR is the percentage difference between the luminous flux of a light source and the amount emitted from the luminaire it is part of.

LOR = Lumen Output (luminaire)

Lumen Output (light source)

Luminaire Lumens (Llm)

This is the final measurement of light emitted from the luminaire. This will take into account the luminous flux from the light source and the LOR of the luminaire.