How to Measure Light

Luminous flux

The luminous flux describes the quantity of light emitted by a light source. It is a key component in determining luminous efficacy, which is the ratio of the luminous flux to the electrical power consumed (lm/W). Efficacy is an indicator of a light source's economic efficiency.

Abbreviation: Φ Phi

Unit: lm Lumen

Luminous intensity

The luminous intensity 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 (LDC).

Abbreviation: Ι

Unit: cd Candela

Illuminance

Illuminance describes the quantity of luminous flux falling on a surface. It decreases by the square of the distance (inverse square law). Relevant standards specify the required illuminance.

Illuminance: E(lx) = luminous flux (lm) / area (m2)

Abbreviation: E

Unit: lx Lux

Luminance

Luminance is the only basic lighting parameter that is perceived by the eye. It specifies the brightness of a surface and is essentially dependent on its reflectance (finish and color).

Abbreviation: L

Unit: cd/m2