Terminology

Luminous Flux

Luminous flux is a quantitative expression of the brilliance of a source of visible light, which is electromagnetic energy within the wavelength range of approximately 390 nanometer s (nm) to 770 nm. This quantity is measured in terms of the power emitted per unit solid angle from an isotropic radiator, a theoretical point source that radiates equally in all directions in three-dimensional space. The standard unit of luminous flux is the lumen (lm).

Luminous Intensity

Luminous intensity is an expression of the amount of light power emanating from a point source within a solid angle of one steradian. The candela was defined and adopted as the standard unit of luminous intensity. One candela (1 cd) is the magnitude of an electromagnetic field ( EM-field ), in a specified direction, that has a power level equivalent to a visible-light field of 1/683 watt (1.46 x 10 -3 W) per steradian at 540 THz.

Luminance

Luminance is the luminous intensity, projected on a given area and direction. Luminance is an objectively measurable attribute. The unit is ‘Candela per Square Meter’ (cd/m2). So, different monitors can be adjusted to the same luminous intensity by measuring the luminance in cd/m2.

Illuminance

Illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception.

Light Effect

Light Effect is the rate of the electronic power turning into the light power, It is measured as “ lm/W ”.

Colour Temperature

Colour temperature is a simplified way to characterize the spectral properties of a light source. While in reality the colour of light is determined by how much each point on the spectral curve contributes to its output, the result can still be summarized on a linear scale. Standard unit for colour temperature is Kelvin (K).

Colour Rendering

Effect of an illuminant on the colour appearance of objects by conscious or subconscious comparison with their colour appearance under a reference illuminant. The Standard unit for colour rendering is Ra.