Photometry and Radiometry

The candela (cd) is the SI unit of luminous intensity, which measures the perceived brightness of a light source in a particular direction.

Luminous flux (Φ), measured in lumens (lm), quantifies the total amount of visible light emitted by a source in all directions.

Illuminance (E), measured in lux (lx), is the SI unit that describes the amount of light incident on a surface per unit area.

Luminance (L), measured in candelas per square meter (cd/m²), quantifies the perceived brightness of a surface in a particular direction.

Illuminance (E) is calculated by dividing the luminous flux (Φ) by the area (A) over which the light is distributed.

Irradiance (E), measured in watts per square meter (W/m²), quantifies the power of electromagnetic radiation incident on a surface per unit area.

Radiant intensity (I), measured in watts per steradian (W/sr), describes the power of electromagnetic radiation emitted by a source in a particular direction.

Radiant flux (Φ), measured in watts (W), quantifies the total amount of radiant energy emitted by a source in all directions.

Radiance (L), measured in watts per square meter per steradian (W/(m²⋅sr)), describes the radiant power emitted by a surface per unit area per unit solid angle.

Irradiance (E), measured in watts per square meter (W/m²), quantifies the amount of radiant energy incident on a surface per unit time.

Irradiance (E) is calculated by dividing the radiant flux (Φ) by the area (A) over which the radiant energy is distributed.

Luminous efficacy (η), measured in lumens per watt (lm/W), quantifies the ratio of the luminous flux (Φ) to the radiant flux (Φ).

Luminous efficacy (η) is inversely proportional to color temperature (T), meaning that higher color temperatures generally result in lower luminous efficacy.

Spectral irradiance (Eλ), measured in watts per square meter per nanometer (W/(m²⋅nm)), describes the distribution of radiant energy across different wavelengths.

Spectral irradiance (Eλ) is inversely proportional to color temperature (T), meaning that higher color temperatures generally result in lower spectral irradiance at longer wavelengths.