# Photometric quantities and units

### From ColorWiki

Any radiometric measure can be converted to the corresponding photometric measure by computing the effect of the stimulus defined in radiometric terms on the CIE standard observer. This is defined by the spectral luminous efficiency curve V for photopic vision.

**Luminous Energy**is the photometric quantity that corresponds to the radiometric quantity*radiant energy*- is denoted by Q
_{v} - its SI unit is the lumen seconds (lm s)

- is denoted by Q

**Luminous Flux**is the photometric quantity that corresponds to the radiometric quantity*radiant flux*- is denoted by P
_{v} - its SI unit is the lumen (lm); 1 watt = 683 lumen

- is denoted by P

**Luminous Exitance**is the photometric quantity that corresponds to the radiometric quantity*radiant emittance*- is denoted by M
_{v} - its SI unit is the lux (lx)

- is denoted by M

**Illuminance**is the photometric quantity that corresponds to the radiometric quantity*irradiance*- is denoted by E
_{v} - its SI unit is the lumen per square meter (lm/m
^{2})

- is denoted by E

**Luminous Intensity**is the photometric quantity that corresponds to the radiometric quantity*radiant intensity*- is denoted by I
_{v} - its SI unit is the candela (cd)

- is denoted by I

**Luminance**is the photometric quantity that corresponds to the radiometric quantity*radiance*- is denoted by
_{v} - its SI unit is the candela per square meter (cd/m
^{2})

- is denoted by

For the purpose to investigating light and color, the most important photometric quantity is the **luminance**, because it is directly correlated with visual perception.

## Bibliography

- G. Wyszecki, W.S. Stiles:
*Color Science: Concepts and Methods, Quantitative Data and Formulae*Wiley 1982 (II ed.) - CIE:
*International Lighting Vocabulary*1970 (III ed.)