Photometry – Precise measurement of light

Photometry describes the scientific discipline for measuring light radiation as accurate as possible taking account of the spectral response function of the human eye.

The visible portion of the optical spectrum is designated as light and covers the wavelengths from 380 to 780 nm. A distinction is drawn between day vision (photopic vision) and night vision (scotopic vision) of an empirically determined standard observer, or in other words the average human being.

The spectral response function for photopic vision is defined by the V(λ) curve. It shows maximum sensitivity at 555 nm. Extremely precise spectral correction of the detector to the V(λ) curve is therefore essential to obtain accurate results when using a photometer for integral measurements.

The key quantities used in photometry are listed in the following table:

QuantitySymbolFormulaNameUnit
Luminous intensityII = Φ/Ωcandela[cd]
Luminous fluxΦΦ = IΩlumen[lm]
LuminanceLL = I/Acandela per square meter[cd/m²]
IlluminanceEE = Φ/Alux[lx]
RetroreflectionRR = I/Ecandela per lux[cd/lx]
Color appearancex,y,zcolor coordinates[1]


Photometers are assigned different classes of quality in standardization. Limits defined within these classes for individual and overall errors must not be exceeded. In addition to the f1’ of V(λ) correction, other characteristics, e.g. the influence of non-uniform illumination of a photometer head f9 and the modulation of light source f7, must be taken into account.

The Optronik Line photometers and measuring systems meet the requirements for laboratory measuring instruments with maximum accuracy. The following list includes a selection of the relevant standards and technical publications:

  • DIN 5032, Part 7: Photometry, classification of illuminance and luminance measuring equipment
  • DIN – EN 13032, Part 1: Measurement and presentation of photometric data of lamps and luminaires
  • CIE Publication No. 69: Methods of Characterizing Illuminance Meters and Luminance Meters; Performance, Characteristics and Specifications
  • CIE Publication No. 84: The Measurement of Luminous Flux
  • CIE Publication No. 121: The Photometry and Goniophotometry of Luminaires
  • CIE Publication No. 54: The Measurement of Retroreflection
  • CE-3874 – GTB Photometry Accuracy Guidelines

Luminous intensity (candela)

The basic unit in photometry is luminous intensity (candela). It is defined as the luminous flux (lumen) radiated in a solid angle dΩ. The solid angle dΩ is calculated from a surface element dA of a spherical surface and the distance r² of this surface from the center of the sphere.