National Physical Laboratory

Spectral total flux and total luminous flux

Spectral and luminous flux scales are now disseminated directly using the NPL goniospectroradiometer.

Goniophotometer and Goniospectroradiometer

Spectral Total Flux

The NPL goniospectroradiometer incorporates a modified array spectrometer and a series of rotary stages to allow measurement of the spectral radiant intensity distribution of a variety of different types of light sources from 350 nm to 830 nm. When combined with a measurement of the power consumption, the spectral radiant intensity, spectral total flux and associated luminous quantities enable the efficacy and energy efficiency of a source to be determined. Spectral total flux measurements are traceable to the UK's primary standard for the measurement of optical radiant power (cryogenic radiometer) through the NPL2003 spectral irradiance scale.

Total Luminous Flux

The NPL goniophotometer replaces the array spectrometer with a photometer placed at the centre of the covered mirror. This provides measurement of the total luminous flux of a source, in addition to measurement of the spatial distribution of the light emitted by the source (luminous intensity distribution). NPL's UKAS accredited measurement service for total luminous flux calibrations of tungsten-based lamps is traceable to the UK's primary standard for the measurement of optical radiant power through lamps calibrated on NPL's luminous intensity scale.

The Advantages of Goniophotometric Methods over Sphere-based Measurement

One significant advantage of goniometric methods is that they provide information about the spatial distribution of the source as well as the integrated properties. Associated source properties such as chromaticity and correlated colour temperature distributions can also be calculated from the radiant intensity data.

Correlated colour temperature

Facility Details

Photometric and Radiometric Properties

  • Spectral Total Flux
  • Luminous Intensity Distribution
  • Total Luminous Flux
  • Correlated Colour Temperature
  • Luminous Efficacy


  • 350 nm to 830 nm


  • Tungsten flux standards
  • Compact fluorescence lamps
  • Solid-State Lighting products


Customer Service tel: +44 20 8943 6151

Last Updated: 15 Jan 2013
Created: 10 Aug 2009


Please note that the information will not be divulged to third parties, or used without your permission