National Physical Laboratory

Products & Services

Radiometry is the measurement of optical radiation and cryogenic radiometry, initially developed at NPL, is now widely used by most NMIs as the basis of all optical radiation scales and in particular the SI unit, the candela.

As well as providing top-level traceability via calibrated detectors, NPL is also working on the application of filter radiometry to the determination of high temperature fixed points based on eutectics.

Radiometry and Detectors products & services

  • NPL's spectral responsivity facility disseminates the response of a working standard from the known behaviour of a transfer detector.
  • The accurate measurement of laser power and energy is important in many fields of science, technology and medicine, where lasers are widely used.
  • NPL's laser sources are power-stabilised to permit accurate calibration of optical detector responsivity, including radiometers and laser power meters.
  • NPL's laser stabilisation facility can be used to externally stabilise a variety of laser sources at known wavelengths, for radiant intensity.
  • NPL is able to offer expertise regarding quantities including luminous intensity and illuminance, luminance and correlated colour.
  • Used in the realisation of spectral scales, detectors can be supplied as calibrated transfer standards to disseminate responsivity from a primary standard to a customer detector.
  • NPL's cryogenic radiometer is the primary standard for the measurement of optical radiant power. It uses the electrical substitution technique, whereby the optical power incident on an absorbing cavity is compared with the electrical power required to heat the cavity to the same temperature.
  • NPL's uniformity and linearity of response facility is able to characterise working standard detectors for both spatial uniformity, across the detector's surface, and the linearity of response to different incident power levels.
  • The facility is used to calibrate both radiance and irradiance sources such as integrating spheres, strip lamps, and FEL and Polaron irradiance lamps.


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