National Physical Laboratory


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 research

  • The Cryogenic radiometer is the primary standard that underpins NPL's optical radiation scales and is used as a basis for the realisation of the SI unit of Luminous intensity, the candela.
  • Filter radiometry is at the heart of optical radiation measurement. It is the tool that provides the traceable link from the cryogenic radiometer to the ‘real world’ of polychromatic sources.
  • NPL is leading an international project that will improve the International Temperature Scale realisation and dissemination at high temperatures. This will be based on new fixed-points. The thermodynamic temperature of these fixed points will be determined radiometrically.
  • Spectral radiance and irradiance primary scales at NPL are directly linked to NPL's cryogenic radiometer through the use of calibrated filter radiometers.


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