National Physical Laboratory

Radiometry & Detectors

Earth
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The characterisation of optical radiation detectors is important in a wide range of applications, such as environmental measurements; colour processing; food; transport; chemical and pharmaceutical industries; and the medical and communications sectors. Cryogenic radiometry, initially developed at NPL, is now widely used by most NMIs as the basis of all optical radiation scales, including the SI base unit, the candela.

As well as providing traceability via calibrated detectors, NPL also provides a wide variety of other services, ranging from measurements of important parameters such as detector linearity and uniformity, through to the design and supply of bespoke radiometric measurement facilities.

NPL is also working on the application of filter radiometry to the determination of high temperature fixed points based on eutectics and the development of improved approaches to traceability for the most demanding application areas, particularly Earth observation and climate change.

  • 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.
  • 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.

Radiometry & Detectors case studies

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.

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