National Physical Laboratory

Primary Standard Cryogenic Radiometer

The basis for the traceability of all optical radiation standards, assuring accuracy into the hands of the end user - the customer.

Primary Standard Cryogenic Radiometer

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. High Tc superconducting leads to the cavity heater ensure true equivalence of electrical and optical power.

The cavity is made of electroformed copper to reduce its mass and is coated internally with NPL super black. The design of the cavity in combination with operation at helium temperatures ensures an optimal response to incoming radiation. The cavity is isolated from fluctuations in the cold head of the cooler by a reference block maintained at a constant temperature. The accuracy of the system is further enhanced by operating the cavity within an isothermal shield.

The thermal link between the cavity and reference block is designed to optimise the temperature rise for a specific power level. A balance is achieved between the need for a high impedance to maximise the sensitivity and the need for a short time constant. This link is adjustable but is set at the time of order.

Purchasing a cryogenic radiometer allows a laboratory to establish their own scales traceable to a primary standard. These scales are usually disseminated using trap detectors as a transfer standard and solid state working standards. The cryogenic radiometer is used with a variety of laser sources.


Operating Details
Cavity operating temperature 12 K
Absolute accuracy ± 0.005 %
Power Requirements 220 V - 240 V, 50 Hz, 1.9 kVA
208 V - 220 V, 60 Hz, 2.0 kVA
Cold head service interval > 15 000 hours
Absorber service interval 15 000 hours
Water cooling 3 l/min minimum at 20°C
Radiometric Properties
Receiver response 1.2 K/mW approximately
Maximum power 2 mW
Resolution at maximum power 1 in 105
Time constant  70 seconds
Brewster window transmission 400nm to 800nm 99.96%
Cavity Absorption 200nm to 2000nm >99.99%
Vacuum pumping station The detector comparitor stage
The detector comparitor stage
Detector comparator stage
Laser stabilisation facility
Brewster window assembly optimised for mid infrared use
Brewster window transmission measurement system

NPL's support includes the installation of the facility and training in its use

Software download for monochromator based cryogenic radiometer (ASR Control Software) - requires username and password

Contact us for further information

Call the Instruments team on: 020 8943 6539

Last Updated: 21 Aug 2017
Created: 31 Jul 2007


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