National Physical Laboratory

Cryogenic Radiometry - Underpinning Our Optical Radiation Scales

Primary Standard Cryogenic Radiometer
Primary standard cryogenic radiometer.

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.

NPL's current primary standard cryogenic radiometer can measure the power of an intensity stabilised laser beam to better than 1 part in 10 4or 0.01%

The first ever operational cryogenic radiometer was designed and built at the NPL in the 1970s to measure radiation emitted from a black body. Over the past 25 years NPL has continually pioneered the development of cryogenic radiometry.

First Operational Cryogenic Radiometer (Schematic) First Operational Cryogenic Radiometer
First operational cryogenic radiometer of Quinn and Martin.

Electrical Substitution Radiometry

Electrical Substitution Radiometry - The Principle Behind the Cryogenic Radiometer
A cryogenic radiometer is based on the principal of electrical substitution radiometry (ESR). An ESR uses a thermometer to measure the temperature rise of a thermal detector, relative to a constant temperature heat sink, during alternate optical and electrical heating cycles. By adjusting the electrical power so that the detector temperature rise is the same for both types of heating, the optical power can be considered equivalent to the electrical power.

The Problems Associated with Room Temperature ESR's
It is difficult to achieve high accuracy measurements when operating an ESR's at ambient temperatures. Fluctuations in background radiation and loss of energy in the electrical connections to the heater circuit are just a couple of the issues associated with room temperature ESR's.

The Benefits of Cooling ESRs to Cryogenic Temperatures
There are a number of advantages that can be harnessed by cooling an ESR to cryogenic temperatures. When an ESR is operated below 20K (that is around -253 °C) it is necessary to operate it in a vacuum, this means that the instrument can be designed to prevent background radiation from reaching the radiometers detector. Super-conducting wires can be used to connect the electrical power circuit, ensuring that all the electrical power is dissipated in the detector and not lost down the connecting leads. By cooling a radiometer to low temperatures the specific heat capacity of the absorbing material is reduced by around 1000 times. The advantage of this is that relatively large, highly absorbing cavities can be constructed with a short time constant allowing measurements to be performed over a short time scale.

Dissemination of Optical Scales from the Cryogenic Radiometer
The cryogenic radiometer is used to calibrate NPL's secondary standard detectors. These detectors are then used to transfer our scales to different laboratory and industrial measurement uses: Power meters, photodiodes, thermal detectors, photon counters and thermal imagers etc.

Further Development of Cryogenic Radiometers
The NPL is constantly pioneering developments in cryogenic radiometry. Current research includes the development of an Absolute Radiation Detector to link our optical scales to a fundamental physical constant, the Stefan Boltzman constant. We are also developing a new radiometer to accurately measure the output from a grating monochromater.

Last Updated: 29 Jul 2014
Created: 23 Jul 2007

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