National Physical Laboratory

Supply of standard NPL fixed point blackbody sources

The NPL primary standard silver fixed-point blackbody source
The NPL primary standard silver fixed-point
blackbody source

Above the freezing point of silver the International Temperature Scale of 1990 (ITS-90) is realised and disseminated using radiation thermometry and the Planck law of radiation. The temperature T90 is defined as the ratio of the spectral radiance of a blackbody at T90 to that of a blackbody at the freezing-point of either silver (961.78 °C), gold (1064.18 °C) or copper (1084.62 °C).

NPL can supply primary standard fixed-point blackbody sources for Ag, Au and Cu to enable users to conveniently realise the ITS-90 in the laboratory. These sources, which are designed, built and tested by NPL, are certified by comparison with primary standard fixed-points, ensuring direct traceability to National Standards.

Fixed point blackbodies of this design are also available for zinc (419.527 °C) and aluminium (660.323 °C). In addition, NPL can supply secondary standard fixed-point blackbody sources for the freezing points of indium (156.5985 °C), tin (231.928 °C), zinc, aluminium and silver.

Above the Cu freezing point, eutectic-point cells containing a mixture of metal and carbon can be used as reference fixed points.

Technical information

The Ag, Au and Cu primary standard fixed-point blackbody sources consist of a graphite crucible containing an ingot of high purity (99.9999 %) metal. A high emissivity (> 0.99995) graphite blackbody cavity is immersed into, and is in thermal equilibrium with, this metal. The crucible is contained in a 58 mm diameter outer graphite casing to protect the assembly from oxidation. A removable rhodium disc defines the blackbody aperture to facilitate alignment. Blackbody sources of this design containing Zn or Al can also be supplied.

The secondary standard In, Sn, Zn, Al and Ag fixed-point blackbody sources are of a similar design to the primary standard sources but have a larger aperture, and therefore a lower emissivity, and do not have a rhodium disc.

The Ag, Au and Cu fixed-point blackbody sources typically achieve melting and freezing plateaux of approximately 30 minutes duration, depending on the furnace conditions, with plateaux of several hours possible for the lower temperature fixed-points. The uncertainty in the radiance temperature is assessed for each individual source, and is typically less than 0.1 °C.

Dimensions of the NPL blackbody assemblies are shown below. Some adjustment is usually possible to meet customer requirements.


Fixed point Crucible outer
diameter / mm
length / mm
diameter / mm
Zn, Al, Ag, Au, Cu
(Primary standard)
42 110 3 >0.99995
In, Sn, Zn, Al, Ag
(Secondary standard)
42 114 9 ~0.9999

All NPL fixed-point blackbody sources are supplied with a calibration certificate and operating/assembly instructions. Sources can be ordered separately, or complete with a three-zone electrically heated furnace and all the ancillary components necessary to establish the freezing point.

Other calibrated artefacts for temperature measurement

In addition to fixed-point blackbody sources, NPL can supply a range of other calibrated artefacts for temperature measurement. These include metal-carbon high temperature fixed points, fixed points for platinum resistance thermometer and thermocouple calibrations, variable temperature blackbody sources, triple point cells, pyrometric lamps, and calibrated platinum-rhodium thermocouples. Information about these artefacts is available by clicking on the links.


Customer Services tel: +44 20 8943 6315

Last Updated: 12 Dec 2013
Created: 11 Sep 2007


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