National Physical Laboratory

Calibration of radiation thermometers

High temperature graphite blackbody source
An NPL high-temperature graphite blackbody source

Radiation thermometers are calibrated using blackbody sources - with emissivity of (or close to) 1. This is a convenient ‘reference condition’ from which calibrations at other emissivities can be readily calculated.

Blackbody source uses concept of truly blackbody radiation inside a closed cavity at a uniform temperature.

Intensity of the radiation is governed by the Planck law: it depends only on the wavelength and temperature, and is independent of the emissivity of the materials of which the cavity is made.

 

 

Immediate practical difficulty

The immediate practical difficulty is that if we make a hole in the cavity to observe the radiation, we perturb the field and the radiation we see is no longer the ideal: the emissivity of the partially open cavity is always less than 1. Nevertheless, if the cavity has good geometrical design and is large compared with the aperture diameter, and if the materials used have high surface emissivities (i.e. they are intrinsically good radiators), then very high cavity emissivities (>0.9999) can be achieved.

In fixed-point blackbodies the cavity is typically made of graphite and is almost completely surrounded by an ingot of a pure metal, so that during a melt or a freeze the temperature is uniform and fixed at a known value. Such blackbodies are used in the ITS-90.

Most blackbody cavities use an oxidised alloy, or a metal painted black, and are contained in a temperature-controlled furnace or fluid bath. The temperature is measured using a calibrated sensor, such as a platinum resistance thermometer or a thermocouple, which provides traceability to the ITS-90.

Alternatively the cavity and sensor can be calibrated as a system, by comparing the radiated output with that of a reference blackbody.

At very high temperatures blackbodies are generally made of graphite which is directly heated by passing a large current through it. These blackbodies must be used in an inert atmosphere to prevent oxidation.

Cavities are large and slow to use so common to use blackened plates, usually with a grooved pattern cut into them, as near-blackbody sources. The plate is heated from behind, with a sensor embedded near the surface to monitor the temperature.

How well the plate approximates to an ideal blackbody depends on the design, but the surface emissivity is likely to be substantially less than 1, and there will also be temperature gradients across the plate, and between the sensor and the surface. As a result it is necessary for the plate to be calibrated by comparing its output with that of a reference blackbody.

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