Case study – National Physical Laboratory

1. Introduction

A large aperture, low temperature range (from -10°C to +80°C) blackbody source with high emissivity (>0.999 over the 3 to 13 μm spectral range) was designed by the National Physical Laboratory, Temperature and Humidity Group, and was constructed by Grant Instruments within a large, custom-made stirred liquid bath. Grant Instruments were approached since they had previously constructed a number of similar, although smaller aperture-size, sources for NPL.

This new blackbody source will assist in the development of a new capability at NPL by enhancing the calibration services for thermal imagers: the source’s aperture size means that it covers almost the complete field-of-view (90° x 64°) of the thermal imagers found in the market. Up to now a smaller (70 mm diameter aperture) blackbody source has been used for thermal imager evaluation. This new asset will underpin core National Measurement Institute (NMI) function by providing top-level measurements nationally and internationally, as only a few NMIs utilise this type of blackbody reference source.

Full characterisation of the source, including its temperature uniformity and stability, was carried across its entire temperature range, and calibration with traceability to the International Temperature Scale of 1990 (ITS-90), to confirm that all requirements of the design specification had been met.

2. Blackbody design

The new blackbody source comprises a large cylindro-conical cavity, coated on the inside with high emissivity paint. The aperture size can be reduced if required using exchangeable aperture rings of differing sizes. The source is mounted into a customised, temperature-controlled, stirred liquid bath. The temperature of the source is measured using a thermometer, calibrated with traceability to the ITS-90, inserted close to the rear of the cavity.

3.Validation measurements

The source was validated in terms of temperature uniformity and temperature stability across the entire temperature range. It was also validated in terms of radiance (apparent) temperature of the blackbody versus the temperature of the contact thermometer within the source by comparison with established NPL blackbody sources. For good blackbody sources (i.e., sources which have a high emissivity and good temperature uniformity) the blackbody radiance temperature should be in good agreement with the temperature measured by the contact thermometer.

4. Results

The results of the validation measurements confirmed that the source met the requirements for temperature stability and bath uniformity at all temperatures at which it was tested. The results of the radiance temperature validation measurements showed excellent agreement between the radiance and contact thermometer temperatures and confirmed the satisfactory performance of the source. The source has now been integrated into the established facility and will be used for commercial activities related to the calibration of infrared thermometers and thermal imaging systems.