Absolute characterisation of the radiation thermometer LP3

Project Description

The cooperation in research aims at the absolute calibration of the radiation thermometer LP3. The intermediate steps are :

• Set-up of a laser-based facility for supplying a stable and intense lambertian spectral radiance using a Ti-Sa laser and an integrating sphere.
• Absolute calibration of this facility using trap detectors traceable to a cryogenic radiometer
• Absolute calibration of the spectral responsivity of the radiation thermometer
• Comparison of the absolutely calibrated radiation thermometer with absolutely calibrated filter radiometers

Final Report 2008-03-25

The KE LP5 pyrometer has been equipped with a specially designed wedged interferential filter centred at 800 nm with a bandwidth of about 20 nm. Wedging the filter should help reducing the interference pattern which has been observed on different instruments (PTB, NIST, KRISS) when trying to measure the spectral responsivity of the pyrometer using a tuneable laser-based monochromatic source.
One of the other difficulties of this measurement method is to stabilise the power on a sufficiently long term in order to achieve the complete scanning of the filter bandwidth. On the short term, this stabilisation is now achieved at LNE-INM to the level of a few 10^-5.
The method used to achieve this stabilisation level is based on a simple acousto-optical modulator (AOM). The tests showed an important sensitivity of this modulator to polarisation and to the wavelength of the laser. LNE-INM has consequently developed a double-pass AOM less affected by the above mentioned effects and showing a limited deviation of the laser beam with changing wavelengths over the spectral band 780 to 820 nm.
PTB supplied a reference trap detector with absolutely calibrated spectral responsivity for comparative measurements traceable to the PTB spectral responsivity scale. The objective is twofold: providing LNE-INM a provisional traceability while its own cryogenic radiometer is being replaced and allowing, once this new cryogenic radiometer is installed, a fast and efficient validation and comparison to PTB’s scale.

The work performed in the frame of this project is part of the CCT-WG5 project on High-Temperature Fixed Points, as well as the EURAMET project 926. We will therefore report in Euramet project 926 for the next steps of this work in the future.