The double wavelength technique is a novel technique to determining thermodynamic temperature that uses an entirely different calibration chain to filter radiometry. This means that the technique can test for systematic effects common to all filter radiometric measurements – a valuable metrological tool, as long as its uncertainties can be similar. In particular it is being developed for our work in the new developments to the international temperature scale and the kelvin and the introduction of new high-temperature fixed points.

Filter radiometry provides thermodynamic temperatures traceable to a cryogenic radiometer. The double wavelength technique’s traceability chain is ultimately back to wavelength/frequency (only), with the radiometric measurements purely relative.

If there are two blackbodies at temperatures T₁ and T₂, measured at two wavelengths λ₁ and λ₂, then the ratios of radiance of the blackbodies at each of these wavelengths

can be measured, providing two equations and two unknowns (T₁ and T₂). The equations can be solved to obtain the two temperatures.

The radiometric measurements required are purely relative – the ratios of the radiances of the two blackbodies at each of two wavelengths. The only absolute measurement is that to do with determining the wavelength.

Experimentally the technique is challenging – highest accuracies are obtained if the longer wavelength is in the infrared, where noise levels are higher and linearity poorer. More critically the wavelength selection, even with a monochromator, will never be ideal and bandwidth corrections must be made. These can be included in the analysis by using numerical techniques to solve the full simultaneous equations.

The technique is being tested experimentally, using Cu-point (standard ITS-90 fixed-point) and Re-C (eutectic fixed-point) blackbodies and using 700 nm and 4500 nm as the two wavelengths.