Measuring absolute infrared spectral radiance with correlated photons: new arrangements for improved uncertainty and extended IR range

New experimental configurations of a system to measure absolute infrared (IR) spectral radiance using correlated photons are presented. The method has the remarkable feature that it allows spectral radiance to be measured in the IR in an intrinsically absolute manner, using only uncalibrated, visible detectors. Previous tests of this method have demonstrated its feasibility by making radiance measurements at wavelengths as long as 5 µm with relative standard uncertainties of less than 2 %. A new configuration of the previously tested LiIO 3 down-conversion crystal is proposed that allows measurements with better efficiencies and lower uncertainties. A second proposal to measure radiance at 8 µm using a new IR crystal is also discussed.