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

New experimental configurations of a system to measure absolute infrared (I R) 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, visibl e detectors. Previous tests of this method have demonstrated its feasibilit y by making radiance measurements at wavelengths as long as 5 mu m with rel ative standard uncertainties of less than 2 %. A new configuration of the p reviously tested LiIO3 down-conversion crystal is proposed that allows meas urements with better efficiencies and lower uncertainties. A second proposa l to measure radiance at 8 mu m using a new IR crystal is also discussed.