MEASURING ABSOLUTE INFRARED SPECTRAL RADIANCE WITH CORRELATED VISIBLEPHOTONS - TECHNIQUE VERIFICATION AND MEASUREMENT UNCERTAINTY

An experimental system in which correlated photons for radiometric mea surements were used has been set up at the National Institute of Stand ards and Technology. We use visible-FR pairs of correlated photons pro duced by means of optical parametric downconversion to measure the rad iance of a high-temperature IR source at 3.415 and 4.772 mu m in an in trinsically absolute manner (i.e., without requiring any externally ca librated radiometric standard). To our knowledge, this is the only rad iometric method with which one measures radiance directly, instead of using radiant power and aperture geometry measurements to deduce radia nce indirectly. This technique has an additional unusual characteristi c: It allows absolute radiometric measurements of IR radiation to be m ade with high-quality visible detectors. We compare measurements made with this technique with radiance measurements made with conventional means tied to existing radiometric standards. These comparisons show a n average agreement to within similar to 3% between the two methods. T he results demonstrate an accuracy consistent with the estimated uncer tainty of the current measurements. This is the first time to our know ledge that this method has been used to provide absolute radiance meas urements of a source that has been calibrated conventionally, revealin g unexpected systematic effects and allowing estimates of the ultimate accuracy of this method. In addition, these measurements are further into the IR than any previous measurements of this process and have pr oduced the highest thermally stimulated downconversion signal yet seen .