Absolute Radiometry Using Correlated Photons

Abstract

Correlated photons have shown promise in several areas of radiometric measurements¹. In particular, several groups have demonstrated that correlated pairs of photons can be used to determine the absolute quantum efficiency of photon counting detectors without reference to any externally calibrated detector standard^2–6. The ultimate goal of our work is to determine the maximum accuracy that can be achieved with this method, as well as to develop additional radiometric applications of correlated light. To carry out a test of the method, we have set up a system at the National Institute of Standards and Technology employing this method to map out the spatial variations of the absolute quantum efficiency of a photomultiplier (PMT) photocathode. To our knowledge, this is the first time that this method has been used to provide a detailed spatial response map consisting of direct absolute measurements at each spatial position, rather than a relative response map which is tied to an absolute scale at only one or a few points. Toward the goal of determining the ultimate accuracy of the method, the measured efficiency was compared with an estimate using the specifications supplied with the detector. Further comparisons are planned employing an independent calibration to verify the method to better accuracy. In addition, the effects of the pulse height distribution of the PMT, the discriminator threshold level setpoint and the PMT supply voltage on the detection efficiency were investigated.