Tests of an omnipresent standard for absolute spectral radiance measurements

Correlated pairs of photons can be used to access what is effectively an omnipresent absolute standard of spectral radiance. The process of spontaneous parametric downconversion, in which pump photons are converted into pairs of photons, can be thought of as being stimulated by an omnipresent and omnidirectional one photon per mode vacuum background. This background has units of spectral radiance and can be expressed in terms of fundamental constants. An unknown radiance can be determined by comparison to this background radiance. The comparison is made by adding the unknown radiance into downconversion process so as to stimulate the downconversion process over that level produced by the vacuum background only. This is done by inputting the unknown radiance into the system so as to overlap spatially and spectrally a portion of the output light. The process is monitored, not by observing the input light beam, but by observing only the light correlated to that unknown radiance. (This makes possible an additional advantage of this measurement technique; it allows IR radiance to be measured by monitoring a visible beam.) The ratio of the increase in the correlated signal is the absolute spectral radiance of the unknown source expressed in units of photons per mode. Initial studies of feasibility and accuracy have been performed. IR radiance has been measured to wavelengths of 5 mm with better than 3% uncertainty. We present the status of work to further improve the uncertainty of this method. # 1999 Elsevier Science B.V. All rights reserved.