An experimental system using correlated photons for radiometric purpos
es has been set up at the National Institute of Standards and Technolo
gy (NIST). We use pairs of correlated photons to produce spatial maps
of the absolute efficiency of a photomultiplier photocathode at four w
avelengths. We also compare this technique with measurements carried o
ut by conventional means tied to existing radiometric standards. These
initial comparisons show an average agreement within similar to 0,6%
between the two methods. The results show that correlated photons can
be a useful tool in radiometry and that the method holds enough promis
e to warrant further studies.