Measuring the absolute photo detection efficiency using photon number   correlations
  We present two methods for determining the absolute detection efficiency of photon-counting detectors directly from their singles rates under illumination from a nonclassical light source. One method is based on a continuous variable analogue to coincidence counting in discrete photon experiments, but does not actually rely on high detector time resolutions. The second method is based on difference detection which is a typical detection scheme in continuous variable quantum optics experiments. Since no coincidence detection is required with either method, they are useful for detection efficiency measurements of photo detectors with detector time resolutions far too low to resolve coincidence events. 
Michael Lindenthal, Johannes Kofler