Abstract: The spontaneous parametric down-conversion (SPDC) light field is generated by the combined action of a monochromatic flux of pumping photons and quantum vacuum noise on a nonlinear crystal with no center of symmetry.Its intrinsical quantum origin determines its important role in the area of non-classical field research.In this paper,the coherence characteristics of the SPDC light field is investigated theoretically,and the application of SPDC light field to absolute measurement of quantum efficiency of photo detectors is studied experimentally.Finally,it points out the wide application prospect of SPDC light field application.