We have characterized an independent primary standard method to calibrate detection efficiency of photon-counting detectors based on two-photon correlations. We have verified this method and its uncertainty by comparing it to a substitution method using a conventionally calibrated transfer detector tied to a national primary standard detector scale. We obtained a relative standard uncertainty for the correlated-photon method of 0.18 % (k=1) and for the substitution method of 0.17 % (k=1). From a series of measurements we found that the two independent calibration techniques differ by 0.14(14)%, which is within the established uncertainty of comparison. We believe this is the highest accuracy characterization and independent verification of the correlated- photon method yet achieved. (c) 2007 Optical Society of America.