  Single-photon detectors are a pivotal component in photonic quantum
technologies. A precise and comprehensive calibration of the intrinsic
detection efficiency is of utmost importance to ensure the proper evaluation of
the performance in view of the specific technological application of interest,
such as the protection against security breaches in quantum cryptographic
solutions. Here we report on a systematic study on and comprehensive analysis
of the estimation of the intrinsic detection efficiency of two commercial
single-photon detectors based on single-photon avalanche diodes (SPADs) for
various mean photon numbers and at high laser pulse repetition rates using
different techniques. We observed an unexpected and signifucant drop in
intrinsic detection efficiency at detection rates of 10 % and higher relative
to the maximum detection rate. It is demonstrated that for data analysis a
statistical model for the detection rate conveniently can be used if no
timestamped data are available. We conclude that the full characterization of
single-photon detectors used in critical applications should include the
sensitivity of their intrinsic detection efficiency to high event rates.
