  In this paper, we present a new experimental apparatus for the measurement of
the detection efficiency of free-space single-photon detectors based on the
substitution method. For the first time, we extend the analysis to account for
the wavelength dependence introduced by the transmissivity of the optical
window in front of the detector's active area. Our method involves measuring
the detector's response at different wavelengths and comparing it to a
calibrated reference detector. This allows us to accurately quantify the
efficiency variations due to the optical window's transmissivity. The results
provide a comprehensive understanding of the wavelength-dependent efficiency,
which is crucial for optimizing the performance of single-photon detectors in
various applications, including quantum communication and photonics research.
This characterization technique offers a significant advancement in the
precision and reliability of single-photon detection efficiency measurements.
