  We present an optical setup that can be used to characterize the thicknesses
of thin NbN films to screen samples for fabrication and to better model the
performance of the resulting superconducting nanowire single photon detectors.
The infrared transmissometer reported here is easy to use, gives results within
minutes and is non-destructive. Thus, the thickness measurement can be easily
integrated into the workflow of deposition and characterization. Comparison to
a similar visible-wavelength transmissometer is provided.
