Superconducting nanowire single photon detectors (SNSPDs) have low dark counts, improved gain stability, and high resolution compared to traditional infrared detectors. Recent work at National Institute of Standards and Technology (NIST) and National Aeronautics and Space Administration (NASA) has incorporated SNSPDs into arrays and extended the response into the mid-infrared to use for spectroscopy and hyperspectral imaging. We are developing novel methods to spectrally calibrate and measure stability of these detectors in this challenging wavelength range from 3 μm to 25 μm. We present our design of a novel cryogenic apparatus uniquely focused on making quantitative efficiency measurements of these quantum detectors by directly comparing to a reference calibrated blocked-impurity-band (BIB) detector so they can be used by researchers from federal agencies, universities, and industry.