While infrared signals at the levels of watts per square centimeter and milliwatts per square centimeter are commonly measured and calibrated with accuracies of a few percent and less, the measurement and calibration of infrared signals with values of nanowatts per square centimeter, picowatts per square centimeter and lower values may have errors of 100% or more. These large errors for low signals involve such problems as chopper blades, baffle systems, stay radiation, detector optics, attenuators, spectral mismatch, experiment planning and blunders.
Frederick O. Bartell,
"Infrared Calibration: Very Low Signals May Have Very Large Errors", Proc. SPIE 0499, Optical Radiation Measurements, (8 February 1985); doi: 10.1117/12.971080; https://doi.org/10.1117/12.971080