Cryogenic test setups with controlled stray light environments capable of reaching ultra-low radiative background levels are required to test far infrared (FIR) and submillimeter (sub-mm) wave radiation detectors for future space based observatories. In recent experiments (Nature Commun 5:3130, 2014), in which 1.54 THz radiation was coupled onto an antenna-coupled kinetic inductance detector (KID), we found a higher than expected optical loading. We show that this can be explained by assuming heating of the metal mesh IR filters and re-radiation onto the KID. Note that the total power from the cryogenic black body source used in the experiments (at T = – K) is much larger than the power inside the – THz band we use to calibrate our detector. The out-of-band radiation can have up to 5 orders of magnitude more power than inside the – THz band of interest. A strategy to mitigate the filter heating problem is presented, and when it is implemented, the validated upper limit for stray light at the detector level is down to few aW.
