Left: laboratory instrumental setup: the NIKA cryostat, the HWP in a fixed position, the polarizer mounted inside the cryostat and a Martin-Puplett interferometer. Right: instrumental setup for polarization measurements at the telescope with the last two mirrors of the optics chain and the polarization module with the HWP and the stepper motor mounted in front of the entrance window of the cryostat. The polarizer is tilted by approximately 10 deg with respect to the optical axis to avoid standing waves.