The Polarized Instrument for Long-wavelength Observation of the Tenuous interstellar medium (PILOT) is a balloon-borne astronomy experiment designed to study the linear polarization of the Far Infra-Red emission, 240 μm (1.2 THz) and 550 μm (545 GHz) with an angular resolution of a few minutes of arc, from dust grains present in the diffuse interstellar medium, in our Galaxy and nearby galaxies. The polarisation of light is measured using a half-wave plate (HWP). We performed the instrumental tests from 2012 to 2014 and are planning a first scientific flight in September 2015 from Timmins, Ontario, Canada. This paper describes the measurement principles of PILOT, the results of the laboratory tests and its sky coverage. These include defocus tests, transmission measurements using a Fourier Transform Spectrometer at various positions of the HWP, and identification of internal straylight.
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