Most flight instruments that use cryogenics or non-cryogenics require physical isolation of their internal instruments (sensors, detectors, spectrometers) from the external environment. This isolating window film also needs to be transparent in the operational frequency of the internal instruments to detect electromagnetic waves. As such, the film needs to have the necessary strength to hold the vacuum pull and provide a low transmission loss.
Most instruments use thicker materials using polyethylene or polypropylene to provide enough material strength; however, thicker films introduce significant loss in the transmission band. For instruments that require a window diameter of greater than 2”, traditional window films are thicker to hold larger force.
A new class of polymer cyclic olefin copolymer (COC) commercially known as thin TOPAS® was used to develop an instrument window for far-IR application. The TOPAS film is high-temperature vacuum formed to reduce the material stress and to avoid micro-cracks during the vacuum pull. TOPAS is a low-loss material in the far-IR range and has advantages over other commonly used films such as thick, high-density polyethylene and polypropylene. The vacuum forming process allowed the window to define a deflection radius for a given pressure and window diameter. This process significantly reduces the stress on the film.
TOPAS has higher transmission (~90%) compared to commonly used far-IR window films (83%). Furthermore, TOPAS has twice the tensile strength (7600 psi) of high-density polyethylene (4600 psi).