Antireflection coated, wedged, single-crystal silicon aircraft window for the far-infrared

Operating far-infrared remote-sensing instruments from inside a pressurized cabin of an aircraft requires a window with high transmittance. Furthermor e, the radiometric properties of the window, such as the transmittance and the emitted radiation (i.e., temperature distribution), have to be known. T he design of a wedged, antireflection coated single crystal silicon aircraf t window, its modeled transmittance spectrum, and the applied coating techn ique are presented here. Measurements of the window transmittance with the 2.5 THz heterodyne spectrometer TeraHertz OH-Measurement Airborne Sounder ( THOMAS) and a Fourier-transform spectrometer are presented, showing a trans mittance of about 90% around 84 cm(-1). The window was designed and built f or the 2.5 THz OH-Sensor THOMAS, operated on the DLR research aircraft FALC ON. The transmittance of 90% means a substantial improvement compared to th e window used previously, With this new window, systematic errors in the me asured atmospheric radiance could be lowered, making the retrieval of atmos pheric parameters easier. Several successful Eights with the new window up to an altitude of 43000 ft have already been performed.