Reflectivity of black-chromium coating at a wavelength of 121.6 nm
This paper concerns the light-absorptive black-chromium (BC) coating intended for reduction of stray light in the satellite-borne optical instruments and particle-analyzing apparatus operating in open space under intensive solar UV illumination. Resolution of this problem is also important in connection with development of the extra-atmospheric Far UV astronomy. The total hemispherical reflectance (THR) measurements of the BC coating samples were carried out at a wavelength of 121.6 nm -- the most intensive line in the short-wave part of the solar UV spectrum which creates the main contribution to the background noise of detectors in space apparatus. THR of the samples tested at a wavelength of 121.6 nm was as low as 2%. The samples of the BC coating were subjected to standard mechanical and environmental tests simulating the shipping, storage, launching, flight, and operating conditions of the space equipment. THR measurements of the samples were made following exposure to each test. As a result of mechanical and environmental testing the following features of the BC coating were discovered: (1) the coating stands up well to all types of mechanical effects (vibratory loads, linear overloads, and impacts) to which the space equipment may be subjected both when shipping to the space- vehicle launching site and also when launching; (2) the most undesirable effect for the undamaged surface of the coating is humidity -- this causes an increase of reflectivity with a factor of 1.2 on average; (3) the most dangerous outcome for the BC coating is the effect of rubbing -- the reflectivity is increased about two times at the place of contact. Advantages of the BC coating consists in its low reflectivity both in the vacuum UV and within the visible ranges, as well as in the short-wave infrared range. The BC coatings is also assumed to possess following valuable attributes: (1) at grazing incidence its reflectivity might be less than that of other types of coatings; (2) low fraction of the incident radiation reflected into the specular peak relative to the THR.
This paper was published in SPIE Proceedings Vol. 3784