Alternatives to SiO x for protective scan mirror coatings in remote sensing instruments

Abstract

Mirrors in remote sensing instruments require durable dielectric coatings, both to prevent oxidation of the reflective surface and to protect it during cleaning. IR absorption bands within widely-used SiO x coatings produce scene radiance and instrument background variations as a function of scan mirror angle which motivate the search for possible substitute materials. In this work several candidate coatings are evaluated including CeF 3 , HfO 2 , MgF 2 SrF2, and Y 2 O 3 . This evaluation consists of reflectance, adhesion, and durability measurements of mirrors with an aluminum reflective surface over-coated with these materials. S-polarized and P- polarized reflectance measurements are presented between 2 and 20 micrometers for incidence angles between 40 and 50 degrees. This angular range is sufficient to scan the earth disk from geostationary orbit. Additional measurements at 45 degrees incidence are presented between 2 and 55 micrometers , covering the IR wavelength range of interest for earth radiation budget sensors. Comparisons are drawn with measurements of scan- mirror witness samples from the imaging and sounding instruments used in the Geostationary Operational Environmental Satellite (GOES). These witness samples exhibit reflectance variations arising from IR absorption bands in the SiO x protective coatings used in these mirrors. The spectral characteristics of several of the alternate materials are found to be quite attractive, however durable coatings of some of these materials require elevated deposition temperature which are incompatible with the nickel-coated beryllium scan mirror substrate construction used in GOES. This work present the achievable reflectance and durability of these alternate dielectric protective coatings at the deposition temperature constraints imposed by the scan mirror substrate. The prospects for substituting one of these coatings for SiO x are evaluated, and contrasted with the capability of radiometric calibration techniques to deal with the reflectance variations produced by SiO x coatings.