SPECULAR REFLECTANCE PROPERTIES OF SILVERED POLYMER MATERIALS

The specular reflection properties of transparent cast polymer sheets and extruded polymer films, silvered and unsilvered, have been characterized with a newly designed specular reflectometer. The results obtained with this instrument are either absolute reflectance or a measure for the Fourier transform of the reflection function of the specimen in one dimension. Cast polymer sheets are investigated before and after silvering, and silvered polymer films are evaluated by mounting them with an adhesive onto aluminum or glass substrates, or by suspending the thin, silvered polymer as a taut membrane. Silvered polymers have attained a specularity such that over 90% of the incident beam is contained in a 1-2 mrad full-cone angle when mounted on a good substrate or suspended as a membrane. This value is well within the current goals for solar concentrators but silvered polymer mirrors are currently less specular than glass mirrors. The image quality of these mirrors does not change significantly over the wavelength range 400-1000 nm and angles of incidence between 20 degree and 60 degree . A principal limiting factor to the initial specularity of the polymer mirrors was waviness and/or curvature of the surface. Hence, the material being used as a substrate plays an important role in the optical performance of the mirror.