A method and apparatus for protecting an optical transmission measurement when sensing transparent materials. Sensing apparatus located in a housing directs a light beam at an upward angle to a sheet of transparent material and detects downward surface reflections of the beam from the transparent material. The light beam and the reflections pass through a transparent protective layer on the housing. A flow of clean air is passed between the protective layer and the transparent material to remove particles and liquid from the protective layer and from the space between the protective layer and the transparent material. Preferably, the protective layer is either made from a hydrophobic material or has a hydrophobic surface coating to facilitate blowing liquid and particles from the surface.