Abstract: A sensor system comprised of a diffuse light source and a sensor mounted on a transparent layer of material that is adjacent to a fluid. The refractive index of the transparent layer is greater than the refractive index of the fluid, causing a portion of the diffuse light source to be reflected from the interface of the transparent layer and the adjacent fluid. A sensor is placed at a location to intercept the reflected light. The diffuse light source and the sensor are optically coupled to the transparent layer with a transparent material with a refractive index greater than that ofthe fluid. A light absorbing coating is placed on the surface ofthe transparent layer in order to absorb extraneous light from internal scattering of light from the fluid and light external to the sensor system. The light absorbing coating may be applied in a manner that masks the light sensor region in order to vary the sensor's output relative to the fluid refractive index or concentration.