Abstract: An innovative portable reflected light sensor for non-destructively measuring characteristics of performance enhancement coatings applied to substrates such as solar photovoltaic panels is described. The innovative portable sensor provides a light source and a photodetector for measuring light incident on a substrate surface from the light source, and reflected to the photodetector. The spot size of the illuminated region of the substrate is at least 1 cm2 in area, thus averaging over a relatively wide portion of the substrate surface relative to existing fiber optic devices. A single measurement may then be representative of the coating. The innovative portable reflected light sensor is adapted to measure substrates in the field, and is especially adapted for assessing coating quality during the coating process. The innovative sensor also comprises a signal processing circuit that performs analysis of the measurements and feeds back status of the coating to the operator for coating process control.