Abstract: A device for non-destructive surface photovoltage testing of silicon-on-sapphire (SOS) wafers employs the electrically equivalent of an inserted capacitance in series in a measuring circuit. A pair of thin irridescent coated (TIC) glasses, are positioned on each side of an SOS wafer with an electrical lead attached to each of the conductive coating of tin oxide and indium tin oxide of the TIC glasses. The glass surface (the non-conductive side) of each TIC glass is positioned adjacent to the sapphire layer and silicon layer respectively. An incident light illuminates the sapphire layer and subsequently illuminates the silicon-sapphire interface with a predetermined wave length of light wherein the silicon absorbs all of the light within about 100A of the silicon-sapphire interface. Thus, the incident light is transmitted through the conductive layer of the first TIC glass which is in electrical contact with a detection circuit.