A photodetector apparatus is provided having an output only when exposed to electromagnetic radiation above or below a specified cut-off frequency. At least first and second light sensor means produce electrical signals responsive to electromagnetic radiation of first and second overlapping frequency ranges, respectively, and peak electrical responses at different first and second peak frequencies, respectively. Electrical means compare the electrical signals from at least said first and second light sensor means and produces an electrical signal preferably of only one polarity and equal to a product of a first constant times the amplitude of the electrical signal produced by said first light sensor means minus a product of a second constant times the amplitude of the electrical signal produced by said second light sensor. The apparatus is particularly useful in ultraviolet detection where the sensor means are comprised of separate silicon carbide semiconductor bodies each containing a PN junction.