Abstract: An upconversion system, including: a glow discharge device (GDD) configured to detect signal source radiation, wherein the signal source radiation is at least one of millimeter wave (MMW) radiation and Terahertz (THz) radiation; and a photodetector configured to measure intensity changes in visible light emitted by said GDD as a result of absorption of the signal source radiation. The upconversion system optionally includes a processor operatively coupled to the photodetector. The processor is configured to analyze imagery captured by the photodetector, and to compute at least one parameter of the signal source radiation based on the imagery.