Photoionization of excited states is the mechanism of detection in gas tube UV photon counting detectors of high sensitivity and variable spectral response. Because of bias to the prebreakdown region, noise current is on the order of only 10-14 A(DOT)Hz(-1/2). Spectral response varies according to gas quantum structure and pressure. High gas pressures shift spectral response to shorter wavelengths while lower pressures shift it to longer wavelengths and broaden it. One detector version is therefore almost solar blind. Another exhibits almost flat spectral response over most of the near UV band. In all cases, response falls off strongly with increasing wavelengths at longer UV wavelengths. Picowatt UV radiant powers yield about 1000 counts per minute. Response gamma is fairly constant up towards the microwatt radiant power range before falling off.