An array of vacuum photodiode detectors has been used to monitor discharge equilibrium, stability, and cleanliness in the Macrotor tokamak. These detectors use the photoelectric effect on small tungsten plates to measure UV emission in the band &lgr;∼200–1200 A˚, and so are sensitive mainly to impurity line radiation in Macrotor. The response of this system to controlled impurity contamination experiments and to disruptions is described. The design, construction, and background problems associated with these detectors are discussed in detail.