UV disinfection depends on lamp output. Although fluorescent UV lamps have long life, they will degrade with time and bulb contamination. Monitoring lamp output with Philadelphia-based Solar Light Co.’s GLM10–a stable, calibratable UV detector–assures lamp efficacy. The GLM’s efficient one-piece design for both readout and detector uniquely suits it to operate in a pressurized water environment. Percent of initial lamp intensity is read on an LED.