Variation in ultraviolet (UV) irradiance is one of the prime causes of variability in tests that use UV lamps to simulate sunlight degradation of polymers. Control of irradiance in ASTM G53 fluorescent UV devices previously consisted mainly of periodic replacement and rotation of aging lamps. A newly developed electronic irradiance control system was evaluated for it's effectiveness in controlling UV variation caused by; (1) lamp aging, (2) lot-to-lot variation in lamps, (3) test temperature, and (4) room temperature. In addition, a system of UV baffles and multiple sensors was examined for its effectiveness in reducing variation in UV intensity at different points in the test specimen plane.