A fluorescent lamp utilized in a document scanning system is environmentally and thermally stabilized by means of a bifurcated heater control assembly. A heater blanket is wrapped around the entire surface of the lamp including the end areas surrounding the filaments but exclusive of the aperture through which light is emitted. The size and temperature of the lamp cold spot are selected emperically to maximize output power and minimize illumination transients during power on. The heater jacket has thermostat control set points which, in both the lamp standby and energized states, maintain the blanket and lamp assembly at a constant elevated temperature which is above that achieved by lamp heat alone. Since the temperature distribution is always determined by the blanket and never by the lamp, changes in temperature or temperature distribution (and hence illumination distribution) as the lamp transitions from standby to full on are minimized.