An apparatus and method is disclosed which stabilizes the light incident a light guide end over a guaranteed operating lifetime of a lamp used to illuminate the light guide end. The apparatus and method minimize variation in illumination incident a light guide end without, for example, controlling the electrical input to the lamp. The invention is especially effective for light guides having a narrow outer diameter, and compensates not only for movement of a region of maximum luminosity, as occurs in lamps where the cathode is partially consumed during operation of the lamp, but also for the lamp luminosity decreasing with age.