Illumination System. A variety of light sources for optical microscopy are available. ï The low-voltage tungsten filament lamp used primarily with bench microscopes has adequate intensity for observation, but not for photography. Altering the current to the bulb controls light intensity. ï Carbon-arc illumination systems, once common on metallographs, have been replaced by arc or filament light sources. ï The xenon-arc light source is prevalent because of its high intensity and daylight colour characteristics. Light intensity, however, can be adjusted only by the use of neutral-density filters. ï Tungsten-halogen filament lamps are also widely used for their high intensity and high colour temperature. Light intensity can be controlled by varying the current or by use of neutral-density filters. ï Other light sources, such as the zirconium-arc, sodium-arc, quartziodine, or mercury-vapor lamps, are less common.