In most fluorescence microscopy applications, the number of photons reaching the eye or other detector, such as a video camera or photomultiplier, is usually very low. These lamps are filled with high pressure gases and should be handled with caution. The most common lamps are the mercury burners, ranging in wattage from 50 watts to 200 watts and the xenon burners ranging from 75 watts to 150 watts. The mercury burner lamp in Figure 1 consists of two electrodes sealed under high pressure in a quartz glass envelope which also contains mercury. The power supply should have a timer to enable you to keep track of the number of hours the burner has been in use.
Arc lamps lose efficiency and are more likely to shatter, if used beyond their rated lifetime.
At other wavelengths of visible light, the intensity is steady but not nearly so bright, but still usable for blue excitation. It also should be understood that mere wattage is not the only consideration for determining brightness. Also important in your selection of a burner is whether or not the spectral intensity peaks of the burner's wavelengths match the excitation requirements for the fluorochromes staining the specimen. Whenever an illumination system is being evaluated, it is necessary to consider the entire system including collector lenses and the use of aperture and field diaphragms in securing K hler illumination. Short-gap xenon burners are usually more desirable because the size of the arc is such that its light may be more readily included within the back aperture of the objective, thus avoiding waste of light intensity.