The lamp housing contains both a field diaphragm and collector lens, in addition to a tungsten-halogen bulb. A separate voltage regulator provides an adjustable direct-current voltage source for the lamp that varies between 5 and 12 volts. Modern microscopes usually contain a light source built into the base of the microscope, along with the collector lens and field diaphragm. Adjustment of microscopes with either internal or external light sources for K hler illumination is similar in theory, however there are certain differences in the procedures that will be described below. We suggest consulting the owner's manual for instructions about how the lamp is to be centered and for other important information about the illumination source.
Some manufacturers provide a frosted glass filter for diffusing light emitted from the bulb.
This filter is usually located in a filter holder adjacent to the lamp housing where it is attached to the body of the microscope. Remove this filter before proceeding with lamp alignment. We have constructed an interactive Java tutorial that reviews the basics of filament alignment. Correctly manipulating the combination of lamp housing adjustments should ensure that the lamp filament is centered in the optical pathway and is focused in the aperture diaphragm of the substage condenser. Figure 3 illustrates a typical view of the microscope light port when the filament is made visible using the procedures outlined above.