Detailed explanation of various types of illumination sources for optical microscopes
While early microscopes relied on oil lamps and natural sunlight, their primitive (but often very accurate) microscopes provided external sources of illumination. They often employ rather ingenious methods, such as collecting light from a large white board or the reflection of diffused sunlight on an overcast day. Unfortunately, these methods do not provide reliable illumination and often the area of the field of view illuminated greatly exceeds the numerical aperture of the objective, causing glare and water immersion.
Modern microscopes usually have an integral light source that can be controlled to a relatively high degree. The most common source for today's microscopes is an incandescent tungsten-halogen bulb positioned in a reflective housing, projecting light through a condenser lens to concentrate it below the stage. Lamp voltage is controlled via a variable rheostat, usually integrated into the microscope stand. Shown in Figure 1 is a typical lighting and housing. The light bulb is a tungsten-halogen lamp that operates on a direct current (DC) voltage of 12 volts and produces up to 100 watts of power for lighting. Lamp voltage is controlled by a DC power supply, usually built into the microscope housing, with a voltage control knob, usually a potentiometer mounted somewhere on the microscope stand. These bulbs generate a considerable amount of heat during operation and the housing is provided with some layers of heat sinks to help dissipate the excess heat. The position of the bulb is controlled by a series of knobs on the side of the illuminator housing, or pre-centered specifically for the housing. Light from the lamphouse is directed through a condenser lens onto the microscope base, and then passed through a sintered glass diffuser often before being focused by an aperture diaphragm on the condenser.
Incandescent Lamp - Incandescent lamps based on tungsten lamps are used as the primary source of illumination in modern microscopes, except for fluorescence microscopy investigations. These lamps are thermal radiation emitting a continuous spectrum of light, extending upward from about 300 nm to 1200-1400 nm in intensity at wavelengths centered in the 600-1200 nm region, most of which are illustrated in Figure 2. They are simple in design, construction and operation, consisting of a closed glass bulb filled with an inert gas and containing a tungsten filament energized by a DC current. Light bulbs produce a lot of heat and light, but only 5 to 10 percent of the energy output accounts for light. Tungsten lamps (but not tungsten-halogen lamps) are similar in operation to ordinary household light bulbs, as such, tend to suffer from some disadvantages such as blackening of the inner envelope with age and the evaporation of slowly deposited tungsten. The color temperature and brightness of these lamps vary with applied voltage, but average values range from approximately 2200 K to 3400 K. Photomicrographic color films used in these lamps must use a microscope that produces a color temperature that matches the lamp. Voltage membrane emulsions, typically at 3150 K and 3250 K. Somewhere in the range between, usually, the color temperature must be photomicrographically fine-tuned for this balance by inserting filters into the light path. The color temperature of the film emulsion for lighting.
Tungsten lamps vary widely in their design and manufacturers and offer a wide variety of models featuring various envelope shapes, mounting fixtures and filament arrangements. A typical selection of tungsten lamps used in optical microscopy is shown in Figure 3. The bulb in (a) is a 6-12 volt square tungsten lamp with a bronze bayonet base and is designed to be used with the end of the cylindrical bulb facing the condenser lens. (b) The rounded envelope bulb also has a bronze bayonet base, but this less powerful 6 volt bulb can position the project light or place it sideways or eventually. The bulb in (c) also has a circular envelope, but is fitted with a threaded base. Operating voltage between 6 and 30 volts, this bulb is designed to be used on one end.
