Incandescent tungsten-based lamps are the most important Microscope light source used in modern and common microscopes, with the exception of those intentional for fluorescence microscopy investigations. These lamps are thermal radiators that emit a continuous spectrum of light extending from about 300 nanometers to upward of 1200-1400 nanometers, with a majority of the wavelength intensity centered in the 600-1200 nanometer region. Their design, construction, and operation are simple consisting of an enclosed glass bulb filled with an inert gas and containing a tungsten wire filament that is energized by a DC electric current. The bulbs produce a tremendous amount of heat and light, but the light accounts for only 5 to 10 percent of their energy output. Tungsten lamps (but not tungsten-halogen) are similar in operation to common household light bulbs and likewise tend to suffer several drawbacks such a decreased intensity with age and a blacking of the inside envelope as evaporated tungsten is slowly deposited. The color temperature and luminance of these lamps varies with the applied voltage, but standard values range from about 2200 K to 3400 K. When these lamps are used in photomicrography with color film, the microscopists must use a lamp voltage that produces a color temperature matching that of the film emulsion, usually somewhere in the range between 3150 K and 3250 K. Frequently, the color temperature must be fine-tuned for photomicrography by inserting filters into the light path that balance the illumination for the color temperature of the film emulsion.

Tungsten lamps are different far and wide in their design and the various set of models obtainable by manufacturer’s characteristic a variety of envelope shapes, mounting fixtures, and filament arrangements. A typical selection of tungsten lamps used in optical microscopy for its microscope light source. The bulb is a 6-12 volt square tungsten filament with a bronze bayonet platform that is designed to be used with the end of the cylindrical bulb facing the collector lens. The rounded-envelope bulb in addition has a bronze bayonet base, other than this less-powerful 6 volt bulb can be positioned to project light either sideways or end-on. The bulb as well has a rounded envelope, but is outfitted with an Edison screw-thread base. In the company of a working voltage between 6 and 30 volts, this bulb is designed to be used end-on. These bulbs are having an extended glass envelope. It is equipped with a brass bayonet base and is a common design used in a number of microscopes built in Eastern Europe. Modern tungsten-halogen lamps (sometimes referred to as a quartz-iodine bulb) are now standard equipment on most microscopes for its microscope light source manufactured in Japan, the United States, and Western Europe. Tungsten-halogen lamps have compact bulbs that bring in a number of advantages in excess of normal incandescent lamps, in the largest part notably their brilliant light, smaller dimension, uniformity of illumination, longer lamp life and greater economy. Unlike tungsten-filament incandescent lamps, tungsten-halogen lamps have halogens added to the filler glass. The halogens (usually iodine) ensure that all vaporized tungsten is returned to the filament and not deposited upon the glass envelope.