Typical modern polarized and brightfield microscopes have a lamphouse, which contains a 50 to 100-watt high-energy tungsten-halogen lamp, attached to the base of the microscope. A transformer providing direct current (DC) voltage to the lamp is usually built directly into the microscope base and is controlled by a potentiometer positioned near the lamp switch in bottom of the base, the lamp voltage control. Between the lamphouse and the microscope base is a filter cassette that positions removable color correction, heat, and neutral density filters in the optical pathway. Also built into the microscope base is a collector lens, the field iris aperture diaphragm, and a first surface reflecting mirror that directs light through a port placed directly beneath the condenser in the central optical pathway of the microscope. These components control the size, intensity, and distribution of light in the illumination field.

The entire base system is designed to be vibration free and to provide the optimum light source for Köhler illumination. In general, the modern microscope illumination system is capable of providing controlled light to produce an even, intensely illuminated field of view, even though the lamp emits only an inhomogeneous spectrum of visible, infrared, and near-ultraviolet radiation. In some polarized light microscopes, the illuminator is replaced by a plano-concave substage mirror. Almost any external light source can directed at the mirror, which is angled towards the polarizer positioned beneath the condenser aperture. This configuration is useful when an external source of monochromatic light, such as a sodium vapor lamp, is required. Because the illumination intensity is not limited by a permanent tungsten-halogen lamp, the microscope can be readily adapted to high intensity light sources in order to observe weakly birefringent specimens.
Polarizers Polarized light microscopy was first introduced during the nineteenth century, but instead of employing transmission-polarizing materials, light was polarized by reflection from a stack of glass plates set at a 57-degree angle to the plane of incidence. Later, more advanced instruments relied on a crystal of doubly refracting material (such as calcite) specially cut and cemented together to form a prism. A beam of white unpolarized light entering a crystal of this type is separated into two components that are polarized in mutually perpendicular directions. One of these light rays is termed the ordinary ray, while the other is called the extraordinary ray. The ordinary ray is refracted to a greater degree in the birefringent crystal and impacts the cemented surface at the angle of total internal reflection. As a result, this ray is reflected out of the prism and eliminated by absorption in the optical mount. The extraordinary ray traverses the prism and emerges as a beam of linearly polarized light that is passed directly through the condenser and to the specimen. Several versions of this polarizing device which was also employed as the analyzer were available, and these were usually named after their designers.

The most common polarizing prism was named after William Nicol, who first cleaved and cemented together two crystals of Iceland spar with Canada balsam in 1829. Nicol prisms were first used to measure the polarization angle of birefringent compounds, leading to new developments in the understanding of interactions between polarized light and crystalline substances. Other prism configurations were suggested and constructed during the nineteenth and early twentieth centuries, but are currently no longer utilized for producing polarized light in most applications. Nicol prisms are very expensive and bulky, and have a very limited aperture, which restricts their use at high magnifications. Instead, polarized light is now most commonly produced by absorption of light having a set of specific vibration directions in a dichroic medium. Certain natural minerals, such as tourmaline, possess this property, but synthetic films invented by Dr. Edwin H. Land in 1932 soon overtook all other materials as the medium of choice for production of plane-polarized light. Tiny crystallites of iodoquinine sulphate, oriented in the same direction, are embedded in a transparent polymeric film to prevent migration and reorientation of the crystals.
Land developed sheets containing polarizing films that were marketed under the trade name of Polaroid®, which has become the accepted generic term for these sheets. Any device capable of selecting plane-polarized light from natural unpolarized white light is now referred to as a polar or polarizer, a name first introduced in 1948 by A. F. Hallimond. Today, polarizers are widely used in liquid crystal displays (LCDs), sunglasses, photography, microscopy, and for a myriad of scientific and medical purposes. Light exiting the port in the microscope base is first passed through a neutral linear Polaroid HN-type polarizer to create plane-polarized light having a vibration vector that is confined to a single plane. H-films are produced by stretching a sheet of polyvinyl alcohol to align the long-chain polymeric molecules, which are subsequently impregnated with iodine. These films are less effective polarizing devices than a calcite prism, but do not restrict numerical aperture. Typically, a pair of crossed polarizing H-films transmits between 0.01 percent and 40 percent of the incident light, depending upon the film thickness.
On most microscopes, the polarizer is located either on the light port or in a filter holder directly beneath the condenser. The microscope has a rotating polarizer assembly that fits snugly onto the light port in the base. The polarizer can be rotated through a 360-degree angle and locked into a single position by means of a small knurled locking screw, but is generally oriented in an East-West direction by convention. Other microscopes typically have the polarizer attached to the substage condenser assembly housing through a mount that may or may not allow rotation of the polarizer. Some polarizers are held into place with a detent that allows rotation in fixed increments of 45 degrees. Polarizers should be removable from the light path, with a pivot or similar device, to allow maximum brightfield intensity when the microscope is used in this mode. Light diffracted, refracted, and transmitted by the specimen converges at the back focal plane of the objective and is then directed to an intermediate tube, which houses another polarizer, often termed the “analyzer”. The analyzer is another HN-type neutral linear Polaroid polarizing filter positioned with the direction of light vibration oriented at a 90-degree angle with respect to the polarizer beneath the condenser. By convention, the vibration direction of the polarizer is set to the East-West (abbreviated E-W position). The same convention dictates that the analyzer is oriented with the vibration direction in the North-South (abbreviated N-S) orientation, at a 90-degree angle to the vibration direction of the polarizer.