Polarization

In reflected light polarization instruments, one places a polarizer in the illumination axis and an analyzer (another polarizer) in the viewing axis, above the illuminator. The polarizers are crossed at 90 degrees such that when looking at a perfectly formed mirrored surface, all light would be extinguished by the second polarizer. A rotatable quarter wave plate is inserted between the end of the lens and the object. Usually the polarizer is oriented at the most favorable point of extinction with respect to the beam splitter and the analyzer is rotatable to further "tweak" the results.

If you choose to add a "polarization" option, it requires an illumination polarizer, an analyzer above the zoom, a quarter wave plate (optional), and a shorter version of an Adapter (RA) because the analyzer "eats up" 50.8mm of the optical path.

About Polarization:

Polarization is a contrast enhancement technique that can reduce back reflections and bring out salient features not readily visible with normal viewing.

It is important to understand the results of polarization first and then you can learn the actual mechanism by which it happens.

First, mentally picture the wave theory of light (which continually is being questioned and modified). Tie a rope to a tree and rapidly oscillate the loose end in a vertical direction. This vertical direction, or whichever direction you choose to shake the rope, is called the "plane of oscillation". The result will be a series of waves traveling down the rope much like those at the shore. The amount of waves per given time relates to frequency, and the actual spacing between waves is called wavelength. The visible light spectrum ranges from the long wavelength, low frequency, reds to the shorter wavelength, higher frequency, blues.

If you were an illumination source, you would have an infinite number of ropes pointing in all directions, with random orientations of their planes of oscillation (randomly polarized). A lens system picks up these waves reflecting from a point on the object and manipulates them so as to place them at an image plane (visual or sensor).

A polarizer is a unique type of filter in that, rather than filtering wavelengths (color), it only passes waves whose planes of oscillation are aligned favorably with part of its internal structure. The result is a beam where all the ropes are parallel to each other, with the waves moving in the direction of travel. Should this beam strike another polarizer, with the same orientation as the first, it would pass thru undisturbed. However, if the second polarizer was oriented 90 degrees to the first, there would be no ropes left to pass thru the new polarizers preferred orientation and no light would be transmitted (called extinction).

There is another element called a quarter wave plate which has the unique feature of taking the polarized light (ropes all parallel) and circularly polarizing the beam (sort of a spiraling effect). When this beam reflects off a specular (shiny) object, the spiraling reverses, and upon re-striking the quarter wave plate, is extinguished. This technique is useful in eliminating reflections from wafers and circuit boards. At other times, the quarter wave plate is used to add contrasting coloration to appropriate objects.

Everyday examples include polarized sunglasses and CRT screens. Because light becomes partially polarized when reflecting off a surface (angle related), the addition of a polarizing material (sunglasses) may cancel the reflections. On the CRT screen, a quarter wave film is used to circularly polarize the incoming outside light and then extinguish it on its way back, before it an reach the operators vision.

A number of manmade and natural materials have molecular structures that alter these "parallel ropes". Many materials exhibit birefringence, where the polarized beam is split with part of the beam's ropes not only rotated about itself but the peak of the waves now occur at a different point in time from the from the original beam (phase shift). When these beams, coming from the object, try to pass thru the analyzer, only portions of each (ropes with planes of oscillation favorable to the analyzer molecular structure) will pass. At the image plane, there will be information that shows structural characteristics unobservable under normal lighting.

Image: Polarizer