Polarized Light Microscopy

One of the microscopy techniques that can be beneficial in a forensic examination involves the use of polarized light (light in which the electromagnetic waves all vibrate in the same plane). The use of polarized light microscopy can not only detect the presence of small pieces of evidence including fibers , crystals, and soil, but can help identify this trace evidence based on the distinctive appearances of different materials under the polarized illumination.

The basis of polarized light microscopy is the wave nature of light. From its source, a beam of light moves outward. Similar to the waves in a pond that move outward from the point of entry of a rock, light waves consist of a series of alternating crests and troughs. These crests and troughs can be oriented vertically, horizontally, or in any other plane in between. In general, this form of light, which is known as unpolarized light, can be thought of as vibrations in the horizontal and vertical planes.

Unpolarized light can be transformed into polarized light. The most common means, which is used in microscopy and even in polarizing sunglasses, is to pass the unpolarized light through a special filter. This Polaroid filter, or polarizer, blocks the vibrations in either the horizontal or vertical plane while permitting the passage of the remaining plane of light. The light emerging from the filter represents the polarized light.

The construction of the filter allows for this selectivity. Within the filter, molecules comprising long carbon chains are arranged in the same direction. The effect is visually akin to the pattern of a picket fence. If the alignment is horizontal, then the "polarization axis" will be vertical. The filter will block all light waves that are vibrating in the horizontal plane, while permitting waves vibrating in the vertical plane to pass through. Alignment of the filter molecules in the vertical direction produces a horizontal polarization axis, so that only waves vibrating in the horizontal plane will pass through the filter.

Some polarization light microscopes are equipped with two filters that can be rotated to permit the sensitive tailoring of the light wavelengths that emerge (since, in reality, waves vibrate in other than the horizontal and vertical planes). If these filters are in exact opposition (i.e., vertical polarization axis superimposed on horizontal polarization axis) then the passage of all the light is blocked and no image is seen. At other filter configurations, different vibrational forms of the light will pass through.

Polarized light has a number of uses other than for microscopy. One of the most appreciated is three-dimensional (3-D) movies. The use of two slightly offset projectors casts two movie images on the screen. One is aligned horizontally and the other is aligned vertically. By wearing the distinctive 3-D glasses, which contain polarization filters, the viewer experiences a sense of depth in the viewed image.

Polarized light microscopy can be used with different types of materials. Materials such as cubic crystals and glass that is not under stress are symmetrical in their optical properties. Light impinging from any direction on these so-called isotrophic materials will behave the same. In contrast, anisotrophic materials have optical properties that vary depending on the orientation of the object in the light beam and on the vibrational property of the light (unpolarized, polarized, horizontally- or vertically-polarized). In the latter, which includes almost all solid materials, the appearance of the object can vary depending on the above parameters.

These different appearances can be exploited to determine the compositional nature of the object being examined. For example, as an object is reoriented, areas of brightness can appear or the color can change. These changes can be directly related to the height differences of the surface and on the presence of differently composed regions. An experienced forensic microscopist can learn a great deal about a sample from these patterns.

As one example, chrysotile, crocidolite, and amosite forms of asbestos can be differentiated from one another based on their microscopic appearance under polarized light. This can be important in a forensic examination, since the chrysotile form of asbestos does not pose the health threat that the latter two forms do. Without the rapid discrimination power of polarized light microscopy, such an assessment could not be made.

Polarized light microscopy can also be done using light that passes through thin and transparent objects (transmitted light) and light that has reflected back off from the surface of an opaque object (reflected light). Thus, the technique can be used to examine the surface of objects like rocks, computer chips, and fibers.

Other potential forensic uses of polarized light microscopy include the determination of the mineral content of a rock chip, the identification of natural and synthetic polymers, and the identification of nylon fibers.

see also Alternate light source analysis; Fluorescence; Monochromatic light; Scanning electron microscopy; Trace evidence.