Basics of Fiber Microscopy
The Fiber Reference Image Library is built upon images of individual fibers or groups of fibers taken under a microscope using differing techniques. Interpretation of these images relies on knowledge of microscopy and how these techniques each provide different information.
Brightfield image
Using standard techniques of illumination, some of the light that is impinged on a fiber specimen is absorbed. The image observed has a bright background with features of the fiber seen as levels of dark color.
Darkfield image
Using a different configuration of light impinged on the fiber specimen, the light reflected from the surface of the fiber is observed. The image has a dark background and features of the fiber are seen as levels of brightness.
Polarized light image
Employing techniques of polarized light microscopy with two polarizers crossed (one above the fiber specimen, the other below the fiber specimen), the image observed has a dark black background. Depending on the molecular order within the specimen, the features of the fiber are seen as brilliant colors of the spectrum (the Michel-Levy chart). In fibers with less order, a paler color is observed.
Differential interference contrast image
With this form of microscopy, polarized light techniques are used with the addition of a compensator which can enhance small differences in structure of the specimen.
Polarized light image with addition of the Red I plate.
By aligning the fiber length and width in parallel or perpendicular to the long axis of the Red I plate, a known amount of retardation of light is added and the consequences observed. This is useful in determining the sign of elongation of the fiber, which can be a distinguishing feature. Properly aligned and observed the color difference at the nodes of bast fibers is indicative of the spiral direction of the fibrils.
Other techniques.
Additional methods employed in identifying the fibers in the Fiber Reference Image Library include the observation of the Becke line, thereby determining the refractive Index of the fiber relative to the mounting medium, the use of alternative mounting media including air mounting of hair fibers for better observation of the scales on the surface, and microsolubility, in which a fiber is dry mounted on a slide and a chemical agent is introduced. The consequent behavior of the fiber is observed under the microscope.
Useful reference texts
Petraco, N. and Kubic, T. Color Atlas and Manual of Microscopy for Criminalists, Chemists, and Conservators. CRC Press Boca Raton. 2004. This book alone is invaluable in explaining fiber microscopy and in providing useful examples.
McCrone, W.C., McCrone, L.B., and Delly, J. G. Polarized light Microscopy. McCrone Research Institute, Chicago, 1984.
Useful websites
The following websites are useful for explaining microscopy and differing techniques, but have less information on fibers in particular.
Microscopy University.
https://www.microscopyu.com/
Molecular Expressions, Optical microscopy primer,
https://micro.magnet.fsu.edu/primer/