Contrast techniques in microscopic observation

Conoscopic light microscopy is performed as a complement to the orthoscopic light method to characterize birefringent materials on thin slides with parallel faces, because knowing the neutral axis orientations and path difference numerical values between both related polarized rays would not be sufficient to identify materials without any uncertainty. This method consists in illuminating a uniform region of the specimen with a cone-shaped polarized light (so-called conoscopy) and then in observing through an analyzer the interferences said to be “in convergent light” which are formed at infinity or at the focus of the objective [ Born 1999^[1]]. For practical purposes, the cone-shaped light beam should be made by Köhler illumination with a very open aperture diaphragm; the field diaphragm should be closed enough to limit the illumination to the chosen region in the middle of the field. The interferences figure in convergent light will appear on the image focus of the objective of the microscope. Observing the interferences figure through the eyepiece of the microscope requires adding a convergent objective, called “Bertrand lens”, to the body of the microscope, which will project the interferences figure at infinity (Figure 4). It is important to highlight that this method does not give an image of the preparation but a complex figure (Figure 5cd) which will permit the characterization of birefringent properties of the specimen's uniform region which had been chosen with orthoscopic illumination.