Sunday, March 11, 2007

Specimens that are nearly transparent and colorless may be almost invisible when viewed in the stereo microscope using traditional transmitted brightfield illumination techniques. This occurs because light diffracted by minute specimen detail is a quarter-wavelength out of phase with direct light passing through the specimen when both are recombined in the intermediate image plane, a classical phenomenon that seriously reduces contrast in brightfield images. Oblique illumination is similar in many aspects to the darkfield technique except that, instead of the specimen being lighted from all directions at oblique angles, light is projected from only a single azimuth. Simple diascopic bases are often equipped with a tilting mirror that can be adjusted to provide a certain degree of oblique illumination, but the light is not easily controlled and does not provide a uniform field of view. More complex microscope stands (or bases) have additional control possibilities, including tilting mirrors that are not restricted to a single axis and sliding mirror assemblies that can be inserted and removed from the light path. Any of these techniques can provide acceptable to excellent results on a wide variety of specimens.