Oblique and Axial Illumination

by John S. Wojtowicz, US


The compound microscope most commonly used by amateurs is designed for transmitted illumination. Much of the "unprepared" world is opaque and unless the material can be cleared and sectioned it is invisible to such an instrument at higher powers. Magnifications involving the 4X to 10X objectives are usually not a problem as the working distance of those objectives is great enough to allow room for an incident illuminator and the optical aberrations introduced by the lack of a cover slip are negligible.

It is otherwise with high magnifications, and several lighting techniques are available. The microscope pictured is equipped for axial illumination. The stand is similar to a biological microscope with a number of significant differences. The three objectives 8X, 20X, and 40X are corrected for use without a cover glass and for an optical tube length of 215mm rather than the standard 160mm. Like so much else in microscopy these standards vary with the manufacturer. There is no substage condenser, just a mirror for the occasional transparent subject viewed at low magnification. The illuminator is located between the objective and the binocular head. The light is reflected onto the subject by a half silvered mirror located just above the back lens of the objective. This acts as a beam splitter, allowing some of the light to be reflected downward towards the subject, yet also allows the image forming light rays to pass through the mirror. Thus, the lens itself acts as the condenser. Since the illuminating and image forming light rays cross, the glare can be considerable. The mirror can be withdrawn if desired when oblique or transmitted illumination from a separate light source is used.

The illuminator has both field and aperture diaphragms, but their positions are reversed compared with a biological microscope. The aperture diaphragm located near the light source can be used to increase depth of field at the expense of resolution as in transmitted light instruments. The field diaphragm is critical for the control of glare. In fact I find I frequently close it down so that it blocks out much of the available field of view to get better lighting on details of interest. This is a particular advantage since my camera crops off the edges of the total field of view in order to obtain an acceptably flat field for photographs.

A primary use of such a microscope is the study of highly polished and chemically etched metal surfaces. Another common use is the inspection of flat surfaces with fine detail such as microchips. The photographs below are of the E in the motto "E Pluribus Unum" on a Lincoln Penny. Including the shadow, the E is 0.3mm high. The oblique illumination photographs used a Nicholas Illuminator from a stereomicroscope as the light source. This provides a parallel beam of light much larger than the field of view, and the only control (besides the lamp position) varies the voltage to control the light intensity. As can be seen in the photographs, the diaphragms in the axial illuminator provide control over the depth of field and superior glare control compared to the Nicholas illuminator, especially for high power objectives with scant working distance. These are important for the intended uses of the instrument.

Axial illumination using 8X, 20X and 40X objectives.

Common biological subjects are generally not especially flat, and the lower powers are by far the most frequently used for opaque subjects. The Nicholas Illuminator is only one of many possible oblique light sources where the working distance allows, and the choice can have a dramatic impact on the view. Ordinary desk lamps can be used with either frosted or clear bulbs, the Mini-Mag flashlights are focusable, and can also be used without the reflector for "raw" lighting. These different lighting techniques complement each other, their versatility is surprising, and the necessary equipment obtained at little cost.

Oblique illumination using 8X, 20X and 40X objectives.

Comments to the author John Wojtowicz welcomed.

An article on basic incident illumination using a compound microscope is in the Micscape Library


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Published in the April 1999 edition of Micscape Magazine.

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