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Illuminator for dark field microscopy.

Dark field microscopy of opaque objects is typically done using special objectives with integral illumination optics and special source optics. Such apparatus is costly, especially if an entire microscope with dark field capability must be purchased. Dark field illumination can also be obtained with commercially available fiber rings. However, these rings do not fit all objectives, and they illuminate over a narrow range of angles, which is sometimes undesirable. Described here is a simple dark field illuminator that can be used with nearly any microscope. This device (Fig. 1) is merely a cylinder with a diffusely reflecting interior that is closed at the top of the objective and open at the bottom. The cylinder has one or more ports in its side, through which light is introduced with a fiber bundle. The light is scattered many times from the microscope objective and the cylinder wall, becoming nearly uniform when it emerges from the cylinder. The light reaches the object from beyond the collection cone of the objective, providing dark field illumination. The working distance of the objective determines the range of illumination angles, which can be adjusted by raising and lowering the cylinder. For objectives with moderate working distances, the illumination can be quite diffuse. The performance of this illuminator does not depend critically on its shape or alignment. It can be constructed of any material, as long as the inside wall is a good diffuse reflector. An aluminum cylinder painted inside with standard flat white paint works very well. The diameter of the clearance hole can be large enough to fit a set of objectives, provided that too much light does not escape. To avoid stressing the objective to which it is affixed, the illuminator is held in place with plastic screws. The outside diameter of the illuminator should be reduced at the top so it can fit between the objectives on a turret. The bottom is beveled so the sample can be