Another technique commonly used for contrast enhancement is dark-field illumination (Abramowitz, 1987b; Davidson, 1999). In this case, the sample is
illuminated at an angle that cannot be accepted by the objective's aperture. In this case only the highly diffracted rays enter the objective. Hence only highly scattering or diffracting structures can be observed using this technique. Dark-field illumination requires blocking off the central light that ordinarily passes through and around the specimen, allowing only oblique rays from every angle to reach the specimen. This requires the use of special condensers that allow light rays emerging from the surface in all azimuths to form an inverted hollow cone of light with an apex centered in the sample plane. If no specimen is present and the numerical aperture of the condenser is greater than that of the objective, the oblique rays cross and all such rays miss entering the objective. In this case the field of view is dark. Because only the rays diffracted or refracted from the specimen reach the objective, this technique gives a high contrast image of the structures in the sample that diffract or refract the light.
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