annular illuminationmedical dictionary

<microscopy> The result of placing a stop in the first focal plane of the condenser to produce an illuminating cone of light with all the light flux near the surface of the cone. The central part of the cone will be dark. This arrangement is often used for a darkfield effect in low-power work by transmitted light. The condenser must be correctly focused and centreed. By reflected light, darkfield illumination is attained with an annular con-denser fitted around the objective for opaque objects. Annular illumination implies that the object is lighted from all sides.

(05 Aug 1998)