The difference in physical properties such as velocity of wave propagation, elasticity, refractive index, absorbance, etc., for the same material measured along different directions annular illumination 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 centered. 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.
Used to restrict electron beams and filter out unwanted scattered electrons before image formation. In optics an aperture is an opening that restricts the size of the light beam that enters or leaves a lens or lens system and is often controlled by an iris aperture diaphram. Its correct placement can be a very critical matter. Aperture size is measured variously according to the field of use of the lens, often by its simple diameter as is that of the telescope. The free aperture of a lens is that which is unrestricted except by its own diameter.