(Science: microscopy) This distance is measured from the upper focal plane of the objective to the image formed by the objective alone upon removal of the eyepiece. A light microscope so arranged that the specimen, usually a solid-liquid colloid or suspension, is illuminated by a strong pencil of light at right angles to the microscope axis. The visibility is limited by the intensity of the light source. A laser can be used as the source. It is used to detect the presence of light-ultramicroscopic particles within the range of about 0.005,um to 0.2 ~m. Not to be confused with an electron microscope.