As with many ophthalmic instruments, it is instructive to consider the illumination and viewing systems of the slit-lamp biomicroscope separately. The illumination system is a light source that is restricted by an adjustable aperture producing a slit of variable height, width, and orientation, to produce an optical section through the eye. The viewing system is a binocular stereomicroscope with individually focusable eyepieces. To vary the magnification of the viewing system, there may be a Galilean magnification changer, a rotating drum of Galilean telescopes that can be oriented either forward to provide higher magnification or backward to provide lower magnification. Alternatively, there may be 2 sets of eyepieces or objective lenses, or a zoom system in which lenses are moved back and forth to change the magnification.
The illumination system and the viewing system with its various levels of magnification are mounted on separate arms. In ordinary usage, these rotate about the same vertical axis in a parfocal arrangement, so that they both focus precisely over a common pivot point. This arrangement allows the examiner to study the eye in direct illumination. Purposefully separating the illumination and viewing arms from their coupled alignment to horizontally and/or vertically decenter the beam allows for indirect illumination. Variations of these illumination techniques allow for examination of the anterior segment in a variety of ways, some of which are described next and illustrated in Figure 8-11 (see also BCSC Section 8, External Disease and Cornea).
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.