The slit-lamp biomicroscope has 2 rotating arms—1 for the slit illuminator and the other for the biomicroscope—mounted on a common axis. The illumination unit is essentially a projector with a light beam that is adjustable in width, height, direction, intensity, and color. The biomicroscope is a binocular Galilean telescope with multiple magnifications. A headrest immobilizes the patient, and a joystick lever and adjustable eyepieces allow the examiner to focus the stereoscopic image.
The illumination and microscope arms are parfocal, arranged so that both focus on the same spot, with the slit beam centered in the field of view. This setup provides direct illumination, and purposeful shifting of alignment allows for indirect illumination. Variations of these illumination techniques, using both dark-field and bright-field contrast, are used to examine the anterior segment of the eye.
, WaringGOIII, eds. 2nd ed. Philadelphia: Saunders; 1998:34–81.