The slit lamp is an instrument consisting of a high-intensity light source that can be focused to shine a thin sheet of light into the eye. It is used in conjunction with a biomicroscope. The lamp facilitates an examination of the anterior segment and posterior segment of the human eye, which includes the eyelid, sclera, conjunctiva, iris, natural crystalline lens, and cornea. The binocular slit-lamp examination provides a stereoscopic magnified view of the eye structures in detail, enabling anatomical diagnoses to be made for a variety of eye conditions. A second, hand-held lens is used to examine the retina.

Direct diffuse illumination
If media, especially that of the cornea, are opaque, optical section images are often impossible depending on severity. In these cases, direct diffuse illumination may be used to advantage. For this, the slit is opened very wide and a diffuse, attenuated survey illumination is produced by inserting a ground glass screen or diffuser in the illuminating path."Wide beam" illumination is the only type that has the light source set wide open. Its main purpose is to illuminate as much of the eye and its adnexa at once for general observation.

Focal illumination
Observation with an optical section or direct focal illumination is the most frequently applied method of examination with the slit lamp. With this method, the axes of illuminating and viewing path intersect in the area of the anterior eye media to be examined, for example, the individual corneal layers