The slit lamp, often called a biomicroscope, is a specialized instrument that provides a high-magnification, three-dimensional view of the eye’s structures. It combines a binocular stereomicroscope with a bright, adjustable light source, allowing for a detailed examination of the eye from the front surface to the back. Its primary purpose is to examine the anterior segment, which includes the eyelids, conjunctiva, cornea, iris, and lens. By manipulating the light beam, the eye care professional can create an optical “slice” through these transparent tissues, revealing abnormalities and assessing ocular health.
Primary Components of the Slit Lamp
The slit lamp is composed of three interconnected systems: illumination, observation, and mechanical support. The illumination system generates and precisely controls the light entering the eye. It features a high-intensity light source (typically LED or halogen) coupled with an adjustable slit aperture that controls the width and height of the light beam, allowing it to be narrowed to a fine line. This system also includes various filters, such as a cobalt blue filter used with fluorescein dye, and a red-free filter that highlights blood vessels against a darker background.
The observation system is a binocular stereomicroscope, providing a magnified, three-dimensional view of the ocular structures. This stereoscopic vision provides the necessary depth perception to accurately locate abnormalities within the eye’s layers. Examiners can quickly change the magnification, typically ranging from 6x for a broad overview to 40x for viewing fine details.
The mechanical system provides the stability and movement necessary for a thorough examination. The patient’s head is stabilized by a chin rest and a forehead support, ensuring the eye remains steady during high-magnification viewing. The main control is the joystick, which allows the examiner to smoothly maneuver the entire apparatus in all three spatial dimensions (forward/back, left/right, and up/down) to focus and scan the eye.
Preparing the Patient and Instrument
A successful examination begins with the proper preparation of both the patient and the instrument. Before the patient is seated, the examiner should ensure the instrument’s surfaces, particularly the chin rest and forehead support, have been sanitized. The examiner must also adjust the oculars (eyepieces) to correct for their own refractive error and to match their interpupillary distance. Adjusting the oculars ensures the examiner sees a clear, single image, preventing eye strain.
The patient is then positioned with their chin firmly on the chin rest and their forehead pressed against the support strap. The height of the chin rest must be adjusted until the patient’s outer corner of the eye (lateral canthus) aligns precisely with the height marker on the side of the headrest. This alignment ensures the eye is centered in the microscope’s optical path for clear imaging. The examiner should also pull the slit lamp back before the patient sits down to avoid accidental contact.
Core Illumination and Magnification Techniques
The power of the slit lamp lies in the ability to manipulate the light beam to highlight different parts of the eye. Diffuse illumination is used first for a broad, low-magnification survey of outer eye structures, such as the eyelids and conjunctiva. This technique uses a wide, open beam of light directed at an oblique angle (typically 45 degrees) to provide an overall illuminated view.
The most fundamental technique is direct focal illumination, which involves narrowing the light beam into a thin, vertical slit. When focused onto the eye, this narrow beam creates an optical section—a transparent, three-dimensional slice through the cornea or lens. This optical section is invaluable for assessing the depth of any abnormality, such as a foreign body or a cataract. The examiner uses the joystick to move the slit beam across the tissue, scanning for structural changes.
Indirect illumination is employed when the examiner focuses the microscope on a structure adjacent to where the light beam is directly striking the eye. The structure is viewed using light scattered or reflected from nearby tissue. This technique is useful for observing subtle details or abnormalities that might be obscured by the brightness of the direct light beam.
Specular reflection is a specialized technique used to examine the surface integrity of the cornea and lens. It requires positioning the illumination beam and the viewing microscope at equal and opposite angles relative to the eye’s surface. When correctly aligned, a bright, reflective sheen appears on the surface, known as the zone of specular reflection. This reflection allows for high-magnification observation of the corneal endothelium, the innermost layer of the cornea, where the cells appear in a mosaic pattern.
Extended Use: Viewing the Posterior Segment
While the standard slit lamp exam focuses on the anterior structures, the posterior segment (including the retina and optic nerve) can also be viewed using auxiliary lenses. These are high-plus, non-contact lenses, commonly 78 diopters (+78D) or 90 diopters (+90D), held directly in front of the patient’s eye. The lens functionally neutralizes the focusing power of the cornea and lens, allowing the slit lamp’s microscope to image the back of the eye.
To perform this examination, the slit lamp’s illumination and viewing systems are aligned coaxially, pointing straight through the pupil. The auxiliary lens is introduced a short distance in front of the eye (typically 5 to 11 millimeters). The examiner then slowly pulls the entire slit lamp unit backward using the joystick. This movement brings the magnified, real, and inverted image of the retina into focus, allowing for stereoscopic assessment of the optic nerve and macula. The 78D lens is often preferred as it offers a good balance of magnification and a wide field of view.