A slit lamp microscope is used by ophthalmologists to directly examine the eyes of a patient below the magnification of a binocular microscope by creating a stereoscopic, erect image. The slit lamp microscope produces a narrow beam of strong light that can illuminate the patient’s cornea, aqueous humor, crystalline lens, anterior vitreous layer, or other see-through ocular tissue. A form of oblique microscopy, the slit lamp microscope uses an imaging method similar to optical sectioning. Identical paired 10x and 15x eyepieces fit the binocular tubes of the Microscope. Thus, the ophthalmologist controls the magnification during an eye examination. Objectives for the slit lamp microscope are relatively low-powered 1.0x and 1.6x lenses, which merge with the eyepieces for total magnifications ranging from 10x to 24x. The numerical aperture of 0.04 for the lower power objective and 0.07 for the 1.6x lens are particularly very low to ensure maximum illumination and infiltration into the eye tissues, with a more concentrated beam of light. A relatively long working distance of 101.5 millimeters is provided for the patient’s comfort and to make easy accurate diagnoses, even for the deeper eye layers.
A 6-volt, 30-watt tungsten-halogen bulb provides the light for the slit lamp illumination system that features a brightness adjustment dial. The slit adjusts from 0.0 to 9 millimeters in width and ranges from 0.2 to 9 millimeters in length, depending on the setting of the five-position rise. Utmost rotation of the slit from the first vertical position is 90 degrees in either direction via the slit rotating lever, which is mounted above the lamp housing. Green and cobalt blue filters are provided as accessories for the illumination and can be bring into the optical path by moving the filter change-over lever. The gelatin green filter blocks red light, which aids observation of the blood vessels in the sclera and conjunctiva membranes.
Serving of the blue filter, when coupled with an applanation-tonometer, facilitates the measurement of intraocular pressure and the finding of glaucoma. To position the lamp, the focusing rod is erected and it is adjusted until the black surface faces the operator. For the fine motion of the slit image, a lever that resembles a video game joystick is tilted and for common motions, the lever is moved in the desired direction. The lamp housing is adjusted vertically and is centered until the image of the slit is projected onto the black surface and there is even illumination crosswise in the field of view.
The slit lamp microscope is mounted on a flat metal base plate for steadiness. The entire visual diagnostic apparatus is raised or lowered by two foot pedal switches mounted on a grouping power control unit and telescoping instrument base. The table adds stability and helps control unwanted vibrations during eye exams. For finer vertical adjustments to match the patient’s height and eye position, the level adjustments for the instrument body and the jaw rest are moved up or down, until the slit image is projected on the patient’s eye correctly. Once the patient’s head is set in opposition to the forehead and jaw rest, the width of the slit is adjusted by turning the slit-width change knob that is mounted on top of the joystick lever, and which is located nearby the ophthalmologist on the microscope base. A fascination lamp enables the patient to focus the eye in a standard direction and distance, while a reference line on the post allows the doctor to examine the patient in a usual or standardized position. The lens, when placed in front of the patient’s eye, allows the doctor to focus from the rear inner part region of the eye.