Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor
Low–power clinical biomicroscopy of the eye is an essential tool to diagnose ocular disease, and Ophthalmologists routinely use a slit lamp to examine almost all of their patients. At its root, the adjustable slit is simply a way to allow variable angles of illumination. However, this surprisingly simple concept has enabled the slit lamp microscope to become an indispensable instrument for every Ophthalmologist. Slit lamps have evolved significantly since their advent in the early 1900s, and many of today’s slit lamp biomicroscopes are sophisticated instruments building on the discoveries of the past century.
One of the first individuals to apply microscopy to the living eye was Purkinje, who in the 1820s studied the iris with an adjustable microscope by illuminating the field of view. The uniocular slit lamp was born years later when Louis de Wecker would combine an eyepiece, objective and adjustable condensing lens within a tube. Wecker’s invention was improved upon by Siegfried Czapski, who added binocularity to the microscope. However, none of the units constructed by these individuals had sufficient and adjustable illumination to be of much clinical benefit. Allvar Gullstrand, an Ophthalmologist and 1911 Nobel laureate, developed a first true slit lamp to illuminate the eye. Then, Henker and Vogt improved upon Gullstrand’s device in the 1910s by creating an adjustable slit lamp and combining Czapski’s microscope with Gullstrand’s slit lamp illumination. The modern slit lamp biomicroscope was born, a powerful diagnostic tool capable of stereoscopically examining optical sections of the anterior segment in great detail. The instrument was not only important as an essential diagnostic tool in the clinic, but also served to greatly advance the scientific knowledge fueling Ophthalmology. Alfred Vogt was instrumental in driving this knowledge base through innovative and meticulous use of the slit lamp. Vogt would use the slit lamp biomicroscope to study a vast array of diseases and document his findings in the highly influential publication “Lehrbuch und Atlas der Spaltlampenmikroskopie des Leibenden Auges” in the 1930s. The slit lamp microscope was thus accepted as an essential diagnostic tool by the Ophthalmology community, and over time Eye MDs invented new ways to reach beyond the cornea and anterior segment. For example, the slit lamp, in conjunction with certain contact lenses, could be used together to examine the anterior chamber angle in great detail.
Today, the two main components of the modern slit lamp microscope are the illumination system and observation system. The illumination system on most slit lamps consists of two different designs. The first design, the Haag-Streit type illumination, allows de-coupling in the vertical meridian. Such vertical de-coupling is particularly useful when performing gonioscopy to minimize reflections and for indirect funduscopy to gain increased peripheral views. The second design, the Zeiss type illumination system, does not allow de-coupling in the vertical meridian. Many ophthalmologists argue that the Zeiss illumination’s advantage is that it confers lightness and compactness onto the slit lamp to make it somewhat easier to use. In either case, the illumination systems of today are both capable of producing a homogenous, aberration-free beam of white light. Most slit lamps use halogen bulbs to yield shorter wavelengths of light, which allow better visualization of smaller structures compared with longer wavelengths of light (i.e. tungsten bulbs).
The second main component of slit lamps is the observation system. Modern slit lamp microscopes can magnify images between 5X and 25X, with some microscopes allowing magnification to 40X and even 100X. Magnification is generally achieved by three methods, namely a flip-type, a Galilean rotating barrel, and a continuous zoom system. However, magnification of the slit lamp is less important than its resolution. A slit lamp’s resolution is dependent on the wavelength of light used, the refractive index between the eye and objective, the working distance, and the diameter of the objective lens. In practicality, the first three of these factors are not easily modifiable, but the objective lens diameter can be modified to increase resolution. However, a very large diameter lens will also introduce optical aberrations. The observation system is also influenced by the proximity of the patient’s eye to the examiner’s eyes. This necessitates a convergence system for binocular viewing, and most modern slit lamp biomicroscopes are designed with 10 to 15 degrees of convergence to minimize eye strain for the examiner. These features in the illumination and observation systems must be considered when deciding on the best slit lamp to match a particular clinic.