There are many other characteristics that are important to professionals in the field, but light source is a hot topic for many. Most modern slit lamps offer a halogen or LED source option for illumination. LED bulbs last much longer and many examiners feel these offer a “cooler” light to make viewing the retina easier. LED does not emit heat like halogens can, making them more comfortable for the patient.

Most slit lamps offer cobalt blue, red free, neutral density, heat absorbing, grey, yellow filter and/or diffuser. Whether you are assessing the tear film quality and break up (cobalt blue), or differentiating pigment lesions from blood vessels in the retina (red free), be sure to review the filter capabilities of the slit lamp you are purchasing to find the one that best fits the needs of your practice. Refer to the comparison chart below for more information.

This is the adjustment feature of the slit (light). Similar to the magnification power, the slit length/width should be adjustable to use different methods of observation. Van Herick's technique requires a 60° slit and 6x-10x magnification into the limbus to measure the angle of the anterior chamber. A 45°-60° angle and 16x-20x magnification is needed to perform either the conical beam or corneal cross-section techniques. Depending on the exams your practice is performing, you will want to understand the width/length features of your slit lamp.

The higher the slit lamp magnification power, the better! At least 20x is required on most slit lamps; however, obtaining a slit lamp with 40x magnification is best for performing a more thorough examination. It is ideal to begin the exam with a larger field of view and gradually increase the magnification power for areas of particular interest.

Slit lamps come in one of two illumination types-the tower illumination type (a.k.a “Haag Streit”) and the integrated illumination type (a.k.a “Zeiss”). What's the difference? The Haag Streit style provides the light source from the top of the instrument, while the Zeiss style supplies the light source from the base. It is ultimately a practitioner's preference as to which illumination type is needed. There are a number of illumination techniques that are used during a typical examination; i.e. retro, diffuse/direct, focal broad beam, focal slit illumination, specular reflection, and sclerotic scatter—all allowing the examiner to view the various structures and tissue of the eye correctly.