to Consider in Choosing a Biomicroscope
HARUE MARSDEN, OD, FAAO
Using a biomicroscope
in contact lens practice is paramount to comprehensive care and lens fitting. The
slit lamp examination requires an instrument that can comfortably, conveniently
allow you to evaluate the health of patients' eyes and the fit of their contact
It All Starts with the Optics
The optical properties of the biomicroscope can affect your visual
comfort depending upon whether the oculars use parallel or convergent optics. Some
slit lamps claim variable interpupillary distances of 47mm to 85mm. However, with
parallel optics it's oftentimes more difficult to fuse the image, especially if
you have a narrow interpupillary distance or poor divergence ranges.
Variable magnification is beneficial to appropriately assess details
of the cornea. Some bio-
microscopes have two magnifications (10x and 16x) while
others offer three to six various magni-fication levels (6.3x, 10x, 16x, 25x, 30x,
40x). Additionally, you can replace the eye pieces with oculars of various magnification
or reticules for measurement.
The capability of your biomicroscope to convert to an ocular photography
system may also be a feature to consider when selecting a slit lamp. Many slit lamps have adapters that allow you to attach
a commercially available camera either to the beam splitter or to the ocular itself.
Positioning is Important
The ability to position the lamp housing in front of the oculars
without obstructing your view is also important in the optical system of the
To be able to adequately manipulate the illumination, slit width and filters improves
your efficiency. Each slit lamp features a slit that rotates to assist in measuring
toric lens rotation or corneal vascularization. Some are located just below the
light source while others are on the lamp housing itself. A protractor is generally
included to help assess rotation in degrees.
Also consider the ability to vertically and horizontally adjust
the lamp for a smooth, efficient exam. With
some biomicroscopes, you make all adjustments by moving the joystick front to back
and side to side, with vertical adjustments made by rotating the joystick clockwise
and counter clockwise. In other instruments, the vertical adjustment is on an outer
ring surrounding the joystick that you rotate to adjust the lamp up and down.
Nuances for the CL Exam
You should be able to easily control the illumination while conducting
the slit lamp examination. Some slit lamps have multiple specific illuminations
located on the console with the power switch, while others have a rheostat that
is conveniently located adjacent to the joystick.
Colored filters aid in examining the cornea and fitting lenses.
The red-free (green) filter can accentuate corneal pigmentation and vessels, which
can aid in measuring vascular encroachment. You can use the cobalt blue filter with
sodium fluorescein to detect corneal staining and also to assess the fitting relationship
of a GP lens. You can enhance this evaluation by placing a yellow filter over the
slit lamp objective lenses.
The Right Slit Lamp for You
Careful consideration of the optical properties
and physical manipulation of the biomicroscope can improve your efficiency in
conducting an examination.
Dr. Marsden is an associate
professor at the Southern California College of Optometry and is the chief of the
Cornea and Contact Lens Service.
Contact Lens Spectrum, Issue: August 2005