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Spectacle-mounted Telescopic Lenses: A
Practitioner’s Guide
Maximizing success in
prescribing and fitting customized spectacle-mounted telescopic systems can be
enjoyable, challenging… and profitable.
By Bruce P. Rosenthal, OD, FAAO
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The clinician
should consider distance telescopic systems when the low vision
patient mentions that she or he enjoyed going to the theater, museums,
shows, playing bridge, or sporting events, but has given these
activities up because of vision problems.
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Variables
in Selecting a
Spectacle-mounted Telescope |
• Type –
Galilean or Keplerian.
• Amount of magnification needed.
• Size of patient pupil.
• Dimension of exit pupil of telescope.
• Ability to include a prescription.
• Available as a bioptic/full diameter/reading
telescope.
• Fixed or variable focus.
• Weight of system.
• Light-gathering quality.
• Binocularity.
• Cosmetics.
• Manual or autofocusing.
• Cost of system. |
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Factors to Consider in the Successful Fitting of Spectacle Telescopic
Systems |
• Identifying important and achievable patient objectives.
• Selecting suitable patients.
• Obtaining the relevant findings of the low vision evaluation.
• Understanding the variables and parameters of telescopic systems.
• Knowing the range of available telescopic systems.
• Making the proper selection and accurate fitting of the telescopic
system. |
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Successful fitting usually means bringing the eye as close to the
ocular as possible to provide the largest field of view.
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Most clinicians specializing in
low vision feel comfortable in prescribing near low vision devices, as do many
new practitioners to the field. But when it comes to distance or intermediate
low vision needs, they often rely on simple handheld telescopes or stock
off-the-shelf telescopes.
Developing one’s proficiency with a variety of low vision aids will certainly
enhance the practice’s profile and service levels to low vision patients.
The following are some ways to maximize success in prescribing and fitting
customized spectacle mounted telescopic systems. Some of these special devices
will not only help patients achieve special objectives, but will make your
practice more enjoyable and challenging … and increase income too.
Getting Started
The careful recording of a patient’s history is a vital part of obtaining
distance objectives and patient selection, and is the first step in successfully
prescribing spectacle-mounted telescopic systems. The clinician should consider
distance telescopic systems when the low vision patient mentions that she or he
enjoyed going to the theater, museums, shows, playing bridge, or sporting
events, but has given these activities up because of vision problems.
Contraindications include a history of severely decreased vision, poor contrast,
rapidly progressive vision loss, hemorrhaging, or multiple pathology.
Evaluating Low Vision
Visual acuity and contrast sensitivity are two important tests of visual
function that will guide the telescopic system prescription and the power of the
system. The ETDRS visual acuity charts, available from Lighthouse International
and Precision Vision, are the preferred choices because they are standardized
and offer high contrast. The Vision Contrast Test System (VCTS) from Vistech and
Stereo Optical or the Pelli-Robson (PR) charts from Clement-Clark are good ways
to test the contrast sensitivity function (CSF). A poor candidate would have a
response of less than 3 on the first three lines of the VCTS and 5 triplets or
less on the PR chart.
How to Pick a Telescope
There are many variables to consider when choosing a spectacle-mounted
telescopic system. Telescopic design is a key one. The Galilean telescope is
made up of a high minus ocular (eyepiece) and a high plus objective. It is
lightweight, compact, provides an erect image, has a large exit pupil, has good
light gathering qualities, but is generally limited to 3.0x magnification.
The Keplerian telescopic system is heavier than the Galilean telescope because
the eyepiece and objective are composed of two plus lenses. As a result, it is
necessary to use erecting prisms or mirrors in order to re-erect the image,
which consequently adds to the weight.
The size of the exit pupil of the telescope is also a factor. The Galilean
telescope has a relatively large exit pupil (the image of the objective as seen
through the ocular). Successful fitting usually means bringing the eye as close
to the ocular as possible to provide the largest field of view. Keplerian
telescopic systems are usually more difficult to fit because the exit pupil is
generally smaller than a Galilean and must coincide with the entrance pupil of
the eye.
Selecting a System
Once you’ve chosen the basic design, you’ll want to choose a device. The
Beecher, Ocutech, Designs for Vision, and Eschenbach Optik spectacle-mounted
telescopes systems should fit almost any patient who is a candidate.
Beecher telescopic devices are lightweight, Keplerian mirror-design systems.
They are available as distance monocular or binocular systems in powers of 4,
5.5, 7, 8x, and 10x magnification, and a near power of 5.5x. The 4, 7, and 10
have objectives that are 20, 30, and 35mm respectively, which enhances the light
gathering ability of the system.
Here are some tips to successfully fit the Beecher mirage telescopic spectacle:
• Use extended eyepieces to get the exit pupil closer to the eye.
• Utilize Rx stock eyepieces, which are available in powers of +/- 4, 6, 8, 10,
and 12 diopters.
• Employ the Lighthouse silicone adjustable comfort bridge that comes in 8mm,
11mm, and 14mm tabs.
• Hold a transilluminator light in front of the objective to make sure that the
exit pupil of the telescope is aligned with the entrance pupil of the eye.
Ocutech, Inc. offers a variety of telescopic systems, including manual,
autofocusing, and binocular arrangements. Vision Enhancing System® (VESII®) is a
monocular, across-the-bridge, Keplerian telescopic system that is available in a
4x and 6x manual system. Ocutech also offers the VES®-Autofocus, a 4x autofocus
that has a continuous and immediate focus from 12" to infinity.
The VES-K® is a new “K mounting style” system mounted on conventional metal
frames. The VES® Mini is another Keplerian telescopic system that can be mounted
in a conventional frame as a binocular system.
Since the VES® - Autofocus, VES®-K and VES®II are Keplerian telescopic systems,
it is important to make sure that the exit pupil of the telescope coincides with
the entrance pupil of the eye. Use a transilluminator in front of the telescope
to make sure that the beam of light falls in the center of the pupil.
The newly redesigned Designs for Vision “Robin fitting system” is designed to
ensure maximum success in fitting bioptic telescopic systems, both monocularly
and binocularly. Fitting frames that hold 2.2, 3, and 4x Galilean telescopes, as
well as 2x to 6x Keplerian telescopes, eliminate much of the guesswork in
successful fitting.
Here are some ways to successfully fit telescopic lenses with the Robin system:
• Select an appropriate size Yeoman special fitting frame, and move the fitting
slots so they center over the patient’s pupil. Check the pupillary reflex with a
trans-illuminator.
• Thread the Galilean telescope in the fitting slot and recheck the pupillary
alignment by holding the transilluminator in front of the telescope and looking
at the pupillary reflex. Make sure it is centered.
• Order the appropriate-size Yeoman frame and specify monocular PDs as well as
jumbo pads.
• Make sure that the patient understands that the final telescopic system will
look virtually the same as the fitting system.
Eschenbach Optik and Designs for
Vision have recently collaborated on an exciting new method of fitting the
Galilean telescopic system. This new fitting system is available in Galilean
design of 2, 3, and 4x powers. Eschenbach’s frame No. 1631 holds the telescopic
lenses to determine the appropriate PD. Fitting frames, in which the telescopic
lenses can be snapped into, provide a close exit pupil for maximum field of
view. The system, which can be ordered in the bioptic and field diameter
position, comes both monocularly and binocularly in a stylish metal frame.
Prescribing and fitting low vision telescopes for distance and intermediate can
be professionally challenging and fun. The next time you have a suitable
candidate, leave the handheld and off-the-shelf telescopes alone, and try one of
the professional devices above. You and your patient will be glad you did.
Bruce Rosenthal, OD, is the Chief of the Low Vision Programs at Lighthouse
International in New York City and Adjunct Professor at the Mt. Sinai Hospital.
He is a Low Vision Diplomate of the American Academy Optometry and on the Low
Vision Executive Council of the AOA.
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