New Spectacle Telescopic Lens Design and Fitting Protocol
By: Michael R. Politzer, O.D., F.A.A.O., F.C.O.V.D.
Many innovations in the field of low vision rehabilitation have provided patients with optical aids that meet their need for independence. These low vision aids fall into various categories that meet activities of daily living requirements such as mobility, reading, interacting with other people or self care skills.
Low vision patients are expressing their need for more independence. They are healthier and living longer. They are better educated and more savvy and want to be able to drive, access the Internet, recognize objects and people's faces in various social settings, read, play cards and pay their bills. In essence, they want to maintain an independent lifestyle and they want what they want and they want it now. However, what they want is currently not available.
Patients are confused, worried and frustrated and often ask "Doctor; can you just make me a pair of glasses that I can see through". They realize that their eyes are changing, that they are possibly getting worse and have a desire to "go back to the way it used to be". The reality is that just a "pair of glasses" will not meet all of their needs.
Patients with central vision pathology have difficulty adapting to and using currently available spectacle telescopic designs. It is more difficult for them to find the exit pupil and keep it aligned while using the telescope or telemicroscope.
Telescopes and telemicroscopes that are currently available characteristically have small exit pupils because they are designed for the static eye. With a large objective lens and small ocular lens the field of view is reduced. Patients report that static eye systems, which use a frosted housing unit, tend to increase internal light scatter and create glare. The systems that use a black housing unit reduce internal light scatter with glare and enhance image quality. However, this type of housing causes objects being viewed to appear darker and are cosmetically objectionable.
The size of the exit pupil traditionally has been limited by the size of the objective lens and the magnification of the telescope. The diameter of the exit pupil is determined by dividing the diameter of the objective lens by the magnification of the telescope.
Eg. A 2.0X telescope with a 40mm objective lens would then have an eye piece lens and exit pupil diameter of 20mm.
The field of view of a telescope is limited by the diameter of the objective lens at a given magnification. The exit pupil diameter is inversely proportional to the magnification and the image brightness of the telescope is determined by the size of the exit pupil relative to the size of eye pupil. Fixed eye designs show that enlarging the exit pupil beyond the size of the eyes entrance pupil will have no effect on increasing the telescopes field of view or its image brightness.
However, according to Marvin Hutt, Ph.D., Senior Adjunct Associate Professor of Optical Engineering at the Stevens Institute of Technology, New Jersey
"Design of low powered telescopes to maximize the ease of use regarding less long term strain on the eye requires that the eye swim in an enlarged device exit pupil".
"While the classical optical design calls for a static eye pupil with an iris diameter set by ambient lighting, the philosophical impact of the device exit pupil being close to the iris diameter of the eye is a tunnel appearance of the field. This leads to long term eyestrain and a physiological reduction in the apparent view through the system".
"In wearing these devices the patient will almost invariably prefer the system where the eye swims in an enlarged exit pupil even though from a non-moving eye model, the over sized device exit pupil is not necessary."
The Politzer Telescopic Lens Series (PTS) has created a new set of spectacle telescopes that are easier and more effective for the patient to use.
Despite traditional optical design concepts, enlarging the exit pupil does impact ease of use, field size and image brightness. Objective comparative measurement of the visual field size between the Designs for Vision standard 2.2X Bioptic I and the PTS 2.2X Bioptic I indicate a twelve percent increase in the horizontal visual field of the PTS over the standard model.
Additionally, with a larger exit pupil the fitting is easy and very little training is required to teach the patient how to view through the telescopic system
The clinical process of prescribing and fitting the PTS is relatively easy. As with any low vision aid examination, begin by determining the best possible spectacle correction. Trial frame the various PTS systems with and without the spectacle correction to determine which combination subjectively gives the patient the best distant vision. Then demonstrate the system chosen by having the patient look out a window or go outside while wearing a demonstrator system of the same design and magnification power. Repeat this process utilizing reading caps to determine the best vision for intermediate and near viewing distances.
Potential uses of the PTS include:
- Watching TV
- Seeing Faces
- Sitting on the porch and watching traffic
- Watching outdoor activities
- Using the computer
- Playing cards
- Finding objects on the shelf
- Setting the dials on the stove, washer or thermostat
- Seeing the controls on the microwave
- Seeing the preacher at church
- Seeing grandchildren play
Patients appreciate being able to experience the effectiveness of a "real" system. This process helps them to understand what their system will look like and how it will work in the real world.
Metal frames with a frame pupillary distance (PD) equal or close to the patients PD are recommended. Adjustable pads allow for the lens system to be properly aligned vertically and horizontally and placed close to the patient's eyes (8-10mm vertex distance) for enhanced ease of use. And, the PTS design series has now been expanded to include 2.2, 3.3 and 4.0X bioptic micro-spiral models with plus lens, prism and split field caps available for all full field and bioptic model one designs.
The PTS design series is fast becoming an integral part of low vision practice. Patients are using the PTS and enjoying it. They continue to find new and innovative uses for this aid and do not object to the issues of size, weight or appearance. The major issue of cost has been resolved with the use of flexible funding sources that provide interest-free financing to qualified patients.