Telescopic lenses for low vision patients can be very useful for those who need different levels of magnification for different types of tasks without having to use multiple pairs of glasses.
Low vision can be very frustrating for patients as it impacts their ability to perform a wide range of tasks in their daily lives. Fortunately, there are tools to help regain that ability, such as telescopes.
Telescope glasses are another way to help low vision patients partake in the activities they want to do more easily, by enabling them to use the same device for different distances.
The magnification from standard corrective lenses requires things to be viewed at a specific focal length, which can be ill-suited to certain tasks which must be done at a specific distance, whether near or far, such as reading, looking at screens, cooking, or viewing traffic signals. Telescopic aids provide the wearer with a way to quickly adjust the level of magnification needed according to the distance.
Telescopic glasses for low vision can be either focusable or fixed-focus. Focusable glasses can be adjusted on their own, while fixed-focus glasses use reading caps meant for specific distances.
There are two main types of telescopes for low vision patients: Galilean and Keplerian Telescopes
Galilean telescopes are made up of 2 lenses. The objective lens, which is a convex (plus) lens, which is close to the object you are viewing, and the ocular lens, which is a minus lens and closer to the eye.
The amount of distance between the two lenses is determined by the differences in their focal lengths so as to produce a real image. When compared to Keplerian telescopes, Galilean telescopes are lighter, shorter, and cheaper, which also makes them a good first-choice for children
For patients with peripheral field loss, these are also ideal, though in those cases the lens order is reversed to provide a wider visual field.
Keplerian telescopes, which are also known as prismatic or astronomical telescopes, are optical systems which use two convex (plus) lenses. In these telescopes, the objective lens is of a smaller diopter power than the ocular lens, and the distance between the two is the sum of their focal lenses. While this produces a real image, it ends up inverted, requiring a prism to reverse the image. This makes Keplerian telescopes longer, heavier, and more expensive, but they provide a larger visual field and better optical quality than Galilean telescopes.
Beyond the two basic types of telescopes, they can be configured in a wide range of ways for different patient needs.
Some of these configurations include:
Reading Telescopes: Reading telescopes are specifically designed to allow the wearer to see a magnified view of an entire column, allowing them to see more words at a more comfortable reading distance.
Full Diameter Telescopes: These telescopes are designed for distance tasks performed while the wearer is stationary, such as watching TV. Due to their level of magnification, attempting to walk while wearing them would be like walking while staring through binoculars, which is not advised.
Bioptic Telescopes: These occupy more of a middle ground, and provide assistance with distance vision while allowing movement, thanks to the higher placement of the telescopes.
Spiral Galilean Telescopes: This design provides focusing ability to galilean telescopes, allowing the wearer to have greater flexibility of use without requiring reading caps.
Spiral Expanded Field Telescopes: These keplerian telescopes maximum flexibility, allowing greater focusing and visual field options.
Additional Telescope Variants
There are different telescope types in regard to how they are utilized, such as hand-held, spectacle mounted, and clip-ons.
Hand-held telescopes are simpler, lighter and cheaper, and are best suited for short activities or for children. One disadvantage of handheld telescopes is that they require prolonged holding of the object, which can be physically exhausting for some people.
Spectacle mounted telescopes provide the advantage of being hands-free, and are better for tasks that require more attention to detail. However they generally are higher cost than handheld.
Clip-ons share the advantages of both previous types, especially in regards to weight, but they can reduce visual field somewhat and scratch the lenses if you aren’t careful.
Another distinction between types of telescopes is between monocular and binocular types. Monocular telescopes are ideal when there is a significant difference in visual acuity between the two eyes. Less expensive, lighter, and more discreet, monocular telescopes can be used with the dominant eye.
Where there is a similar visual acuity in both eyes, binocular telescopes are preferred for increasing the visual field. Though these do weigh and cost more.