Magnification and Low Vision
NEW Amazing New Movie on Macular Degeneration
The lower power magnifier will have a larger diameter or field of view but but will create a smaller image.
|A higher power magnifier will have a smaller field of view but the image will be larger. Note, the larger phone numbers by the magnifier above|
The Basics of
Magnification and Low Vision
Magnification is a major tool of low vision rehabilitation. But how do we obtain that magnification? Moving closer to an object provides magnification. The aged related macular degeneration patient might move closer to the television. The low vision child may hold reading material only a few inches away. Magnifiers, strong bifocals, telescopic systems, microscopic systems, closed circuit television systems, large print, and computer screen magnification programs may all provide effective magnification for low vision patients.
Magnifiers are one of first low vision devices selected by patients, but unfortunately without professional guidance, patients frequently obtain magnifiers that are too low in power, too heavy and often without needed internal illumination. Small pocket magnifiers might be used to read a price tag or a menu, while larger illuminated stand magnifiers may be used at home or work. Magnifiers with brighter halogen and xenon illumination often improve reading, particularly in age related macular degeneration patients.
Strong bifocals, dedicated reading glasses and microscopic eyewear may improve reading. Patient acceptance, however, may depend on their willingness to hold reading materials close. The Designs for Vision Type R‚ microscopic bifocal provides outstanding optics, but may require a working distance of only a few inches. Near telescopes, like those worn by surgeons and dentists, are used to read at greater distances and are mounted within or attach to eyewear.
To magnify distance objects, telescopic systems are frequently used. These may be handheld or mounted in eyewear called bioptics. Thousands of mildly visually impaired patients in the United States use bioptics to drive, watch television or do other distance tasks. Beecher Researchís telescopic system, the Beecher Mirage‚, is a lightweight binocular system that is worn like eyewear for distance viewing. We have found it to be an exceptional aid to view television even in patients with visual acuities below 20/600.
Telescopes may be attached to prescription eyewear to create what is called a bioptic system. Bioptic eyewear design has evolved from large telescopic tubes to smaller, more cosmetic telescopes. Modern bioptic design includes many, small lightweight systems that have improved patient acceptance such as the Vision Enhancement System‚ introduced by Ocutech ten years ago.[i]
Recently, Ocutech has introduced an auto focus version, the VES-Autofocus‚. It is a monocular 4X bioptic telescope mounted above the eyeglass frame. Powered by a rechargeable battery, it contains two computer chips and an infrared focusing system similar to that found in an auto focus camera. VES-Autofocus can focus automatically from 12 inches to infinity in a fraction of a second, and has dramatically changed the purposes for which we prescribe bioptics. Rather than being used for one or two specific tasks, these systems are being worn fulltime and more importantly, the patients using this system regain a sense of personal space and an ability to better interact with people. They can again see facial expressions and notice body language, subtle yet important aspects of everyday communication.[ii]
Magnification from telescopes, magnifiers and microscopic lenses is limited by their design, and can rarely be used successfully above magnification of 20X. Closed circuit television systems are capable of higher levels of magnification and can manipulate the brightness and contrast of the image. Video magnification has been reported in the literature for over four decades. [iii] [iv]
The concept of a CCTV is simple. A video camera is used in real time to capture the image of the reading material and display it on a monitor. For example, the poor contrast of newspaper print can be enlarged 40X, but can simultaneously be converted to white letters on a black background. The CCTV evaluation should include testing with real world items including newspapers, telephone books, utility bills, an addressed envelope, a handwritten letter, a greeting card, magazines, a checkbook, photos, a can of food, a medicine bottle, the patientís fingernails, a tax form, an address book, a recipe, and jewelry. Filling an insulin syringe or performing glucometry should be demonstrated for the insulin dependent diabetic. Preferences for image size, type of contrast, color and the ability to operate the system should be determined. Motion problems from moving the image on the screen may be reduced with larger monitors that allow displaying more text at one time.
New innovations in CCTV technology are expanding their use and availability. The Jordy‚ system, introduced by Enhanced Vision Systems is a wearable full color CCTV system that includes a camera and two LCD screens mounted in a headset. It can operate on a battery or direct current. This system provides up to 27X magnification through the headset providing dramatic improvements in distance vision for many profoundly impaired patients. It can also be placed in a stand with a X-Y table and connected to a television monitor for functioning like a traditional CCTV. The Jordy‚ has the potential to aid profoundly impaired patients. When worn on the head, the 27X maximum magnification far exceeds that of the typical bioptic or monocular telescope. The Jordy‚ may also be attached to a larger television monitor such as a 32-inch portable to provide much higher levels of magnification than most console CCTVs.
Other innovations include the Flipperport by Enhanced Vision Systems and the ZACC Plus‚ by OVAC, which include tilting cameras that allow viewing across the room. The Personal Vision Assistant‚ (PVA) employs a detachable mirror, which magnifies the face for shaving or applying makeup. Also, the cost barrier for CCTV technology has been broken through the introduction of small inexpensive handheld CCTVs that attach to any television.
[i] Greene HA, Pekar J, Brilliant R, Freeman PB, Lewis HT, Siwoff R, Paton C, Madden DJ, Westlund R. The Ocutech Vision Enhancing System (VES): utilization and preference study. J Am Optom Assoc 1991 Jan;62(1):19-26.
[ii]Greene HA, Beadles R, Pekar J. Challenges in applying t\Autofocus technology to low vision telescopes. Optom Vis Sci 1992 Jan;69(1):25-31.
[iii] Potts AM, Volk D, West SW: A television reader as a subnormal reading aid. Am J Ophthalmol 47:580-581, 1959.
[iv] Genensky SM, Baran P, Moshin HL, et al. A closed circuit TV system for the visually handicapped, American Foundation for the Blind Research Bulletin 19:191, 1969.