Reading again is one of the most frequently reported goals of patients with chronic vision impairment or “low vision.” Many low vision reading devices are available on-line, through vision rehabilitation programs or low vision retail outlets. This article will review basic reading devices that are prescribed by doctors of optometry.
Low vision reading devices can be prescribed for monocular or binocular use. When in doubt as to whether binocular reading is best, it is easiest to demonstrate the device binocularly then cover each eye independently to determine which provides the optimal visual experience for the patient. For ease of understanding, I will list the devices as either (B) binocular (M) monocular (O) optionally can be used either way. The monocular devices are monocular by design, which by its nature eliminates concern over convergence demand.
Lenses are typically ordered in dioptric power and telescopes/microscopes in magnification (X). Magnification (M) for a reading lens is determined by the formula M=P/4, where P is the dioptric power of the lens/add. For example, a +8.00 add has a 2X magnification. Microscopes are measured in magnification rather than dioptric power. The same formula applies so a 3X microscope would be 12 diopters/4. The European labeling system is sometimes different which is why many low vision doctors of optometry use dioptric power instead of magnification to avoid confusion
As the lens power increases in a binocular system, the near focal point gets closer and the amount of convergence required for comfortable vision increases. This is mitigated by prescribing base in prism that is split between the two eyes. The formula for the prism is 2+ the dioptric power of the add. A + 8 reading add would require a net amount of prism of (2+8) =10 prism diopters (5 base-in prism prescribed for each eye). The advantage to a prismatic lens is affordability, binocularity and the ability to place the patients proper lens correction in the prescription. Prismatic glasses are available as a 1/2 eye as well as a full diameter frame.
Microscope lenses (O)
A microscope has a compound lens. When two lenses are put together aberrations are minimized. The doctor of optometry can prescribe higher powered lenses with significantly less distortion. There is a practical limit to the amount of prism that can be used in a microscopic so this limits magnification in a binocular system to about 3X.
When prescribed monocularly there is no physical limit to the carrier unit of the compound lens. Much higher magnification can be obtained such as 4-6X when prescriptions are monocular. At high magnifications, there is a very short working distance making illumination and maintaining the focal distance challenging for many patients. Microscopic lenses have excellent quality optics. They are affordable and very portable. Like prismatic prescriptions, they require accessory lighting and the patient must be instructed to read at a close working distance.
Reading Telescopes (B)
Reading telescopes are task-specific. They can be ordered for focal distances from 10” to 30”. Reading telescopes are angled inward and typically have a fixed focal point for the working distance. In order to have the largest reading area available, telescopic magnification is usually 1.7X or less. Additional magnification can be obtained with a closer working distance (less than16”) using a fixed focal distance or a reading cap mounted on a distance telescope.
Monocular Telescopes (M)
A monocular telescope often works best when the visual acuity is significantly better in one eye. The telescopic component does not need to be angled for reading and a blackout cap or opaque balance lens is used in the contra-lateral eye to avoid interference with the better-seeing eye if the telescope is binocular. Caps of different powers including split caps can be ordered in order to adjust the focal points for several distances. Sometimes even with a monocular telescope the patient will have a more comfortable viewing experience with the contra-lateral eye blacked out.
High Add Bifocal (O)
Adds greater than +3.00 diopters over the distance correction can also be utilized for reading. They are typically used for reading price tags, menu items, cell phones and money as well as continuous text. High add bifocals are usually fit higher than standard bifocals to reduce vertical prism. Patients should also be cautioned about the risk of falls when walking.
Wearable Low Vision Technology (WT)
Doctors of optometry also dispense wearable technology (WT) to make it easier for those with progressive eye diseases to see and have a higher quality of daily life. Medical devices, such as IrisVision, NuEyes, and eSight, are worn as glasses or worn over glasses for individuals with macular degeneration.i Many of these devices have advanced optical character recognition that can assist the patient by scanning text and reading it aloud.
Lastly, if you don’t prescribe low vision devices, the AOA website or goggle searches can be used to identify doctors of optometry who provide low vision rehabilitation devices and services. Low vision doctors of optometry will report back to the patients primary doctor of optometry so that team based care can be sustained for the patient.
Gregory Evans O.D.
member AOA VR committee