focus on low vision

The Low Vision Microscope

Alex Yoho, ABOM

For those not familiar with low vision, the term microscope brings to mind a scientist inspecting a cellular sample on a glass slide. In the low-vision world, however, a microscope is just a high-power plus lens. In general, any lens over 12D is called a microscope.

The microscope comes in several different forms. It can be as simple as a single-vision lens, but regular spectacle lenses in powers stronger than +12.00 are very thick and heavy. Also inherent in standard spectacle lenses are distortions as the eye gazes close to the periphery of the lens.

ASPHERIC OPTIONS

Instead, an aspheric lenticular could be a better option. Though the usable portion of the lens is smaller, the image quality will be much better. Finished, uncut aspheric lenticular lenses are available for those who do their own edging.

Another aspheric option is the unilens. This is a 22mm diameter stick-on lens that can be added to an existing spectacle lens and positioned by the practitioner. They come with an adhesive ring and can be held with a suction cup for positioning.

Lastly, there are multi-element lenses made of glass and usually around the size of a quarter. As with any multi-element magnifier, the optics are very good. The lenses reduce chromatic distortion and have a nice flat field. They are either mounted through a carrier lens, having a lenticular appearance, or they may occupy the entire aperture of a spectacle. The spectacles must be round in this case. Some can even incorporate the patient's prescription.

BENEFITS FOR READING

Microscopes are a valuable option for reading. Students find them useful and they are easily used by younger patients. Because of their very short focal length, they will only work with the reading material close to the nose.

To start a patient out with a microscope, have them touch the reading material to their nose and slowly move it away until the reading material focuses.

The main reason for failure is the inability of the patient to maintain the proper working distance. It may be necessary to use a reading stand in conjunction with the microscope. A reading stand that will move the material from side to side is best as moving the head tends to make tracking harder. Another challenge with such a short focal distance can be getting enough light on the subject matter because the head blocks most of the ambient light.

Incandescent lights are probably out of the question because of heat, but LED illumination is a good option as they have gotten much brighter recently and provide a cool source of light. Several colors are available as well, which can improve contrast.

Microscopes can greatly assist low vision patients with reading-related activities. Image courtesy of Eschenbach

MONOCULAR OPTIONS

Binocularity is not possible in these higher magnifications, so the eye with the best acuity is used monocularly.

It is not necessary to occlude the unused eye because everything will be blurred as the material being viewed is held at the focal length of the microscope. However, there are some that find light entering the unused eye bothersome, so occlusion is not totally out of the question.

Some individuals with two usable eyes may wish to have a microscope in each eye so they can alternate from time to time to avoid tiring too quickly. They must understand that they will not achieve binocularity and may require a movable occluder to avoid distractions from the unused eye.

If binocularity is desired, one may opt for a prismatic half-eye spectacle. However, these are not available beyond a +14D, whereas the monocular microscope can be +40D and sometimes higher.

Obviously, the short focal length makes anything but reading impossible. To get more working distance, a telemicroscope is an option that can let fingers into the focal plane and allow the patient to manipulate items. Depth of focus is critical in higher powers, so a reading stand or other device that can hold the work being viewed is essential.

Microscopes can be a great addition to your arsenal of low vision aids and should not be neglected. For severely impaired vision, it may be the only option that works.

Most of the traditional sources for low vision aids will have microscopes as an option. Keep in mind that there are several different forms available on the market that may serve different needs. EB