Assessing Vision Enhancement Devices

Description: Familiarity with devices and techniques of vision enhancement will assist people with partial vision to determine the most comfortable and efficient methods to access visual tasks. For many people whose vision is not fully correctable with standard eyeglasses, devices and/or techniques cannot improve visual acuity. These devices and/or techniques, instead, enhance the image by:

1) making it appear larger; 2) improving the contrast; 3) optimizing illumination; or 4) minimizing glare.

I. Principles of Image Enlargement

Image Enlargement and Viewing Distance:

Think of these as one in the same. If a person gets 4 times closer to an object, it appears 4 times larger on the surface of the person’s retina. If a person uses a 4X telescope, the object appears 4 times larger (or 4 times closer). As the power of a hand-held or spectacle-mounted magnifying lens increases, the distance the lens is from the object decreases. (E.g., a 2X magnifier has a focus distance of 10 inches; a 5X magnifier has a focus distance of 2.5 inches.) When a person is viewing a PC monitor or a video magnification system (also called a CCTV), and the person leans forward to get his or her eyes closer to the monitor, the person is enlarging the image size of the viewed object on the surface of the retina.

Image enlargement vs. field decrease:

It’s exponential! When a person is 1’ from a task, the person sees ¼ of the area than if the person was 2’ from the task; 1/16 of the area than if the person was 4’ away, etc.

(OR)

When the person uses a 4X telescope, the person’s visual field is 1/16th the size than when the person views without a telescope at the same distance; with 6X, it is 1/36th the area.

As the power of a hand-held or spectacle-mounted magnifying lens increases, the diameter of the lens decreases. (E.g., a 5X magnifier has a maximum lens diameter of 2-1/4 inches.)

(OR)

When an image on a computer monitor is enlarged two times, only one quarter of the total screen will be visible at any one time.

Perception of Movement: When 4X image enlargement is used, perception of movement increases 4 times.

For example: If a person wears a head-mounted 8X telescope, if the person makes a ¼ head turn, the person perceives as much motion as two complete head turns.

Telescopic devices over 10X often need to be steadied on a tripod to be effective. Perception of movement increases as the distance between the person and the viewed object decreases.

People using video magnification systems may need to move the viewing table more slowly as magnification increases to avoid motion sickness. People may search a page at low magnification, so that they can move more quickly around the page, and then increase the size of the print to read when they thing they have found what they need to read.

II. Assessment

Visual Threshold vs. Preferred Letter Size – Use near and distance charts to determine image enlargement needs. Typically for as comfortable viewing as possible, the letter size must be at least twice as large as the smallest size print that can be read.

Assess the person doing a distance task without a device:

For example, how close does a student need to get to a whiteboard before words on the board can be read? If the person can see the whiteboard at a distance of 1’, the person should be able to see it at 4’ with a 4X telescope, 6’ with a 6X telescope, etc.

Assess the person doing the specific task(s) with potential devices. Consider:

Duration – Can the person use the device for the average duration of the task? Observe and ask the person whether signs of strain or fatigue are occurring during and at the end of a task: Inattention, irritability, rubbing of eyes, tearing eyes, head- or stomachache.

Speed – Does the task have a time limit? (For example, overheads being shown in a mainstream class.) Can the person perform the task well enough to keep up with any necessary time constraints?

Ergonomics - Is the person able to maintain good posture while using the device? Head upright (line of sight between 45° to 90°) atop a vertical, untwisted spine with feet resting flat on the floor, hands slightly lower than elbows

Personal Ability – Is the person able to both use and maintain the device? Assess initial ability and ability with training.

Portability – Is portability important? Is there potential for loss or damage of the device? Is this a significant consideration?

Motivation - Once the person has used the device, ask the person how he/she feels about using the device in the practical situations. Will significant others support the person’s use?

Safety Hazards/Durability – Does the device create any potential safety hazards? Wires to trip over? Heat generation? Will it stand the rigors of use?

III. Other Considerations

Contrast Enhancement – People with reduced visual acuities often benefit from contrast enhancement. Some devices can enhance contrast by making dark colors appear darker, while lighter colors appear lighter. The light on a hand-held lens magnifier is one example of a simple way to improve contrast. Most telescopes decrease contrast by making the viewed image appear darker. The wider the diameter of the objective lens (where light enters the telescope), the brighter the image.

Glare Reduction - Glare sensitive people may benefit from devices that reverse dark and light areas. When print is white, and background is black, a smaller total area is white, and this reduces glare. This can significantly increase both acuity and reading duration.

Compare image enlargement devices and techniques with tactual or auditory access modes to determine the most comfortable, efficient way to access a particular task. Some people use visual devices for short, spot tasks like reading a name, address, phone number, label or price tag. They use tactile or auditory modes of access for lengthier tasks.

IV. Explanation of Visual Acuity

When visual acuity is measured, it is usually expressed by noting the distance a person is from a target, and the size of the target. “20/20” visual acuity means that when a person is standing 20 feet from an eye chart, the smallest size letter that can be read is the “20-foot” letter, the size that most people are able to read after their eyes are corrected for any focusing problems. If a person has a 20/100 visual acuity, even when eyeglasses are correcting any focusing problems, this person can only read the “100-foot” letter 20 feet from the eye chart. The “100-foot” letter is five times larger than the “20-foot” letter. (A person with 20/20 vision can read the “100-foot” letter when standing 100 feet from the eye chart; that’s why it’s called the “100-foot” letter.)

Because size and viewing distance are related, to estimate the amount of image enlargement a person would need to achieve the equivalent of 20/20 vision, divide the letter size by the viewing distance. Example: 20/100 vision = 5X magnification; 5/200 vision (the viewing distance is 5 feet) = 40X magnification. To convert “X” into diopters (the standardized measure for the power of a lens), multiply “X” by 4, example: 5X = 20 diopters.

This simple calculation is an estimate, a place to start. A magnifier should be tested using samples of gradually decreasing print sizes from 72-point print down to 5-point print. The goal of a person with “typical” reading tasks is generally to see 10-12-point print as comfortably as possible. Most clients will do better with less magnification; they may prefer a blurrier image, in exchange for a wider field of view or a longer working distance. Some clients may prefer magnification that gives them the equivalent of 20/20 vision for the sharpest image possible. Once a preferred magnifier is found, test it again with print samples that are similar to the tasks that a person needs to accomplish -- newspaper print, for example.

L. Burkhardt, Braille Institute, 323-663-1111, ext. 1315, 03/05, Page 1