Tuesday, 11 February 2014

Magnification and magnifiers

Many people with low vision find magnifiers useful to help them do short everyday tasks such as reading their post or instructions on a packet. Magnification increases the retinal image size. For people with a scotoma this may make an object easier to see, because although the retinal image size increases the area of visual loss remains the same size (Figure 1).

Figure 1: A schematic and simplified representation of how magnification can help a person to read short text.

1. Relative size magnification

This is a linear relationship: doubling the size of the object makes the image on the retina twice as large, creating x2 magnification. This form of magnification is usually limited to about 2.5x because of the physical limitations of enlarging an object. Examples of this type of magnification are large print books, watches or timers (Fig. 2).

2. Relative distance magnification

This is also a linear relationship: halve the distance of the object and the retinal image becomes twice as large, creating x2 magnification. For example, viewing the television from 2m rather than 4m gives x2 magnification (Figure 3).

This type of magnification can also be used for near tasks, e.g. bringing print closer to the eye from 40cm to 10cm gives x4 magnification.

Children and young adults can use accommodation to provide this form of magnification, mainly for short duration near tasks. Myopes who take off their glasses can achieve some magnification without the need for accommodation.

Plus lens magnification

A plus lens creates magnification by allowing the person to adopt a closer viewing distance. When the plus lens is placed so that the object viewed is at the anterior focal point of the lens, the object is focused clearly on the retina and accommodation can be relaxed. Most hand and stand magnifiers work on this very simple principle. The plus lens can be close to the eye, in a spectacle lens, or remote from it, in a hand or stand magnifier.

Limitations of plus lens magnifiers

Field of view: Patients often ask for larger magnifiers, hoping that this will

Figure 2: Making things bigger creates relative size magnification.

Figure 3: Moving things closer creates relative distance magnification.

Figure 4: A wide range of hand magnifiers is available, including folding and illuminated versions

increase their field of view. However, as the power of a magnifier increases, the diameter of the lens decreases, due to the weight of the lens and physical constraints in manufacturing. Instead, they should be encouraged to hold the magnifier as close as possible to the eye, thereby increasing the field of view.

Short working distance: Although the distance from the eye to the magnifier can be varied, the distance from the magnifier to the object is often very short, especially with stronger magnification. This makes it difficult to place implements such as a pen or screwdriver under stronger magnifiers, and directing adequate light on to the object can be problematic.

Hand magnifiers

Hand magnifiers are useful for short ‘survival’ tasks such as looking at packets or the dials on a cooker. Most people find them socially acceptable and they are easy to carry in a pocket or handbag. There are countless designs available at low cost in a wide range of powers, and many are internally illuminated (Figure 4). People with hand tremors or grip problems may, however, find them impossible to use.

Stand magnifiers

Stand magnifiers allow the maintenance of a precise magnifier-to-object distance, which is advantageous because of the small depth of focus of plus lens magnifiers. This means they are particularly useful for sustained tasks or where there are physical difficulties, such as tremor. The most commonly prescribed stand magnifiers are internally illuminated because the stand can obstruct light from getting to the object (Figure 5). Some lower-powered stand magnifiers allow tools, such as a pen, to be used (Figure 6). The disadvantage is that they are very bulky.

Spectacle-mounted plus lens magnifiers

The best optical solution to the difficulties of plus lens magnifiers is to mount them in spectacles: this gives the best magnification and greatest field of view. However, the majority of patients do not like any magnifier that focuses less than 25cm from the spectacle plane. For people who are able to accept shorter working distances, spectacle-mounted plus lenses are sometimes tolerated because they give the best magnification and field of view, and allow their hands to be free (Figure 7). Spectacle-mounted low vision aids can be prescribed monocularly or binocularly

Figure 5: Illuminated stand magnifiers are the most commonly prescribed stand magnifiers. 

Figure 6: A pen may be used under some low powered stand magnifiers.

Figure 7: Spectacle-mounted low vision aids allow the person to do tasks that need both
hands free, but only at a short working distance.

if prisms are incorporated to help convergence. Over +10.00DS, the person is unlikely to maintain binocularity. As well as providing magnification, some allow for the correction of refractive errors; high powered bifocal near additions are also available.

3. Real image magnification

Optical magnifying systems are limited to a magnification of about x20. Real image magnification produced electronically is available in much larger magnifications of x50 and over.

Closed circuit televisions

Closed circuit televisions (CCTVs) produce real image magnification electronically using a camera to create a magnified image on a monitor screen. They are usually used for near or intermediate tasks.

In theory, CCTVs should be the solution to all the frustrations of low vision aid users. They can produce high degrees of magnification, contrast reversal and enhancement, zoom facilities and binocularity of the image with none of the postural difficulties of many other magnifiers. In practice, however, they are expensive, quite difficult to use and often bulky. Only a small proportion of the low vision population use CCTVs, and most do so for longer, sustained reading tasks while they use optical low vision aids for short, survival tasks.

The most common type of CCTV is a TV screen mounted above an ‘X-Y’ table where the object is placed or held (Figure 8). Standard CCTVs cost about £1,500 but many models are much more expensive. TV readers are more affordable (£100 to £500). They consist of a hand-held camera which is plugged into the patient’s own television (Figure 9). The magnification is limited, often fixed at one value and dependent on the size of the television screen. Although they are cheap and quite portable, they are difficult to manipulate.

In recent years a number of head mounted CCTVs have been developed, such as the Jordy. The camera and TV screens are mounted in a virtual reality-type headset, and the control box is attached to the belt. These remain very expensive, heavy, difficult to use and cosmetically poor and, as yet, they cannot be used when walking around.

Unlike optical low vision aids, CCTVs are not provided on the NHS. Employment and education services will usually provide them if deemed necessary for the person’s

Figure 8: Various models of CCTV are available. The material to be viewed is placed on an X-Y table.

Figure 9: A TV reader.

Figure 10: A flat field magnifier.

work or schooling. Older people usually have to purchase their own. Many public libraries, some voluntary organizations for blind people and some social services departments have them available for trial use. Most manufacturing companies will let people try the CCTV in their own home for a short period before purchase. Due to the great expense and difficulty involved in using CCTVs, this approach should be strongly recommended to patients. 

Flat field magnifiers

These are single lenses of hemi-cylindrical or hemispherical form, designed to be put flat onto the object (usually text). The thicker the magnifier is in relation to its radius of curvature, the higher its magnification. This is unlikely to exceed x3 because of size and weight. Flat field magnifiers are very useful for children with a visual impairment as they look like a paperweight or ‘crystal ball’ (Figure 10).

4. Angular (or telescopic) magnification

Telescopes and binoculars are very effective in producing magnification for distance, while allowing the person to stay at their chosen distance from a task, such as viewing a street sign or blackboard. They can also be used for near tasks. Their main  disadvantage is restricted field of view. Also, distortion of space and movement perception prohibits walking around while using the telescope. Their use requires considerable manual dexterity, skill and practice, particularly to follow moving objects. Only a very small proportion of people with low vision use them (Figure 11).

Low vision therapy

Although as yet there is no conclusive evidence1, it is thought that people may benefit from training which maximizes the usefulness of low vision aids and vision in daily life. Some rehabilitation workers are trained to provide low vision therapy, which may take place outdoors with distance aids or in the person’s home environment.

Figure 11. Devices that produce angular magnification. A distance Galilean telescope used for TV viewing, terrestrial telescope and a pair of binoculars