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
