Developments in optical aids for the visually impaired

The main means we have at our disposal to assist visually impaired people to make the best use of their residual vision are magnification, illumination, and contrast. Developments in the design and manufacture of low vision aids give practitioners an unprecedented choice of equipment to recommend to their patients. New systems for illumination provide better, cheaper and more efficient units. Advances in optical design make for better image quality, and lighter units.
It is, however, a shame that there seems to have been no movement yet from the major manufacturers to standardise the magnification values for their products. Despite EN ISO 15253:2000, which discourages the use of Trade Magnification (F/4 + 1), many manufacturers still use this value.
The interest of some UK practitioners is beginning to move towards bioptic telescopic units with the possibility that the UK will join some states in the US in allowing their use for driving. While driving with LVAs is likely to be controversial, there is no doubt that bioptic units do provide some patients with greater mobility and a better quality of life.
Bioptic mounting involves fitting a telescopic lens very high in a pair of spectacles (see Figure 1) so that a patient with a central field defect can use the normal, lower part of the spectacle lens and eccentric viewing for general purposes. By nodding the head down slightly they can see detail by using the telescopic unit. A well-adapted patient may move continuously from one type of vision to the other.

HAND MAGNIFIERS
Hand magnifiers are probably the most widely used LVAs, and most practitioners will have come across the COIL range, particularly the aspheric hand magnifiers.
These still have their place in the LVA practice, but some interesting newcomers are the aplanatic units manufactured by Eschenbach and Schweizer. These have very good aberration control and are quite modestly priced. The lens system consists of two plano-convex lenses with the convex surfaces facing each other so the outer surfaces are flat. The practical advantage to this is ease of cleaning.
Folding magnifiers are often required for spot reading tasks such as shopping and several designs are available from various suppliers. My particular favourites are the Eschenbach Mobilent range which are available in X4, X7 and X10 nominal magnification. They fold up into an integral case, and there is the facility for the magnifier to be worn on a cord around the neck for easy access (Figure 2).

BRIGHTFIELD MAGNIFIERS
Dome and bar magnifiers are available from many manufacturers. The new CTP COIL VTM magnifier was originally designed for dyslexia, but has also found its place in the general LVA armoury. Shaped brightfield magnifiers such as the Eschenbach 1426 and 1436 allow for good posture while reading and a wide field of view, as well as exhibiting the usual good light-gathering characteristics we normally associate with this type of magnifier.

STAND MAGNIFIERS
Stand magnifiers are often preferred in cases where hand tremor might be a problem. They can also be used for writing, for example doing crossword puzzles. The CTP COIL range of 'tilt' stand magnifiers available in X3 and X4 (trade magnification) are very useful for this type of task.

ILLUMINATED MAGNIFIERS
These are highly effective in assisting patients with many eye conditions, particularly age-related maculopathy. Traditionally, illumination has been obtained by using bulbs. The advent of light emitting diodes (LEDs) has brought a new dimension to these appliances. LEDs have a much greater efficiency than bulbs (Table 1). They will provide much more light for a given quantity of electrical power. This property is utilised in two ways Ð more light, and longer battery life. Eschenbach, COIL (Figure 3) and Schweizer produce LED-illuminated magnifiers. Eschenbach offers the facility of a yellow filter covering for the LED, which is often useful for patients who have some degree of media opacities.
LED-illuminated magnifiers are available over a wide range of powers, from 1.3X to 12.5X nominal magnification. Until very recently the power source for LED units had always been conventional batteries, but now CTP COIL has introduced its Autotouch LED illuminated range (Figure 4). These are available in X3 to X11 (trade magnification). They feature internal rechargeable batteries and a very new concept. Instead of a mechanical switch, the light turns on automatically when the unit is picked up, and off when it is put down, thus saving battery power. This is achieved by touch pads that react to the moisture of the skin. One is always concerned about teething trouble with new technology, but the concept is a very attractive one.
Schweizer has a new handle design in its modular LED Mobil range, with a swivel joint. This allows an individually adjusted position for the user. Five interchangeable heads are available, covering the range X2 to X10 nominal magnification.

SPECTACLE-MOUNTED TELESCOPES
A major development in distance vision telescopes has been the introduction of diffractive optics by Eschenbach.
The Eschenbach 1623 is a Galilean telescope. The objective lens is a front-surface aspheric, with a diffractive back surface. The result is an incredibly light, wide-angle X2 distance-vision telescope, which can be mounted in any suitable spectacle frame. The aperture of the unit is almost rectangular, rather than circular. This gives a wider field in the horizontal meridian than in the vertical meridian. A 22