Ординатура / Офтальмология / Английские материалы / Low Vision Aids_Chaudhry_2006

50 Low Vision Aids

Optical Design of the Magnifiers

Used as Low Vision Device (Fig. 6.8)

1.They can be spherical, aspheric, biaspheric and doublet.

2.They can also be illuminated or non illuminated form.

3.Higher the power the better is the optical quality required. Spherical magnifiers are cheaper ones. Higher the powers poorer will be the optics of the lens because of all types of possible aberrations a lens can have.

4.Aspheric lens has advantage or reducing thickness and peripheral distortions.

5.Biaspheric magnifiers have advantage of eliminating aberrations from both the surfaces.

6.Aplanatic or doublet magnifiers give a flat and wide distortion free field and good edge to edge clarity.

Fig. 6.8: Higher power smaller field of view

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STAND MAGNIFIERS (Fig. 6.9)

Stand –mounted magnifiers may be technically simpler for the patient to use than the hand magnifier because they are pre focused and rest on the rigid mount. Illuminated magnifiers have an added advantage of non glare light source. Stand magnifiers range from 2X to 20X. The patient needs to place the stand magnifier on the reading material and move across the page to read. A reading stand is recommended with this kind of magnifier. The presbyopes should read with his reading glasses.

They are a choice for patients with constricted or central field loss.

Uses

•Reading from a book or newspaper

•Looking at a picture or diagram.

Advantages

•It has a fixed focus easy for patient to see through

•It works good for patients with tremors, arthritis and constricted fields

•They may have their own light source.

Fig. 6.9: Stand magnifier

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Disadvantages

•The field of view is reduced

•It required good hand coordination

•Too close to reading posture is sometimes painful for long hours.

Optical Quality of the Stand Magnifier

It has to have the same properties as hand magnifier lenses.

TELESCOPES (Fig. 6.10)

Telescopes have a small but an important place in low vision. Telescopic systems magnify the apparent size of the distant objects making them appear closer to the patient than they actually are.

Binoculars magnify distant objects. Binoculars placed into eyeglass frames are called telescopic spectacles. They can be

Fig. 6.10: Spectacle mounted telescopes, uniocular, binocular

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monocular (one eye) or binocular. The telescope can be placed in various positions:

•Telescopes with magnification powers from 2X to 10X are prescribed. They are prescribed for distance, near and intermediate tasks. The patient has to spot the object he wishes to see and then brings the telescope from the side to the eye.

They are generally used for:

•Signs

•Finding and recognizing people or animals

•Reading from a blackboard in school. Reading from a blackboard from a distance > 2 m

•Finding an entrance to a building

•Watching games

•Looking at objects you cannot get close to, e.g. top of a tree, animals

•Traffic signals, street signs bus numbers

•Television

•Or reading at normal distance etc.

Types of Telescopes Include

1.Hand held monocular (Fig. 6.11)

2.Clip-on, spectacle mounted, monocular or binocular

3.Bioptic designs.

Fig. 6.11: Hand-held monocular telescope

54 Low Vision Aids

The optics is based on two principles:

•Galilean

•Keplerian.

Galilean or Keplerian Telescopes

These are the two basic designs based on the principle of angular magnification. The main difference is:

There are four types:

1.Regular field—a “Galilean” system available as fixed focus or variable focus.

2.Wide angle—a “Galilean” system available only as fixed focus.

3.Expanded field—a “Keplerian” variable focus system.

4.Micro-spiral—a “Galilean” variable focus system.

•Magnification and field of vision Telescopes vary in power from 1.7 to 8.0X.

•Field of view decreases with power.

•Field of view also varies with design of the telescope Higher the magnification lesser is the field of vision. This limits the level of magnification one can go upto, e.g. A 3X telescopes gives 12.5 degrees of field and a 4X telescope gives only 10 degrees of field of vision.

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•The trial should be done on the better eye.

•Children can be prescribed a spectacle mounted telescope, both uniocular or binocular.

Advantages

•It is the only possible device to date which enhances distance vision

•Can be used in a classroom for blackboard reading or outdoors.

Disadvantages

•The major drawback with telescope is the restriction in field of view

•The second is the appearance and the apprehension

•Patients with constricted fields do not benefit much by them

•The focusing requires coordination of hands and eye, which is sometimes difficult for the patient to focus

•They are usually expensive and costly for many of our patients.

Bioptic Telescopes (Fig. 6.12)

A bioptic telescope is a system where the telescope is attached to the top of a pair of eyeglasses, which allows the wearer to switch their sight between their “regular vision” and the magnified vision of the device by just a slight tilt of the head, similar to how one uses a bifocal.

Fig. 6.12: Ocutech bioptic telescopes the VES® autofocus, the VES®-K

56 Low Vision Aids

Magnifiers for Specific Needs and Demands

(Figs 6.13 to 6.15)

Fig. 6.13: Various other types

Fig. 6.14: Various other types of magnifiers

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Fig. 6.15: Bar reader, paperweight, fresnelbook magnifier

Telescopes for Near/Intermediate Tasks

Telescopes can also be prescribed for near and intermediate distances.

For near tasks they have an advantage over the reading spectacles – the reading distance is more with them compared to reading spectacles. The acceptance of these may be less due to its cumbersome looks and reduced field of vision.

Telescopes also have been used for intermediate tasks like computers.