Refraction and Devices

In order to understand problems related to the optics of the eye, such basic facts as the structure of the eye as an optical system, the function of lenses and lens systems and the use of low vision devices need to be known. Some basic facts are discussed in Eyes and Vision. There is a thorough discussion of optical and non-optical devices in special education by Heather Mason et al (1998, ISBN 1-85346-412-0). It would also be an advantage to have a specific handbook on low vision optometry for therapists and educators.

It is essential that the therapists and teachers know the child's refractive error and how much of it is corrected with different glasses, at which distances the glasses give the best image, at which distance the magnifier is most effective, and how well the child can use the devices.

If the teacher of a low vision child in the integrated or inclusion education, in a "mainstream classroom", has not had a basic course in refraction and low vision aids, the information in "Eyes and Vision" gives an introduction. A local optometrist (in some countries the term optician is used) may explain the structure of the devices and how the child can best use them. "Eyes and Vision" can also be used to teach the child about vision and vision impairment so that his/her impairment and disability become more understandable, helping the child to learn to live with them.

It is worthwhile studying with the child each new optical and non-optical device (especially new glasses) to know what to expect in the classroom and in physical education. The child can learn about refraction and glasses by investigating his/her devices. An important concept is the focal length of a lens. The focal length of a lens is 1 meter (=100cm) divided by the refractive power of that lens. When a plus lens, e.g. a usual low vision reading glass of +6.0 is placed in front of an emmetropic eye, (= no refractive error) the image is clearest at 17cm, the focal length of the lens (100cm divided by 6= 17cm) and closer, if the child can accommodate. When the child and the teacher know the working distances of the devices, the teacher can support the child in positioning the texts and other materials at the correct distance so the child is using the best possible image quality.

Optics of "simple" magnifiers are not simple because different magnifiers have the image visible at different distances from the lens. Quite often the child needs to change between the use of high plus reading glasses and a strong magnifier or to look at the black board. The low vision optometrist (where available) tries to find a solution that would work but observations in the class room and at home are needed to properly evaluate the usefulness of the devices.

The ergonomic needs of low vision children are different from those of normally sighted children due to the use of strong reading glasses, magnifiers, telescopes and increasingly, electro-optical magnifying systems combined with the use of computers. Teaching of optimal ergonomics is based on thorough understanding of the limitations and strengths of the child's vision, and on knowledge about the structure and function of visual and non-visual aids. Ergonomics should be an integral part of low vision education so children can finish school with the highest marks/grades and no problems with posture.

In developing countries the teacher or the therapist may need to try to assess the refractive errors if optometric services are not available. In the measurement of the refractive power of an eye one can do a lot with a few lenses, a ruler, a near vision card and a piece of paper. Kits with a few lenses, magnifiers and telescopes, produced locally, are available in many developing countries.

The evaluation is based on the use of strong plus lenses to make the person temporally myopic (short-sighted), so that the distance at which the image is clearest can be measured. When a strong plus lens, e.g. +6.0 is placed in front of an eye, with for example a refractive power of +1.0, the image is clear at 20cm distance. The 20cm distance corresponds to the focal length of a +5.0 lens, thus the eye itself must be +1.0.

If the child is a high hyperope (long-sighted), the +6.0 lens may only correct the long sightedness and may not make the person myopic. In that case another and, if needed, a third plus lens is added to bring the clear image within near vision. Some children have been operated for cataract but may not have glasses or these have been broken or lost several years ago. These children may need stronger than +20 diopter glasses. - Some aphakic children (aphakic=operated for cataract) may have a small hole in the secondary cataract that functions as a pin hole and allows fairly good image quality at all distances without corrective lenses (which is a surprising observation for even an experienced refractionist).

When the +6.0-lens is used to estimate the refraction of a myopic eye, the image may be clear at 10-12 cm distance, which corresponds to -10 to -8.50 lens. Thus the eye itself is myopic in the range -2.50 to -4.0 diopters. Without the +6.0-lens, the degree of myopia can be calculated by dividing 100 cm with the distance of the farthest clear focus.

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