What is eye strain?
It is a condition in which the eyes are tired, with dull pain behind the eyes, usually accompanied by a low headache in the forehead. Vision, though clear, is not comfortable, especially for reading.
What are the common causes of eye strain?
The most common reason for eye strain is an uncorrected error of refraction. More often than not the uncorrected refractive error is a slight astigmatism. It is ironical that large degrees of errors of refraction cause no strain as the eyes do not attempt to see clearly. Strain only occurs in low errors, when the eye attempts to see clearly and “strains” to do so.
The other important cause of eye strain is illumination. If it is inadequate, or even too much, it can cause problems.
Alternative causes are poor reading habits or posture, and poorly printed books with too small, illegible letters.
Television and movies are also potent causes of eye strain if indulged in to excess.
How should illumination variables be adjusted to prevent eye strain?
Normal vision has the ability to adapt to a wide range of illumination—but upto a limit.
Illumination of an object depends upon the kind of work being done at the moment.
(a) Reading: The best illumination is a 60 watt milky bulb, 4 feet from the reading surface, the light preferably falling over the left shoulder. However there are two important variables which would still cause eye strain even in the presence of adequate light: (i) glare from the printed page, especially if it has been printed on a glossy paper; and (ii) insufficient surrounding illumination. If the surroundings are dark it becomes difficult to concentrate on the reading object. The surrounding illumination should be just a little less in intensity than the object being read.
(b) Sewing: Light should be direct. Indirect light does not cause shadows and therefore obscures the thread used for sewing, because of lack of contrast. A fundamental fact must be remembered: the finer the work to be seen or done, the higher must be the illumination. Eye surgeons who deal with minute micro-sutures use high intensity lamps which give a highly focused intense light to be able to see the sutures and place them well.
(c) Watching television: The television should be adjusted to the minimum brilliance at which it can be seen clearly. The room light must be kept on so that the surrounding illumination is maintained at a low level (vide infra).
(d) Cooking in the kitchen: Ceiling recessed bulbs are considered the best. Bulbs give yellow light which do not distort the colour of the food. Fluorescent tubes, though convenient, cause a change in colour and a flicker effect when a spoon is stirred in a pot or a knife is used, which tends to distort vision.
The light intensity needed for cooking is much less than needed for sewing or reading, and too powerful a light is not required.