The eyes might be exposed to a wide range of electromagnetic radiation from the shorter wavelength ultraviolet rays through the wavelengths of visible light to the longer infrared waves, X-rays and microwaves. X-rays pass straight through the eye without being focused by the optical media and, in large enough doses, can cause generalised damage. It is important to realise that therapeutic but not diagnostic doses of X-rays tend to cause cataracts and the eye must be suitably screened during treatment. Excessive doses of X-rays also cause stenosis of the lacrimal canaliculi, destruction of the secretory cells within the lacrimal glands and retinal neovascularisation. As one might expect, visible light does not normally damage the eyes, although an intense light source can be absorbed by the pigment epithelium behind the retina and converted to heat, producing a macular burn. After eclipses of the sun, there are usually a number of patients who arrive in the casualty departments of eye hospitals with macular oedema and sometimes serious permanent impairment of visual acuity. Sun gazing, with consequent retinal damage, has been reported after taking lysergic acid diethylamide (LSD).
The laser beam provides a source of intense light, which is used widely in ophthalmology as a deliberate means of producing gentle burns in the retina or making holes in the lens capsule after cataract surgery. However, uncontrolled use of lasers can cause blinding foveal burns as the subject tends to look directly at the beam momentarily, until they realise what it is. Ultraviolet rays, which are shorter than visible light, do not normally penetrate the eye but in large enough doses produce burns of the eyelids and cornea. On the skin this is seen as erythaema and later pigmentation, and on the cornea a punctate keratitis is seen with the slit-lamp. Ultraviolet damage of this kind is seen after gazing with unprotected eyes at welder's arcs, after exposure of the eyes to sunray lamps, and after exposure to the sun under certain conditions such as in snow on mountain tops. All these types of ultraviolet injury show a delayed effect, the symptoms appearing 2 h or 3 h after exposure and lasting for about 48 h. There is usually severe pain and photophobia so that it might not be possible to open the eyes, hence the term "snow blindness". The use of locally applied steroid and antibiotic drops hastens recovery.
Unlike ultraviolet light, infrared rays penetrate the eye and can cause cataract. A specific kind of thermal cataract has been well described in glass-blowers and furnace workers but this is now rarely seen because of the use of protective goggles. Microwaves, in the form of diathermy, can cause cataract but the eye must be in the path of the beam if damage is to occur, and microwave ovens would not be expected to be dangerous in this respect. Concern is quite often expressed in the press or elsewhere about the possibility of radiation damage to the eyes from visual display units. Such damage has never been demonstrated any more than it has from the face of a television set. Someone not used to working with a visual display unit who is suddenly made to spend several hours a day in front of one might experience eyestrain, especially if incorrect spectacles are worn.
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