Figure 3-37 Photochromic lenses. (Courtesy of Tommy Korn, MD.)
Ultraviolet-absorbing lenses
The spectrum of UV light is divided into 3 types: UVA contains wavelengths of 400–320 nm, UVB contains wavelengths of 320–290 nm, and UVC contains wavelengths below 290 nm. The ozone layer of the atmosphere absorbs almost all UVC coming from the sun. Most exposure to UVC is from manufactured sources, including welding arcs, germicidal lamps, and excimer lasers. Of the total solar radiation falling on the earth, approximately 5% is UV light, of which 90% is UVA and 10% UVB.
The amount of UV light striking the earth varies with season (greatest in the summer), latitude (greatest near the equator), time of day (greatest at noon), and elevation (greatest at high elevation). UV light can also strike the eye by reflection. Fresh snow reflects between 60% and 80% of incident light; sand (beach, desert) reflects approximately 15% of incident light; and water reflects approximately 5% of incident light.
Laboratory experiments have shown that UV light damages living tissue in 2 ways. First, chemicals such as proteins, enzymes, nucleic acids, and cell-membrane components absorb UV light. When they do so, their molecular bonds (primarily the double bonds) may become disrupted. Second, these essential biochemicals may become disrupted by the action of free radicals (such as the superoxide radical). Free radicals can often be produced by UV light in the presence of oxygen and a photosensitizing pigment. For a fuller discussion of free radicals, see BCSC Section 2, Fundamentals and Principles of Ophthalmology.
Because it may take many years for UV light to damage eye tissue, a tight linkage between cause and effect is difficult to prove. Therefore, proof that UV light damages the eye comes primarily from acute animal experiments and epidemiologic studies covering large numbers of patients.
The available data on the effects of exposure to UV light have suggested a benefit to protecting patients from UV light after cataract surgery. Some surgeons routinely prescribe UV-absorbing glasses after surgery. Intraocular lenses incorporating UV-absorbing chromophores are now available. For further information regarding the effects of UV radiation on various ocular structures, see BCSC Section 8, External Disease and Cornea; and Section 12, Retina and Vitreous.
Almost all dark sunglasses absorb most incident UV light. The same is true for certain coated clear-glass lenses and clear plastic lenses made of CR-39 or polycarbonate. One suggestion has been that certain sunglasses (primarily light blue ones) may cause light damage to the eye. Proponents of this theory contended that the pupil dilates behind dark glasses and that if the sunglasses do not then absorb significant amounts of UV light, they will actually allow more UV light to enter the eye than if no sunglasses were worn. In fact, dark sunglasses reduce light levels striking the eye on a bright, sunny day to the range of 2000–6000 foot-lamberts. Such levels are approximately 10 times higher than those of an average lighted room. At such light levels, the pupil is significantly constricted. Thus, contrary to the preceding argument, dark sunglasses used on a bright day allow pupillary dilation of only a fraction of a millimeter and do not lead to light injury of the eye.
Special Lens Materials
It is important for the ophthalmologist to be aware of the variety of spectacle lens materials available. Four major properties are commonly discussed in relation to lens materials:
1.Index of refraction. As the refractive index increases, the thickness of the lens can be decreased to obtain the same optical power.
2.Specific gravity. As the specific gravity of a material decreases, the lens weight can be reduced.
3.Abbe number (value). This value indicates the degree of chromatic aberration or distortion that occurs because of the dispersion of light, primarily at the fringes of the lens. Materials with a higher Abbe number exhibit less chromatic aberration and thus allow for higher optical quality.
4.Impact resistance. All lenses dispensed in the United States must meet impact-resistance requirements defined by the US Food and Drug Administration (FDA) (in 21CFR801.410), except in special cases wherein the physician or optometrist communicates in writing that such lenses would not fulfill the visual requirements of the particular patient. Lenses used for occupational and educational personal eye protection must also meet the impact-resistance requirements defined in the American National Standards Institute (ANSI) high-velocity impact standard (Z87.1). Lenses prescribed for children and active adults should also meet the ANSI Z87.1 standard, unless the patient is duly warned that he or she is not getting the most impact-resistant lenses available.
Standard glass
Glass lenses provide superior optics and are scratch resistant but also have several limitations, including low impact resistance, increased thickness, and heavy weight. Once the standard in the industry, glass lenses are less frequently used in current practice; many patients select plastic lenses. Without special treatment, glass lenses may be easily shattered. Chemical or thermal tempering increases the shatter resistance of glass, but if it is scratched or worked on with any tool after tempering, the shatter resistance is lost. Farmers appreciate photoreactive glass for its scratch resistance and easy care. Welders and grinders are better off with plastic, as small hot particles can become embedded in glass. Persons with myopia who desire thin glasses may choose high-index glass. The highest-index versions cannot be tempered and require that waivers be signed by patients who accept the danger of their breakage. High-index glass does not block UV light unless a coating is applied. (Characteristics of standard glass lenses are as follows: index of refraction, 1.52; Abbe number, 59; specific gravity, 2.54; impact resistance, pass FDA 21CFR801.410 if thick enough and chemically or heat treated.)
Standard plastic
Because of its high optical quality and light weight, standard plastic (also known as hard resin or CR39) is the most commonly used lens material and is inexpensive. Standard plastic lenses are almost 50% lighter than glass lenses owing to the lower specific gravity of their material. They offer UV protection and can be tinted easily. A scratch-resistant coating is usually advisable because of the ease with which plastic lenses can be scratched. The index of refraction is not high, so the lenses are not thin. CR-39 lenses do not have the shatter resistance of polycarbonate or Trivex. (Characteristics of standard plastic lenses are as follows: index of refraction, 1.49; Abbe number, 58; specific gravity, 1.32; impact resistance, pass FDA 21CFR801.410.)
Polycarbonate
Introduced in the 1970s for ophthalmic lens use, the high-index plastic material polycarbonate has a low specific gravity and a higher refractive index, which allow for a light, thin lens. Polycarbonate is also durable and meets the high-velocity impact standard (ANSI Z87.1). One disadvantage of this material is the high degree of chromatic aberration, as indicated by its low Abbe number (30). Thus, color fringing can be an annoyance, particularly in strong prescriptions. Another disadvantage is that polycarbonate is the most easily scratched plastic, so a scratch-resistant coating is required. Also, if polycarbonate is cut too thin, it can flex on impact and pop out of the frame. (Characteristics of polycarbonate lenses are as follows: index of refraction, 1.58; Abbe number, 30; specific gravity, 1.20; impact resistance, pass FDA 21CFR801.410 and ANSI Z87.1.)
Trivex
Introduced in 2001, Trivex is a highly impact-resistant, low-density material that delivers strong optical performance and provides clear vision because of its high Abbe number. Its impact resistance is close to that of polycarbonate, and it blocks all UV light. Its index of refraction is not high, however, so the lenses are not thin. Trivex is the lightest lens material currently available and meets the high-velocity impact standard (ANSI Z87.1). Trivex material allows a comparably thin lens for the ±3.00 D prescription range. A scratch-resistant coating is required. (Characteristics of Trivex lenses are as follows: index of refraction, 1.53; Abbe number, 45; specific gravity, 1.11; impact resistance, pass FDA 21CFR801.410 and ANSI Z87.1.)
High-index materials
A lens with a refractive index of 1.60 or higher is referred to as a high-index lens. High-index materials can be either glass or plastic and are most often used for higher-power prescriptions to create thin, cosmetically attractive lenses. The weight, optical clarity, and impact resistance of highindex lenses vary depending on the specific material used and the refractive index; in general, as the index of refraction increases, the weight of the material increases and the optical clarity (Abbe number) decreases. None of the high-index materials passes the ANSI Z87.1 standard for impact resistance. Plastic high-index materials require a scratch-resistant coating.
Strauss L. Spectacle lens materials, coatings, tints, and designs. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2005, module 11.
Figure 3-38 (Courtesy of Tommy Korn, MD.)
Figure 3-39 (Courtesy of Tommy Korn, MD.)
Figure 3-40 (Illustration developed b y Tommy Korn, MD.)
Therapeutic Use of Prisms
Small horizontal and vertical deviations can be corrected conveniently in spectacle lenses by the addition of prisms.
Horizontal heterophorias
Asthenopic symptoms may develop in patients (usually adults) if fusion is disrupted by inadequate
vergence amplitudes; if fusion cannot be maintained, diplopia results. Thus, in patients with an exophoria at near, symptoms develop when the convergence reserve is inadequate for the task. Some patients can compensate for this fusional inadequacy through the improvement of fusional amplitudes. Younger patients may be able to do so through orthoptic exercises, which are sometimes used in conjunction with prisms that further stimulate their fusional capability (base-out prisms to enhance convergence reserve).
Symptoms may arise in some patients because of abnormally high accommodative convergence. Thus, an esophoria at near may be improved by full hyperopic correction for distance and/or by the use of bifocal lenses to decrease accommodative demand. In adult patients, orthoptic training and maximum refractive correction may be inadequate, and prisms or surgery may be necessary to restore binocularity.
Prisms are especially useful if a patient experiences an abrupt onset of symptoms secondary to a basic heterophoria or heterotropia. The prisms may be needed only temporarily, and the minimum amount of prism correction necessary to reestablish and maintain binocularity should be used.
Vertical heterophorias
Vertical fusional amplitudes are small (<2.00Δ). Thus, if a vertical muscle imbalance is sufficient to cause asthenopic symptoms or diplopia, it should be compensated for by the incorporation of prisms into the refractive correction. Once again, the minimum amount of prism needed to eliminate symptoms should be prescribed. In a noncomitant vertical heterophoria, the prism should be sufficient to correct the imbalance in primary gaze. With combined vertical and horizontal muscle imbalance, correcting only the vertical deviation may help improve control of the horizontal deviation as well. If the horizontal deviation is not adequately corrected, an oblique Fresnel prism may be helpful. A brief period of clinical heterophoria testing may be insufficient to unmask a latent muscle imbalance. Often, after prisms have been worn for a time, the phoria appears to increase, and the prism correction must be correspondingly increased.
Methods of prism correction
The potential effect of prisms should be evaluated by having the patient test the indicated prism in trial frames or trial lens clips over the current refractive correction. Temporary prisms in the form of clip-on lenses or Fresnel press-on prisms can be used to evaluate and alter the final prism requirement. The Fresnel prisms have several advantages: (1) they are lighter in weight (1 mm thick) and more acceptable cosmetically because they are affixed to the concave surface of the spectacle lens, and (2) they allow much larger prism corrections (up to 40.0Δ). With higher prism powers, however, it is not uncommon to observe a decrease in the visual acuity of the corrected eye. Patients may also observe chromatic fringes.
Prisms can be incorporated into spectacle lenses within the limits of cost, appearance, weight, and the technical skill of the optician. Prisms should be incorporated into the spectacle lens prescription only after an adequate trial of temporary prisms has established that the correction is appropriate and the deviation is stable.
Prism correction may also be achieved by decentering the optical center of the lens relative to the visual axis, although a substantial prism effect by means of this method is possible only with higherpower lenses. Aspheric lens designs are not suitable for decentration. (See earlier discussion of lens decentration and the Prentice rule.) Bifocal segments may be decentered in more than the customary