In certain work sites, the eyes are at risk to mechanical dangers such as flying particles, chips and even molten splashes. Protection of the eyes is therefore important and industrial safety lenses in single vision and multifocals are available.

For further information, call the SA Optometric Association at (011) 805-4517 or e-mail them at pro@saoa.co.za

A spectacle lens is a scientific instrument that is able to refract (bend) light. Lenses come in a wide variety of strengths. The majority of lenses dispensed fall within a lower lens power range. There are, however, occasions when a strong lens needs to be prescribed. These lenses are often thick and bulky in dimensions. To reduce the mass of the lens and to ensure an attractive appearance, special materials can be employed that have a high refractive index, resulting in a lens that is far thinner, has less mass and is consequently lighter in weight than an equivalent lens made up of glass with a standard refractive index. Different high refractive index materials can be employed, depending on the advantages required.

High index lenses made from specialty plastics allow thinner, lighter lenses than traditional glass or plastic. These lenses are geared towards those with very strong prescriptions, both nearsighted and farsighted. Glasses correct vision by bending the light that passes through them. A stronger prescription requires more bending than a weaker one, resulting in a thicker lens. High index lenses are made of denser material than conventional lenses, bending more light through less thickness, resulting in ultra-thin, lightweight glasses.

When viewing conventional glasses, one can easily see lenses designed to correct nearsightedness. The lenses are thicker at the edges than in the middle. The stronger the prescription, the thicker the edges become.

Most stylish frames are too thin to hide the thick edges of nearsighted lenses. Thick lenses also distort the eyes, making them look unnaturally small. High index lenses eliminate both problems.

Those with extreme farsighted vision have lenses that are thicker in the middle, with thin edges. The thick middle has the opposite effect of a nearsighted lens, making the eyes look unnaturally large. High index lenses again offer a solution.

All lenses are categorized by how much light they can bend using an index of infraction. This index compares light speed through air to light speed through the lens material. Glass bends light at an index ratio of 1.52, while conventional plastic has an index of 1.50. Lenses with a greater index are considered high index lenses. There are several plastic materials now available in high index lenses, with index ratios between 1.53 and 1.71. The higher the index, the thinner, lighter, and more expensive the lenses. High index glass lenses are also available, with a ratio of 1.90. However, glass lenses are not as light or durable as plastic lenses. In fact, high index glass is actually heavier than standard glass, cancelling much of the benefit of the thinner lens.

High index lenses can correct astigmatism and are available as bifocals or trifocals. They look nice in thin, wire, and “frameless” frames. The index that will best suit you is determined by your prescription, but don’t assume a vendor will make the best choice for you. Some suppliers of high index lenses do not sell all types, and you could end up with a lower index lens than is ideal. Ask your optometrist which index is best. High index lenses are available from many optical centres. They can make wearing glasses enjoyable, improve self-image, and restore confidence. They are durable and shatterproof, perfect for children and active adults. After you’ve worn a pair of high index lenses, chances are you’ll never go back to standard glasses again!

Anti-reflective coatings
At the interface between air and the lens surface (glass or plastic), reflections occur that reduce the light transmission and can cause noticeable secondary disturbing and interfering ghost images. These negative effects of reflections can be reduced or even eliminated by special and specific metallic oxide coatings applied to the lens in a vacuum plant. This will improve the transmission of light through the lens and will minimise secondary images and so lessen the disturbances caused by surface reflections.

Absorbent coatings
Any glass lens can be given any desired absorbent effect by coating it with a layer of metal oxides. This coating is available in a range providing from 10% to 85% light reduction, together with elimination of ultraviolet and infra-red radiation. These coatings guarantee uniform transmission of light over the entire lens, and aesthetically the lens presents a uniform colour appearance.

The eye is sensitive to light radiation in the visible spectrum from 370 nano-metres (blue) to 760 nm (red). Below and above these frequencies, the radiation flux can be damaging to certain tissue cells of the eye and protection is needed against these harmful wavelengths. This can be achieved by the tinting of lenses both in glass and plastic.

Fixed tints
Harmful radiations can be absorbed by the addition of specific chemicals in the glass substance or by the application of certain dyes to plastic lens material. Tints can range from the very light, with weak absorption, to the very dark, with up to 85% absorption of the visible light spectrum and the elimination of harmful and dangerous ultraviolet or infra-red radiation, or both.

Photochromatic / Variable Tint
This permits the lens to darken in bright light and lighten in the dark. Lenses with this characteristic are day temperature sensitive and the reaction is greater when the temperature is lower. These lenses darken on exposure to ultraviolet light, whilst infra-red radiation reverses the process, causing the tint to fade.

A standard lens diameter is currently set at a minimum of 65 mm and is sufficient to satisfy the majority of fittings in the various frame styles. Occasionally, however, a lens larger than the standard size needs to be employed because of the frame size or shape, or the interaction between the user’s inter-pupillary distance and the frame selected.

Single vision

These lenses come in four varieties:

Spherical

This lens has a single point focus and is employed to correct spherical focus errors such as myopia (short-sightedness) or hyperopia (far-sightedness).

Cylindrical

This lens has no point focus but a line focus and is employed to correct astigmatic focus errors.

Sphero-cylindrical

This lens also has no point focus but has two line foci and is employed to correct a combined spherical and cylindrical focus error. The above-mentioned lenses are suitable over the entire visual range, from distance to close.

Ready made readers (untested spectacles for reading bought over the counter) would accommodate many patients. In fact, many optometrists make available readers in their practices. Nevertheless, the following needs to be considered:

If glaucoma is diagnosed sufficiently early, the condition can be treated. The inherent danger in utilising readers is that they can be purchased in increasing strengths which may, for a period, alleviate the reading difficulty but, at the same time, disguise the creeping seriousness of glaucoma and prevent its early detection.

Reader lenses are the same power for each eye. Should patients require different powers, the assumption that the readers are equally effective is incorrect.

Readers take the form of spherical lenses and don’t take into account astigmatic correction.

The lenses are set at an average interpupillary distance (PD), which may induce prismatic effect if this is not the patient's PD. This may lead to temporary symptoms of eyestrain, tiredness and inefficiency when reading.

The profession of optometry has introduced a number of initiatives to accommodate economically compromised communities throughout South Africa. Therefore, the low cost of readers should not be seen as the deciding factor.

Ready made readers

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Combined focal length lenses

Bifocal

This lens permits the user to see clearly in the distance and at close range. It has a visible line demarcating the distance and near sections.

Trifocal

This lens permits the user to see clearly in the distance, at intermediate distances and at close range. It has two visible lines demarcating the distance, intermediate and near sections.

Progressive lenses

This lens is designed to give continuous and uninterrupted vision from a distance to all closer seeing distances. There is a gentle and gradual power increase from the distance portion of the lens to the near portion.

Progressive lenses, also called progressive addition lenses, progressive power lenses, graduated lenses and varifocal lenses, are ophthalmic spectacle lenses used to correct presbyopia and other disorders of accommodation. A gradient of increasing lens power is added to the correction for the other refraction error, going from a minimum or nothing at the top of the lens to maximum magnification at the bottom of the lens. A wearer can then adjust the lens power required for clear vision at different viewing distances by tilting his or her head to place the line of sight through different parts of the lens.

Progressive addition lenses avoid the discontinuities in the visual field created by bifocal and trifocal lenses. The lenses are also more cosmetically attractive. The lenses suffer the disadvantage of creating regions of distortion and blur away from the optic axis, yielding poor visual resolution. Although manufacturers are constantly striving to minimize these distortions, some wearers cannot tolerate the lenses.

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Essilor lay a strong claim to have invented varifocals in 1958 with their first design of Varilux lens. Early varifocals were relatively crude designs but modern sophisticated varifocals have gained much greater patient acceptance and include special designs to cater for many separate types of wearer application, e.g. for use on with computers, offering enlarged near and intermediate areas.


The varifocal lens give a progressive vision from near to far, for the PRESBYOPIC eye. When the designer reduces the amount of astigmatism which occurs in the lower portion of the lens - in an attempt to speed the subject's adaptation to progressive lens wear - by spreading the astigmatism into the distance portion as indicated in the following figure, this arrangement results in a soft progressive design. There can be no doubt that when the addition is low and. hence, the surface astigmatism is low. The soft progressive design has proved to be the most successful in enabling rapid wearer acceptance of progressive power lenses.


Many manufacturers now produce progressive lens series that are deliberately soft in design for the low-addition lenses in the series, the design tending to become harder as the additions increase. These are known as multi-design series.The following figure illustrates how the power law differs with a multi- design series for the additions. + 1.00. +2.00 and + 3.OOD. It is also seen that the length of the progression zone reduces as the addition increases for these lenses.


These features of modern progressive power lenses, together with the attempts to ensure that prismatic effects are similar at corresponding points on the lens. so-called horizontal symmetry of the design, have ensured that the vast majority of wearers will adapt to progressive lens wear.

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Design diagrams

Traditional progressive lens

Minimum height 24 mm

Modern design

Minimum height 24 mm

Top modern design

Minimum height 22 mm

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Although it is easy to draw some conclusions about the likely performance of one design over another by inspection of iso-cylinder diagrams it is more difficult to pontificate over acceptance of the design from these diagrams. The last figure indicates quite clearly that the soft design has a narrower intermediate channel and a narrower near portion than the lens that is described as a hard design. The widths of these areas could be measured and expressed, for example, in the same way as we would express the diameter of a bifocal segment. However the significance of this information is not immediately apparent. The author wears several different progressive lens designs.

Each with different characteristics in their intermediate and near portions, and has no strong preference (at least, which is related to the optical performance of the lenses for any one design over another.)
Needless to say the advantages of progressive designs over other forms of multifocal correction should be spelt out in simple terms.

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Glass ophthalmic Lenses
Any prescriptive lens can be hardened by means of a thermal or chemical process whereby the lens is toughened to offer eye protection. Besides the hardening process, other physical conditions of the lens must be met before such a lens can be certified as being a safety lens, such as minimum edge or centre thickness. Safety lenses can be fitted in a fashionable frame but where called for, an industrial safety frame is preferable in industry.

Plastic or CR39 0phthalmic Lenses
Plastic lenses are impact resistant, making them a better choice for children, active adults, sportswear, and safety wear. It takes a far greater force to break a plastic lens than a comparable glass lens, and while glass shatters, plastic will more likely dent, or if the force is great enough, crack without shattering

Polycarbonate ophthalmic lenses
Polycarbonate is a versatile, tough plastic used for a variety of applications, from bulletproof windows to compact disks (CDs). The main advantage of polycarbonate over other types of plastic is unbeatable strength combined with light weight. While acrylic is 17% stronger than glass, polycarbonate is nearly unbreakable. Bulletproof windows and enclosures as seen inside banks or at drive-throughs are often made of polycarbonate. Add to this the advantage that polycarbonate is just one-third the weight of acrylic, or one-sixth as heavy as glass, and the only drawback is that it is more expensive than either acrylic or glass.

Compact disks and digital versatile discs (DVDs) are perhaps the most readily recognized examples of polycarbonate. If you’ve ever archived files on a writable CD, then later tried to break it before throwing it away, you know just how tough polycarbonate can be!

Clear polycarbonate is used to make eyeglasses because of its excellent transparency, durability, and high infraction index. This means that it bends light to a far greater degree than glass or other plastics of equal thickness. Since prescription lenses bend light to correct vision, polycarbonate lenses can be far thinner than glass or conventional plastic, making polycarbonate the ideal material for heavy prescriptions. Thin polycarbonate lenses correct poor vision beautifully without distorting the face or the size of the eyes, yet this extremely thin lens is virtual indestructible, an important safety factor for children and active adults.

Polycarbonate lenses are also used in quality sunglasses that incorporate filters to block ultra-violet (UV) rays and near-UV rays. The lenses can also be polarized to block glare, and their high impact resistance makes them perfect for sports. Many sunglasses manufacturers choose polycarbonate because it can be easily shaped without problems like cracking or splitting, resulting in extremely lightweight, distortion-free, fashionable glasses that feature all of the health benefits doctors recommend. This material outperforms treated glass, because of its outstanding strength.

Materials employed in the generating of spectacle lenses are basically those of glass (inorganic) or plastic and polycarbonate (organic) compounds.

The advantages of plastic lenses are:

• reduced weight when compared with matching similar glass lenses
• great impact resistance and consequently unlikely to break
• ease of tinting.

The advantage of glass lenses is that they do not scratch easily. Tinted glass lenses also absorb light more effectively.