Buying guide for ophthalmic and sun glasses
Eye glasses and contact lenses | Sunglasses | Contact lenses | Buying guide
Material
At present, three types of material are used in the fabrication of ophthalmic lenses:
- Mineral matter (glass);
- Organic matter (plastic);
- Polycarbonate.
Mineral matter (glass)
Before the invention of plastic, all corrective lenses were made of mineral matter (glass) Glass itself has little resistance to impact and may fracture into many tiny pointed fragments. These bits of glass can injure the eye.
There are tempering processes which increase the resistance to impact. These processes involve heating or chemical treatments but they don’t render the glass unbreakable. Glass is also heavier than the other materials.
Crown glass lenses transmit about 91.6% of the light.
Glass lenses having a higher density (index 1.6 and 1.7) are used for the strongly myopic because they are thinner and thus lighter.
Organic material (plastic)
Plastic material is a resin also known as CR-39 (or diallylglycol carbonate) Ophthalmic lenses made of plastic have numerous advantages: they are much more resistant to impact, 50% lighter and have less tendency to fog. They also absorb ultraviolet rays up to 350 nm (nanometres)
Plastic lenses, however, are susceptible to scratching but a scratch resistant coating can be applied to them. For the same optical strength, the plastic lenses are thicker than the mineral ones.
The great majority of prescription lenses are currently made of plastic especially since thin plastic lenses for strong prescriptions have recently become available.
Plastic lenses transmit about 92.2% of the light, or about 0.6% more than the glass ones. They cause less glare and internal reflections (in the lens) are reduced.
Plastic lenses are also more susceptible to distortion if they are inserted in frames that are too tight. These lenses must be cut to the exact dimensions required prior to being installed in the eyeglass frames.
Polycarbonate
Polycarbonate is also a plastic lens. Lexan is a polycarbonate the main characteristic of which is its resistance to impact. This lens is by far the most resistant of all. The first bullet-proof windows in the 1960’s were made of polycarbonate. The polycarbonate lens is 10% and 15% thinner than glass and plastic lenses respectively.
It is, however, a relatively soft plastic which scratches easily but the addition of a scratch resistant coating makes it a number one choice in lenses. Moreover, it is the ideal industrial lens even though the availability of such lenses still leaves something to be desired.
Polycarbonate is recommended in situations where ocular protection is a major factor; athletes, children and individuals who have only one eye (monocular patients) are ideal candidates for polycarbonate lenses.
Polycarbonate also has the advantage of absorbing ultraviolet rays up to 380 nm.
Source : Association des Optométristes du Québec
Tinted lenses
To look better, to see life through rose coloured glasses, or blue, or green...
Tinted lenses are also known as absorbent lenses. They fall into two classes: according to their color or according to how much light they let through.
The relative absorption of each colour can also be indicated in different ways :
- Directly by the transmission
- by the letters A, B, C, or D
- By the numbers 1,2,3,or 4 (from lightest to darkest)
Glass ophthalmic lenses (mineral material) are made of crown glass and transmit 91.6% of the light. Nearly 8.4% of the light is lost through reflection. Crown glass absorbs ultraviolet rays below 290 nm but it is the rays in the 290 to 380 nm that are the most harmful to the eyes. Infrared rays are not affected by glass.
For this type of lens, the material itself can be tinted by the manufacturer. The obvious advantage is that the tint will not deteriorate but the disadvantage is that colouration of the glass will vary with its thickness. The tint will be darker wherever the glass is thicker, something that is not very aesthetic in cases of high optical strength lenses. Glass lenses can also be surface tinted with metallic oxide coating.
Plastic ophthalmic lenses (resin or organic material) offer a greater variety of tints as they are surface tinted by dipping in an appropriate dye. The longer the period of immersion, the darker the lens will be. Progressive shades (from dark to light) are also easy to achieve in this manner.
Plastic lenses have little effect on infrared rays and only absorb the ultraviolet up to 350 nm.
Which tint and for what application?
A question frequently asked of the optometrist is: which tinted lens offers the best protection or what fashion shades can be tolerated without affecting the eyesight?
For normal conditions, light transmission between 15 and 30% (70 to 85% attenuation) is adequate to combat glare effects. Those who are exposed to the sun for longer periods should wear glasses with 15% transmission or less.
Obviously, tinted glasses should not be worn under shady or low light level conditions. Studies show that wearing tinted glasses while driving in low light conditions can reduce one’s eyesight by as much as 60%.
As for reflections caused by lighting systems, light tints can attenuate some reflexions but not as well as anti-reflective treatments.
Grey
Grey tint is the most popular for protection against the sun and for good reason. The main characteristic of grey is its uniform transmission across the visible spectrum. Consequently, it does not change the natural appearance of colours.
Yellow
Yellow tint is subject to discussion. It has traditionally been the choice of marksmen who report that their shooting improves with this tint. Studies, however, show that this is not always the case. This tint, also known as “sodium yellow” absorbs the blue light of the visible spectrum and can be useful to reduce glare from a very blue sky, for example. Yellow tint is also used to improve contrast both in hunting and skiing near the end of the day but the effect of this shade does not appear to last for more than 30 to 45 minutes.
Green
Green tint offers a transmission curve very similar to that of the human eye. This tint is especially popular with the military. It provides good absorption for both infrared and ultraviolet rays but only for glass lenses since green plastic lenses have little effect on the infrared.
Brown
Brown tint is often used for sunglasses. This tint offers some of the yellow’s characteristics in that it also absorbs the blue wavelengths. Grey remain, without a doubt, the most effective as far as the rendition of colours is concerned.
Pink
Pink tint is often used indoors under unfavourable light conditions such as brilliant fluorescents or glare in the workplace. Thanks to pink’s absorption of ultraviolet rays up to 350 nm, older individuals suffering from cataracts may find comfort in the pink tint. Also, those who have undergone cataract surgery can benefit from the absorption of some ultraviolet radiation. Anti-UV treatment, however, has the same effect without the need to add colour. Normally, only pale pink tints are recommended as dark pink alters the perception of colours and causes ocular discomfort.
What to choose? Discuss these aspects with your optometrist. He will be in a position to answer all your questions concerning the colour or colours that could best suit your needs.
Source : Association des Optométristes du Québec
Tempering
Are your lenses safe? Here is how they can be made more resistant.
All ophthalmic lenses must meet specific impact resistance norms. All conventional lenses (non industrial) must be at least 2 mm thick at the center and the average thickness between the center and the edge of the lens must not be less than 1.7 mm. The thickness at the edge must not be less than 1mm at its thinnest point.
In the case of industrial eye protection glasses, the thickness at both the center and the edge must not be less than 3.0 mm.
Plastic and polycarbonate lenses are more impact resistant due to the nature of their composition. They are even less susceptible to breakage than the safest of the glass ones.
There are two ways to treat glass lenses to increase their impact resistance, but in no case can the glass be said to be unbreakable. No such glass lens exists. This is why glass lenses are not to be used in industrial environments, for whatever the reason.
Mineral lenses can be heat treated (thermal tempering) or chemically treated (chemical tempering) to improve their resistance.
The two procedures are as follows:
Thermal tempering (heat)
After the lens has been cut to the required dimensions and the edges have the desired shape, it is heated in a vacuum to a temperature nearing its softening point, the ideal temperature being in excess of 600 degrees Celsius. The lens is then cooled rapidly (either by a jet of cold air or by immersion in a cold liquid) in order to create tension between the internal and external molecular layers. This process causes a slight deformation of the surface of the glass.
The main disadvantage to heat tempering is only acceptable if the lens is free of significant scratches. If a lens is deeply scratched, it must be changed in order to offer maximum safety.
Chemical tempering
The lens is placed in a very hot chemical bath where an ion exchange takes place on the surface of the glass. Glass contains sodium ions and the lens is submerged in a bath of potassium salts. At a temperature of 440 degrees Celsius, the sodium ions escape from the surface of the glass and are replaced by potassium ions. As the potassium ions are bigger than the sodium ions, a compression is developed in a thin surface layer, thus increasing the mechanical resistance of the glass.
The lens remains in this chemical bath for 15 to 17 hours to allow the ions exchange to take place as deeply as possible in the glass. Contrary to thermal tempering, there is no way to confirm its presence so one must rely on the honesty of the laboratory.
Resistance to shock is best provided by chemical tempering and approaches that offered by organic material lenses. The chemical process also offers superior optical qualities since the temperatures used are lower.
Source : Association des Optométristes du Québec
treatments
Scratch resistant treatment
Because plastic lenses are more easily scratched than the mineral ones, manufacturers have developed a surface treatment which makes the surface harder and thus more resistant to abrasion. There are essentially four ways of applying this treatment:
- by dipping
- by centrifical application
- during the moulding process
- by vaporization under vacuum
The dipping process appears to be the best choice as it provides a scratch resistant layer that is thicker and therefore more resistant. The lack of standardized tests can, however, present certain problems in the quality evaluation of this product.
Anti-reflective coating
Because the density of ophthalmic lenses is very different from that of air, a part of the light is reflected by the two surfaces of the lens. These reflections can cause problems such as ghost images, glare and unaesthetic concentric circles as in the case of strong myopia.
In addition to the reflections caused by the surfaces of the lens, its edges (thick) are responsible for the concentric circles along the frames for the myopic with medium or strong prescriptions.
It is important to understand that the anti-reflective treatment does not completely eliminate the ghost images but it does reduce their intensity. Furthermore, the anti-reflective treatment increases the amount of light passing through the ophthalmic lens. There are different versions of this treatment namely the monolayer (a single layer) and the multilayer (several thin layers). The monolayer treatment increases the transmission from 92% to 98% and the multilayer treatment reaches nearly 99% transmission. The more layers, the better the transmission and anti-reflective properties.
The lens must be carefully cleaned and placed in a vacuum in order to remove all air that could interfere with the metallic deposit (magnesium fluoride for mineral matter). The lens is heated to 300 degrees Celsius and the fluoride, vaporized at a temperature of 2500 degrees Celsius, deposits on the surface of the lens.
Photochromic lenses
Photochromic lenses that darken according to their exposure to light were created during the 1960’s.
This process is possible through the addition of silver crystals, sensitive to ultraviolet light between 300 and 400 nm, to the liquid monomer. This is similar to what happens when light falls on photographic film emulsions which contain the same type of crystals.
More recent photochromic lenses are sensitive to the visible spectrum as well as the ultraviolet. There is a variety of shades but, for the reasons explained earlier, grey and brown shades are the most commonly used.
Certain factors affect the performance of the photochromic process. These lenses will darken less on very warm days whereas the process will be 8 to 10% more efficient. Furthermore, these lenses will not darken to their maximum until they have been exposed to the sun a number of times. In effect, they have an exposure memory and if they are put away for a certain period of time they will lose their exposure memory and will have to be exposed to the sun a few times to regain their normal cycle. Also, the thicker the lens is, the greater the darkening effect will be. Photochromic lenses absorb a good part of the ultraviolet rays but do not absorb the infrared.
Photochromic lenses transmit nearly 87% of the light when they are pale and 42% when they are dark. A darker version transmits a maximum of 40% and only 12% when they are darkened. They absorb ultraviolet rays between 290 and 315 nm but less than 3% between 315 and 380 nm.
Polarized lenses
Some individuals are bothered by glare caused by reflective surfaces such as snow, roads, water etc. Tints uniformly reduce glare in the field of view but are not particularly effective against light rays reflected by such surfaces. Light reflected in this manner is partially polarized, which is to say that its major plane of vibration is the horizontal whereas light waves are normally free to vibrate in all planes. Polarized lenses contain a filter that only lets through light vibrating in the vertical plane.
These lenses offer definite advantages to boaters, motorists and skiers who are bothered by light reflected off the water, ground or snow.
Polarized lenses are composed of a thin layer of polarized plastic laminated between two layers of cellulose acetate and pressed to the desired form. For glass lenses, the polarized plastic layer is laminated between two layers of glass. These lenses are also available in prescription form but they are relatively expensive.
Anti-ultraviolet treatment
Since most lenses, tinted or otherwise, do not have adequate absorption between 300 and 400nm, it was necessary to find an effective way to block these rays that are dangerous for the eyes. The simplest way to do this is with a surface treatment containing ultraviolet blockers. This is easily done by dipping the lenses.
Source : Association des Optométristes du Québec
the lenses
Single vision
This consists of a lens with a single correction for a given distance: for myopia, hypermetropia and/or astigmatism. This lens is said to be single vision because its optical power is the same for the whole lens.
Bifocals
A person may require two different corrections, one for distance vision and one for near vision, in his lenses. The upper area of the lens corrects the distance vision and the lower corrects the near vision for various purposes. This happens because the eye has difficulty in focusing close-up. As this problem adds to problems of refraction at a distance, two or more regions, or segments, with different optical powers are used allow comfortable vision for tasks at different distances: reading, sewing, close-up work etc. There are a number of different types of bifocals on the market.
We talk of round segments, ST-25 segments, executive segments and trifocals. These different segments are used according the wearer’s occupation. With the exception of progressive lenses which are more a question of aesthetics, these other shapes really depend on the occupation and type of work involved.
Progressive lenses
For some time now, a new type of lens having an invisible segment has been available. These are called “progressive” lenses because the optical power changes from the upper region to the lower. The optical modulation allows distance vision as clear as the near or intermediate (60 – 80 cm) vision. This is a more functional type of lens because it allows the wearer a “continuous” vision.
It can focus on objects situated at various distances from the eyes, while the bifocal lens only permits sharp vision at two given distances namely those for which the upper and lower segments have been calculated. This is also more aesthetically appealing because the lower segment is not apparent.