Sunlight filters: eye protection and improvement of visual performance
Dr. Graziano Marusi.
Quality Control Manager
INTERCAST EUROPE S.p.A.
Personal eye and face protection WG UNI Coordinator
Solar Radiation can be subdivided into three main sectors:
UV radiation, between 280 and 380 nm
Visible radiation, between 380 and 780 nm
Infrared radiation, between 780 and 2000 nm
Fig. 1
The intensity of solar radiation at different wave lengths depends on altitude, latitude and weather conditions. Conventionally, the spectrum of solar radiation at the ground level is defined as shown in figure 1.
The degree of dangerousness of solar radiation depends on its energy (therefore on wave length). The shorter the wave length the more dangerous.
The dangerousness of electromagnetic radiation is linked to its capability to interact with the organic molecules which, in our case, make up the visual apparatus.
Ultraviolet radiation
Although it is the solar spectrum component that is present in the smallest quantity, it is the most dangerous component since it has sufficient energy to break chemical bonds, thus causing irreversible damage. As ultraviolet radiation is a dangerous but not necessary component for vision, the objective of sun filters is its total elimination. The damage caused by ultraviolet radiation mainly affect the cornea and crystalline, since the retina is screened almost totally by crystalline. Cataract is among the commonest pathologies affecting the crystalline.
Visible radiation
Obviously, this is the most important component for vision, and it ranges from violet to red. The violet-blue component (HEV) has a very great energy (it is the closest component to ultraviolet), being sufficient to cause cumulative damage to the retina. The HEV component is the radiation with the highest energy to reach the retina (ultraviolet is blocked by the crystalline). So far, no evidence has been found of direct damage to the eye at the normal amounts present in solar light, however there is substantial scientific evidence of cumulative damage to retina, commensurate to the exposure time and intensity of radiation.
Since it is a fundamental component for correct vision of colours and images, it should not be completely eliminated: instead, the right balance between effective protection and correct colour vision should be sought. Although they eliminate all Blue light, the so-called Blue Blocker filters are characterised by a very strong colour distortion which makes it difficult for people to wear sunglasses for a long time, thus annihilating the advantages of protection. The damage caused by blue light is cumulative, therefore the limited use annihilates the effectiveness of protection.
Infrared radiation
It is the lowest energy radiation, therefore the less dangerous. In practice, it consists of thermal energy, which is not sufficient to break chemical bonds. Its possible dangerousness is connected with the exposure to very intense artificial infrared sources (e.g. in foundries or during welding processes). There is evidence that, in some cases, infrared radiation can act as a catalyst increasing the damage caused by Ultraviolet radiation. However, this benefit is useful only if it has not been eliminated.
Filters designed to protect the eyes from solar radiation
Ultraviolet rays
Research in the field of protection has been focused initially on the most dangerous component of Solar Radiation: ultraviolet rays.
In the absence of any effect on vision, the objective of all manufacturers has been its complete elimination. This has been accomplished by now by almost all organic filters available in the market, while it has not been so for glass filters due to the difficulty in including pigments absorbing UV rays without affecting excessively the filter colour (above all in the case of grey or green glass filters).
In order to increase protection, UV ray absorption has been extended up to 400 nm, to include a small area of the visible spectrum which has limited effects on vision. For this reason, in spite of the fact the standards require that ultraviolet radiation be stopped at 380 nm, they commonly refer to UV400 filters.
Infrared rays
As far as infrared radiation is concerned, since it is not considered dangerous, no specific effort has been made in this respect, except for a few applications for high mountain use or commercial use. It is extremely difficult to obtain the elimination of the infrared component, therefore also in filters for industrial use, against Infrared Radiation, it is always only partial infrared radiation attenuation.
In addition, when the infrared radiation is filtered, it is inevitable to obtain also an attenuation of the red colour radiation, to the detriment of visibility of traffic signals and danger signs.
Visible
It is the most delicate area of operation, since any action has simultaneous influence on colour vision and protection, with sometimes opposite effects (see the extreme case of Blu Blocker). For this reason, this is the area on which most studies are being concentrated now, that was addressed in more recent times and that is still evolving.
In this case the objective – from the protection viewpoint - is to reduce the amount of Violet-Blue light (radiation between 380 and 500 nm) that reaches the eyes. The reduction of this components leads to a brownish colouring of the filter and relative emphasising of the Red-Brown colours.
Since damage is cumulative, it is very important to consider – besides the protection factor – also comfort. A comfortable filter is preferably worn for longer time, since the amount of Blue light received is reduced along the time. Therefore, the best filter should give the highest degree of protection and, at the same time, be as comfortable as possible so that the consumer is lead to use it for a longer time.
One of the approaches attempted to optimise the balance among protection, comfort and colour vision takes into account the sensitivity curve of human eyes to solar radiation. This curve shows that the eye is little sensitive to violet components between 380 and 430 nm (and, as such, the have little impact on the vision mechanism) which are at the same time the visible radiation with the highest energy.
Therefore, the ideal choice is to obtain a transmittance curve starting from 0 to 380-400 nm, and progressively rises up to 500 nm, thus opening progressively a wider and wider window as the radiation becomes less dangerous and – at the same time – more important for the colour vision. Also natural melanin – a component of the skin and eyes – is based on this principle. As a consequence, the simplest and most effective solutions seems to be melanin-based lenses.
Fig. 2 – Comparison of a transmittance curve of a melanin lens with the V sensitivity curve.
Gradient filters
An important help to protection is given by gradient filters. Thanks to their particular design, they provide effective protection from sky glare, although they maintain an optimal luminance level towards the ground and to the front. Among the applications destined to benefit greatly of gradient filters, the most important is driving. gradient filters allow for a considerable attenuation of sky light, a good road luminance level and optimal visibility of the dash board.
Effects of sunglass design and dimensions
As to the protection from ultraviolet rays, the sunglass design and size are very important factors. Small-sized and/or regular flat flamed sunglasses, allow considerable amount of reflected light to reach the eye, thus annihilating most of the screening effect. There is evidence that, due to the effect of reflected light, up to 2-5% of ultraviolet radiation can reach the eye.
Improvement of visual performances
Through an appropriate transmittance curve of solar filters, visual performances can be improved. A major range of action covers the improvement of contrast and the reduction of glare.
Attenuation of Blue light
One of the most common approaches is the attenuation of blue light. The blue component of light is focused with greater difficulty by the eye and, at the same time, it has a tendency to attenuate the colour contrast. Paradoxically, when blue lenses are worn, the eye has a black and white perception of what it sees.
Based on this principle, many lenses for sporting activities or driving have been designed, which are usually tinted brown in colour.
Increase of chromatic contrast
In this case the principle used is completely different, and it is based on the analysis of the colour perception curves of human eye as defined by CIE 1931.
Sensitivity curves show that there are sufficiently wide overlapping areas between chromatic receptors. In these areas it is difficult for the brain to rapidly define the differences in colour and, consequently, the chromatic contrast.
Through the use of special equalising filters, the overlapping areas can be reduced, and receptors can be characterised more neatly. In this way, confusion areas are attenuated, and the chromatic contrast is improved, while response time is reduced.
This type of filters has been used in competitions under low contrast conditions with significant improvements in terms of performances.
Fig. 3 – Coordinates of a tristimulus curve CIE 1931.
Among the benefits of this technique there is the possibility to obtain a variety of colour (among which grey), while maintaining a high level of performance.
Fig. 4: transmittance curve of a high contrast lens compared with tristimulus curves.
Fig. 5: Tristimulus curves modified by effect of a high contrast filter.
For a better comprehension of the benefits in terms of improvement of the chromatic contrast, one can examine the relative coefficients of transmittance of the colours Blue, Red, Green and Yellow, as defined by the European Standard EN 1836. These coefficients were defined by the standard to provide an indication of the degree of attenuation of road sign colours as compared to the background by effect of the filter used. Factors smaller than one indicate an attenuation of specific colour. Greater values indicate enhancement of the specific colour and, consequently, better visibility.
Normally, a grey filter has factors equal or close to one, while in the other cases the increase of any of the factors is compensated by an equivalent attenuation of others.
In the case of high contrast filters which use the principle described in this paragraph, a generalised increase of the relative transmittance factors is obtained instead, without affecting any of them.
| Tv | Red Q | Yellow Q | Green Q | Blue Q | |
| Grey HC filter | 15 | 1.54 | 0.96 | 1.04 | 1.59 |
| Standard grey | 15 | 0.98 | 0.96 | 1.04 | 1.26 |
| Standard brown | 15 | 1.42 | 1.27 | 0.83 | 0.85 |
| Blue Blocker | 14 | 1.99 | 1.61 | 0.60 | 0.61 |
Conclusions
To obtain maximum protection from a solar filter, one should:
To improve visual performances, one should:
Richard W. Young, Ph.D.
Professor emeritus, UCLA
Member, Jules Stein Eye Institute
The message I have to share with you is an important one. It
describes what I believe to be the greatest discovery of the 20th
century affecting visual health. It sets the stage for a new era
in the next century. And as it turns out, it depends importantly
on the participation of eyewear profession and press.
The message has three parts. First, the problem of
sunlight-related eye deseases is serious. Indeed, it represents
an impending visual health crisis. Second, the precise cause of
the problem has been identified. Third, the means of protection
has also been discovered, and is really available.
The sunlight-related eye diseases include age-related
cataract, age-related macular degeneration, pterygium, cancer of
the skin surrounding the eye and photokeratisis. Everyone is at
risk. No one, rich or poor, anywhere on earth, is immune to the
tragedy of sunlight-related eye diseases.
Furthermore, these are major eye diseases. Cataract and macular
degeneration are the principal causes of vision loss in our
society and the primary causes of blindness in the world.
The most common of the sunlight diseases is cataract. Cataract is
costly in human terms of lost vision and also in monetary terms.
Fifty billion dollars were spent for cataract surgey during the
decade ending in 1992, with well over a million cataract
surgeries performed annually. A person with a normal life span is
now more likely to have a cataract operation than any other
surgical procedure.
Even worse is macular degeneration, the major cause of blindness
in elderly Americans. Macular disease produces a degeneration of
the light-sensitive retina in the worst conceivable
location-directly in the center of the visual field. Wherever the
person looks, there is always a blind spot blocking the desired
view. Reading is impossible. This is a career-ending disease and
nothing can be done to restore the lost vision.
What does the future hold if present trends continue? In the year
2040, despite an estimated 100 millions cataract operations
during the intervening years, 25-36 millions Americans will
suffer from cataract and another 15-20 millions will have macular
degeneration.
The problem is serious.
The sunlight-related diseases have different symptoms and affect
different parts of the eye. Cataract is a cloudiness of the lens.
Macular degeneration destroys the central part of the retina.
Cancer of the circumocular skin, pterygium and photokeratitis are
afflictions of the front of the eye. But they all belong to the
same family of diseases, because they are all share the same
primary causal factor: sunlight.
Having estabilished that sunlight was the source of these eye
diseases, vision scientists set out to learn what parts of
sunlight were hazardous.
Sunlight is radiation-pure, radiant energy. There is a range or
spectrum of energy particles-low energy in the infrared, more
energy rising continuously through the visible spectrum through
the red, orange, yellow and green, high energy in the blue and
violet and highest of all in the invisible ultraviolet.
It turned out that the hazardous component of sunlight was all
concentrated in the same place, at the high-energy end of the
spectrum, in the blue-violet and ultraviolet radiation.
Ultraviolet radiation proved to be the cause of all the
sunlight-related eye diseases axcept for macular degeneration.
The retina of eye, the site of macular degeneration , was spared
ultraviolet radiation's deleterious effects because UV is
absorbed in the anterior part of the eye, where it causes the
sunlight-related eye diseases situated there. (That is why we
can't see UV; it does not normally reach the light-sensitive
retina). It is the high energy violet and blue part of the
sunlight spectrum that proved to contain the radiation that harms
the retina.
The good news is that protection against all of these
devastating eye diseases can be achieved simultaneously without
disturbing normal vision, simply by using the right kind of
sunglasses. The key idea is to absorb the hazardous radiation in
the sunglass lens, before it can strike the eye and produce
damage. Since the harmful components of sunlight have been
identified, it is now possible to design a lens that can protect
and preserve visual health.
The scientific prescription for a protective sunglass lens begins
with 100% UV absorption-all of the UV, both A and B, up to 400
nm. UV is useless for vision and harmful to every part of the eye
that absorbs it. Complete UV protection should be built into all
eyewear, not just sunglasses. There is no justification for
exposing delicate eye tissues to ultraviolet radiation.
The second part of the prescription protects the retina against
macular disease: the sunglass lens should absorb practically all
of the violet/blue radiation. Why not all of it? Absorbing 100%
of violet/blue light has an unwanted side-effect: it produces
severe color distortion. Blue and violet appear grey, yellows
fade, purple looks red. Many people will not wear sunglasses that
disturb color vision.
Fortunately, scientists have found that as much as 96% of the
violet/blue radiation can be eliminated without affecting color
vision. This is a most favorable result, because it means we can
have both protection and perfectiv natural visual function.
By using sunglasses lenses that absorb 100% of the UV and up to
96% of the violet/blue we have a simple, safe, effective,
inexpensive, readily available means of protection against all of
the sunlight-related eye diseases.
Wearing the right kind of sunglasses is the most inexpensive kind
of visual health insurance imaginable. It makes sense to use our
scientific knowledge to protect our precious eyesight.
Use of protective sunglasses should begin early in life and be
continued throughout the lifespan. Cataract and macular
degeneration are age-related diseases: they characteristically
occur late in life because they are diseases of aging
(deteroration of the normal structure). When aging eventually
interferes with function, we call it a disease. Slowing the rate
of aging by minimizing exposure to UV and violet/blue radiation
can pastpone the onset of the sunlight-related eye deseases. A
20-year delay would practically eliminate cataract and macular
disease as significant causes of visual impairment in the United
States.
Preserving visual health with protctive sunglasses is a concept
that has now been supported by the American Optometric
Association, the American Academy of Ophthalmology, the American
Public Health Association, and Prevent Blindness America.
The discovery of the cause and means of protection against
humanity's main eye afflictions presents an enormous and
unprecedented opportunity for the eyewear industry, particularly
for the manufacturers and distributors of sunglasses and the news
media that cover these professions.
First, there is the opportunity to raise the status and improve
the image of the industry by actively pursuing the link between
sunglasses and visual health. By joining forces with the eyecare
establishment the industry can become a partner in the movement
to prolong visual health.
Secondly, there is the opportunity to increase the market for
sunglasses. People who might otherwise not purchase sunglasses
are more likely to do so if they realize that it is a simple,
safe, effective and inexpensive way to protect their eyesight and
that of their family members.
Thirdly, there is the opportunity for both the industry and the
related press to make a positive contribution to the visual
health of the public. By making available a wide range of sunwear
of the right kind, and by prominent use of the linkage of
sunglasses and visual health in marketing, the industry can
enhance the public's understanding of this issue. The press is
particulary well positioned to deliver the important message of
the seriousness of sunlight-related eye diseases and the means of
protection that are available.