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Controlling Light: Transmission, Reflection and Absorption by Spectacle Lenses

2 CPD in Australia | 0.5G in New Zealand | 30 April 2018
Figure 1. The light spectrum


By Nicola Peaper

German abstract artist, Hans Hofmann said, “In nature light creates the colour. In the picture, colour creates the light.” As optometrists we have the power to control light transmission, reflection and absorption… and in
doing so enhance our patients’ visual experience, their comfort and quality of life. It all comes down to choosing the most effective lens solution.

LEARNING OBJECTIVES

  1. Appreciate how different materials transmit various wavelengths of light and the effect this may have on vision, contrast and colour perception.
  2. Understand the various uses of anti reflection coatings to increase or reduce transmission of light through a lens.
  3. Understand the difference polarisation, tinting and photochromic treatments have on contrast and colour perception.

 

Light is essential for vision, colour perception and some aspects of health however, over the last five years, awareness of the negative consequences of both artificial light and sunlight has increased. Patients require protection from glare and possible light-induced ocular damage while maintaining contrast and colour perception. This article will describe and compare current solutions that control light by transmission, reflection and absorption.

The Light Spectrum

It is not the intention of this article to discuss in depth the possible damage and health benefits of various wavelengths in the electromagnetic spectrum, indeed in a paper on UV radiation1 Coroneo listed over forty ophthalmic conditions in which sunlight has been implicated in pathogenesis over the eyelid, anterior and posterior eye. It is, however, necessary to be aware of the way that different wavelengths are transmitted through the ocular media. The light spectrum can be split into non-visible and visible (Figure 1).

In the young eye with clear media the transmission can be represented by Figure 2. As the lens progressively yellows with age, the shorter wavelengths do not reach the retina.

Figure 2. Light transmission in the eye

Infra Red Wavelengths

Infra Red (IR) can also be categorised into zones according to the biological effects, but it is rare, if ever, for natural solar radiation to exceed the maximum permissible exposure level for each category.2

Visible Wavelengths

Visible light can be split into the various colours. The eye does not respond equally to all wavelengths of light. When the visual response (luminous efficiency function) to daylight is plotted, it forms a bell curve with the maximum response in the green/yellow portion of the spectrum. The peak moves steadily towards the blue end of the spectrum in mesopic and scotopic conditions (Figure 3). If recommending blue light protection, consider the fact that these wavelengths are important for vision in low light levels.

Figure 3. Visual response in different light conditions

For colour perception to be maintained, a spectacle lens should transmit all wavelengths of visible light equally and then the material is referred to as being white. Any discolouration of a material means that certain wavelengths are being absorbed and colour perception will be affected. High index materials easily oxidise and yellow with age unless an antireflection coating is added.

Discomfort caused by visible light is referred to as glare and can be split into categories depending on the effect on the eye:3

Disabling Glare

Disabling glare occurs when there is excessive, intense light when facing the sun. It is often caused by reflected light from shiny surfaces such as roads, water, sand and snow. Disabling glare can block vision as the intense light can significantly reduce contrast on the retinal image.

To diminish the effects of blinding or reflected glare, a polarised lens should be recommended. These absorb both unwanted radiation depending on the tint density, generally either 65 per cent or 85 per cent, and plane polarised light from reflecting surfaces.

Light can be represented by wave motion that vibrates in all directions. When incident upon a horizontal surface, such as a road, the reflected light wave vibrates in a direction parallel with the surface. A polarising filter set vertically will therefore absorb the reflected glare (Figure 4).

Figure 4. Polarisation of reflected light

When checking spectacles fitted with polarising lenses, always check the axis of polarisation is vertical and that the axis is the same in both eyes.

As reflected light is reduced and so contrast is increased a polarised solution is ideal for sports, fishing and driving in bright sunlight (Figure 5).

Figure 5. Glare reduction from polarisation

Some occupations that rely upon reading instrumentation behind polarised glass or looking out through toughened glass preclude the use of polarising lenses. Examples include pilots and drivers of heavy machinery. In these cases, a solid tint can be used. To have the best glare protection, the tint colour should reduce the shorter, blue wavelengths of light as they scatter the most easily. Colours such as yellow, amber and red will give the best contrast although brown may be preferred as a compromise between cosmetics and contrast (Figure 6). If colour perception is the most important aspect for the patient, then a grey tint will cause the least disruption of colours.

Figure 6. Tint colour recommendations for good contrast and glare reduction

When prescribing any type of sun protection, advice should be given not only on the level of protection provided, but also on the suitability for driving.

  • Any lens with an absorption of greater than 20 per cent is not suitable for night driving.
  • Any lens with an absorption greater than 92 per cent is not suitable for driving at all.4

Taking into account light levels in a car, tints of between 60 per cent to 80 per cent absorption are more advisable.

Whenever ordering a solid tint for sunglass purposes, ensure that UV absorption is included.

A back surface antireflection coating should always be recommended with a sunglass lens as:

  • The pupil will be enlarged due to the tint, which will reduce the eyes’ natural protection. A back surface coating with UV protection (see later) is essential protection.
  • The light reflections from the back surface of the lens will distract the wearer from the reduced intensity image seen through the tinted lens.

The most efficient solution to disabling glare is a polarising lens with a back surface antireflection coating that reduces UV reflection.

If polarising materials are precluded by the visual task, a contrast tint with UV absorption and a back surface antireflection coating that reduces UV reflection should be recommended.

If colour perception is important to the patient, then a grey lens solution with appropriate UV protection should be recommended.

Discomforting Glare

Discomforting glare is caused by normal sunlight conditions and depends on the individual’s light sensitivity. As such it can be experienced regardless of weather conditions and often occurs when moving from one lighting condition to another. It causes squinting and eye fati gue.

While fixed tints offer immediate relief from bright light, in variable light conditions they may be too light or too dark. Self-adjusting photochromic tints work best in these circumstances.

Photochromic glass first appeared in 1964, although the speed and density of tinting took a further 10 years of research and development. The basis of the material is that silver halide crystals are added to the glass. UV and short wavelength visible light cause silver particles to disperse in the glass causing a tint. In the absence of UV light, the silver recombines with the halogens to produce clear glass. The problem with molecules being distributed through the material is that uneven tinting occurs in high powered lenses, specifically high minus.

The first photochromic plastics materials appeared in the 1970s, but problems with inactivated colour, maintaining the same colour during the whole lightening and darkening period, and final colour, meant that these materials were not widely used until the mid 1980s (Figure 7).

Figure 7. Early plastic photochromic materials had a brown residual tint and changed colour as they darkened

The principle behind photochromic plastics is a colourless organic molecule which, when excited by UVA irradiation, beaks a chemical bond in the molecule. The accompanying structural and electronic change shifts the absorption spectrum into the visible region – the molecule is now coloured (‘activated’). The molecule relaxes back to the colourless (‘inactivated’) state predominantly by thermal rearrangement (‘fading’) (Figure 7).

The initial molecules produced colours ranging between violet and green but to achieve a grey colour, a molecule ranging from yellow to orange was needed. In the 1990s, naphthopyrane molecules, which result in colours from yellow to orange depending on the molecular structure, allowed grey to be developed (Figure 7).

It has taken 25 years, and the brain power and work of many chemists, to develop technology that maintains a colour shade produced from several different coloured molecules, and is able to change at the same rate, including a rapid fade back time.5

The environment for these molecules; ie. the plastic material, has an important influence on the performance of the photochromic lens and has been similarly optimised. The photochromic organic molecule must fit into the matrix of the plastic material. If there are different colour molecules, they all have to be incorporated into the plastic material in the right concentration.

Photochromic low and mid-index polymers are usually either methacrylate-based polymers with homogeneously dispersed photochromic dyes (‘in mass’) or derivatives of CR39, which are then imbibed with photochromic dyes (‘on surface’). For these types of polymers, there is no way to attain a refractive index of 1.6 or higher. High-index plastic lenses are based on completely different chemical polymers and all attempts for mass tinting or imbibing these types of lenses failed. The higher index materials tend to have a thin layer of photolacquer applied to the surface.

Present day photochromic plastics are available in a wide variety of colours, including high contrast colours, and polarising versions are available. The level of polarisation depends on the degree of activation.

The general drawback of these products is that they react mainly to UV light and so do not react fully behind the windscreen of a car. They also tend to be heat sensitive, reacting less in high temperatures. One way around both of these problems is to recommend a material that holds a residual tint of around 40 – 50 per cent. These not only provide a good compromise, but are excellent for sports such as golf where light concentration varies during the day.

When recommending photochromic lenses as a solution to discomforting glare, choose clear to 85 per cent for general purpose use and 45 per cent to 85/90 per cent for sports use. Recommend a dual sided antireflection coating for general use and a back-surface coating for sports use, both with back surface UV protection.

Distracting Glare

Distracting glare causes eye fatigue and annoyance and is often associated with light reflected from the back surface of a lens or internal reflections from a lens, especially when driving at night. The solution to this is an antireflection coating.

Consider what happens when light is incident on a lens surface (Figure 8).

Figure 8. Light incident on a lens has multiple reflections from both surfaces

Light is reflected from both the front and back surface. This will cause a reduction in contrast as the transmitted light is reduced, and there are annoying reflections from both surfaces of the lens. Depending on the refractive index, transmittance reduces from 92.4 per cent with 1.5 index to 88.2 per cent with 1.67 index. To cancel reflection, a clear coat that is one quarter wavelength thick needs to be deposited on the surface of the lens. This coat will produce a wave that is half a wavelength out of phase with the reflection from the lens surface. The two waves cancel out to produce a reflection free surface.

As the wavelength of visible light varies from 380nm to over 700nm, it is not possible for one layer to cancel out reflections for the entire visible spectrum. Indeed, the first coatings were single layer aimed at reducing reflections around the centre of the spectrum at around 550nm. This coat stopped yellow/green reflections but allowed considerable reflections of red and blue. This gave the residual colour of amethyst but still allowed reflectance of around 4 per cent.

To reduce reflections over the whole of the visual spectrum, a sequence of layers for different wavelengths is used. These combinations tend to produce coatings that have a residual minimal reflectance around the green area of the spectrum, which gives them a green hue.

More recently, antireflection coatings have been developed to reflect specific parts of the visible light spectrum and to reduce reflection of non-visible wavelengths such as UV.

To improve the quality of vision, by reducing reflection and increasing transmission, an antireflection coat should always be recommended.

Visible Wavelengths (Blue)

Recently there has been a lot of discussion about blue light. This has been triggered by our increased exposure to LED light sources, particularly in the evening and at night.

Discussion revolves around three aspects of blue light:

  1. Visually, the short wavelengths of blue light scatter easily to produce discomforting glare and reduce contrast. This has significance when using digital devices and also driving in low light conditions and at night.3
  2. Wavelengths around the blue turquoise part of the spectrum stimulate the intrinsically photosensitive retinal ganglion cells (melanopsin-containing retinal ganglion cells) to affect circadian rhythm and also stimulate pupillary reflex.
  3. The shorter, therefore higher energy blue wavelengths below 450nm may cause photochemical damage to the retina with long term exposure. However, a 2016 review of current knowledge of the effects of blue light exposure on ocular health concluded: LEDs with an emission peak of around 470–480nm should be preferred to LEDs that have an emission peak below 450nm… exposure to blue light from LEDs in the range 470–480nm for a short to medium period (days to a few weeks) should not significantly increase the risk of development of ocular pathologies, this conclusion cannot be generalised to a long term exposure (months to years)… additional studies on the safety of long-term exposure to low levels of blue light are needed to determine the effects of blue light on the eye.6

There are three ways to reduce the amount of blue light reaching the eye:

(i) An Anti-Reflection Coating

Most lens manufacturers produce a coating option. The coat reflects blue light and so has a blue residual hue. The colour is indicative of the wavelengths reflected and none of these coatings block blue light completely. Indeed, on average they reduce blue light by about 10 per cent, but individual manufacturers should be consulted for details. As they reduce the transmission of blue light, along with the blue surface hue, the lens will have a residual amber colour.

These coatings are ideal for digital devices as they are designed to reduce specific wavelengths of blue light. They are not as suitable for night driving as they will have back surface and internal reflections.

(ii) A Tint

Tints to reduce blue light are yellow/amber in colour. The amount of blue light absorbed depends on the density of the tint. A tint with an efficient antireflection coating is a more efficient way to reduce blue light while driving. For driving at night, the density of tint is governed by legal requirements.

(iii) The Material

Recently, lens manufacturers have produced materials to block the shorter wavelengths. Materials to block up to 420nm are available. The question that these materials address is “at what wavelength does light become safe?”; ie. if UV is considered to stop at 380nm, is light of 385nm absolutely safe? The decision as to which wavelength to block to is a compromise between possible protection and residual colour. The longer the wavelength blocked to, the more amber, less cosmetically acceptable the lens will be.

It should also be considered that these materials block 100 per cent of the wavelength specified and as such should have a highly efficient antireflection coating that is biased towards the yellow/red area of the spectrum.

UV Wavelengths

UV can be broken down as follows:

  • UVC 100 – 280nm. Blocked by the earth’s atmosphere. No ocular absorption.
  • UVB 280 – 315nm. Absorbed into the cornea. UVB can directly damage skin cells’ DNA and are the main rays that cause sunburn. They are thought to cause most skin cancers.
  • UVA 315 – 380nm. Transmitted into the ocular media. The longer the wavelength, the deeper the penetration. These rays are linked to long-term skin damage such as wrinkles, but they are also thought to play a role in some skin cancers.

N.B. According to the Cancer Council of Australia “both UVA and UVB can reach the earth's surface and are classified as human carcinogens”.

UV contributes to the formation of age-related cataract, pterygium, cancer of the skin around the eye, photokeratitis and corneal degenerative changes.

When advising a spectacle solution to protect the eye from UV light, it is essential to consider the three ways that light reaches the eye:

(i) Direct Light

Direct light from above or the sides of the spectacle appliance – very little can be done to protect from this with a spectacle appliance.

(ii) Transmission Through the Lens Material

Lens materials absorb a certain amount of the light that is incident on its surfaces and passing through the substrate. Absorption depends on the molecules that make up the material. Most plastics materials are made of molecules that will absorb the shorter wavelengths of UV while transmitting the longer wavelengths of visible light. It is important to note that generally 1.5 index clear plastic materials only absorb UV to 350nm. It is possible to add UV absorbers to the lens to make the material absorbent to 380nm but not all lens manufacturers do this, and those who do may distinguish between stock and grind products.

Another way to offer 100 per cent UV absorption is to use a UV dye. This will leave a residual yellow colour, signifying that some blue light is also being absorbed along with the UV.

UV protection is an important consideration when ordering a tinted prescription sunwear lens. Again, the tinting process differs between manufacturers. When receiving an order for an eighty-five per cent tint, some will automatically include UV absorption in the tinting process, while others will not.

The AS/NZS 1067:2016 Standard for Eye and Face Protection – Sunglasses and Fashion Spectacles, that dictates UV transmittance should be 100 per cent to 400nm, does not cover prescription sunglasses. It is, however, reasonable that patients will assume that prescription sunglasses will offer 100 per cent UV protection from transmitted light. It is therefore essential to know what UV protection your lens supplier provides with both their materials and tinting process.

Polarising filters, photochromic solutions and high index clear lenses generally absorb 100 per cent of UV light.

(iii) Reflection From the Back Surface of the Lens.

Because light is incident on the lens from behind, it will be reflected on to the nasal eye and adnexa. 

Coroneo1 suggested that peripheral light focusing, by the cornea of light incident from behind, on the lens and corneal limbus may contribute to cataract and pterygia formation. In the same way, depending upon the angle of incidence, light reflected from the back surface of a lens may be focused on areas of the eye that are normally protected from direct light. (e.g. the limbal stem cells, which may give rise to pterygia formation, would normally be protected by the superficial limbal cells). The inferior nasal cortical lens, where cortical cataract formation is common, would also be protected from direct light by the iris (Figure 9).

Figure 9. UV light reflects onto limbal stem cells and lens cortex

Basal cell carcinoma (BCC) is the most common cancer in the world. Eighty per cent of BCCs occur in the head and neck region, of which 20 per cent occur on the eyelids, with >50 per cent on the lower lid; 30 per cent on the medial canthus; 15 per cent on the upper lid; and 5 per cent on the lateral canthus. The infrequent involvement of the upper lid may be due to the protection by the eyebrow. In contrast, the frequent involvement of the lower lid may be the result of light reflection by the cornea onto the lower lid margin.7 The spectacle lens will also reflect UV onto the nasal area of the lower eyelid.

Back surface UV reflections can be minimised by the addition of an antireflection coating that allows UV to pass through the surface. Antireflection coatings were originally developed to reduce reflections of visible light from the surface of a lens. In the last few years, coatings have been introduced that reduce the reflection of UV light and allow it to pass into the lens substrate. This comes in the form of an extra layer in the AR stack on the back surface of the lens. Most lens manufacturers produce two coatings of this type:

  • Clear lens coating
  • Sunglass coating – there may be residual colour to sunglass coatings as they also tend to extend into the blue area of the spectrum.

To give the maximum protection from UV light, a material that blocks UV transmission – such as high index, polarised or photochromic – should be dispensed, in conjunction with an antireflection coating that stops UV reflecting from the back surface. The material or coating in isolation will not give the desired level of protection.

Conclusion

When advising a visual solution to a patient, it is important to consider the environment in which the spectacles will be used. The choices made by the practitioner to control light transmission, reflection and absorption will impact on the patient’s vision and comfort through contrast and colour perception. We should also be mindful that the choices made can have the potential to limit ocular damage. This is clear when controlling UV light for example, but in some instances, such as the control of blue light, it is less obvious and the value of control or maintaining a neutral position is something that is still hotly debated. The ability to change the patient’s visual experience, their comfort in their environment and maximum enjoyment of their lifestyle, make the control of light transmission, reflection and absorption a most interesting aspect for the ophthalmic practitioner. 

 

    

Nicola Peaper BSc (Hons) Opthalmic Optics, Cert 4 TAE, qualified as an optometrist in the UK in 1985 and practiced in private and corporate practice in the UK for 20 years. In 2001/2 she was employed as Ophthalmic Advisor to Kensington, Chelsea & Westminster Health Authority.

Since moving to Australia in 2005 she has worked in fitting laboratories advising on procedures and quality. In roles as State and National training manager, she has gained extensive experience in presenting the technology behind, and the prescribing and fitting of, ophthalmic lenses.

Nicola Peaper is currently Professional Services Manager for Rodenstock Australia.  


Nicola Peaper acknowledges the contribution of Helen Venturato whose CPD accredited presentation, “What You Don't Know Can Hurt You” provided information for this article.
References
1. Coroneo M. 2011. ‘Ultraviolet Radiation and the Anterior Eye’. Eye and Contact Lens Vol. 37 no.4:214-224.
2. Jalie M. 2008. ‘Ophthalmic Lenses and Dispensing’. Elsevier Butterworth Heinemann. 97 – 98.
3. Occupational Dispensing - A reference guide to professional advice and solutions for the workplace. ABDO May 2013.?
4. AS/NZS 1067.1:2016. Eye and face protection – Sunglasses and fashion spectacles
5. Zinner h, Schuster H, Weigand U, Muller W. 2011. ’25 years of Colormatic lenses’. Optician 23.09.11 p 12 – 16
6. Tosini G, Ferguson I, Tsubota K. 2016. ‘Effects of Blue Light on the Circadian System and Eye Physiology’. Molecular Vision 22:61-72
7. Yingyun Shi, Renbing Jia, and Xianqun Fan. Ocular basal cell carcinoma: a brief literature review of clinical diagnosis and treatment. Onco Targets Therapy. 2017; 10: 2483–2489

' Patients require protection from glare and possible light-induced ocular damage while maintaining contrast and colour perception '