Continuing Education
Polarisation and polarising lenses
In this CET article on polarisation and polarising lenses, Andrew Keirl revisits the theory of polarisation and discusses some of the polarising products available (C1841)
Figure 1. An electromagnetic wave
The term polarisation has two meanings. Firstly, it is used to describe a particular property of light - for example, linear, elliptical, circular or partial polarisation, and secondly, to describe the production of polarised light by means of scattering, reflection, selective absorption or double refraction. In optometry and ophthalmic dispensing, we are usually interested in polarisation by reflection.
Light is an electromagnetic wave. Such a wave is illustrated in Figure 1 which shows the electric and magnetic fields. The electric and magnetic fields are perpendicular to the direction the light wave moves, and the electric field lies in one plane. Light is often described in terms of a wave motion. Using the wave analogy, the polarisation of light waves can be best illustrated by the shaking of a rope (Figures 2a to 2d). Plane or linearly polarised waves are those in which the oscillations, which make up the wave, occur in only one direction and are perpendicular to the motion of the wave. A wave generated on a rope by shaking the end up-and-down is a plane-polarised wave with the plane of polarisation vertical (all the oscillations are in the same plane). The rope is travelling outwards but the waves on the rope are oscillating (moving) up and down. If the rope is shaken side to side a horizontally plane-polarised wave is produced. Moving the end of the rope in a circle at a constant speed can produce a circularly polarised wave.
| Figure 2. The polarisation of light waves can be best illustrated by the shaking of a rope |
Polarisation is a phenomenon associated with transverse waves only. In fact, one of the properties of a transverse wave is that it can be polarised. Waves can be described as transverse if the oscillating elements move in a direction perpendicular to the direction in which the wave travels. A ripple on a pond or a wave on a rope are easily visualised examples of transverse waves. Transverse waves cannot propagate in a gas or a liquid because there is no mechanism for driving motion perpendicular to the propagation of the wave. In order for a transverse wave to propagate in a medium, the medium requires shear strength - a characteristic of solids but not of liquids or gases. Longitudinal waves cannot be polarised since the oscillations occur in the direction in which the wave is travelling. Sound waves, therefore, cannot be polarised.
| Figure 3 |
Most light as it comes from the sun or other incandescent or fluorescent source is unpolarised. It is a mixture of light polarised in different ways, in different directions and to different degrees. Having said that, linearly or plane polarised light is a surprisingly common phenomenon in everyday life. Light reflected from any smooth surface, such as a wet road or a polished table top, is partially linearly polarised. The light of the blue sky, which is sunlight scattered through an angle, is polarised. Insects such as honeybees can detect the polarisation of the sky and use its direction in relation to the sun for navigation.
An unpolarised wave is much more difficult to imagine, but it can also be formed on a rope if the end is shaken randomly in all directions, horizontally, vertically and in all diagonals.
| Figures 4 and 5 |
POLARISED AND UNPOLARISED WAVES
As opticians, we are of course interested in light waves. However, it is useful to imagine transverse waves as waves travelling along a rope.
Figure 3 shows a transverse wave oscillating in the y-direction (in this case vertical) while Figure 4 show waves oscillation in the z-direction (horizontal). In both cases the waves are travelling along the x-axis.
| Figure 6 |
Oscillations are not limited to the y or z-directions. The wave can make oscillations at any angle in the y-z plane as long as the oscillations are at right angles to the direction in which the wave is travelling (Figure 5). If all the oscillations occur in one plane, as shown in Figures 3 to 5, the wave is said to be polarised. If oscillations occur in many or random directions, the wave is described as unpolarised. Figures 6a and 6b summarise the difference between polarised and unpolarised waves. Staying with our rope analogy for the moment, Figure 7 shows a polarised transverse wave travelling along the rope in the x-direction, with its oscillations in the y-direction. In Figure 7, the rope meets an opaque surface with a slit cut into it. In Figure 7(a) the slit is aligned parallel to the y-axis, since the oscillations of the rope are parallel to the slit the waves pass through. In Figure 7(b) the slit is positioned perpendicular to the oscillations, along the z-axis. In this case, the waves cannot be transmitted through the slit.
| Figure 7 |
Although the mechanical analogy used above is helpful in understanding the phenomena of polarisation, it cannot be transferred directly to the comparable situation involving light waves.
POLARISER ACTION
There are a number of different devices that can act as polarisers for light. In all of these devices, often referred to as filters, an incident wave is resolved into two orthogonal components, one parallel to and one perpendicular to a specific pass direction or transmission axis. The component parallel to this direction is transmitted, and the other component is either absorbed or redirected in some way. If the incident wave is unpolarised, and if there are no losses by absorption, exactly half of the light energy is transmitted by the device. In reality, there is always some absorption and less than 50 per cent of the incident energy is transmitted. The plane of polarisation of the emerging wave is parallel to the pass direction (transmission axis) of the polariser.
POLARISTION OF LIGHT WAVES USING POLAROID
Figure 8
Polaroid (a trademark of the Polaroid Corporation) is the name of a type of synthetic plastic sheet which is used to polarise light. The original material, patented in 1929 and further developed in 1932 by Edwin H Land, consists of many microscopic crystals of iodoquinine sulphate (herapathite) embedded in a transparent nitrocellulose polymer film. The needle-like crystals are aligned during manufacture of the film by stretching or by applying electric or magnetic fields. With the crystals aligned, the sheet tends to absorb light, which is polarised parallel to the direction of the crystal alignment, but transmits light which is polarised perpendicularly to it. This allows the material to be used as a light polariser. This material, known as J-sheet, was later replaced by the improved H-sheet Polaroid, invented by Land in 1938. H-sheet is a polyvinyl alcohol (PVA) polymer impregnated with iodine. During manufacture, the PVA polymer chains are stretched such that they form an array of aligned, linear molecules in the material. Because of the way that a polarised light wave interacts with these molecules, the sheet of Polaroid only transmits the components of the light with the electric field vector perpendicular to the molecular alignment. The direction which passes the polarised light waves is called the transmission axis. The Polaroid sheet blocks the electric field vector component that is parallel to the molecular alignment. In Figure 8, light from a tungsten filament lamp is unpolarised. This light is directed on a sheet of Polaroid whose transmission axis is vertical. Light that emerges from the filter is polarised in the same direction as the transmission axis of the Polaroid.
| Figure 9 |
Polarising lenses are glazed with their transmission axis vertical. Polarising lenses are often supplied with engravings along the horizontal meridian to assist in the glazing process. The tolerance for the correct orientation of the transmission axis is usually ±3 degrees. Looking at the reflected light from any horizontal surface and rocking the head from side to side can assess the effect of a polarising lens. The maximum reduction of glare should occur with the head in the normal position.
MALUS' LAW
Whatever the state of polarisation of the incident light on a plane-polarising filter, the emerging wave is plane-polarised. If the emergent light is then incident on a second identical filter (an analyser), the finally emerging wave is also plane-polarised with the plane of polarisation parallel to the pass direction of the analyser (second filter). So, if the pass directions of the two filters are parallel, the wave emerging from the polariser passes straight through the analyser without further loss - the transmitted amplitude is a maximum. If the pass directions of the two filters are perpendicular, the pair are said to be crossed and the wave emerging from the polariser is totally blocked by the analyser - no wave is transmitted. A polariser/analyser pair is commonly found in practice in the form of a strain tester.
So how much light is transmitted when the transmission axis of a filter (analyser) is at an angle to the plane of polarisation? This is illustrated in Figure 9 which shows unpolarised light being polarised by passing it through a polariser. The polarised light illustrated then travels to the analyser, the transmission axis of which is at an angle <03B8> with the plane with the plane of polarisation of the incident beam. The beam that is transmitted from the analyser is polarised in the same direction as the transmission axis of the analyser and the intensity of the transmitted beam depends on the angle between the transmission axes of the polariser and analyser. This can be easily demonstrated by viewing light reflected from a horizontal surface such as a table-top surface through a polarising filter and then rotating the filter. A change in brightness of the reflected light will be observed, which is due to the change in angle between the plane of polarisation and the transmission axis of the polarising filter. For any angle (other than perpendicular or parallel) the amplitude emerging from the analyser is resolved into components parallel and perpendicular to the pass direction of the analyser, and only the parallel component is transmitted.
The intensity (or energy) of a wave is proportional to the square of the amplitude. Almost all detectors, including the eye, respond to the intensity of light, not its amplitude. The intensity of light emerging from a polariser/analyser pair is directionally proportional to the square of the cosine of the angle between the pass directions of the two filters. The intensity of the beam I transmitted through an analyser is given by the equation:
I = I0 cos2<03B8>
where I0 is the intensity of the incident light and I the intensity of the transmitted light. This equation is known as Malus' law and gives the intensity of the light transmitted through an analyser. If a sheet of Polaroid is used to reduce the intensity of a beam of polarised light by 50 per cent, the transmission axis of the Polaroid will need to be at an angle of 45¡ to the plane of polarisation of the beam.
POLARISATION BY REFLECTION
When light is incident normally at the surface of glass, plastics or other electrical insulator, it is partially reflected and partially transmitted. The fraction of the incident intensity which is reflected is termed the reflectance. In the case of normal incidence, the reflectance (symbol <03C1>) depends on the refractive indices of the material either side of the reflecting surface and is given by the equation
If the angle of incidence is anything other than 90¡ the reflectance depends on both the angle of incidence and the state of polarisation of the incident wave. Light reflected from a dielectric surface is partially and sometimes fully polarised. As mentioned above, when light is incident on a reflecting surface separating two media, some is refracted and some is reflected. The refracted light is partially polarised but the reflected light can be partially or totally polarised. The degree of polarisation by reflection is determined by the angle of incidence of the wave and the refractive index of the reflecting media.
| Figure 10. Brewster's Law |
The physicist Sir David Brewster noted that for a certain angle of incidence, monochromatic light was 100 per cent polarised after reflection.
Light undergoing refraction was partially polarised, but the reflected element was completely polarised parallel to the reflecting surface.
Brewster also noted that at this angle of incidence, the refracted and reflected beams were perpendicular. In summary, if an unpolarised wave is incident on the interface between two media such that the sum of the angle of incidence and the angle of refraction is 90¡, the reflected wave will be 100 per cent plane-polarised and the plane of polarisation will be perpendicular to the plane of incidence. The angle of incidence when this occurs is given the symbol ip which in turn can be found by the equation:
The above statement and associated equation is known as Brewster's law and the angle ip is known as the Brewster angle (Figure 10). The maximum effect or complete polarisation is found for reflection at a particular angle of incidence (the Brewster angle) for a particular pair of media. For an air/water surface where the refractive index of air is 1.0 and refractive index of water n2 is 1.33, this is found as follows:
As the refractive index of a material varies with the wavelength of incident light, the Brewster angle also depends on wavelength. It therefore follows that a beam of light does not have a unique polarising angle.
The term 'glare' is often used to describe light polarised by reflection. How often have has the phrase 'polarising lenses cut out reflected glare' been used in practice? The term glare is usually thought of as the presence of a bright light source that prevents us from seeing detail, causes visual discomfort or results in a lower performance in a visual test for example, visual acuity or contrast sensitivity.
There are three types of glare. Disability glare is described as a very bright light source that impairs the ability to see detail without necessarily causing visual discomfort. It is usually associated with excessive light received by the eye, either directly from a source or by reflection.
Discomfort glare is glare that causes visual discomfort without necessarily impairing the ability to see detail or interfering with visual performance. Veiling glare is glare caused by light scatter, resulting in a reduction in contrast. The quantity of light or luminous flux emitted from a light source is expressed in units of lumens. This is a measurement of the rate of flow of energy. Discomfort glare results if the luminous flux emitted from a source is approximately 3,000-9,000 lumens and disabling glare results if the luminous flux emitted is in excess of 10,000 lumens.
Polaroid, NuPolar and other polarising filters are designed to reduce the effect of reflected glare from horizontal surfaces. Sunlight reflected from wet roads, pond or river surfaces and from snowfields is strongly plane-polarised, with the electric vector in the horizontal direction and the effect is most pronounced when viewing at or near the Brewster angle.
When looking into a pond, the eye receives a weak signal from below the surface and a stronger light signal by reflection from the surface. The result is that the reflected light swamps the light from below the surface making the viewing of sub-surface objects difficult. The light reflected from the surface is strongly plane-polarised, and the light from below is unpolarised. If the scene is viewed through a polarising filter with its transmission axis vertical, the reflected light will be blocked and the weaker light from below the surface transmitted. The sub-surface detail will be seen without difficulty.
OTHER METHODS OF PRODUCING POLARISED LIGHT
Birefringent (double-refracting) materials such as calcite have different refractive indices for perpendicular polarisation components. This means that an unpolarised beam incident on a calcite crystal will be split into two beams polarised at right angles to each other. So if a small object is viewed through a crystal of calcite, two images are seen. Dichroic crystals act in the same way as Polaroid as their crystal structure allows only light with electric field components parallel to the crystal axis to be transmitted.
ADDITIONAL APPLICATIONS OF POLARISATION
Photoelastic stress analysis. Many normally isotropic materials, such as stress-free glass, become anisotropic and therefore birefringent when placed under stress and the birefringence caused is proportional to the stress involved. This phenomenon is called photoelasticity. If a double refracting material is placed between a polariser/analyser pair, the plane-polarised light emerging from the polariser may be converted to circularly, elliptically or plane-polarised light. Some of this light will pass through the analyser and the field viewed will no longer be uniformly dark as the parts of the material under stress become visible. Since birefringence is wavelength dependent, the patterns seen are often very colourful. This phenomenon is used in engineering to analyse stress in complicated structures and in optometry and ophthalmic dispensing to detect stress within optical components. Stain patterns in air or oil-drenched thermally toughened lenses can be tested with a polariser/analyser pair or 'strain-tester'. The patterns are often used to confirm the degree of toughening that the lens has received. The patterns are quite characteristic, but, in general, the strain pattern observed depends on the power and shape of the lens. The same device can be used to detect the strain produced by the incorrect glazing of spectacle lenses in a metal frame. Toughened car windscreens also produce a strain pattern when viewed through polarising lenses.
Optical activity. Optically active materials can change (rotate) the plane of polarisation of a beam of light. This process is due to the molecular structure of such materials and has been observed in crystalline materials such as quartz and liquid organic compounds such as sugar solutions. The most common use of optical activity is in liquid crystal displays (LCDs). A typical LCD consists of a small cell of aligned crystals sandwiched between two transparent plates between a polariser/analyser pair. LCDs can be found in digital watches, ATMs, car instrument panels, mobile phones and computer screens. Patients often comment that they cannot see LCDs on their car instrument panel when wearing polarising lenses. This is because the transmission axes of the LCD and the polarising lenses coincide. Polarising instrument manufacturers appear to be aware of the advantages of polarising lenses for driving and are adjusting the orientation of the transmission axes of their LCDs to an oblique meridian in order to keep screens and displays readable.
BENEFITS OF POLARISING LENSES
Polarising filters are particularly useful for motorists as they reduce glare from bright sunlight reflected from road surfaces. They are also useful for many sporting activities. Anglers in particular benefit greatly from the use of such lenses. The main clinical advantage of polarising lenses is the alleviation of possible asthenopia due to the reduction/elimination of glare from reflected light.
Drivers appear to be subjected mostly to discomfort glare which can be problematic when the sun is low in the sky, especially during the late autumn to early spring. In such a situation, discomfort glare can cause momentary dazzle, disturb vision and lower performance when driving.
A recent wearer trial was conducted in Northern Ireland to examine and quantify the gain (if any) of a polarising lens compared to a tinted CR39 lens. Thirty professional ametropic drivers were recruited for this double masked test where both pairs of lenses were glazed into identical frames. The CR39 lenses were tinted with a colour equivalent to the polarising lenses. The wearers had each pair for a two-week period commencing with either the polarising lenses or the CR39 tinted lenses. After the trial, the subjects had to give, with reasons, a preference.
Twenty-eight wearers completed the test and 21 of the 26 who expressed a preference chose the polarising lenses because they provided good vision in all weather conditions and offered more comfortable vision due to glare reduction. Nineteen of the 26 wearers chose the polarising lens for global comfort and for enhanced colour reproduction. The subjects in the trial noted that the polarising lenses were not only beneficial when looking directly at the road surface, but also when looking at the windscreen and at the car in front. The benefits of polarising lenses for driving are well known, but the results of this short trial are a useful reminder of the visual advantages of polarising lenses.
Polarising lenses are of course used in water sports and by skiers. They also provide excellent colour definition and contrast and give good UV absorption. Other clinical uses of polarising lenses include:
If a tinted lens is to be used for reading outdoors, the patient will benefit from a reflection-free coating applied to the back surface of the lens.
Light reflected form the back surface of the lens will result in discomfort glare and the term discomfort ratio has been used to describe the glare experienced in situations such as reading through a tinted lenses outdoors. The discomfort ratio (DR) is a ratio of the reflectance from the back surface of the tinted lens and the transmission of the lens. For example, if an uncoated tinted lens has a LTF of 15 per cent and a reflectance from the back surface, the discomfort ratio is:
If the back surface of the same lens is now reflection-free coated, the DR, assuming a 1 per cent reflectance, reduces to 7 per cent. Discomfort glare becomes less of a problem with the coated lens and reading outdoors is more comfortable.
BLUE LIGHT
Direct sunlight is always subject to scatter, with the blue portion of the visible spectrum being subject to the greatest scatter. This affects colour perception, reduces contrast and causes glare. Blue light is responsible for veiling glare, which is a result of scatter from dust and moisture particles in the air. Most of the glare we experience from sunlight also comes from blue light. An attenuation of the blue portion of the spectrum results in a marked enhancement in contrast. The blue light portion of the visible spectrum is most likely to scatter in the eye, causing discomfort and a reduction in contrast.
Historically, yellow or 'contrast' filters have been used by skiers and in hazy conditions. Such filters have about 90 per cent transmittance for wavelengths above 500nm but cut off all wavelengths below about 425nm. If such a filter is used in hazy daylight conditions the blue light scattered by the atmosphere will not reach the retina to cause veiling glare, so the resultant retinal image will have a higher contrast. Another problem with blue light is that due to the relative myopia of the eye for short-wave radiation, blue light is 'bundled' in front of the retina and superimposed on the other colours. This is sometimes referred to as blue blur and is another reason why removal of the blue region of the spectrum enhances contrast. Modern sunglass lenses often incorporate blue light filters to enhance contrast and visual comfort.
PRODUCT REVIEW
| Figure 11. Construction of the Serengeti polarised Drivers lens |
Serengeti lenses are available from Bushnell Performance Optics. The Serengeti brand was created in 1985 in Florida and has attempted to position itself within the 'top of the range' sunglass sector, targeting in particular, professional drivers and pilots. Serengeti lenses derive from a technology that was initially developed to manufacture windshields in the car industry to enhance ocular comfort when driving and improve visual performance.
There are several different lenses available but all Serengeti lenses are photochromic and are available in glass and polycarbonate. The mineral (glass) lenses are manufactured by Corning using borosilicate glass. In addition, all Serengeti lenses feature a patented technology called Spectral Control which effectively filters and fine-tunes the full spectrum of incident light by selectively blocking and filtering each wavelength. All Serengeti lenses also block at least 95 per cent of blue light (useful in a moist environment - fog, rain etc) without distorting colours and have an anti-reflective treatment applied to the inner surface of the lens. Certain lenses in the range also feature a polarising filter.
Strata is a glass, polarising, photochromic lens available in two colours, 555nm (a grey-green lens) with a 15 per cent to 8 per cent transmission and Drivers (a brown-orange lens to improve contrast) with 18 per cent to 10 per cent transmission. The polarising film is inserted directly between two layers of glass and is therefore integrated into the lens. The construction of the Serengeti polarised Drivers lens is shown in Figure 11. Four other glass lenses are available which are photochromic but not polarising. These are Drivers (this term refers to the lens colour) with a transmission of 24 per cent to 9 per cent, Drivers Gradient (transmission 24 per cent to 9 per cent), 555nm (transmission 15 per cent to 9 per cent) and Sedona. Sedona is a rose-coloured lens which, according to the manufacturer, gives superior image definition in all types of day light.
The polycarbonate range of Serengeti lenses has been given the name Polarmax which, as with all Serengeti lenses are photochromic. They are also polarising. The two polycarbonate lenses available are the Polarmax Drivers lens (orange-brown with a transmission of 17 per cent to 9 per cent) and also Polarmax CPG (cool photo grey) with a transmission of 18 per cent to 9 per cent. Due to the high impact resistance and low density of polycarbonate, Serengeti recommends the Polarmax range for sports enthusiasts.
Certain Serengeti lenses are also available to prescription. All the glass lenses in the range are available in single-vision form, albeit in a limited range of powers and greater in minus powers than plus. Cylinders are available to 3.00 dioptres. Polarmax lenses are not available to prescription and a limited range of progressive power lenses are available with Drivers Drivers Gradient (non-polarising) filters.
| Figure 12. Maui Jim sunglass models in lens colours Maui Rose and HCL Bronze |
Maui Jim plano and prescription sun lenses are perhaps not as well known as Serengeti but they are also pitched at the quality end of the market. The range again addresses the problems of reflected glare, colour discrimination and visual comfort and the product literature describes three 'types' of glare - reflected, direct and bounce-back. Reflected glare refers to light polarised by horizontal, flat, reflecting surfaces.
Direct glare refers to light from above and below and bounce-back glare refers to light being reflected from the back surface of the lens when the light source is from behind or from the side. The construction of Maui Jim lenses relates directly to these three glare 'sources'. The lenses are a multi-layer design and contain three patented elements to combat the stated glare problems. A polarising filter absorbs a stated 99.9 per cent of reflected glare to maximise the amount of usable light that reaches the eye. A bi-gradient mirror eliminates overhead glare. And finally, a reflection-free coating is applied to the back surface of the lens to eliminate the so-called bounce-back glare. Two patented polarising films are used in the manufacture of the lenses. The polarising film used in the Maui Jim glass lenses is referred to as PolarisedPlus2. The glass lenses either side of the polarising film include three rare earth elements - praseodymium, erbium and neodymium. These are incorporated directly into the glass at the time of manufacture and are designed to enhance colour discrimination.
The product literature suggests that wearers will enjoy deep saturated colours, a high level of contrast and enhanced depth perception. There are three colours available in the range of Maui Jim glass lenses - Neutral Grey, Maui Rose and HCL Bronze (High Contrast Lens). The use of rare earth elements should in theory make each of the three glass lens colours equal in terms of contrast and colour enhancement. The Neutral Grey lens has an 8 per cent LTF. It offers good contrast and the highest levels of light reduction in the series. It should be suitable for most light conditions. It is worth remembering that neutral grey lenses tend to 'flatten' light evenly across the visible spectrum. If the manufacturer's claims are to be believed, this flattening should not occur with the PolarisedPlus2 Neutral Grey lens as colours should be richer and contrast improved.
| Table 1. NuPolar availability |
The HCL Bronze glass lens has a 12 per cent light transmission. The product literature suggests that this lens is ideal for low light conditions. It should be noted that PolarisedPlus2 and the use of rare earth elements apply to the Maui Jim glass lenses only.
| Figure 13. Construction of the NuPolar lens |
Skylet is a high quality dip coating for the Clarlet range of lenses available from Carl Zeiss. Skylet lenses have been available for some time and were designed for situations in which greater protection is required from UV and glare, along with high contrast. Skylet lenses have reduced transmission in the short-wave portion of visible radiation resulting in improved contrast.
Three Skylet lenses are available. Skylet fun is a category 2 filter with a transmission of 70 per cent. It has an orange-brown tint and is particularly suitable for diffuse weather conditions and hazy visibility. Skylet road is also a category 2 filter with a transmission of 80 per cent. It has a brown tint and is suitable for general sun protection. Skylet sport is a category 3 filter with a transmission of 90 per cent. If is a dark brown lens and is recommended when dazzling sunshine and glare is a problem. Skylet has recently become available in polarising form marketed as SkyPol. It is available in CR39 in the three Skylet colours.
NuPolar polarising lenses are manufactured by Younger Optics USA and are available from Norville. Like all modern polarising lenses, the polarising film is cast within the lens itself and will never delaminate. They have a stated polarisation efficiency of 98-99.9 per cent and provide 100 per cent UV protection below 380nm. NuPolar meets or exceeds ANSI 80.1 impact standards and FDA impact resistant regulation 801.410.
Scratch resistant, reflection-free and mirror coating can be supplied as options. However, any lens that will be used for sun protection will benefit from a back-surface reflection-free coating. NuPolar availability is summarised in Table 1.
In addition to the NuPolar range of polarising lenses, Norville also supplies polycarbonate polarising melanin lenses manufactured by Vision-Ease. These are available in brown and have a 'warmer' colour than the NuPolar brown polarising lenses. Single-vision and progressive designs are available with 15 per cent LTFs. Norville also offers a 1.67 polarising single-vision lens.
Essilor has also recently entered the polarising market with single vision and Varilux Panamic lenses, now available in polarising form. Both lens options are available in Orma and Airwear. The polarising element is incorporated into the lens by placing, during moulding, a polarising film 0.4mm from the front surface. A 99.99 per cent polarising efficiency is claimed. Essilor polarising lenses are available in either brown (average LTF 16 per cent) or grey (average LTF 18 per cent). Lenses can be Crizal coated on the back surface to reduce discomfort glare when the sun is positioned behind the wearer.
SUMMARY
Polarising lenses have many advantages when compared to a standard tinted lens. Reflected glare is troublesome to motorists and motorists who switch to a polarising lens from a dip-dyed plastics lens immediately notice the difference.
Glare at the wheel also raises the issue of safety, particularly when driving into low sun. Visual hazards associated with many recreational activities are alleviated by the use of a polarising lens, resulting in enhanced ocular comfort and increased performance. Polarising lenses not only eliminate reflected glare but increase visual clarity, enhance contrast and improve colour definition. Most lenses available offer protection from UV below about 380nm and lenses with features such as spectral control offer additional visual benefits such as the reduction of veiling glare resulting from blue light.
So who is the polarised patient? Well, the simple answer is everyone! So why doesn't everyone wear polarising lenses? The answer to this question may lie in the abilities of practice staff to successfully communicate the features and benefits of these lenses. Product knowledge and confidence in the products available are vital skills, but perhaps most important is the skill of 'top-down selling'. It is important to offer all customers the best, but it is imperative that practitioners and practice staff start at the top. If there is price sensitivity, the appropriate action can be taken, but going the other way is much tougher.
From the practitioner's point-of-view, 'add-on' dispensing is often perceived as the easier option as the patient is very much in control. He or she will tell you when to stop adding-on and from that point on there is very little the practitioner can do about it. Make sure that you discuss both the benefits and the features. The feature of polarising lenses is that they reduce reflected glare. But what's in it for the patient? Time should be taken to explore the patient's lifestyle so that all potential benefits are exploited. It is important to be able to offer a polarising lens in the same design as the patient's everyday clear pair, for example, both single vision or both progressive. Single-vision and progressive power designs are widely available, but Norville's extensive range of polarising lens options are useful tools in the armoury.
Acknowledgements
The author wishes to thank the following for their help in obtaining information on the products mentioned: Andy Hepworth (Essilor); Bill Hatton (Maui Jim); Claire Walker (Carl Zeiss); Dawne Warren (Serengeti); Paul Walden (Norville); Thanks to Paula Stevens for reading a very early draft of this article.
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