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TRANSCRIPT
Coatings
OPHTHALMICOPTICS FILES
Why protect lenses against abrasion ? 14
What if the lens has an antireflection coating ? 14
How do anti-abrasion coatings work ? 17
Why antireflection coating ? 21
The technology of antireflection coating 27
Anti-tarnish coatings 33
INTRODUCTION 2
TINTING, PHOTOCHROMISM AND SPECIAL FILTERS
I
SUMMARY
II
III
General remarks 6
Fixed tint lenses 8
Variable tint lenses 10
Special filters 13
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PROTECTION AGAINST ABRASION
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ANTIREFLECTION COATING
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CONCLUSION 35
INTR
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INTRODUCTION
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This volume of the Ophthalmic OpticsFiles presents a study of the Coatingsapplied to ophthalmic lenses. By “Coatings”,we mean all manufacturing proceduresand the resultant products designed toenhance the performance of ophthalmiclenses, independent of their power. ThisFile is divided into three parts :
I - tinting (in the broadest sense of theword), including photochromism andspecial filters,
II - protection against abrasion,
III - antireflection coating and associatedanti-tarnish coating.
Each part consists of an analysis of thespecific function of each type of coating,and a description of the technology usedto meet the given objectives.
It is important to retain a global view ofthe ophthalmic lens. Indeed, this is anincreasingly complex product because ofthe combination of diverse Materials andCoatings, which are being more and moreoften integrated into the manufacturingprocess (figure 1). Consequently, coatingsare gradually being considered not somuch as optional “extras”, but rather asessential lens components. Indeed, theemphasis is on a greater interactionbetween the various components toobtain the final performance of the lens.For an overview of the general structureof modern lenses, we thus recommendreading of the Ophthalmic Optics File on“Materials”.
COATINGS
COATINGS
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Figure 1 : A coated plastic lens is a complex system.
TOP COAT
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SUBSTRATE
Figure 1
THE LENS AS A SYSTEM
SUPPLEMENTREVIEW OF BASIC NOTIONS CONCERNING THE NATURE
OF LIGHT
4
The following description of Coatings draws on notionsrelating to the “structure of matter” presented in theOphthalmic Optics File on “Materials”. It also refers tovarious aspects concerning the nature of light ; themain characteristics of light are reviewed below.
At the end of the 19th century, the physicist Maxwellestablished that, in a general fashion, the materialobjects which exist in the Universe may exert electricand magnetic influences on each other. Since theseinfluences are inversely proportional to the distancebetween the objects, they may be exerted over verylong distances.
These phenomena have been attributed to anextremely fast oscillation of the electrons present ineach atom. This oscillation, called an electromagneticwave, is able to propagate in a vacuum or in amedium, and to exert electric and magnetic influencesby exciting electrons present in the atoms of bodieslocated far from each other (this is why oscillations ofelectrons of the Pole Star still have enough influence toexcite the electrons in our retina).
The sum of these two phenomena - the oscillation ofelectrons in atoms, and the propagation of electro-magnetic waves over a long distance - is calledelectromagnetic radiation.
A wave is a perturbation of space which is charac-terized by the fact that it periodically resumes thesame value during its displacement. Its essentialvalues are as follows :- wavelength l in m (or multiples) : this is the distancecovered between 2 identical successive states,- frequency n in Hertz (Hz) : this is the number of timesto the same state per second,- velocity (in m.s-1), in the case of electromagneticwaves, velocity - in a vacuum - is c = 300 000 km/s= 3 108 m.s-1 ; these 3 values are related by the basic formula : l5 c/n.
Matter which fills the Universe consists of atoms andtheir electrons which vibrate and continuously emitelectromagnetic waves. The frequency of these wavesvaries enormously, depending on temperature (ratio of1 to about 1020) . This range of frequencies is calledthe electromagnetic spectrum, or the electromagneticwave spectrum. It is made of zones which have beenhistorically defined by their frequency/ wavelength (figure 2).
The electromagnetic radiation emitted by the Sun islargely absorbed by the Earth’s atmosphere. Solarradiation which reaches the surface of the Earth ismade up of the following rays :- ultraviolet rays, called “UV-A” (315 to 380 nm), well-known for their tanning effect, and “UV-B” (280 to315nm), which cause sunburn and ocular disorders(e.g. snow blindness). Ultraviolet radiation contains athird area called “UV-C” (200 to 280 nm) ; this verydangerous component is fortunately stopped by theOzone layer surrounding the atmosphere.- visible light made up of waves which, after goingthrough the intra-ocular media, stimulate the retinalreceptors; their wavelength ranges from l= 380 nm(violet) to l = 780 nm (red).- infrared rays with wavelengths ranging between l=780 nm and l = 2000 nm. Infrared radiation goes upto l = 1 mm, but is stopped by the water vapor in theatmosphere.
Visible light is an area corresponding to a specific andremarkable band of waves within the vastelectromagnetic spectrum which fills the Universe.These waves are remarkable in that they interact withour eye, and thus allow us to see the world.
Natural light emitted by the Sun, and perceived by thebrain as white, is made of a continuous spectrum ofradiations, each of which creates perception of a color.This range of colored rays can be seen in a rainbow,where each ray is characterized by its specificwavelength l (figure 2).
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lumièrevisible
SUPPLEMENTREVIEW OF BASIC NOTIONS CONCERNING THE NATURE
OF LIGHT
5
Figure 2 : Electromagnetic waves and light.
Figure 2
COSMICRAYS
RAYSg
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ULTRA-VIOLET
INFRARED RADAR HERTZIANBEAMSMICRO-WAVES
TV FM OC PO GO
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visiblelight
ULTRAVIOLET
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A/ General remarksThe human eye possesses anumber of natural anatomicalor physiological defense mecha-nisms, ensuring protection against
light. These mechanisms include the closing reflexmovement of the eyelids, the reduction in pupillarydiameter, the filtration of transparent media (tears,cornea, aqueous humor, lens), retinal adaptation toluminous intensity, etc. But this natural defense maybe insufficient, in which case extra protection isobtained through the use of a filtering lens, eitherpermanently, to improve wearer comfort, or spe-cifically, to protect against harsh luminous radiation.The filtering lens has a twofold role : to reduceintensity of the light which reaches the eye, and toeliminate dangerous rays by absorbing them. It mayhave a permanent tint, i.e. present as a single colorlens with a full or graduated tint, or its tint may bevariable, i.e. photochromic.
Any light filter can becharacterized by its phy-sical light transmissionproperties - transmissionfactor τ, transmission
curve and U.V. cutoff - and by its ensuing physiologicalproperties : relative transmission factor in the visibleτv. The latter factor complies with a standardizedinternational definition and is used to classify lensesinto 5 categories based on their light transmission :from 0 for the lightest to 4 for the darkest lenses(table 1). The classification criteria do not only refer tothe lens transmission properties in the visible range,but also in the UV-A and UV-B ranges. These criteriaare established for 2.0 mm thick plano lenses and anormal light incidence.
Table 1 : Lens classification as a function of luminous transmission.
1/ The principleunderlyingprotection
2/ Lens classificationas a function ofluminoustransmission
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Filter Ultraviolet range Visible range Former Frenchcategory of the spectrum of the spectrum classification
Maximum value Maximum valueof solar UV-B of solar UV-A Light transmission Transmission intransmission transmission interval visible τVτSUVB τSUVA
280 nm - 315 nm 315 nm - 380 nm from to from toUV-B UV-A Intensity
% % % % % %
0 τV 80.0 100.0 A 78.1 90.0
1 τV 43.0 80.0 AB 61.0 78.1
2 0.125 τV 18.0 43.0 B 32.2 61.0
30.5 τV
8.0 18.0 C 16.15 32.2
4 1.0 3.0 8.0 D 5.0 16.15
7
The tint of a lens is deter-mined by the chromaticcomposition of the light it
transmits (except for mirrored lenses). It results fromthe summation of visible radiation by the receivingobserver’s eye.
It is difficult to precisely assess lens transmissionproperties based on tint alone. Nevertheless, certaingeneral principles hold good (see figure 3) :- a grey tint transmits visible radiation more evenly,- a brown tint is more absorbent in the blue-greenrange of the spectrum than in the orange-red range,- a green tint is more absorbent in the orange-redrange than in the blue-green range,- the tint intensity shows the extent of absorption inthe visible range,- tint provides no indication of absorption in the ultra-violet or infrared ranges.
Conversely, it is difficult to predict the color of a lensfrom its transmission curve. The choice of tint is afunction of absorption properties required, and maybe determined by the wearer’s ametropic tendency (amyope may prefer brown, and a hyperope may prefergreen), but is also a function of the wearer’s own taste.
Figure 3 : Transmission curves.
TINTING, PHOTOCHROMISM
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3/ Lens tint and transmission
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a) for different tintsgreybrowngreen
b) for different intensitiesof transmission τV : 15 %, 30 %, 60 %, 70 %.
Figure 3
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B/ Fixed tint lensesThe filtering properties and characteristics of thevarious tints have been described above. This sectionwill focus on the techniques used to tint glass andplastic lenses, and on their respective properties.
Solid tinting
Solid tint glass lenses are manufactured directly fromrough blanks of tinted material supplied by the glass-making industry. The drawback of this method is thatpowerful lenses present an uneven tint. Indeed, sinceabsorption is exponentially proportional to thicknessof the material (Lambert’s law), minus lenses aredarker at the edge than at the center and, conversely,plus lenses are darker at the center than at the edge.This major drawback has been the downfall of suchlenses, which have now practically disappeared fromthe market. The only area where solid tint glass is stillwidely used is for making plano sun lenses.
Tinting by vacuum coating
Vacuum tinting of glass lenses consists of depositing alayer of light-absorbent metallic compounds on oneside of the lens. The lenses are heated to 200-300°C/440-570 °F, and the coating is applied under vacuumby evaporation (10-5 mbar) of materials such asChromium, Molybdenum or Titanium oxides mixedwith silica, Silicium monoxide, or Magnesium fluoride.Depending on the materials used and the requiredcolor, the coating may consist of a single, thick,continuous layer, or a stack of various alternating thinlayers producing total thickness of the order of amicron (10-6 m). Tint intensity is determined bythickness of the applied layer, and color is determinedby the materials used : oxides generally producebrown tints, while the grey tint is obtained from acombination of metals and transparent compoundssuch as silica. The applied layers are intrinsically ofequal thickness, thus ensuring that the lens tintremains constantly even. The range of available tintsis relatively limited. The technology used to vacuumtint glass lenses is very sophisticated, and is similar tothat used to apply antireflection coating, described inPart III of this File.
1/ Fixed tintglass lenses
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Solid tintingThe only solid tint plastic lensesare plano sun lenses. They are
obtained by polymerization of a monomer whichcontains colored dyes. Moreover, an ultravioletblocking agent is added to the monomer formulationto reinforce protection against this harmful radiation.
Tinting by surface treatment
Plastic lenses are tinted by impregnating theirsurfaces with colored dyes. The lenses are immersedin a solution containing the dyes and various additivesto promote the coloring process. The dyes penetratethe material to a depth of 6 to 10 microns. Tinting isperformed before or after application of a scratch-resistant coating, depending on whether the hardcoating can absorb the colored dyes.
Tint intensity is determined by the nature andconcentration of the dyes, and by the duration ofimmersion of the lens, which is about 1 minute for thelightest tints, and may last as long as 2 hours for thedarkest tints. As the tint is determined by relativeconcentrations of the 3 primary pigments (blue,yellow, red), it is possible to obtain an infinite range ofshades. Moreover, an even, full tint can be applied tothe entire lens, a color gradient can be obtained bymaking tint intensity vary from top to bottom, adouble gradient can be obtained by simultaneouslyvarying tint from both the top and bottom of the lens,or a “rainbow” effect can be obtained by applying adouble gradient tint onto a full tint lens. Graduatedtints are obtained by slowly removing the lens fromthe color bath : the lens is held in a lens holder andtotally immersed (top section downmost) then veryslowly removed : the lower part, which is immersedlongest in the bath, is therefore more stronglyimpregnated with the dyes than the upper part.
There are numerous possibilities for tinting plasticlenses. The procedures are relatively simple : lensesmay be tinted individually, in pairs, or in batcheswhere reference lens tints are copied. The operator’ssensitivity to colors and visual skills are veryimportant : tinting plastic lenses calls for realcraftsmanship !
Figure 4 : Solid tinting vs tinting by surface treatment.
9
2/ Fixed tintplastic lenses
Figure 4
a) solid tinted plus lens
b) solid tinted minus lens
b) plus or minus lenses tinted by surface treatment
TINTING, PHOTOCHROMISM
AND SPECIAL FILTERS
C/ Variable tint lensesThe basic principles of glass and plastic photochromicmaterials have been described in the OphthalmicOptics File on “Materials”. This section will thereforefocus on an analysis of the properties of glass andplastic photochromic lenses.
The light transmission prop-erties of a photochromic lensare precisely described by itstransmission curves and τV
coefficients measured in the clear and dark states.The examples in figure 5 show transmission graphs forglass and plastic lenses measured before activationand after exposure to light for 15 minutes. Thevariation in transmission caused by the photochromicproperty is clearly visible. Moreover, the U.V. cutoff ofthe plastic lens (390 nm) is significantly higher thanthat of the glass lens (345 nm).
Ormex® is a registered trademark of Essilor International.Transitions® is a registered trademark of Transitions Optical Inc.
Figure 5 : Transmission graphs in the clear and dark states.
10
τ (%)
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1/ Transmission in clear and darkstates
a) clear stateglass (IsorapidE 15)plastic (Ormex® Transitions® III)
b) dark stateglass (IsorapidE 15)plastic (Ormex® Transitions® III)
Figure 5
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Photochromic properties ofa lens are generally repre-sented with graphs showing
the lens darkening and fading kinetics. These graphsshow the change in the lens transmission factor τv asa function of time during the darkening and fadingphases. On the examples shown in figure 6 a) and b),the left-hand side of each graph shows the lensdarkening over a period of 15 minutes, and the right-hand side shows the lens fading over a period of 20minutes. The value of τv can be seen to decrease, ata temperature of 2°C / 68°F, from a value close to 90 % to about 30 % after darkening for 15 minutes.Similarly, τv subsequently rises to about 75 % afterfading for 20 minutes. The curve slope shows that dark-ening is significantly faster than fading. Furthermore,the photochromic performance of current glass andplastic lenses can be seen to be practically the same.
Figure 6 : Darkening and fading kinetics.
11
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a) glass photochromic lens
b) plastic photochromic lens
2/ Darkening andfading kinetics
Figure 6
TINTING, PHOTOCHROMISM
AND SPECIAL FILTERS
darkening fading
fadingdarkening
12
The photochromism of a lens isobtained by introducing U.V. light-sensitive substances into the glass or
plastic material. For glass lenses, a single substance isused and the lens color remains relatively constantthroughout the photochromic process. For plasticphotochromic lenses, several photosensitive sub-stances are used, each absorbing a specific part of thevisible spectrum. Since these substances do notnecessarily react at the same rate, the resultant colorcombination may change during the photochromicprocess. This accounts for the marked color variationsometimes observed in early plastic photochromiclenses. Nevertheless, such variations have beenlargely eliminated from materials of the latestgeneration.
Heat is the main factor whichtriggers lens fading and ensuresreversibility of the photochromicprocess. Consequently, ambient
temperature influences the photochromic perfor-mance of a lens, which tends to darken less at hightemperatures than at low temperatures. To describethis discrepancy, the darkening performance of lensesis measured in different simulated climatic conditions.On figure 6, the lower graph simulates a harsh winter(Montreal in winter, at -11°C/12°F), the middle graphsimulates a temperate summer (Paris at +20°C/68°F),and the upper graph simulates a tropical summer(Miami at +35°C/95°F). The difference between
these 3 graphs shows the effect of climatic conditionson the photochromic process for glass and plasticlenses.
The performance of photochromic lenses is measuredin laboratory conditions using sophisticated apparatuswhich artificially recreates the real climatic conditionsin which the lenses are used. The apparatus (figure 7)consists of two arc lamps (A) which reproduce thespectrum of sunlight, a climatic chamber (B) whichreproduces temperature conditions, a spectro-photometer (C) which permanently measures theintensity and chromaticity of the light transmitted bythe lens, and a computer (D) to process the collecteddata.
Figure 7 : Apparatus for measuring photochromic performance.
3/ Tintstability
4/ Sensitivityto climaticconditions
Figure 7
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a) b) c) d) 1.5 clear glass Orma® Lumior polarized slight brown solid tint UVX Kiros RT
Extreme,
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D/ Special filtersThese filters are designed to selectively transmitcertain types of radiation and partially or totallyabsorb others. They may play two different roles :- a protective role, by reducing or eliminating thenoxious effects of certain wavelengths, and/orattenuating luminous energy which enters the eye,- an enhancing role, by selectively transmitting certainwavelengths likely to improve wearer perception.
There are many kinds of filters, including the following :
Filters improving natural U.V.absorption provided by clearglass and plastic materials can be
used to improve protection against this radiation. Toallow these lenses to be worn permanently, theprotective mechanism should only slightly attenuatetransmission of the visible spectrum. With glass, solidtint materials are used because of their better U.V.cutoff : for example, a filtering material with a slightsolid brown tint (figure 8a) raises UV cutoff for 1.5index glass from 280 nm for traditional materials orfrom 330 nm for the most recent materials, up to 350 nm. With plastics, U.V. cutoff of CR 39 (1) (atabout 350 nm) can be reinforced by a coating : forexample, “UVX” coating (figure 8b) is a U.V. blockercombined with a pink amber tint, allowing all UV-Band UV-A to be eliminated by cutting off radiations upto 400 nm. Generally speaking, plastics are clearlybetter U.V. filters than glass materials. Among theformer, photochromic high-index plastics, includingpolycarbonates, are better U.V. filters than CR 39.
These filters absorb ultravioletand blue radiations, and speci-fically transmit the central part ofthe visible spectrum. For example,
the “Kiros” filter (figure 8c), which has a light yellowtint, stops the transmission of blue light andspecifically transmits wavelengths close to maximumsensitivity of the eye. It enhances contrast perception
in cloudy conditions, and is therefore useful for drivers,mountaineers, and hunters. Similarly, Essilor “Lumior”,which is a darker tinted orange-yellow filter, specificallytransmits the central zone of the visible spectrum andfilters all U.V. and blue radiation up to 400 nm. It is usedto improve vision and visual comfort of amblyopic andaphakic patients. These filters are obtained by treatingOrma® (2) plastic lenses.
These filters absorb U.V. light and thelower part of the visible spectrum,and only transmit its upper part. Forexample, “Extreme” coating (figure
8d), which has a dark yellowish-brown tint, cuts off allwavelengths up to 530 nm, thus eliminating all U.V. andthe most active part of the visible spectrum. Its yellowcomponent improves contrast. It is designed to be usedin strong lighting conditions, such as on sunlit snow.
The Essilor “RT” coating, which cuts off all U.V. rays andthe visible spectrum up to 420 nm, allows a reductionin retinal rod cell stimulation and rests the scotopicsystem, while maintaining central visual acuity. Thesefilters are obtained by treating Orma® lenses.
Polarization of light : when a lightbeam is reflected by a polishedsurface, the vibration of the wave
becomes strictly confined to a plane, which isperpendicular to the light incident plane. Certainsubstances can be arranged in optical filters such thatthey absorb the ray of polarized light if correctlyoriented with reference to the polarization plane. Theeffect of these filters is to cancel glare emitted byreflecting surfaces. One particular application consistsof orienting filters of this type so that they absorbpolarized light reflected by water in the naturalenvironment. These polarized filters are useful forwater sports and fishing, since the wearer can observewater without being dazzled by reflections anddisturbed by glare (see Figure 8d).
Figure 8 : Transmission graphs for some of the Essilor special filters.
τ(%)
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1/ Ultravioletlight filters
2/ Contrastenhancingfilters
3/ Highabsorptionfilters
Figure 8
TINTING, PHOTOCHROMISM
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4/ Polarizedfilters
(1) CR 39 is a registered trademark of PPG Industries.(2) Orma® is a registered trademark of Essilor International.
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B/ What if the lens has an antireflection coating ?The basic phenomenon remains the same, but theeffects of the abrasive particles are considerablyamplified by the structure of the antireflectioncoating. Indeed, materials used for antireflectioncoating are mineral materials applied in very thinlayers (less than a micron) to the plastic lens surface.The thinness of the applied layers certainly does notgive them the rigidity of a glass lens; under the localpressure of a grain of dust, these layers undergodeformations which are characteristic for thedominant material in terms of volume, i.e. thepolymer substrate. At the same time, however, thesethin layers continue to behave mechanically as mineralmaterials : when the polymer is subjected tosubstantial deflection, making the mineral layersdeform beyond their breaking point, they will crackabruptly, creating a rather uneven groove on the lenssurface (figure 9).
This effect is obviously catastrophic for a plastic lenswithout a scratch protective coating. Even a plasticlens coated with traditional polysiloxane varnish willshow considerably lower resistance to abrasion andscratches if it has also been antireflection coated,since deformations of the lens/ varnish combinationgo beyond the breaking point threshold of theantireflection coatings.
This phenomenon is moreover amplified by greatervisibility of scratches : the contrast between thebrilliant, deepest point of a scratch, and the matteantireflection coating, is stronger than on an uncoatedlens (it should be pointed out that this phenomenon isalso noticeable, although to a lesser extent, on glasslenses).
However, this complex problem has prompted thedevelopment of a new generation of specific hardcoatings. These nanocomposite varnishes endowantireflection coated plastic lenses with extraordinaryperformance.
Figure 9 : Scratch resistance of a hard substrate - soft substrate.
Figure 9
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A/ Why protect lenses againstabrasion ?Among the everyday enemies of spectacle lenses, thedamage caused by the rubbing of various objects andby abrasive dust particles (chiefly made of silica)wiped across the lenses is certainly one of the worst.This goes for all lenses, glass or plastic, although glasslenses are generally considered much more scratchresistant than plastic materials. This chapter thereforedeals mainly with the abrasion resistance of plasticlenses.
A preliminary observation of “scratched” plasticlenses suggests that two phenomena are at work :- abrasion by the rubbing of “fine” particles, respon-sible for small, fine scratches which are practicallyinvisible to the wearer,- scratching by “large” particles, causing visible splin-tering which is a source of wearer discomfort.
Nevertheless, physical analysis of abrasion andscratching phenomena reveals more complexmechanisms, governed by mechanical properties ofhardness and deformation response of the analyzedmaterials.
The general aim of the present File is to describe, fromthe wearer’s viewpoint, the reasons for developingvarious coating techniques. However, we felt it wouldbe useful to provide an additional, more detaileddescription of one of the basic mechanisms whichcause scratches in the supplement below.
PROTECTIONAGAINST ABRASION
RIGID SUPPORT
MALLEABLE SUPPORT
15
Characterization of the phenomenon ofscratching and abrasion
The implemented mechanism can be described byconsidering an abrasive particle as a point which exerts apressure - called stress - locally, at the lens surface. Thissurface reacts to the stress as a function of its mechanicalcharacteristics. When the stress is eliminated, there is aremnant imprint, the shape of which varies, and whichconstitutes a kind of signature of the interaction between theabrasive grain and the lens surface. This mechanismsimultaneously calls on two mechanical properties -hardness and deformation - which are specific for a givenmaterial. Hence, one can imagine applying identical stresswith the same abrasive point :- on a block of rubber : this will deform in a completelyelastic fashion, and resume its initial state upon removal ofthe point- on a block of glass : this will show very little deformation,then fracture if the stress exceeds a certain threshold,yielding a very visible remnant imprint- on a block of aluminum : this will show greater deformationthan glass because of the nature of the material (plasticbehavior), and the print will retain the shape acquired at thetime of maximum deformation, without any fracture orsplintering.Thus, one can refer to a “law of behavior” specific for eachmaterial.In mechanical physics, this is represented on a graph,plotted as follows :- on the abscissa : % deformation with respect to the initialstate,- on the ordinate : the value “s” of the applied stress(expressed in Pascals, meaning that a stress is consideredas a pressure).
For any material, the law of behavior is represented by acurve which originates at O and terminates at a point R,where breaking point occurs. Abscissa xR of R is called breaking point deformation.Ordinate sR of R is called breaking point pressure.Slope of the curve may be constant (straight line) - indicatinga material with purely elastic behavior : resumption of initialshape after elimination of the stress. Slope of the curve mayalternatively be variable - indicating a material with plasticbehavior, more precisely termed “ visco-elastic “ behavior :there is a remnant imprint after elimination of the stress.Figure 10 shows the laws of behavior for glass and apolymer (type CR 39).
The polymer may be scratched subsequent to rupture, for farweaker stresses than those withstood by the glass.Similarly, before attaining its breaking point deformationthreshold, the polymer may undergo major permanentdeformation, without any rupture or splintering.
To verify these theoretical explanations, it is imperative toobserve numerous lenses which are effectively worn. Thiscan be done using a “scanning electron microscope”, whichallows the observation of scratches with tearing of materialand splintering, i.e. the causal factors in diffraction leadingto wearer discomfort (figure 11).
Figure 10 : Laws of behavior for glass and polymer materials.
SUPPLEMENTCHARACTERIZATION OF THE
PHENOMENON OF SCRATCHINGAND ABRASION
Figure 10
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Figure 11 : Scratch viewed under an electron scanningmicroscope.
An “atomic force microscope” can also be used : this calls onforces of attraction between atoms to draw up a 3D map ofa surface at the nanometric scale (1 nm = 10-9 m). The mapshows defects under a very different light, and highlights themechanisms of tearing of material (figure 12).
Figure 12 : Scratch viewed under an “atomic force microscope”.
Analysis of the scratch phenomenon provides researcherswith clues as to the development of abrasion-resistantcoating prototypes. It must be possible to characterizeperformance of such prototypes.
Characterization consists of performing precisionmeasurement of hardness and deformation-resistance of thecoated lens as a whole, but also of the actual coating. Forexample, this can be done with an instrument which ensures“indentation” of the test surface : a diamond point of knowndimensions is applied to the surface, implementingdetermined load “P”, and residual imprint “A” is measured(figure13).
The diamond tip can also be displaced to inscribe a scratchwith a variable load, and at a set displacement rate.
Figure 13 : Principle of indentation.
Figure 14 shows indentation imprints obtained with a load of300mN, for a thick evaporated silica-based subcoat on theleft, and for a “nanocomposite” varnish on the right
Figure 14 : Indentation photos - mineral hardening layer on theleft - nanocomposite layer on the right.
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C/ How do anti-abrasioncoatings work ?
The first generation of coatings(1970) was exclusively basedon the notion of hardness :plastic lenses scratch easily
because of the softness of the material, whereas glasslenses are almost “unscratchable” because they arehard. Hence, the principle for increasing the abrasionresistance of a lens consisted of applying a mineralcoating to the polymer surface by vacuumevaporation. The coating was made of silica, and theprocedure was commonly referred to as “quartzing”.
From 1975, finer analysis of the behavior of materialsrevealed interworkings of the relation between“hardness” and “deformation”. At the same time,progress in Chemistry allowed harder plasticmaterials, capable of following deformations withoutbreaking, to be applied to the polymer surface. Thisled to the generation of hard varnishes - polysiloxaneor acrylic compounds applied in the liquid phase; thissecond generation of coatings is still widely usedtoday.
Finally, as mentioned earlier, the specific problemcaused by antireflection coated lenses gave rise to anew generation of hard layers. The solution in this caseconsisted of compensating for the difference betweenmechanical properties of the polymers and those ofthe thin antireflection layers, by interposing betweenthese two substances a structure characterized by anintermediate behavior. A new generation of coatings,nanocomposite varnishes, appeared in the earlynineties, and proved to be capable of fulfilling thisfunction. Indeed, their new structure offered a goodmechanical transition - a kind of “shock absorber”effect - between the hard, brittle antiflection coating,and the flexible, deformable polymer.
Furthermore, new forms of mineral hardening layershave been introduced to the market : they are basedon vacuum evaporated silica compounds. As theirmechanical behavior is similar to glass, they stillremain brittle when subjected to strong loads.
The Quartzing procedure, firstused in the seventies, consists
of applying silica (“Quartz”) to a plastic lens byvacuum evaporation (this method will be described inpart III, on antireflection coatings). It is difficult tocombine Quartz and the CR 39 polymer, since thelatter’s dilatation coefficient is almost 20 times that ofQuartz. This induces stress which leads to fracture ofthe layer in the event of thermal shock. When wipingthe lens, if a speck of dust exerts a strong localpressure on the quartzed surface, the layer may break,and very visible scratches may appear. Thesedrawbacks account for the relatively limited success ofthe “quartzing” process.
1/ The principleof anti-abrasioncoatings
2/ Quartzing
PROTECTIONAGAINST ABRASION
a) Traditional varnishes
The advent of polysiloxane varnishes represents oneof the numerous applications of Silicon chemistry.It has already been stated that organic matter ischiefly comprised of C, H, O and N elements, whereasmineral matter consists of various elements,particularly Silicium. The chemistry of Silicons, whichis a branch of organic chemistry, has developedthanks to the extraordinary properties of chemicalcompounds where the Carbon atom has beenreplaced by a Silicium atom. Thanks to Silicium, theseSilicon compounds form a bridge between organic andmineral matter (figure 15).
Thus, polysiloxane varnishes provide extraordinaryperformance in terms of their reinforcement of thesurface hardness of organic materials :- thanks to the presence of Silicium, which ensures anintermediate hardness value, between that of silicaand that of the pure polymer materials, and- thanks to the existence of long hydrocarbon chains,which maintain the elasticity and dilatation coefficientnecessary to ensure cohesion of the “layer/ polymer”system.
These varnishes are applied by two specific methods(described in the supplement, see below) :- varnishing by “dip coating”,- varnishing by centrifugation or “spin coating”.
b) Nanocomposite varnishes
As seen above, the application of a polysiloxanevarnish under an antireflection coating does notprovide enough mechanical support to preventcracking. Hence, the underlying structure on which theantireflection coating is applied must be stiffened,from the core to the outer layers.
Certain manufacturers have modernized the quartzingprinciple to manufacture hard, silica-based mineraltype coatings, even though they still present a risk ofcracking beyond a certain load.
Other manufacturers have focused on research todevelop composite materials called nanocomposites,which are transparent to visible light. These materialsconsist of an organo-silicon matrix in whichnanoparticles have been dispersed, i.e. sub-microscopic particles measuring 10 to 20 nm, whichare therefore smaller than the wavelength of light. Thisrules out the risk of diffusion. These compositematerials generally use very stable particles ofcolloidal silica.
Thus, the principle of building a bridge betweenmineral and organic matter thanks to Silicium atoms is once more operational here, though at an entirelydifferent scale : whereas polysiloxane varnishmolecules contain a small percentage (in mass) of Si-O radicals, nanocomposite molecules are made ofabout 50% silica.
The silica nanoparticles are suspended in a liquid witha similar structure to that of polysiloxanes to form ahomogenous mixture which has the properties of avarnish, and is applied to the lenses either by dipcoating or by spin coating (see supplement). Theliquid film is then polymerized by baking at about100°C/200 °F, to become an organo-silicon polymerin which mineral nanoparticles “float” in an organicmatrix. This nanocomposite coating has remarkableproperties :- high resistance to deep scratching, preventing thesuperimposed antireflection coating from beingdeformed beyond breaking point,- relatively high flexibility, to follow deformations ofthe polymer without separating,- an extremely low friction coefficient, with a resultantincrease in abrasion resistance : angular particlescannot “get a grip” on this kind of surface.
Figure 15 : Mineral structure (lower left) - organic structure(upper left) - silicon (right).
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3/ Varnishing
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Varnishing by dip coatingThis is a dipping process where the lensessimultaneously receive a layer of varnish on eachsurface. The lenses are cleaned using ultrasonicactivated chemistry, then immersed in the liquidvarnish bath from which they are removed at aconstant rate to control thickness of the final coating,which is likewise dependent on viscosity of the liquid.The varnish is then polymerized by baking at atemperature of about 100°C/200°F. It then turns intoa hard, solid film which endows the coated lens withresistance to scratching, its performance being afunction of composition and thickness.
All these operations are performed in controlledatmospheric conditions (clean room), with monitoredtemperature and humidity.
Varnishing by centrifugation or spin coating This procedure is simple, and its principle makes itparticularly suitable for small items which rotatesymmetrically : the lens is attached to a rotary sup-port at a controllable speed, a drop of the liquidvarnish to be spread is applied to the lens center, thenrotation speed is accelerated until a uniform filmcovering is obtained by centrifugation.
This method is readily adaptable to the manufacturingof small series of lenses ; since the actual varnishingoperation is performed very fast, it can be used toapply less stable, more complex varnishes than the dipcoating procedure.
Moreover, because of its simplicity and speed, it isused to varnish concave surfaces in US surfacinglaboratories (where polymerization is often achievedby a few minutes’ exposure to ultraviolet radiation). Onthe other hand, the resulting coatings often show poorabrasion resistance.
Figure 16 : Principle of varnishing by dip coating.
Figure 17 : Principle of varnishing by centrifugation
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This procedure uses an entirelydifferent technology to thosedescribed above. A plasma, i.e.
an electric discharge in a gas at low pressure (as in aneon tube), is created in a vacuum chamber, beforeintroduction of a gaseous monomer (of the “HMDS” -“hexamethyldisilazane” family), rich in siloxanes. Themonomer polymerizes under the effect of the energyprovided by the plasma, and by condensation, formsa solid film on the lenses in the chamber. There areseveral ways to generate the plasma : direct current,microwaves and radio-frequency, etc.
Plasma polymerization is expensive, and the manu-facturing control involved is complex. Furthermore,this type of treatment shows up surface defects. Somemanufacturers use this procedure to coat plasticbifocals. Others have tried to combine it with standardvacuum coating to create a hard sublayer, or a water-repellent layer for an antireflection-coated lens. Forthe time being, reliability is still apparently not easy tomaster.
It is difficult to check the abra-sion resistance of a coated lens.Such control must be quick to
implement, and easy to interpret. Manufacturers havedeveloped methods of testing which consist of sub-jecting samples of a manufactured batch to abrasionor scratch simulations.
Here are a few of the most commonly used tests :- Bayer test : the lens is moved back and forth in arecipient containing an abrasive powder (sand) with aprecisely defined grain. Diffusion of a control lens andthe test sample is measured “before” and “after” thisaction, and results are compared ;- abrasimeter test : a ribbon encrusted with fineabrasive particles (e.g. carborundum) is rubbed on thetest lens a certain number of times, with apredetermined load. Diffusion of transmitted light isthen compared with that diffused by a control lens ;- steel wool test : there are several ways of rubbing atest lens with a fine steel wool pad : using a standardmechanical device for reproducibility, or simplymanually, by way of demonstration. In the latter case,a lens is rubbed by hand with a fine steel wool pad.The test and control lenses are compared visually orwith a standard apparatus for measuring diffusion ;- “Taber” test : a “Taber” type rubber wheel, shapedto match the lens curvature, is applied to the lens witha predetermined load. Diffusion is measured andcompared with that of a CR 39 control lens ;- barrel test : there are several similar tests of thistype : the lenses are set in rotation in a barrel filledwith various precisely calibrated items, and diffusionof transmitted light is measured at regular intervals.
Figure 18 : Exemple of a varnishing machine.
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4/ Plasmapolymerization
5/ Abrasionresistance tests
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The futureOne of the most marked trends in the ophthalmic lensmarket at the end of the 20th century is the gradualand inevitable substitution of glass lenses by plasticlenses. This has accelerated research to improveperformance of the already large family of plasticmaterials. In particular, impressive progress has beenmade with medium-index polymers (n ≤ 1.56) andhigh-index polymers (n > 1.56) : these allow
substantial reductions in lens thickness, making the lenses more cosmetically pleasing. On the other hand,the abrasion resistance of medium- and high-indexplastics is lower than that of CR 39. Consequently, ALLplastic lenses made with these materials must beprotected with an abrasion-resistant coating. Hence,we can reasonably expect rapid progress to be madein this area !
Figure 18
A/ Why antireflection coating ?The parasite effects due tothe reflection of light on bothsurfaces of a lens are varied
and well known (see figure 19).
The most obvious effect is the “mirror” effect observedby the person looking at a spectacle wearer : lightreflections on uncoated lenses make it impossible tosee the other person’s eyes, and are consideredannoying and unattractive (see figure 20). Yet thiseffect is negligible compared with the very real visualdrawbacks that parasite reflections represent for thewearer of the spectacles.
Figure 19 : Lens surface reflections.
Figure 20 : “Mirror” effect of an uncoated lens (on the left).
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1/ The effects ofreflected light
Figure 20
Figure 19
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Figure 21 : Formation of retinal images of separate points.
Figure 22 : Effect of a parasite reflection.
Let us look at what happens when images are formedon the retina. Like any optical system, the eyepresents imperfections, and the image the eye givesof a point on the retina is not a point, but a blur spot(figure 21-1). Thus, the perception of two close pointsresults from the juxtaposition of two more or lessoverlapping blur spots. As long as the distancebetween these two points is big enough, the imagesformed on the retina result in the perception of twopoints (figure 21-2), but if the points are too close, thetwo blur spots tend to merge, and are wronglyperceived as a single point (figure 21-3).
This phenomenon can be quantified by using therelation determining contrast, based on the maximumand minimum illumination values of the spot formedon the retina (figure 21 : segments “a” and “b”), anddefined as follows :
C = a - ba+b
This ratio must exceed a certain value (detectionthreshold, corresponding to an angle of 1 to 2°) for theeye to be able to separate the two neighboring points.
Figure 21
Figure 22
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Let us now take the following situation : a driverwearing spectacles is driving his/her car at night and,in the distance, the wearer clearly sees two cyclistscoming toward his/her vehicle. Let us now supposethat the headlights of a car behind the first car reflecton the rear surface of the driver’s spectacles : thisparasite reflection creates an image on the retina(figure 22) which is added to the intensity of the twoobserved points (i.e. the cyclists’ lamps). Conse-quently, length of segments “a” and “b” increases,causing a reduction in the value of C since thedenominator (a+b) has increased, while thenumerator (a-b) has remained constant. This results ina reduction in contrast, which may go so far as tomerge as a single image the driver’s initial perceptionof two cyclists, exactly as though the angle separatingthem were suddenly reduced !
Clinical studies in this area consist of having wearersof spectacles with uncoated and antireflection-coatedlenses observe contrast gratings, and of measuringtheir contrast sensitivity. Figure 23 shows normalsensitivity (red line) without dazzling, the loss ofcontrast caused by dazzling with uncoated lenses(purple line), and restored contrast obtained inidentical dazzling conditions when lenses withantireflection coating are worn (blue line) .
Similarly, under predetermined dazzling conditions, ithas been possible to ascertain that the visual field ofa subject wearing antireflection-coated lenses isconsiderably larger than that of the wearer ofuncoated lenses.
Another source of disturbance is “phantom images”generated by the double reflection on both surfacesof a corrective lens (figure 24).
This effect is illustrated by the night scene in figure25, observed through an uncoated lens (upperphoto), then through an antireflection-coated lens(lower photo) : the double parasite image of the lightbulbs is clearly visible on photo (a).
Studies in night driving conditions have shown that,compared with uncoated lenses, antireflection-coatedlenses cut down the time necessary to recover normalvision after dazzling by 2 to 5 seconds.
Figure 23 : Contrast sensitivity in dazzling conditions, with andwithout antireflection coating.
Figure 24 : Phantom image generated by the double reflection onboth the lens surfaces.
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Antireflection coatings arebased on the undulating natureof light and on the phenomenonof light interference. Let us take
two sine waves and calculate the sum of their ampli-tudes (segment AB) in the following two cases : on theleft, the amplitudes add up : “constructive” inter-ference occurs when waves have the same position onthe x axis - they are said to be in phase ; on the right,the amplitudes cancel each other out : “destructive”interference occurs, and the waves are said to be inphase opposition.
The principle of antireflection coating consists ofapplying a layer to the uncoated lens surface suchthat the reflected waves on the layer and at the “layer/lens” interface are in phase opposition, thussuppressing light waves reflected by destructiveinterference (figure 26).
For this, the 2 waves must be out of phase by l/2 afterreflection : it is easy to deduce that the thickness ofthe layer must be l/4 in relation to wavelength. Butreal layer thickness must be calculated by taking intoaccount the change in velocity of the wave when itgoes from air to the antireflection coating; thisintroduces the refractive index which is the ratiobetween these two velocities, and gives the followingformula :
e = l ,
4n1
n1= refractive index of the antireflection coating,otherwise expressed as
n1=c, light velocity in vacuumv, light velocity in the layer
2/ Principle ofantireflectioncoating
Figure 25
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Figure 25 : Night scene observed through an uncoated lens andan antireflection coated lens.
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Moreover, for extinction to be total, amplitudes(segment AB above) of the two reflected waves mustbe equal. The calculation based on so-called“Maxwell’s equations stipulates a further condition,which defines the refractive index of the layer :
n1 =√ n , n= refractive index of the lens.
Example : Single-layer antireflection coating on acrown glass lens :
The index of crown glass is n = 1.523 ; calculationdefines the index for the layer as n1 = 1.235, but nosolid material has this index. Hence, the materialwhich comes closest to this value is used, i.e.magnesium fluoride, MgF2, which has an index of n1 = 1,38.
As antireflection coatings are generally calculated forthe central wavelength of the visible spectrum, l =550 nm, the real thickness of the layer becomes e = 100 nm.
Note : according to the principle of energyconservation, if intensity of the light reflected on thetwo surfaces of a coated lens is less than on thesurfaces of an uncoated lens, then transmittedintensity T will be increased by the same amount. Thetransmission value for a lens where each surfacepresents reflection r, is equal to T = 100.(1-r)2.Hence, for example, the above-mentioned crown glass,with uncoated reflection factor r = 4.3%, presentstotal transmission T = 91.6% - with single-layerantireflection coating reflection drops to r' = 1.6%and transmission becomes T = 97%.
Figure 26
Figure 26 : Diagram showing the principle of antireflectioncoating.
ANTIREFLECTIONCOATING
c) Antireflection coating residual color
Figure 27 shows that the amount of reflected light ishigher with wavelengths in the blue (400 nm) and red(700 nm) part of the spectrum. This residual coloredreflection, essentially consisting of blue and red light,gives the lens a well-known “purple” appearance.
Figure 28 shows two graphs which do not have exactlythe same shape throughout the spectrum, but whichare characterized by a very low amount of reflectedintensity : one might therefore expect the residualcolor to be fairly similar. In fact, this is far from true :the lenses corresponding to these two real curveswere fitted to a pair of spectacles (figure 29) whichproved unacceptable for cosmetic reasons. Indeed,the two lenses could under no circumstancesconstitute a pair.
Type of antireflection Reflection Totalcoating per surface, r transmission,τ
Standard efficiency 1.6 to 2.5 % 95 to 97 %
Medium efficiency 1,0 to 1.8 % 96 to 98 %
High efficiency 0.3 to 0.8 % 98 to 99 %
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b) Optimization of performance
The antireflection coating represented in figure 27does not cancel out reflections completely. It allowssome luminous reflection, depending on thewavelength : while this is minimal at about 460 nm, itis much greater at the two extremities of thespectrum. To improve the efficiency of antireflectioncoatings, physicists have refined the principle of lightinterference described above by calculating thethickness and refractive index of several stackedlayers (up to 7 or 8), such that multiple interferencesof the waves reflected on these layers taken two bytwo substantially reduce reflection at different pointsof the visible spectrum.
The efficiency of these multilayer stacks is spectacular(see figure 28), and their application is developingfast, despite the complexity of designing suchsystems. Generally speaking, antireflection coatingsavailable on the market can be classified into threecategories :
a) Specification of theantireflection effect
The antireflection functionis represented by a graphshowing intensity of reflec-
ted light (ordinate) as a function of wavelength(abscissa). This set of data is called a “reflectiongraph” or “spectrogram R (l)”. The reflection graph ofan uncoated lens is the reference, and allows theantireflection efficiency factor to be calculated as theratio of the areas located under each of the twocurves.
Figure 27 : Antireflection coating reflection graph.
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3/ Specification and performanceof antireflectioncoatings
Figure 27
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In conclusion, residual color represents one of theimportant characteristics of antireflection coating, asit affects perception of the product quality, and thewearer’s final choice. It is therefore necessary to beable to control this phenomenon, both at the designphase, and during manufacturing. For this reason,scientific tools have been developed to allow thecalculation of residual color to be deduced from aspectrogram R(l). This measurement has beendefined in the “CIE L*a*b*” colorimetric system forinstance (see supplement section p. 28).
B/ The technology of antireflection coating
The manufacturing technologyfor antireflection coatings con-sists of stacking thin layers with specific characteristics toachieve the overall result. These
characteristics are :- specified refractive index,- absolute transparency,and the method implemented to obtain the requiredconfiguration :- high precision layers of even thickness,- excellent adherence to the lens,- outer surface coating as polished as the uncoatedlens,- optical qualities equal to that of the substrate : noisolated defects caused by dust (“pinholes”), no “haze”effect due to scattering, no pitting due to a materialdeficit (“bubbles”), etc.
Only one technology currently provides a satisfactorysolution to all these requirements. This is vacuumevaporation. Why ?- evaporation allows very pure materials to be appliedto lenses by condensation; the chemical compositionof these materials can be rigorously controlled ; - vacuum evaporation allows layers to be built up withthe required accuracy, however thin they may be (± 5 Å) ;- the vacuum technique guarantees optimal adher-ence, as the lens-layer interface is free from anyresidual contamination.
1/ Vacuumevaporation : the reasons forthis choise
Figure 29
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Figure 28 : Reflection graph for two high efficiency multilayercoatings.
Figure 29 : Difference in appearance of two multilayer AR coatedlenses (right lens = red line, left lens = green line).
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“CIE L*a*b*” Colorimetric system
An x axis called “a*” is plotted in a plane from pointO, and serves to measure the variation from red togreen. A y axis called “b*” measures the variationfrom yellow to blue. A color is defined by a point Pfrom coordinates “a* , b*”, hue “h*” is the angleformed by OP with the a* axis, and saturation“C*”is equal to the length of segment OP.
- hue “h*” translates the sensation of color,
- saturation “C*”, or Chroma, expresses the sen-sation of chromatic purity, i.e. the position on a scalegoing from “achromatic” black/white, devoid of anytonality, to “monochromatic” saturated color, ofcompletely pure tonality. Figure 30 : Colorimetric system L*a*b*
Figure 30
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a) Procedure
Vacuum evaporation consists of bringing a material toa gaseous state by heating it in a rarefied atmosphere.The evaporation rate depends on the temperaturereached by the material. Materials used for antire-flection coating must be heated to 1000°C/1800°F to2200°C/4000°F, to obtain good quality coatings. Toreach these temperatures, the materials are placed ina crucible where heat can be generated in two verydifferent ways :
- heating by the Joule effect this is the effect governed by Ohm’s law U = RI(electric heaters operate according to this law). A “boat”made of refractory metal (tungsten or tantalum) orcarbon is filled in solid material which reaches a hightemperative when a strong current passes into it : thematerial melts, then evaporates toward the lenses ;
- electronic bombardment an “electron gun” is used, based on the same principleas the cathode tube of TV monitors. The gun emits anelectron beam which is electromagnetically focusedon the material to be evaporated, placed in a
refractory crucible. The electrons are blocked by thetarget material, and give up their energy as heat, thusraising the temperature of the material to beevaporated (see figure 31).
b) Measurement and control of the evaporatedthin layers
The thickness of the layer building up on the lensesmust be measured in real time : one of the mostcommon methods consists of weighing the depositedcoating with a microbalance.
A piezo-electric quartz is a quartz crystal capable ofvibrating with a very precise frequency (like the crystalused in a quartz watch). This frequency can bemodified by applying a mass to one of the surfaces ofthe quartz. Hence, the thin layer is applied to a quartzcrystal placed in the vacuum chamber with the lensesto be coated. Thanks to electronic processing, thevariation in frequency is converted into a precisemeasurement of the thickness and rate of applicationof the thin layer.
Figure 31 : Diagram of a vacuum evaporation chamber.
2/ Vacuumevaporation
Figure 31
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What is a vacuum ?
In a container full of gas, molecules are subject toconstant movement, consisting of rectilinear trajec-tories and collisions (with the other molecules orcontainer walls). The mean free path designates themean distance covered by any molecule between twosuccessive collisions.
The kinetic energy due to velocity of the molecules isresponsible for pressure exerted on the containerwalls.
Atmospheric pressure at the Earth’s surface, resultingfrom the action of gravity on the layer of atmosphericair, corresponds to the presence of 2,5.1013 moleculesof nitrogen and oxygen per cm3.
A vacuum has no real physical existence, but is simplya term used to designate low pressure. For example,one generally speaks of a vacuum at the surface of theMoon, whereas in fact this designates a rarefiedatmosphere where there are only 3.105 molecules percm3 !
Let us take the gas container example once more : ifwe reduce the number of molecules present in thecontainer (by “emptying” it), from a certain point intime, they will collide only with the walls, and no longerwith other molecules. At this moment, the mean freepath exceeds dimensions of the container.
A vacuum evaporation jar is a sealed chamber inwhich vacuum pumps reduce the number of gas mole-cules to a value such that the mean free path exceedsdimensions of the jar (“secondary” vacuum). Themolecules present thus propagate without colliding withother molecules, until they meet a wall or, in thepresent case, a lens. This is what happens tomolecules of the evaporated material, which condense
directly on the lens surface, thus creating the thin layer.Antireflection coatings are elaborated in chambersmeasuring approximately 1m3, evacuated to ensure avacuum of about 1.10-6 mbar, corresponding to32000 molecules per cm3. The mean free path is 50m.Pumping time before evaporation lasts about half anhour, and a total evaporation cycle about one hour.
Figure 32 : Atmospheric pressure - high vacuum.
30
Figure 32
New developments for plastic lenses
Progress in the nineties in the electronic industry, inthe area of ion beam bombardment, has allowedantireflection coating manufacturing cycles tointegrate additional operations likely to improveadherence of the thin films (between the filmsthemselves, and between the films and substrate),to increase their density, and to modify theirrefractive index. For example, such operationsconsist of optimizing lens surface properties by“ionic bombardment” - which somewhat resemblesthe use of high pressure hoses to scour a wall - justbefore undertaking antireflection coating (“IPC” orIon Pre-Cleaning procedure). Another operationconsists of “packing” the thin layer with heavy ionswhile it is being applied, to increase its density (“IAD” or Ion Aided Deposition).
These operations are performed with an ion gun setup in the vacuum chamber - ions are particlescomprised of atoms of gas (e.g. Argon) from whichan electron has been extracted.
At another level, namely that of the chemistry of thematerials, progress in the manufacture of ultrapuremineral compounds makes it possible to elaboratestacks with better resistance to chemical corrosionand thermal shock. Such compounds includeTantalum pentoxide (Ta2O5), Zirconium dioxide(ZrO2), Titanium dioxide (TiO2), mixtures ofNeodymium and Praseodymium oxides (Nd2O5 etPr2O5).
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This is an essential aspect ofophthalmic lenses, since theyare treated roughly even
during normal use :
- abrasion (wiping, placing face downwards onfurniture, rubbing inside a handbag, etc.),- corrosion (sweat, humidity, sea air, cleaningproducts, etc.),- thermal shock (car in the sun).Contrary to the uncoated lens, which remains stableover time, coatings may appear perfect at the timethey are manufactured, then show defects after aperiod of several months. Coating procedures musttherefore include accelerated aging tests, results ofwhich should be correlated with actual wearer tests.Although there are many different artificial andaccelerated aging tests, which are specific for eachmanufacturer, they generally consist of subjectingsample lenses to the physico-chemical testsemployed in the optics industry :- boiling salty water,- cold salty water,- steam,- deionized cold water,- deionized boiling water,- abrasion by a felt pen, steel wool, rubber, etc.,- immersion in alcohol and other solvents,- thermal shock,- UV climatic chamber - humidity,- etc.
The highest elaborated technologiesare now put into service of anti-
reflection coating development in order to procure anoptimal and durable transparency to corrective lenseswearers.
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Properties of the thinfilms depend essentiallyon those of the sub-strate to which they areapplied, and several
polymers with specific characteristics have requiredthe development of specific coatings for plasticlenses. This is clearly illustrated by the following twoexamples :
- Glass can be heated up to 300°C/570°F, butplastics heated above 100°C/210°F turn yellow, thendecompose very fast. Magnesium fluoride (MgF2),which is an ideal material because of its very lowrefractive index, and is universally used to coat glass,becomes brittle and has no adherence if appliedbelow 200°C/400°F. Coating plastic lenses thus callsfor more complex stacking procedures with low indexlayers made of silica. This means longermanufacturing times ;
- The heat expansion coefficient for plastic lenses,which is about 20 times that of the materials used tomake the thin layers, is such that shearing stressappears at the contact surface; this may result incracking, and even peeling of the layer, when thecoated lens undergoes a thermal shock (for example,in a frame heater when the spectacles are beingglazed, or on the dashboard of a car left in the sun).The lens surface temperature must be more preciselycontrolled than that of glass lenses, for example usingan infrared thermometer (“optical pyrometer”).
Before applying the thinlayers to the lenses, theirsurface must be cleaned,and any residues from
previous manufacturing steps must be eliminated, toobtain a surface which is practically perfect at amolecular level. Cleaning is performed in tanks ofdetergent solutions activated by ultrasound. Theiraction is based on the phenomenon of cavitation,which consists of inducing powerful, high frequencyvariations in pressure of the liquid ; this has an effectsimilar to that of vigorous brushing.
These ultraclean lenses are loaded into the vacuumchamber in clean room conditions, to avoid any dustdeposit which might pollute the coating and give riseto shiny dots on the lens surface.
3/ Specificcharacteristics of antireflection coatingon plastic
4/ Preparation of the lenses beforeevaporation
Conclusion
The final cleaning then takes place under vacuum, justbefore coating, thanks to a “glow discharge” phase(electric discharge in gas under low pressure), or ionicbombardment (“Ion Pre - Cleaning” or “IPC” - seeinsert “New developments for plastic lenses”).
5/ Reliability ofthe coatings
ANTIREFLECTIONCOATING
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Figure 33
Figure 33 : Artist’s impression of a vacuum chamber in operation.
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Si O2
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C/ Anti-tarnish coatings
Multilayer antireflection coatingscan be easily soiled : for example,
an oily smear forms an additional layer whichinterferes with performance of the antireflectionlayers.
How can this drawback be countered ?
Analysis of the microscopic structure of the thin layersshows that they are relatively porous (at themicroscopic scale) ; it is thus possible for greasypollutants and impurities deposited on the coatedlens to become encrusted in the porosities of theoutermost layer, making it difficult to clean the lens.Researchers have therefore implemented techniquesused in the manufacture of compounds in theelectronics industry. These techniques consist of coat-ing the surface with an extra layer, giving the lens oiland water-repellent properties. Consequently, adher-ence of oily matter and water is considerably reduced.This coating is ultrathin (a few nanometers), and hasno effect on antireflection performance.
Figure 34 : Diagram showing the structure of an anti-tarnishcoating.
1/ Principle
Figure 34
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This coating is prepared fromchemical compounds containingfluorinated or hydrocarbonated
chains. For example, these may consist of fluorinatedpolysilazanes, which have a relatively complexmolecular structure (figure 34). These compoundspossess radicals which act as “hooks” ensuring verygood adherence to silica, the outermost layer of theantireflection coating. Moreover, these compoundspossess Fluorine-rich radicals which manifest strongchemical repulsion of water and greases. Thesecoatings can be applied in two different ways.
- by “dip coating”, using a similar process to thatdescribed for abrasion resistant coating, although it ismuch lighter in this case,
- by vacuum evaporation using a crucible heated bythe Joule effect, placed in the vacuum chamber usedfor antireflection coating. Application of this extralayer by evaporation is performed immediately afterevaporation of the last layer of the antireflectionstack.
The outer layer acts in two complementary ways :
- it prevents grease deposits from settling andremaining on the lens surface, by clogging surfaceporosities ;
- the layer is composed in such a way that it modifiesthe shape of water and grease droplets, reducing theircontact surface by a factor of 2.5 ; this makes it easierto wipe them off the lens.
Thanks to the specific properties of this extra layer,lenses with an antireflection coating are easily cleanedwith a dry cloth. However, regular cleaning with soapand water maintains optimal transparency of thespectacles, and is still recommended.
Figure 35 : Contact angle of a drop of water on a lens treatedwith an “anti-tarnish” coating.
2/ Manufacturingprocedure
Figure 35
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CONCLUSION
Market surveys carried out in severalcountries clearly show the interest for lenseswith “all-in-one” coatings or “integratedcoatings” ; such lenses correspond best tothe wearer’s expectations, as they offermultiple advantages without raising theawkward question of options.
Antireflection coatings are recognized byconsumers as a determinant factor for com-fort and appearance. Similarly, anti-tarnishcoatings have brought about a dramaticimprovement in the cleaning of coatedlenses, even though further research isrequired to definitively rule out this problem.Finally, the increased market share for high-index lenses implies substantial futuregrowth in the area of anti-abrasion coatings.
These different needs allow us to concludethat coatings have a bright future. They arebecoming fully fledged lens constituents, andtheir success has been proved throughoutthe world. Although their use varies from onecountry to the next, there is however a clearoverall upswing. Hence, lens coatings arebound to assume an increasingly importantplace within the general economy ofophthalmic lenses.
© Essilor Internationalapril 1997
All rights reserved. No part of this publication may be reproduced in any form or by any means without the prior permission of Essilor International.
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