OPHTHALMIC OPTICS FILES

Materials

INTRODUCTION 2

CONCLUSION 28

CHARACTERIZATION OF MATERIALS

I

SUMMARY

II

III

Optical properties 6

Mechanical properties 12

Thermal properties 13

Electrical properties 13

Chemical properties 13

A

B

C

D

E

LENS MANUFACTURE

Manufacture of glass lenses 24

Manufacture of plastic lenses 26

A

B

THE BASIC OPHTHALMIC LENS

Glass materials 14

Plastic materials 18

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B

1

INTRODUCTION

2

This volume of the Ophthalmic OpticsFiles presents a study of the Materialsused to manufacture ophthalmic lenses. “Materials” means “all materials used duringmanufacturing”, i.e. all materials whichenter into the composition of the basicophthalmic lens and which, shaped to aspecific geometry, give the lens itscorrective function (function described indetail in other volumes of this series).

The present File is divided into threeparts : the first part consists of an inven-tory of the physical and chemicalproperties used to describe materials, thesecond part presents a structural andfunctional analysis of the materialsavailable, and the third part is a briefdescription of the techniques used tomanufacture ophthalmic lenses.

MATERIALS

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The ophthalmic lens is acomplex systemBefore proceeding with an analytical description ofmaterials, we should point out that today, anophthalmic lens is a very complex product, resultingfrom the combination of various Materials andnumerous Coatings which endow it with specificproperties (figure 1).

Thus, the Material is just one of the lens compo-nents : its role consists not only of participating inproducing the corrective function of the lens, but alsoof acting as a support for one or several coatings. Thestudy of materials is therefore intimately linked withthat of the coatings to be applied to those materials.

Figure 1 : A coated plastic lens is a complex system.

Figure 1

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THE LENS AS A SYSTEM

TOP COAT

AR

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PRIMER

DYES

SUBSTRATE

The analysis of material structures calls upon somenotions of Physics and Chemistry relating to theNature of matter. A brief review of these notions isprovided below.

Modern Physics is based on the fact that, throughoutthe Universe, any material body is made of atoms.These are corpuscles which are perpetually inmovement. They are in turn made of relatedelementary particles called protons and neutrons,around which smaller particles called electrons followa distant orbit.

The radius of an atom measures between 1 and 2.10-10 m - i.e., approximately 1Å (Angström). Atomsare the basic building blocks of all material bodies ;physicists have discovered that there are no more thanabout a hundred types of atoms present throughoutthe Universe. Mendeleev ingeniously predicted the listof these materials, and classified them in a table evenbefore they were actually discovered : thisclassification is known as the Periodic Table of theElements (figure 2).

Figure 2 : Periodic Table of the Elements - (Mendeleev’s table ).

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In material bodies, atoms are assembled in more orless complex structures, which chemists havedescribed as follows :- Simple Bodies made of a single type of atom :Hydrogen, Oxygen, Carbon, Iron, Silicon, Barium,Titanium, etc.- Compound Bodies, made of a single type ofmolecule, which is in turn a rigorously quantifiedassembly of predetermined atoms : water (H2O),sodium chloride (NaCl), ethyl alcohol (C2H6O), silicondioxide (SiO2).

Simple and Compound Bodies are called Pure Bodies.

In particular, they are characterized by the rigorouslyprecise temperatures at which they change state(Melting Point, Evaporation Point); these temperaturesare in fact physical constants of the given material.- Mixtures, consisting of a combination of Simple andCompound Bodies which is not strictly defined, andtherefore cannot be represented by a precise chemicalformula. Mixtures include air, steel, glass, rubber,honey, blood, petrol, etc. Their Melting andEvaporation Points are not precise, and varydepending on their composition.

SUPPLEMENTREVIEW OF BASIC NOTIONS

CONCERNING THE NATURE ANDSTRUCTURE OF MATTER

1 H1

2

3

4

5

6

7

Li

Na

K

Rb

Cs

Fr

3

11

19

37

55

87

Be

Mg

Ca

Sr

Ba

Ra

4

12

20

38

56

88

Sc

Y

La

Ac

21

39

57

89

Ti

Zr

Hf

22

40

72

V

Nb

Ta

23

41

73

Cr

Mo

W

24

42

74

Mn

Tc

Re

25

43

75

Fe

Ru

Os

26

44

76

Co

Rh

Ir

27

45

77

Ni

Pd

Pt

28

46

78

Cu

Ag

Au

29

47

79

Zn

Cd

Hg

30

48

80

B

Al

Ga

In

Ti

5

13

31

49

81

C

Si

Ge

Sn

Pb

6

14

32

50

82

N

P

As

Sb

Bi

7

15

33

51

83

O

S

Se

Te

Po

8

16

34

52

84

F

Cl

Br

I

At

9

17

35

53

85

He2

Ne

Ar

Kr

Xe

10

18

36

54

86

1

2

3

4

5

6Rn

III b IV b V b VI b VII b VIII i b II b

I a II a III a IV a V a VI a VII a O

Ce

Th

58

90

Pr

Pa

59

91

Nd

U

60

92

Pm

Np

61

93

Sm

Pu

62

94

Eu

Am

63

95

Gd

Cm

64

96

Tb

Bk

65

97

Dy

Cf

66

98

Ho

Es

67

99

Er

Fm

68

100

Tm

Md

69

101

Yb

No

70

102

Lr

71

103

6

7

Lu

Figure 2

In our terrestrial environment, it is also possible todefine materials macroscopically, by referring to theiressential atomic constituents. The following classi-fications can thus be distinguished :

- mineral matter, consisting of the Pure Bodies andMixtures which form the rocks of the earth’s crust -called “SIAL” and essentially made up of SIlicon andALuminum - and their derivatives. Their moleculesconsist of various combinations of a small number ofatoms (from one to twenty) listed on the Periodic Tableof the Elements. The main elements encountered in thefield of Ophthalmic Optics are represented by thegreen area of figure 2.

- organic matter, consisting of the Pure Bodies andMixtures which make up the basis of the plant andanimal worlds and derivative bodies, fossil fuels, andsynthetic materials obtained by Organic Chemistry.Organic materials, which are very numerous andcomplex (“the living organs are made of organicmatter”), are made of molecules often comprised of alarge number of atoms (as many as several thousand),although these atoms belong to a very limited numberof species, i.e. basically to C, H, O and N (pink areaon figure 2). Carbon, C, is a kind of skeleton for livingmatter, while H, O and N are the atmosphericelements which make life possible.

The 3 states of Matter.

Imagine a drop of water : this consists of H2Omolecules, which in turn consist of 2 atoms ofHydrogen combined with a single atom of Oxygen. Atroom temperature, these molecules are permanentlyin motion but stay together through the attraction theyexert on each other, which maintains cohesion of thedrop of water - this is the LIQUID state. The molecularagitation is proportional to temperature : if tempe-rature rises, the motion accelerates and distancesbetween the molecules increase. At a certain tem-perature, the forces of attraction are no longer enoughto keep the molecules together, and they disperse in alldirections - this is the VAPOR or GAS state. Althoughthe drop can no longer be seen, the steam isnevertheless still made of H2O molecules. Conversely,if we cool the drop of water, the motion is slowed downuntil the molecules stop overlapping altogether andthe system becomes static and literally frozen,producing ice, the SOLID state of water. Ice ischaracterized by a regular periodic structure calledthe crystalline network. It also exists another solidstructure, called amorphous - or vitreous - state, whichis characteristic of glass). The characteristic of thesolid state is the fact that all molecules are in a fixedposition with respect to each other, enabling solidmatter to transmit forces and movement.

5

COMPLEMENTREVIEW OF BASIC NOTIONS

CONCERNING THE NATURE ANDSTRUCTURE OF MATTER

Figure 3 : Liquid-to-gas change of state : evaporation of water.

Figure 4 : Solid state : ice.

Figure 3 Figure 4

6

The aim of ophthalmic optics is to restore clear visionto an ametropic eye by placing a corrective lensbetween the viewed object and the eye. The dioptricfunction of the lens depends on :- its geometric characteristics : radius of curvature,surface geometry, etc.- the physicochemical characteristics of the material :refractive index, constringence, etc.

Research on Materials is chiefly aimed at obtainingand controlling these physicochemical characteristics.Furthermore, tools and techniques used tomanufacture lenses require precise knowledge of thecharacteristics of the materials used. Extensiveknowledge of these materials is therefore needed inthe following areas :- optical properties, to calculate the dioptric functionand control of optical performances,- mechanical and thermal properties, to describedimensional aspects of lenses and their resistance todeformation,- electrical properties, to physically characterize thematerials,- chemical properties, to account for behavior ofmaterials with respect to external chemical agents.

A/ Optical properties

These are the essential properties of the material. Theoptical properties act on light and, first and foremost,allow the sought corrective effect to be obtained.They correspond to various optical phenomenaencountered at the level of the actual lens : lightrefraction through the surfaces, reflection on eachside of the lens, possible absorption by the material,and also diffusion and diffraction. These phenomenawill be described in detail, and material propertiesassociated with each of them will be characterized.

Figure 5 : Refraction and chromatic dispersion.

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Figure 5

The rays of light which cross thelens undergo refraction - ordeviation - at the level of its front

and back surfaces. Amplitude of these deviations isdetermined by the refractive power of the material,and by the incidence of the rays of light on eachsurface.

a) Refractive index

The refractive index of a transparent medium is theratio n = c / v of the velocity of light in a vacuum (c) tothe velocity of light in the given medium (v). Thisnumber, which has no unit and is always greater than1, quantifies refractive power of the medium : thehigher the refractive index, the more a beam of lightentering this medium from air will be deviated.

At any point of a surface separating air from atransparent medium with refractive index n, thedeviation or refraction is calculated according toSnell’s - Descartes’ law which stipulates that (figure 5) : 1 - the refracted ray lies in the plane formed by theincident ray and the perpendicular to the surface atthe point of incidence2 - angles of incidence i and refraction r, respectivelyformed by the incident and refracted rays with theperpendicular to the surface, are such that

sin (i) = n sin (r)

As the velocity of light in a transparent medium varieswith wavelength, the value of the refractive index is always expressed for a reference wavelength : in Europe and in Japan, this reference is λe = 546.07 nm(Mercury green spectral line), whereas in othercountries like the U.S.A. it is λd= 587.56 nm (Heliumyellow spectral line) . This distinction has no real effectsince it only changes the 3rd decimal of the value ofthe index.

Refractive indexes of materials currently used inophthalmic optics range from approximately 1.5 forstandard glass and plastic, up to 1.9 for the mostrecent glass materials.

b) Chromatic dispersion : Constringence or Abbenumber (*)

The variation of the refractive index with thewavelength of light causes the phenomenon ofchromatic dispersion of white light upon refraction.Indeed, since the refractive index is higher for shortwavelengths, refraction of visible light spreads fromthe red zone towards the blue zone of the spectrum(figure 5).

To characterize the dispersive power of a material, aquantity called constringence or the Abbe number isused (*). In Europe and Japan this is defined by ne ,and in other countries like the U.S.A. it is defined bynd , with the following respective formulae :

where :ne : index for λe = 546.07 nm (green Hg)nd : index for λd = 587.56 nm (yellow He)

nf ’ : .................. λf ’ = 479.99 nm (blue Cd)nf : .................. λf = 486.13 nm (blue H)

nc’ : .................. λc’ = 643.85 nm (red Cd)nc : .................. λc = 656.27 nm (red H)

The Abbe number is inversely proportional tochromatic dispersion of the material, and generallyvaries in ophthalmic optics between 60 for the leastchromatic and 30 for the most chromatic materials.Generally speaking, although there are someexceptions, the higher a refractive index, the greaterthe chromatic dispersion, thus, the lower the Abbenumber (see table 2).

Chromatic dispersion is an important but not anessential characteristic in ophthalmic optics. Althoughit exists in all lenses, this factor is always negligible atthe center, and is only really discernible at theperiphery of lenses manufactured with highlydispersive materials. In this case, dispersion is seen ascolor fringes at the edges of off-axis objects.Experience has shown that chromatism of ophthalmiclenses has no perceptible effects for the majority ofcorrective lens wearers.

(*) See “ Supplement “ section below

ne - 1 nd - 1 ne = or nd =

nf ’ - nc’ nf - nc

7

CHARACTERIZATIONOF MATERIALS

1/ Refraction of light

Figure 6 : Refraction, reflection and absorption of light in anophthalmic lens (1.5 index lens with 15 % absorption).

At the same time as refraction, aphenomenon of light reflectionalso occurs at each of the lens

surfaces. This results, for the wearer, in a loss of lenstransparency, and in undesirable reflections on thelens surfaces. The higher the refractive index of thelens material, the more light is lost through reflection :

Index :

1.5 1.6 1.7 1.8 1.9

Total reflected light :

7.8 % 10.4 % 12.3 % 15.7 % 18.3 %

This unwanted reflection can be almost completelyeliminated by applying an efficient antireflectivecoating (see Ophthalmic Optics File on Coatings).

The amount of light which goesthrough a lens can be reducedbecause of absorption by the

material : this is negligible for an untinted ophthalmiclens, but constitutes an intrinsic function of a tinted orphotochromic lens. Absorption of an ophthalmic lensgenerally refers to its internal absorption, i.e. to thepercentage of light absorbed between the front andrear lens surfaces. For example, 30 % absorptioncorresponds to a 30 % internal reduction in luminousflux, which should be added to the loss alreadycaused by the reflection on both surfaces of theuntinted lens. Lens absorption occurs according toLambert’s law, and varies exponentially as a functionof lens thickness (see “Supplement” section below).

Light transmitted by an ophthalmic lens

The light transmitted by a lens is simply the amountof light that is neither reflected nor absorbed by thelens. Luminous flux fτ , which reaches the eye, cor-responds to incident flux f at the front surface of the lens, attenuated by flux fρ reflected by the 2 surfaces, and by flux fα , possibly absorbed by thematerial, such that fτ + fρ + fα = f. Furthermore,the wearer’s perception results from the combinationof 3 elements : intensity and spectral composition ofthe incident light, absorption and spectral selectivityof the lens, and sensitivity of the eye to the differentvisible wavelengths (see figures 6 and 7 and“Supplement” section below).

8

2/ Reflection of light

3/ Absorption of light

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φρ

1 = 4,0%

φρ

2 = 3,2%

φex

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,0%

φ τ = 77

,8%

φin =

96,

0%

φ = 100%

Figure 6

CHARACTERIZATIONOF MATERIALS

a) Diffusion Diffusion is a phenomenon whichcan be described as a scatteringof light in all directions. It occurs

at the surface of all solids, and inside transparentmaterials. In an ophthalmic lens, there is theoreticallyno surface diffusion since lens surfacing processes(particularly polishing) are performed to eliminate thisphenomenon. It does occur, however, when the lens issoiled by external pollution or when its surface issmeared with oil. Diffusion within the actual lens isalso very limited, but in some cases, may give lensesa yellow or milky appearance. Overall, though, anophthalmic lens diffuses only a very small quantity oflight, generally considered as negligible.

b) Diffraction Diffraction is a phenomenon consisting of a change inthe direction of a light wave when it encounters a smallobstacle (measuring only a few wavelengths). Thisphenomenon is important in ophthalmic optics, sinceit can reveal irregularities on lens surfaces, and inparticular scratches resulting from wear and tear.

9

4/ Diffusion and diffraction of light

τλ

λ(nm)

100

80

60

40

20

0380 400 450 500 550 600 650 700 780750

dλ

(%)νλ

λ(nm)

100

80

60

40

20

0380 400 450 500 550 600 650 700 780750

dλ

(%)

λ(nm)

60

40

20

0380400 450 500 550 600 650 700 780750

dλ

ρλ

λ(nm)

60

40

20

0380400 450 500 550 600 650 700 780750

dλ

ρλ

τv

φλ

λ(nm)

60

40

20

0380 400 450 500 550 600 650 700 780750

dλ

φλ

λ(nm)

60

40

20

0380 400 450 500 550 600 650 700 780750

dλ

νλ

λ(nm)

100

80

60

40

20

0380 400 450 500 550 600 650 700 780750

dλ

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W.m

-2.h

m -1

W.m

-2.h

m -1

W.m

-2.h

m -1

W.m

-2.h

m -1

Figure 7

Figure 7 : Transmission of an ophthalmic lens.

CHARACTERIZATIONOF MATERIALS

Characterization of chromatism :Constringence or the Abbe number ?

Improper use of terminology often leads to use of theterm “Abbe number” to designate constringence,whereas in fact definitions of these two factors areslightly different. While the Abbe number uses thesame formula as constringence - nd - the referenceyellow spectral line is the D medium of the sodiumdoublet with wavelength λD = 589.29 nm, instead ofthe yellow Helium spectral line d, with wavelength λd = 587.56 nm. However, differences between nd and nD (and even ne ) are small and relativelyinsignificant in ophthalmic optics, since they affectonly the first decimal of the constringence value.

Characterization of transmission of anophthalmic lens :

Transmission factor :

The transmission properties of a lens are charac-terized by its transmission factor τ= fτ / f ratio ofthe luminous flux fτ which exits from its back surfaceto the luminous flux f incident reaching its frontsurface (figure 6). Since transmission varies withwavelength, the spectral transmission factor τλ of thelens is determined for each wavelength λ of theincident light.

Transmission curve

The transmission curve describes the physicalproperties of the light filter comprised by the lens,presenting the variation of its spectral transmissionfactor τλ as a function of wavelength (figure 7). Thiscurve allows spectral selectivity of the filter to beobserved, and the physical transmission factor τ ofthe lens to be determined throughout the wavelengthsranging from λ1 to λ2 , using the formula :

λ2f λ.τλ dλ

λ1

τλ = λ2

fλ. dλλ1

where fλ = incident spectral flux.

Relative transmission factor in the visible range τv

This factor is specific to ophthalmic optics, and sum-marizes the physiological properties of the filter as asingle number : the ratio of the luminous flux emer-ging from the lens and of the luminous flux incidentupon the entrance surface, as perceived by the eye, i.e.weighted for each wavelength by relative spectralluminous efficiency Vλ of the eye. This factor iscalculated using the following formula :

780

fλ.τλ Vλ dλ380

τv = 780

fλ.Vλ dλ380

where τλ = spectral transmission factor, fλ= incidentspectral flux, Vλ = relative photopic spectral luminousefficiency of the eye (see detailed illustration, figure 7).This coefficient, τλ , is used to describe and classifysunglasses.

U.V. cutoff :In ophthalmic optics, ultraviolet absorption propertiesare of particular interest : to characterize the “UVcutoff” of a lens, the wavelength up to which the lenstransmits less than 1% light is determined on the lenstransmission curve.

NB : Transmission characteristics of ophthalmic lensesare usually established for lenses of 2.0 mmthickness, under normal incidence.

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SUPPLEMENTCHARACTERIZATION OF THE OPTICAL

PERFORMANCES OF AN OPHTHALMIC LENS

Characterization of reflection of anophthalmic lens :

Reflection factor :

Reflection at the interface of two media ischaracterized by the reflection factor ρ = fρ / f ratioof reflected luminous flux fρ and of incident luminousflux f (figure 6). Since reflection varies with wave-length, the spectral reflection factor ρλ is generallydetermined for each wavelength λ of the incident light.

At the level of one surface separating air from atransparent medium of refractive index n, thereflection factor is obtained using the followingformula, established by Fresnel :

n - 1 2

ρ = ( )n+1

assuming that the light incidence is normal. This factorrepresents the curbing of luminous flux through the surface, and is used as an attenuator coefficient,applied to the incident flux of light : thus luminous flux f crossing one surface of reflection factor ρ losesa fraction of intensity fρ to become f.(1 - ρ) aftertraversing the surface. For an ophthalmic lens, thereflection phenomenon occurs at both the front andback sides of the lens, such that the total reflected fluxis calculated with the formula fρ = f . ρ .(2 - ρ) , whenthere is no absorption.

Relative reflection factor in the visible range ρv

This factor is used in ophthalmic optics to characterizethe visual effect of reflection by the ratio of reflectedlight flux fρ to incident light flux f as perceived bythe eye, i.e. weighted for each wavelength by therelative spectral luminous efficiency of the eye, Vλ. Thisfactor is calculated with the following formula :

780

ρλ fλ Vλ dλ380

ρv =780

fλ Vλ dλ380

where ρλ = spectral reflection factor, fλ = incidentspectral flux, Vλ = relative photopic spectral luminousefficiency of the eye.

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SUPPLEMENTCHARACTERIZATION OF THE OPTICAL

PERFORMANCES OF AN OPHTHALMIC LENS

Characterization of absorption of anophthalmic lens :

Absorption factor

Absorption of a lens is characterized by its internalabsorption factor ai = fa / fin , the ratio of luminousflux fa = fin - fex which is absorbed betweenentrance and exit surfaces to luminous flux fin whichhas passed through the entrance surface (see figure 6). As lens absorption varies with wavelength,the internal spectral absorption factor ai l of the lensis determined for each wavelength l of incident light.

The quantity of light absorbed during traversal of amaterial is given by Lambert’s law, which stipulatesthat layers of material of equal thickness produceequal light absorption (expressed as %), irrespectiveof light intensity. It is thus possible to deduce thatluminous flow fex reaching the lens exit surface isgiven by the formula fex = fin . e-k.x where k is thespecific coefficient of extinction of the material, and xis thickness of the material traversed by the light. Theinternal absorption factor is calculated using theformula ai = 1 - e-k.x; and is applied as an attenuatingcoefficient such that fex = fin. (1- ai ).

Application : calculation of the luminous fluxtransmitted by a lens

Given an incident luminous flux f which reaches thesurface of a lens : - after partial reflection on the first surface, the fluxwhich enters the lens is : f . (1 - ρ),- this flux is attenuated upon traversing the lens, andbecomes : f. (1 - ρ)( 1 - ai ) when it reaches thesecond lens surface- reflection occurs again, such that the flux emergingfrom the lens is : fτ = f (1 - ρ)2 .(1 - ai ).

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B/ Mechanical propertiesMechanical properties are the characteristicproperties of the solid and massive state of materials.They define values relative to mass, volume anddimensions, and resistance to deformation and shock.The following characteristics are generally used :- specific gravity is the ratio of the density of 1 m3 ofa given substance to the density of 1 m3 of water, theusual standard of reference for solids under normalconditions. It is expressed by a number without a unit,- hardness measured in clearly defined systems usingprecision instruments, such as the Knoop, Rockwell,and Barcol hardness tests,- modulus of elasticity E (or Young’s modulus) : ratiobetween a stress and the corresponding deformationwhere the material resumes its initial shape uponremoval of the stress (in Pascal : Pa),- impact resistance : compliance with the testconsisting of dropping a 0.56 ounce (16 g) steel ballfrom a height of 50 inches (1.27 m) - test establishedin the USA by the Food & Drug Administration (FDA),- breaking-point resistance : compliance with the“100 Newton” CEN static deformation test - whichconsists of exerting increasing pressure at a constantrate up to a value of 100 Newtons (procedure beingreviewed by the European Standardization Committee),- breaking-point in traction, compression, anddeflection (Pa),

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C/ Thermal propertiesThermal properties characterize changes of state andthe effect of temperature on materials. The mainthermal properties are as follows :- thermal conductance (W.m

-1.°K

-1)

- specific heat : the ratio of the quantity of heatrequired to raise the temperature of a body by onedegree to that required to raise the temperature of anequal mass of water by one degree (J.Kg

-1.°K

-1)

- linear dilatation coefficient : given for a predefinedtemperature range (10

-7/°K)

- melting point (d°C) - a physical constant for PureBodies - for glass, melting point is defined by thetemperature at which it has a viscosity of 10

2,5poises

- evaporation point (d°C) at a given pressure - e.g. formaterials used for Antireflection Coatings, theevaporation point may be specified under lowpressure (~ 10

-6mbar),

- stress temperature TC (for the lens) : the temperatureat which viscosity reaches the value of 10

14,5poises,

- vitreous transition temperature TG, at which lensviscosity presents a value of 10

13,3poises.

D/ Electrical propertiesElectrical properties characterize effects of electro-magnetic waves and electricity on materials. They aregoverned by complex laws of physics. Some link theoptical properties of solids to their electricalproperties. For this reason, manufacturers of materialsmention the following parameters :- dielectric strength (V.m-1),- dielectric loss factor (tg d x 10

4), at predefined

frequencies.

E/ Chemical propertiesChemical properties characterize the reaction ofmaterials to the chemical substances usually foundduring lens manufacture, in everyday life, or to certainextreme conditions to which materials can besubjected, such as accelerated ageing in order to testtheir reliability. These substances are usually water -hot and cold, salty and fresh - acids, bases, andvarious organic solvents. Finally, internationalstandards also require testing to determine fireresistance of materials used to manufacture oph-thalmic lenses.

13

CHARACTERIZATIONOF MATERIALS

14

A/ Glass materialsGlass is a solid, amorphous material, which is hardand breakable at ambient temperature, and becomesviscous at high temperatures. It is obtained by meltinga mixture of oxides of elements such as Silicon,Calcium, Sodium, Potassium, Lead, Barium, Titanium,and Lanthanum at a temperature of about 1500 °C /2700 °F. Glass does not have a homogeneouschemical structure. Consequently, there is no precisemelting point at which it abruptly changes from thesolid to the liquid state. Furthermore, as temperaturerises, glass softens, its viscosity decreases, and it verygradually goes from solid to liquid, this very gradualchange being characteristic of the so-called “vitreous”state. This unique feature means that glass can beworked while hot, and molded. It is valuable forophthalmic optics for two reasons : it transmits visiblelight, and its surface can be polished and thereforemade transparent.

Glass of index 1.5 is thetraditional material used inophthalmic optics. It ismade of 60 to 70 % Silicon

dioxide, the remainder consisting of various com-pounds such as Calcium, Sodium and Boron oxides. Glass of index 1.6 is currently becoming the newstandard in ophthalmic optics. Its higher index isobtained by adding a significant proportion ofTitanium dioxide to the mixture.

Materials are usually grouped into 2 categoriesdepending on their chemical composition (see table 1) :- “Sodium-Calcium” glass, containing significantproportions of Sodium and Calcium : this is thetraditional material used in optics : its refractive indexis relatively low (ne = 1.525 / nd = 1.523), as is itschromatic dispersion (constringence of about 60).- “Borosilicate” glass, with a high Boron content : thisis the most recent material used to manufacturephotochromic and medium-index glass lenses (ne = 1.604 / nd = 1.600).

Glass can be mass tinted byincorporating metal salts withspecific absorption properties

into its composition : for example, salts of Nickel andCobalt (purple), Cobalt and Copper (blue), Chromium(green), Iron, Cadmium (yellow), Gold, Copper,Selenium (red), etc. These solid tint materials aremainly used for mass production of plano sun lensesor protective spectacles. A few faintly tinted materials(brown, grey, green or pink colors), with specificfiltering properties are also used to manufacturecorrective lenses, but such lenses are in far lessdemand today, a major drawback being that darknessof the tint varies with lens thickness.

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1/ Standard glassmaterials (1.5 and 1.6)

2/ Solid tintglass materials

THE BASICOPHTHALMIC LENS

15

THE BASICOPHTHALMIC

LENS

Photochromism is the propertyof a material to react to theintensity of sunlight by a

modification of its light absorption properties. Itsbasic principle - which is common to glass and plasticphotochromic materials - is to darken under theeffects of U.V. radiation, and to fade under the effectsof ambient heat ; these two actions are reversible, andmay occur indefinitely. This result is achieved by theactivation of molecules of photochromic substancesincorporated into the material. Light absorption isdetermined at all times by the equilibrium betweenthe number of photosensitive molecules activated byU.V. stimulation, and the number of moleculesdeactivated by heat.

The first photochromic glass materials appearedaround 1962, and have been constantly improvedwith each successive generation of lenses. They areobtained by introducing silver halide crystals into thematerial. These crystals react to ultraviolet radiation,such that the lens darkens. At the atomic level, thebasic mechanism of photochromism is an electronexchange between the Silver atom - present as SilverChloride (figure 9) - and its environment. In theabsence of light, the Silver-Chloride bond is ionic, andthe Silver ion is transparent ; the lens is clear. In thepresence of U.V. radiation, the unstable electronleaves the Chloride ion to join the Silver ion whichbecomes metallic and absorbs light : the lens darkens.When the U.V. radiation diminishes, the mobile

electron leaves the Silver atom and returns to theChloride atom, and the lens gradually returns to itsoriginal clear state.

Figure 8 : General principle of photochromism.

Figure 9 : Principle of glass photochromism.

Figure 8

Figure 9

3/ Photochromicglass materials

Chlorine (Cl)

electron

Silver (Ag)

For many years, glassmakershave sought to raise therefractive index of theirmaterials, while maintaining

low chromatism values. To do so, they have regularlyintroduced new chemical elements into the com-position of glass. Thus, around 1975, lensescontaining Titanium with a 1.7 index and cons-tringence of 41 were manufactured ; these werefollowed approximately fifteen years later by lensescontaining Lanthanum with a 1.8 index andconstringence of 34 and, finally, around 1995, bylenses containing Niobium with index 1.9 andconstringence of 30. These materials allow thinnerlenses to be manufactured, but do not achieve asignificant reduction in weight. Indeed, the increasedrefractive index of the material goes together with anincrease in density, which counteracts any reductionin weight one might have expected from the reductionin lens volume .

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4/ High indexglass materials(1.7, 1.8 et 1.9)

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Table 1 : Indicative chemical composition of the main glass materials (in weight)

GLASS MATERIAL TYPES

Component Oxide Sodium-Calcium Borosilicate Titanium Titanium/ Lanthanum/NiobiumLanthanum

1.5 clear 1.5 tinted 1.5 photo 1.6 photo 1.6 clear 1.7 clear 1.8 clear 1.9 clear

Silicium SiO2 70 71 57 48 56 36 29 7

Aluminum AI2O3 1 - 6 - 1 - - -

Boron B2O3 1 - 18 15 6 10 2 17Sodium Na2O 11 12 4 1 9 2 - -

Potassium K2O 5 6 6 5 8 - - -

Lithium Li2O - - 2 2 4 6 4 -Magnesium MgO 1 - - - - - - -

Calcium CaO 9 11 - - - 9 15 14

Barium BaO 2 - - 6 - - - -

Zirconium ZrO2 - - 5 7 1 5 5 8

Titanium TiO2 - - 2 6 15 6 9 9Niobium Nb2O5 - - - 8 - 9 15 21

Lanthanum La2O3 - - - - - 14 21 24

Strontium SrO - - - 2 - 3 - -

Iron Fe2O3 - 1 - - - - - -

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Table 2 : Characteristics of the main materials

PLASTIC MATERIALS

ThermosettingThermo-plastic

Standard index Medium index High index High index

clear photochromic photochromic clear photochromic photochromic clear polycarbonate

grey brown grey brown

Essilor brand name Orma Orma Orma Ormex Ormex Ormex OrmilTransitions Transitions Transitions III Transitions III

Plus Eurobrown

Refractive index ne 1.502 1.502 1.502 1.561 1.559 1.559 1.599 1.591

nd 1.500 1.500 1.500 1.557 1.557 1.557 1.595 1.586

Constringence ne 58 58 58 37 37 37 36 31

nd 59 59 59 37 38 38 36 30

Specific gravity 1.32 1.28 1.28 1.23 1.20 1.20 1.36 1,20

U.V. cut off 355 nm 370 nm 370 nm 370 nm 390 nm 390 nm 380 nm 380 nm

GLASS MATERIALS

Standard index Medium index High indexclear photochromic photochromic clear photochromic photochromic clear clear

grey brown grey brown

Essilor brand name 15 clear 15 Isorapid 15 Isorapid 16 clear 16 Isorapid 16 Isorapid 17 18grey brown grey brown

Refractive index ne 1.525 1.525 1.525 1.604 1.604 1.604 1.705 1.807

nd 1.523 1.523 1.523 1.600 1.600 1.600 1.701 1.802

Constringence ne 59 57 56 41 42 42 41 34

nd 59 57 56 42 42 42 42 35

Specific gravity 2.61 2.41 2.41 2.63 2.70 2.73 3.21 3.65

U.V. cut off 330 nm 335 nm 335 nm 335 nm 345 nm 345 nm 335 nm 330 nm

B/ Plastic materials

Plastic materials can be divided into 2 groups :- thermosetting resins which have the property ofhardening under the action of heat, without thepossibility of subsequent reshaping under heat ; mostof the resins used in ophthalmic optics are of thistype, particularly CR 39,- thermoplastic resins which have the property of soft-ening under the action of heat, and are particularlysuitable for shaping under heat or injection molding :polycarbonate is such a material.

(1) CR 39 is a registered trade mark of PPG Industries.(2) Orma® is a registered trade mark of Essilor International.

Diethylene Glycol bis (AllylCarbonate), better known asCR 39 (1), is the material mostwidely used to manufacture

currently marketed plastic lenses. It was discoveredduring the forties by chemists from the “ColumbiaCorporation” (division of the American PPG -Pittsburg Plate Glass company), hence its name, sinceit was the “Columbia Resin” # 39 of a series ofpolymers studied by chemists for the US Air Force. Itwas used to manufacture corrective lenses between1955 and 1960 (by the firm called LOS - LentillesOphtalmiques Spéciales - , one of the Essilor foundingcompanies), which led to the introduction of Orma®

lenses (2), the first lightweight, impact-resistantlenses.

CR 39 is a polymerizable thermosetting resin, i.e. in itsbasic form it is a liquid (the monomer), which ishardened by polymerization under the effect of heatand a catalyst. Polymerization is a chemical reactionwhereby several identical molecules of monomercreate a new molecule of polymer, with differentdimensions and properties. In the case of CR 39, thepolymer thus obtained is reticulated (i.e. comprised ofa three-dimensional network), making it unmeltable,insoluble, resistant to solvents, and dimensionallystable.

For ophthalmic optics, CR 39 features a number ofproperties which account for its success : a refractiveindex of 1.5 (close to that of standard glass), a densityof 1.32 (almost half that of glass) and a constringenceof 58-59 (thus a very low chromatism), high impactresistance, excellent transparency, and multipletinting and coating possibilities. Its major drawback isits limited resistance to abrasion relative to glass.

Figure 10 : CR 39 molecule : monomer and polymer.

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1/ Standardplastic material(CR 39)

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Figure 10

THE BASICOPHTHALMIC

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Solid tint plastics are exclu-sively used to manufacturesun lenses. They are obtained

by adding dyes before polymerization, and areparticularly suitable for the mass production of planosun lenses, with a wide variety of tints andabsorptions. U.V. absorbers are generally added tothese materials, to improve protection against U.V.radiation.

The first photochromic plas-tics appeared briefly around1986, but they only really

started to develop in 1990, when the first Transitions®

lenses were introduced (3). The photochromic effect isobtained by introducing photosensitive compoundsinto the material. Under the action of specific band ofultraviolet radiation, these compounds undergostructural changes, which modify absorptionproperties of the material. The compounds areincorporated using two main procedures : either theyare mixed with the liquid monomer beforepolymerization, or they are applied by imbibing afterpolymerization (the latter procedure is used forTransitions lenses). Several families of molecules areused to create photochromism, and these families aresubject to several types of structural modifications.Figure 11 shows the case of a Spiro-Oxazine molecule : Figure 11 : Principle of photochromism in plastic material

(example of a Spiro-Oxazine molecule).

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(3) Transitions® is a registered trade mark of Transitions Optical Inc.

Figure 11

2/ Tinted plasticmaterials

3/ Photochromicplastic materials

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WHEN CLEAR

WHEN DARK

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The use of medium index (n < 1.56) and high index(n > 1.56) plastics hasgreatly increased over recent

years. Compared with traditional CR 39, they allowmanufacture of even thinner, lighter lenses. Theirdensity is generally similar to that of CR 39 (between1.20 and 1.40), although their chromatism isgenerally greater (constringence < 45), as is theirsensitivity to heat and, generally, they offer betterU.V. protection (see table 2). The main drawback ofthese materials is that they are easily scratched, and a protective coating must thus be appliedsystematically. They can also be tinted andantireflection coated. These materials are currentlyundergoing intensive development, and hold muchpromise for the future.

a/ Thermosetting resins

Most medium and high index plastics available todayare thermosetting resins. Their refractive index isincreased using one of the two following techniques : - modification of the electronic structure of the initialmolecule, for example, by introducing aromaticstructures,- introduction in the initial molecule of heavy atomssuch as halogens (Chlorine, Bromine, etc.) or Sulfur.The manufacturing process for lenses made of thesematerials is similar in principle to that describedabove for CR 39.

the right part of the molecule rotates under the effectof ultraviolet light, causing the lens to darken.Photochromism of plastic lenses is generally obtainedby using several dyes, and the final product combinesrespective photochromic effects of each of theseagents.

4/ Medium andhigh index plasticmaterials

THE BASICOPHTHALMIC

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Polycarbonate is a relatively old material - discoveredin about 1955 - but its use in ophthalmic optics hasreally grown over the past few years. It has undergonenumerous improvements, particularly for use in thecompact disk industry, and now offers optical qualitywhich is equivalent to that of the other plastics.

In chemical terms, polycarbonate is a linear polymerthermoplastic with an amorphous structure, itscarbonated skeleton being made of a succession ofCarbonate radicals (-CO3) and Phenol (-C6H5OH)(figure 12). It is most often manufactured in solution,according to the following chemical reaction :

Polycarbonate presents a number of particularlyuseful benefits for ophthalmic optics : excellentimpact resistance (more than 10 times that of CR 39),a high refractive index (ne = 1.591, nd = 1.586),very light weight (specific gravity = 1.20 g/cm3),efficient protection against ultraviolet rays (U.V. cutoffat 380 nm) , and high resistance to heat (softeningpoint exceeding 140 °C / 280 °F). Like all high indexplastics, polycarbonate is a soft material which must systematically be protected with a scratch-resistant coating. Its constringence is relatively low (ne= 31, nd= 30), but in practice, this does notnoticeably affect wearers. In terms of tinting andantireflection coating, the current possibilities ofpolycarbonate are rather close to those of otherplastics : since the material itself is difficult to tint,tinting is generally obtained by impregnating atintable scratch-resistant coating applied to the lensback side. Antireflection coatings are applied in thesame way as for other materials.

Figure 12 : Polycarbonate Molecule : monomer and polymer.

b/ Thermoplastic resins : Polycarbonate

Thermoplastics like Plexiglas or PMMA were usedduring the fifties to manufacture the first plasticlenses, but proved too sensitive to abrasion, and weretherefore rapidly replaced by CR 39. Today, however,the development of polycarbonate is bringing thermo-plastics back onto the scene.

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Figure 12

THE BASICOPHTHALMIC

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Depending on the materials considered, the basic lensmanufacturing technique differs substantially, but theoverall principle remains constant : either the lens isdirectly manufactured as a “finished” product, or as aso-called intermediate “semi-finished” product, whichis a thick lens with a completed front surface, but aback surface which still needs to be worked so as toobtain prescription specifications.

A/ Manufacture of glasslensesWhatever the material used, glass lens manufactureconsists of surfacing the front and back surfaces of arough blank supplied by the glass-making industry.This blank is manufactured by molding still incan-descent glass, immediately after removal from theoven in which it is made by melting its variouscomponents. The blank is a very thick lens withuneven surfaces, but a perfectly homogeneous innercomposition. Its front and rear surfaces are thengenerated to obtain the correct curves, yielding thefinished lens.

Surfacing of each of the two lens surfaces can bedivided into three distinct phases (figure 13) :- phase 1 : grinding which consists of milling the lenswith a diamond wheel to give it its thickness and radiiof curvature. After grinding, the lens has acquired itsalmost definitive shape, but still has a rough,translucent surface.- phase 2 : fining or smoothing which consists of refiningthe grain of the lens surface without modifying its radiiof curvature. For this, the firmly held lens is broughtinto contact with a tool which has been fitted with anabrasive pad or disk, the radius of curvature of thetool being strictly identical to that of the sought lens.The lens and tool are set in motion and cooled downwith a lubricant solution. At the end of this operation,which lasts a few minutes, the lens has the exactthickness and curvature required, but the surface isnot yet perfectly polished.- phase 3 : polishing is the finishing operation whichgives the lens its transparency. This stage is similar tothe previous operation, but with a softer polishing diskand abrasive solution with a very fine grain.

Industrially, surfacing of the front lens surfaces(whether their designs are spherical, aspherical,bifocal, or progressive) is a mass production process,whereas surfacing of the back surfaces (which are onlyspherical or toric) is done individually or in series,depending on the quantity.

Figure 13 : Surfacing principle.

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Figure 13

- 1 -Grinding

- 2 -Fining

- 3 -Polishing

B/ Manufacture of plasticlensesThe following sections successively cover manufactureof thermosetting and thermoplastic lenses, since verydifferent techniques are used.

Let us take CR 39 as anexample of a thermosettingresin. The monomer sup-

plied in liquid form by industrial chemicals suppliersundergoes the following manufacturing steps :- preparation of the monomer : filtering, degassing,and addition of a catalyst ;- mold assembly : the molds consist of two glass ormetal parts, assembled either by pressing against acircular ring (the gasket) and tightening with a clip, orby using special adhesive tape ;- filling : the empty space created between the twoparts of the mold is filled with the liquid monomer ;- polymerization : the filled molds are placed in anoven where they undergo a heat cycle for several

hours - or, for certain materials, are exposed toultraviolet radiation for a few minutes - to provokegradual hardening of the resin ;- demolding : the clip or tape is removed and themold parts are separated to release the lens.

This procedure is used to manufacture both “finished”and “semi-finished” lenses, the only difference being inthe mold shape. This remains more or less the samefor most thermosetting resins currently used inophthalmic optics.

Figure 14 : Manufacturing principle for CR 39 lenses.

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Figure 14

1/ Thermosettingresins

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t°

Let us take polycarbonate asan example : the raw materialis made of transparent

granules which melt upon heating, and are then injec-ted into lens-shaped molds. The technology consistsof liquifying the raw material through heating and,using a piston, pushing it into metal or glass molds. Anextrusion screw ensures plastification of the materialin the injection cylinder, as well as playing the role ofa piston, pushing the hot material into the moldcavities via several channels. After injection andcooling time, the two-part molds are opened torelease the lenses (figure 15).

The various manufacturing steps are as follows:- preparation of the material : drying the granuleswith hot air and feeding them into the press ;- setting the press : laying out the molds, settingpressure, mold temperature, injection and coolingtime, and heating of the material (to about 300°C /570 °F) ;- injection : pushing the molten material underpressure into the molds ;- cooling : solidification of the material by conduction

through the molds ;- demolding : opening the press and molds andreleasing the lenses.

This technology allows lenses of any design to be manu-factured, by simply filling different molds in the press.These lenses are either “finished” lenses ready forcoating, or “semi-finished” lenses which requiresubsequent surfacing of the back surface, using tech-niques similar to those adopted for other materials.

Figure 15 : Manufacturing principle for polycarbonate lenses.

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Figure 15

2/ Thermoplasticmaterials

LENSMANUFACTURE

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CONCLUSION

Since they first appeared in the sixties,plastic materials have constantly deve-loped and gradually replaced glassmaterials. Today, they account for mostlenses sold in the industrialized countries.Their success is essentially due to theirmain advantages : lightness and impactresistance. These have been reinforcedby breakthroughs in a number of areaswhere they previously presented draw-backs : efficient scratch resistance, deve-lopment of plastic photochromic materials,higher refractive index materials, andwider range of tinting and antireflectioncoatings. With these enhanced features,today’s plastic lenses will continue to takeover the international ophthalmic lensmarket. Yet glass lenses, with better scratchresistance and higher refractive indexes,will continue to offer a useful alternativeto plastic materials, to ensure constantlyimproved visual comfort for the entirewearer community.

© Essilor Internationalmarch 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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