Tints and Coatings

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<p>The ideal ophthalmic lens corrects refractive error with full __________.</p>

The ideal ophthalmic lens corrects refractive error with full __________.

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UV protectio

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<p>Light Entering the Eye</p><ul><li><p>Imagine the incident light onto a lens surface is “1” (100%), then …</p></li><li><p>1= Reflection (R)+ Absorption (A)+ Transmission (T)</p></li><li><p>The light entering the eye is the transmitted part of it</p></li><li><p>the more reflection and/or absorption, the less transmission</p></li><li><p>Reflectivity: 𝑅 = 100% ∗ (𝑛′−𝑛)<sup>2</sup> / (𝑛′+𝑛)<sup>2</sup> with n’ = refractive index material behind reflection surface and n = refractive index of material in front of reflection surface</p></li><li><p>Reflections increase as refractive index, n increases</p></li><li><p>Can be reduced by applying antireflective (AR) coating ➢Transmission:𝑇λ = 1 / log<sup>-1</sup> 𝐷λ</p></li><li><p>Transmission can be specified by density (Dλ ) per mm from a given density: Dλ = log 1/ T </p></li><li><p>Transmission will be altered in applying/using tints</p></li></ul><p>Light entering the eye is the transmitted part and the increase in refractive index increases the reflections. True or False? </p>

Light Entering the Eye

  • Imagine the incident light onto a lens surface is “1” (100%), then …

  • 1= Reflection (R)+ Absorption (A)+ Transmission (T)

  • The light entering the eye is the transmitted part of it

  • the more reflection and/or absorption, the less transmission

  • Reflectivity: 𝑅 = 100% ∗ (𝑛′−𝑛)2 / (𝑛′+𝑛)2 with n’ = refractive index material behind reflection surface and n = refractive index of material in front of reflection surface

  • Reflections increase as refractive index, n increases

  • Can be reduced by applying antireflective (AR) coating ➢Transmission:𝑇λ = 1 / log-1 𝐷λ

  • Transmission can be specified by density (Dλ ) per mm from a given density: Dλ = log 1/ T

  • Transmission will be altered in applying/using tints

Light entering the eye is the transmitted part and the increase in refractive index increases the reflections. True or False?

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True

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In this session you will learn about tints and coatings that are 'attached' to lenses for various purposes such as reducing glare or protect from potentially damaging light such as UV or IR.

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1
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<p>The ideal ophthalmic lens corrects refractive error with full __________.</p>

The ideal ophthalmic lens corrects refractive error with full __________.

UV protectio

2
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<p>Light Entering the Eye</p><ul><li><p>Imagine the incident light onto a lens surface is “1” (100%), then …</p></li><li><p>1= Reflection (R)+ Absorption (A)+ Transmission (T)</p></li><li><p>The light entering the eye is the transmitted part of it</p></li><li><p>the more reflection and/or absorption, the less transmission</p></li><li><p>Reflectivity: 𝑅 = 100% ∗ (𝑛′−𝑛)<sup>2</sup> / (𝑛′+𝑛)<sup>2</sup> with n’ = refractive index material behind reflection surface and n = refractive index of material in front of reflection surface</p></li><li><p>Reflections increase as refractive index, n increases</p></li><li><p>Can be reduced by applying antireflective (AR) coating ➢Transmission:𝑇λ = 1 / log<sup>-1</sup> 𝐷λ</p></li><li><p>Transmission can be specified by density (Dλ ) per mm from a given density: Dλ = log 1/ T </p></li><li><p>Transmission will be altered in applying/using tints</p></li></ul><p>Light entering the eye is the transmitted part and the increase in refractive index increases the reflections. True or False? </p>

Light Entering the Eye

  • Imagine the incident light onto a lens surface is “1” (100%), then …

  • 1= Reflection (R)+ Absorption (A)+ Transmission (T)

  • The light entering the eye is the transmitted part of it

  • the more reflection and/or absorption, the less transmission

  • Reflectivity: 𝑅 = 100% ∗ (𝑛′−𝑛)2 / (𝑛′+𝑛)2 with n’ = refractive index material behind reflection surface and n = refractive index of material in front of reflection surface

  • Reflections increase as refractive index, n increases

  • Can be reduced by applying antireflective (AR) coating ➢Transmission:𝑇λ = 1 / log-1 𝐷λ

  • Transmission can be specified by density (Dλ ) per mm from a given density: Dλ = log 1/ T

  • Transmission will be altered in applying/using tints

Light entering the eye is the transmitted part and the increase in refractive index increases the reflections. True or False?

True

3
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The Problem - Reflections

  • Lens reflections at the surfaces reduce the optical performance by creating glare, reducing light transmission and contrast

    • Light incident on back or front surfaces of the lens from other light sources can reduce the contrast on the screen and impair visual performance.

    • Light reflection off the back surface of a pair of sunglasses can create a veiling glare which reduces visual performance by reducing contrast. If the reflected light source is bright enough, it can cause discomfort.

  • Lens reflections at the surfaces reduce the optical performance

  • By producing ghost images from light sources (in front and behind the lens)

  • Lens reflections reduce cosmetic appearance of spectacles

  • Bifocals produce unwanted reflections off the segment boundary which are greatest where the thickness is widest

  • Concave lenses with large edge thickness produce “power rings” giving poorer cosmetic appearance

Lens reflections at the surfaces _______ the optical performance.

Reduce

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Reflectance vs Refractive Index - Light Loss

  • Examples of light loss due to reflections on the two surfaces of a lens: The exact equation is

  • 𝑅 = 100% ∗ (𝑛′−𝑛)2 / (𝑛′+𝑛)2 with n’ = RI of material lens and n = refractive index in air

  • For plastic, n = 1.49 R = 0.039 = 3.9% reflected at first surface > remaining light 96.1%

  • Total light loss (1st and 2nd surface) = 3.9% + (3.9% of 96.1%= 3.7%) = 7.6%

    • 𝑅 = 100% ∗ (𝑛′−𝑛)2 / (𝑛′+𝑛)2

    • R = 100% x (1.49 -1.00)2 / (1.49 + 1.00)2

    • R= 0.039

    • R = 3.9%

    • 100% - 3.9% = 96.1%

    • Total light loss = 3.9% + (3.9% of 96.1%= 3.7%) = 7.6%

  • For glass, n = 1.5 → 7.8% of light is lost by reflection

  • For glass, n = 1.6 → 10.4% of light is lost by reflection

  • For glass, n = 1.7 → 12.3% of light is lost by reflection

  • For glass, n = 1.8 → 15.7% of light is lost by reflection

  • For glass, n = 1.9 → 18.3% of light is lost by reflection

True or False?

True

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The Principle of Interference

Terminology:

  • Wave-length – is a Phase (=2π =λ =360degree)

  • Phase difference between two waves of the same wavelength and its consequences:

    • Interference - superimposing waves

    • Constructive interference: when the phase difference is a multiple of λ: m x λ

    • Destructive interference: when the phase difference is a multiple of ½ λ: (m-1/2) x λ

  • Phase change (π or 180o ) on the reflection surface(s)

    • Happens for reflections at the border of an optically rarer vs a optically denser medium

    • Happens not for reflections at the border of an optically denser vs a optically rarer medium

A wave-length is a ______.

Phase

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AR Coating - Thin Film Interference

  • Aim: two reflections are 180o out of phase (path difference =1/2λ) to cancel each other out (destructive interference), when

    • Thickness of the coating is ¼ λ

    • nAR=√𝒏𝑳ens (total cancellation)

  • Since there is no net reflected light energy the lens transmission increases

  • for n = 1.5 the coating must have a refractive index of 1.22, n = 1.7 (1.30), n = 1.9 (1.38)

  • MgF2 (n = 1.38) is commonly used on glass

  • Limitation: With a single layer, only one wavelength can be eliminated: 550nm is mostly chosen (photoscopic sensitivity of the eye)

With a single layer only one __________ can be eliminated at 550nm.

Wavelengtth

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Absorbance

  • Occurs according to the Lambert’s law (the absorbance of a substance is directly proportional to the path length of the light beam and the concentration of the absorbing substance) and varies exponentially as a function of thickness

  • It is negligible in a “white” lens, but constitutes an intrinsic function in tinted and photochromic lenses and filters – lens manufacturer talk about “light reduction”

Absorbance (A) is a measure of how much light of a specific wavelength is absorbed by a sample, it follows _________ and varies exponentially as a function of _________.

Lambert's law; Thickness

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Transmission

  • Represents the amount of light that passes through e.g. a lens

  • Remember the electromagnetic spectrum

    • Ultraviolet C: 200nm to 280 nm (absorbed by the atmosphere)

    • Ultraviolet B: 280 nm to 315 nm

    • Ultraviolet A: 315 nm to 400 nm

    • Visible light: 380 nm to 780 nm

    • Infrared: > 780 nm

  • Ocular transmission:

    • Cornea: 290 nm to 3000 nm

    • Aqueous Humour: 290 nm to 2700 nm

    • Crystalline Lens: 310 nm (young), 375 nm (old) to 2500 nm

    • Vitreous Humour: 290 nm to 1600 nm

  • UVA and UVB: damage to cornea, implicated in cataract and AMD

  • IR (>1400 nm): hazardous => glass blowers cataract

  • Considerations for occupational requirements:

    • UV: dentists,

    • IR: furnace workers

  • UV and IR Blocking filters available

  • IR is a major design feature in tinted car windows

_____ is the amount of light that passes through the lens.

Transmission

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UV and IR Blocking Filters

  • UV-protection:

    • UV blocking filters can be incorporated into spectacle lenses

    • Should block wavelengths at least below 360 nm (ideally below 380 nm)

    • Coated lenses usually have a very pale brown/ yellow appearance

    • CR 39 does not block UV unless UV coating is applied

    • Polycarbonate blocks UV

    • IR (infrared) blocking filters are usually supplied as plano goggles / face shields

    • Best protection from IR - absorbing glass (addition of ferrous oxide) – usually pale green appearance

UV protection filters should block wavelengths below ______.

360nm (Ideally 380nm)

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Polarisation Lenses

  • Light reflecting off surfaces such as water, road surfaces, car windscreens and snow becomes plane polarised (horizontally polarised).

  • The intensity of the reflected polarised light acts as a glare source which can be both uncomfortable and impair vision.

  • A polarising filter will reduce the effect of this glare:

    • A sheet of polyvinyl alcohol is stretched into a thin sheet

    • The molecules are straightened into chains lying parallel to the direction of stretch

    • Impregnated with iodine (electron donor) → polarising filter

    • Polarising sheet sandwiched between two sheets of cellulose acetyl butyrate and pressed to impart a -6 D base curve

    • Method: Polarization by transmission

The intensity of the reflected polarised light acts as a glare source. True or False?

True

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Polarization by Transmission

  • Light is a transverse electromagnetic wave

    • Light is a transverse vibration: meaning vibrations in all directions within the same beam of light

    • Naturally occurring light is said to be unpolarised, i.e., vibrates in all directions

  • Polarisation is the process of transforming unpolarised light into a polarised state

  • When light is polarised, the waves in which the vibrations occur are in a single plane.

  • By transmission:

    • The stretched hydrocarbon chains can be thought of like “wires”.

    • The wave oscillation sets up an oscillation in the electrons within the “wires” which absorb the wave energy in the direction of the “wire” (in the case of the diagram, vertically)

    • The chain molecules are spaced at a distance comparable to the wavelength of light.

Light is a ________ electromagnetic wave and is said to be unpolarised.

Transverse

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Tints

  • Purpose:

    • Eye protection from harmful radiation

    • Glare reduction

    • Cosmetic

    • Psychological

  • Specified by:

    • Colour

    • Transmission/absorption

  • Type:

    • Fixed,

    • graduated,

    • Rainbow,

    • Polarising,

    • Mirror,

    • Photochromic

There are various tint options. True or False?

True

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Manufacturing Tints - Glass

  • Solid: tint material is incorporated within the lens matrix

  • Disadvantage (higher prescriptions): variable tint density dependent on lens thickness (raccooning- high minus lenses or bulls eye effect- high plus lenses)

  • Produced by mixing metallic oxides into glass (e.g. iron=green, nickel=brown, silver=yellow)

  • Advantage: repeatable

  • Equitint:

    • Tint material is vacuum coated onto the top surface of the lens or alternatively a layer of tinted glass is bonded (laminated) onto the top surface of the lens

    • Advantage: uniform tint density across the lens surface,

    • Disadvantage: not repeatable

Iron = green tint, Nickel = brown tint and silver = yellow tint. True or False?

True

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Manufacturing Tints - Plastic

  • Most are produced by dipping the lens into a bath of hot dye (lens absorbs tint into surface layers, longer the time in the bath, the darker the tint becomes.

  • Tint can be lightened using a bleaching agent

  • Usually done manually by female workers

  • Note:

    • Equitint (most cases)

    • Not reproducible as tinting usually carried out by visual matching

    • High index plastics variable ability to be tinted

    • Some hard coats prevent tinting (Vacuum deposition, In mould coating)

Tints for plastic lenses are produced by dipping the lens into a bath of _______.

Hot Dye

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Photochromic Glass Lenses

  • AgCl (Silver halides) and Cu(I)Cl (copper chloride) added to glass during manufacture (“in mass” technology)

  • Trace amounts of gold and palladium are added to shift the colour from grey to brown

  • Photochromic forward chemical reaction:

    • When exposed to UV light, silver ions (Ag+) are reduced to form silver atoms and

    • chloride (Cl-) ions are oxidised to chlorine atoms (gas trapped within the glass).

    • The silver darkens the lens.

  • Photochromic backward chemical reaction:

    • The reverse chemical reaction is only possible because of the addition of copper(I)chloride (CuCl).

    • When UV light is withdrawn, chlorine (Cl) oxidises the copper I chloride (CuCl) to copper II (Cu++) ions which then oxidise silver atoms into silver ions (Ag+).

    • The chloride ions (Cl-) resulting from the first chemical reaction then react with copper I ions (Cu+) and silver ions (Ag+) to form copper I chloride (CuCl) and silver halide (AgCl).

  • The reverse chemical reaction is much slower than the forward reaction. Hence, photochromic lenses darken quickly (approx 1 min) but lighten much more slowly (approx 40 mins)

Photochromic lenses darken quickly and but lighten much more slowly. True or False?

True

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Photochromic Plastic Lenses

  • Also known as “Transitions” despite this being a brand-name created by Essilor.

  • Process is called imbibing (“surface technology”)

    • organic photochromic molecule from organic molecule that rotates to form a shape, reducing light transmission (e.g., naphthopyrans and spirooxazines).

  • A combination of several different spirooxazines, each with different light transmission and temperature dependencies is combined to give a particular brand and colour of plastic photochromic lens

Making plastic lenses photochromic is fone through a process called _______

Imbibing

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Need to Know:

  • Reflections on lenses reduce optical performance (e.g., veil glare)

  • Higher n > more reflection > need for anti reflective coating (AR)

  • Principle of AR is thin-film interference – more layers needed to mitigate reflection caused by multiple wavelength

  • Transmission and absorbance are the measures to describe tints

  • Light that is absorbed by various parts of the eye (i.e., cornea, lens) can cause damage to the part

  • Advise patients on UV protection – UV coating unless polycarbonate lens

  • Polarised lenses help to mitigate glare from water surfaces

  • Tinted plastic lenses are produced by dipping - careful with high index lenses and hard coatings

  • Photochromic plastic lenses are produced by a process called imbibing, organic photochromic molecules (e.g., spirooxazines) change shape to adjust light transmission

Polarised lenses help to mitigate glare from water surfaces. True or False?

True

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<p>What Do We Expect From a ‘Good’ Ophthalmic Lens</p><ul><li><p>Read the slide</p></li></ul><p>Plastic lenses provided better UV protection than glass lenses (&lt; 280nm). True or False? </p>

What Do We Expect From a ‘Good’ Ophthalmic Lens

  • Read the slide

Plastic lenses provided better UV protection than glass lenses (< 280nm). True or False?

True

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Minimizing Reflections - AR Coating (Single Layer)

  • Utilises the principle of thin-layer interference

  • Reflective colour (residual hue) is determined by varying the coating thickness so that it is ¼ λ for a given wavelength

  • Gold AR often recommended for night driving due to shift in spectral sensitivity (scotopic sensitivity curve has a peak at 500 nm)

  • This layer is a thin film – not a solid layer

    • Does not react in the same way as the lens material to temperature changes (heat) – using the lens in a sauna can destroy the AR coating

  • Applied onto the lenses by: vacuum deposition technology

    • Metal oxides (e.g. SiO2, TiO2) are evaporated onto the lens using an electron gun; Before coating, an ion gun bombards the lens with argon ions which make the lens ultra clean (otherwise the coating may not adhere to the lens and peel off)

Single layer AR utilizes the principle of _________.

Thin-layer Interference

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Multi-Layer Anti-Reflection Coatings (MAR)

  • Are able to eliminate reflections of more than one wavelength

  • Most MAR coatings have a green, blue or gold residual hue

  • The dimmer the hue the better the coating performance

  • Advantages:

    • Improved visual performance:– veiling glare reduction and contrast increased (night driving, VDU workers)

    • Improved cosmetic appearance (particularly high lens powers)

    • Mandatory for high index materials (too much light loss without)

    • Improve photochromic performance (due to increased UV transmission)

  • Disadvantages:

    • Difficult to clean: smear easily (hydrophobic coating)

    • Vacuum coated (expensive to manufacture)

    • Very thin so can easily be scratched off (combined hard MAR)

    • Prone to chemical damage (e.g. hairspray) and damage due to heat

    • MAR are achieved by vacuum coating

MAR coatings are able to eliminate reflections of more than one wavelength. True or False?

True

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Mirror Coating

  • Mirror coatings (“silvered”) utilize the same principle as AR-coatings, but aim for “constructive interference”:

  • Same material as anti-reflection coating but the optical thickness of the coating is ½ λ so that the reflected light rays double the reflection

Mirror Coatings utilize the same principle as ________ coatings but aim for _____ interference.

AR; Constructive

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Other Coatings

  • Hard Coating:

    • Increase the surface hardness of plastic improving scratch resistance (i.e. not scratch proof)

    • Between 5-10 x10-3 mm thick (10 - 20 times thicker than MAR)

    • Materials used for coating: diamond-like carbon (DLC) and polycrystalline diamond

    • Lacquer hard coats: (dip or spin) generally tintable (common way)

    • Vacuum deposited: (thinner but harder material e.g. silica) not tintable (often only front surface), high cost but can be combined with MAR process

    • In mould hard coating: material embedded within lens matrix- not tintable

  • Top Coating/Hydrophobic coatings (plastic lenses only)

    • Smearing happens on all lenses but is particularly visible in the presence of an AR coating because where the smear exists the MAR cannot function by interference (patients often complain they cannot keep the lenses clean)

    • hydrophobic (water repellent) properties (e.g. SiO2, Titanium oxide, perfluoropolyether)

    • and oleophobic (fat repellent) properties (e.g. fluoroacrylic copolymer)

    • Ordering coated lenses with combined hard coating, MAR and top coat will increase patient compliance and satisfaction with the coating

Hard coating make lenses _______ not scratch proof.

Scratch Resistant

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<p>European Standard for Sunglasses: </p><p>BS EN ISO 12312-1: 2013+A1: 2015 and BS EN ISO 12312-2:2015</p><p>A better representation of table 1:</p><p>(see image) </p><p>Transmission of 3-8% in category four is  a ________ and is suitable for strong sunlight but not for drivers and road users.</p>

European Standard for Sunglasses:

BS EN ISO 12312-1: 2013+A1: 2015 and BS EN ISO 12312-2:2015

A better representation of table 1:

(see image)

Transmission of 3-8% in category four is a ________ and is suitable for strong sunlight but not for drivers and road users.

Very Dark Tint

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Photochromic Performance and Its Implications

  • Activation speed is much faster than fade back speed

  • Temperature dependent (darker and faster when cold)

  • Enhanced by MAR (increases UV transmission)

  • Not recommended for driving (reaction speed, possibly no reaction at all due to windscreen absorbing most of the UV light) … but since 2019 Transitions “XACTIVE” and “DRIVEWEAR”

  • UV blocking (depends on brand)

  • In mass (glass): tint depth proportional to thickness i.e. darker when thicker and uneven tint depth

  • Imbibing technique (plastic): equitint

Photochromic reactions is darker and faster when it is cold. True or False?

True