Optical Lens Characteristics Quiz

OPT 505: Clinical Skills and Refractive Management - Lens Materials, Tints and Coatings

• The module consists of 6 lectures and 6 lab sessions (2 hours each)
• There are 2 formative OSCEs
• A final OSCE accounts for 15% of the module mark
• A written dispensing scenario is part of the assessment
• A focimetry of bifocals is part of the assessment
• The final and written scenarios are both 20 minutes long
• An example question focuses on taking into account a patient's prescription
• Key competency: Advising on, ordering, and dispensing suitable optical correction, including durability, comfort, cosmetic appearance, age, and lifestyle.
• Aims: Understanding different lens materials, comparing their advantages/disadvantages, analyzing "ideal lens" properties, identifying undesirable effects of lens choices, describing tinting processes, explaining tint advantages/disadvantages, listing regulations on tints/coatings, discussing additional coatings and their uses

The Ideal Lens

• Patient concerns: Looks good, lightweight, good vision, low cost, minimal glare
• Concerns: Stable material, aberration-free, reflection-free, thin, abrasion & impact resistant, easily tinted, UV protection, available in a range of surface processes, retain hard and reflection-free coat easily

Lens Properties

• Thickness
• Index
• Relative curvature
• Density/weight
• UV transmission/absorption
• Reflectance
• Abbe Number
• Aberrations
• Durability Strength
• Hardness
• Impact resistance
• Tintability
• Processing capability
• Ease of manufacture

Key Terms

• Refractive Index (RI): The ratio of light velocity in air to its velocity in the medium.
• V-value (Abbe Number): Indicates a material's dispersion, higher dispersion (bad optically) means smaller Abbe Number.
• Relative Curvature: Used to calculate lens flattening due to high RI materials.

Lens Materials

• Glass: Crown glass, Mid & High index glass, Flint glass*
• Plastic: CR39, Trivex, Polycarbonate, Tribrid, Mid & High Index plastic

Classification of Materials

• Normal index: 1.48 < 1.54
• Mid index: 1.54 < 1.64
• High index: 1.64 < 1.74
• Very high index: 1.74+

Lens Materials and Properties (Table)

• Includes a table with different material properties
• These include refractive index, Abbe number, and density

Aspheric Form

• Advantages of aspheric lenses: Thinner, lighter, flatter, less distortion than an equivalent steep lens, less spectacle magnification.
• Disadvantages of aspheric lenses: More distortion than equivalent steep lens, sensitive to decentration.

Estimating Refractive Index Using Relative Curvature

• Formula: Rc = (nnew mat - 1) / ( nnew mat -1)
• Where Rc is relative curvature and F/r is FOCAL LENGTH divided by RADIUS

Relative Curvature (Table)

• Shows the relationship between refractive index and reduction in thickness.

Choosing Which Lens Material

• Plastic vs. Glass
• Index
• Lens form
• Durability

Choosing the Best Lens for Vision

• Spherical
• Highest Abbe number
• Lower refractive index
• CR39 (Plastic) and Crown Glass are examples

Common Vision Problems

• Monochromatic aberrations
• Oblique astigmatism
• Distortion
• Warping of the peripheral field of vision
• Transverse Chromatic Aberration (TCA)
• Off axis dispersion of white light into the color components

Dealing with Oblique Astigmatism

• Making the lens steeper or using a best form lens reduces or eliminates oblique astigmatism.
• Steeper lenses are thicker and produce more magnification.
• Aspheric lenses can eliminate oblique astigmatism.

Dealing with Distortion

• Reduce the BVD and Use a small lens/frame
• Use best form lens (e.g. point focal)

Transverse Chromatic Aberration (TCA)

• Otherwise known as dispersion
• Only occurs off-axis
• Dispersion of light into colors due to wavelength differences
• Occurs further away from the optical center
• Can be computed using two formulas measured in prism diopters.

TCA Tolerance

• Generally only noticed above 0.1
• Affects higher prescriptions (especially myopes).

Dealing with TCA

• Use a lens with a high Abbe Number
• Good lens centration
• Small vertex distance
• Adjust pantoscopic tilt
• Choose a smaller frame

High Index: Density and Reflectance

• Density increases with index, but not always.
• Reflectance increases with increasing index.
• TCA increases with prescription, lens size, and index.

So which one?

• No perfect lens exists, compromises are necessary regarding appearance, vision, comfort, and side effects.

The Compromise (with high index lenses)

• Thinner lenses with (less volume) high-index may not be heavier.
• Many are willing to compromise to get aesthetic looks with a small frame.
• Plasic is still lighter than glass.

Choosing the Best Lens for Cost

• Easy to manufacture
• Available in a range of powers and forms
• Readily available
• Example: CR39 (1.5 Plastic)

Choosing the Best Lens for Thickness

• Higher refractive index
• Flatter form
• Aspheric form

Choosing the Best Lens for UV Protection

• Blocks 100% of UVA and UVB light
• UV cut-off value (e.g., Trivex™)
• Advantages and disadvantages of different lens materials for UV protection

Choosing the Best Lens for Protection

• Resistance to chemical damage and successful impact test.
• Examples: Trivex, Polycarbonate

Choosing the Best Lens for Comfort

• Lightweight.
• Modern lens materials are unreactive.

Tint and Coating Properties

• Application
• UV status
• Conditions
• Rx considerations
• Pros
• Cons
• Suitability
• Science behind it

Tinting and Coating Lenses

• Light absorbency and blocking, hardness and reflectance vary across lens types.
• Coatings and tints used to modify lens appearance/design.

Tints

• Reasons for adding tints: Reduce glare, increased contrast, harmful radiation protection, cosmetic appearance, dyslexia treatment.

Solar Radiation and Electromagnetic Spectrum

• Optical range: 100-10,000nm
• Visible light range: 380-780nm
• Eye is most sensitive to 555nm (yellow-green)
• Optometrists concerned with high-energy end (UV light).

UV Radiation

• High-energy end of the electromagnetic spectrum.
• Three categories: UVA (315-380nm), UVB (280-315nm), UVC (10-280nm).
• Longer wavelengths penetrate deeper into the skin, shorter have higher energy.

UVA (315-400nm)

• Not absorbed by ozone layer.
• Travels deep into the eye (causing damage to the crystalline lens, retina, and potential cataracts/AMD).

UVB (280-315nm)

• Absorbed by the cornea.
• Responsible for sunburn and skin cancer.

UVC (10-280nm)

• Absorbed by the ozone layer.
• Most active and toxic part of the UV spectrum (artificially created).

UV Cut-off in Lens Materials

• Table of different materials and their approximate UV cut-off values.

UV Blocking Treatments (Clear)

• Clear coatings to increase UV protection.
• Some coatings result in a visible tint.

Visible Light

• Essential for vision
• Reflected light can cause glare.
• Too much bright or concentrated light can cause discomfort.

Glare

• Different types of glare (Distracting/Veiling, Discomfort, Disability, Blinding)
• Methods to reduce glare (Clear/Photochromic/Tinted/Polarized lenses)

Reducing Visible Light Transmission

• Tinted lenses are designed to absorb visible light.
• Various coatings and tints to block visible light: Tinted, Photochromic, Polarized.

Tints - Absorption and Transmittance

• Absorption (Abs): Percentage of light absorbed, darker the tint
• Luminance Transmittance (LT): Percentage of light that passes through, lighter the tint.

Spectral Transmittance

• Spectral Transmittance Factor (STF): Fraction of incident light transmitted at a specific wavelength.
• Can be plotted on a graph
• Used with spectrometers.

Luminance Transmittance

• LTF: Overall effect of a tint on a standard eye viewing a daylight light source.
• Single figure for each tint type
• Only refers to visible light spectrum.

Rules and Regulations for use of Tints

• Standards for safety and performance specifications.
• Compliance with regulations (e.g., ISO 12312, BS EN ISO 8980-3).

Sun Glare Classification (Table)

• LTF of different tint categories.
• Descriptions, usage, and any restrictions

Types of Tints

• Various solid, coated, dip-dyed, and photochromic tints for different types of glass and plastic.

Glass - Solid Tint

• Uniformly distributed tint throughout the material.
• Appearance can be uneven in higher powers.
• Color variations.

Glass - Solid Tint Properties

• Color indicates transmission properties (pink/brown for UV and IR absorption, green for reduced color distortion...).

G15 tint

• Reference to a specific Ray-Ban tint
• Developed for pilots in the 1950s
• Solid tint, high absorption of unwanted light
• Includes ferrous oxide.

Glass - Coated tint

• Reflecting filters to reflect unwanted light.
• Applied in a vacuum.
• Can be applied in various colors.

Glass - Photochromic

• Darkens and lightens based on ambient UV levels.
• Activated by silver halide crystals.
• Various brands and types available.

Glass - Photochromic: Affected by

• Temperature
• Type of radiation
• Lens thickness
• Age of lens
• Amount of light

Plastic - Solid Tint

• Casting material is dyed.
• Tint can appear uneven, especially in higher powers
• Absorbs unwanted light used for specialist tints (e.g., sports).

Plastic – Dip-dyed

• Finished lens immersed in photographic dye at 95°C.
• Dye permeates the surface.
• Even appearance.
• Absorbs unwanted light.
• Not always successful on CR39.

Plastic - Photochromic

• Spiro-oxazines or spiro-pyrans/fulgides added to CR39.
• Exposure to UV causes molecular rotation.
• Absorbs radiation.
• UV removal returns molecules to their original orientation.
• Protection against UVA/B.

Plastic - Photochromic (Brand variations)

• Transition.
• Sunsensors.
• Suntech.
• Drivewear

Plastic - Photochromic (Generations)

• Multiple colors and generations of transition lenses.

Polarising Lenses

• Designed to reduce glare from horizontal surfaces (e.g. roads, water, snow).
• The polarizing effect is strongest at Brewster's angle.

Polarized light - Brewster's Angle

• Angle where complete polarization of light happens.
• Derived from Snell's law.
• Horizontal light is largely what creates glare.

Plastic – Polarised Light

• Large horizontal vibrations create glare.
• Vertical vibrations are useful.
• Polarized filter blocks only horizontal vibrations.

Plastic - Laminated

• Different method for tinting lenses.
• Sandwich of clear and tinted materials chemically bonded.
• Even tint, unaffected by prescription.

Vacuum Coating

• Equi-tint.
• Mirror finishes available.
• Anti-reflective coatings are vacuum coated.
• Variety of colors and semi-mirrored finishes.

Coatings

• Hard coat (scratch resistance)
• Hydrophobic
• Anti-reflection
• Multipurpose

Hardcoat

• Hardcoat added to plastic lenses to toughen the surface.
• Applied by dip coating or spin casting.
• Variety of hardcoat types, inconsistent with different manufactures.

Anti-Reflection Coatings

• Vacuum-coated, work on destructive interference.
• Double-layer coatings are more effective than single.
• Can have additional layers for properties such as hydrophobicity.

Anti-reflection Coatings: Effectiveness

• Effective over a specific wavelength.
• Limited range of useful coatings.
• Material (e.g., CR39 and Crown Glass) and coating index values are important.

Dual Layer AR coatings

• Using multiple coatings with different refractive indices to increase their effectiveness.

Multi-layer Anti-reflective (MAR) Coatings

• Multiple layers for reducing reflections across multiple wavelengths.
• Additional functions: hard coat, hydrophobic layer etc...

Hydrophobic Coating

• Wetting angle is crucial for hydrophobic properties.
• Higher wetting angle means better ability of the water droplets to run off the surface as a droplet.

Specialist Tints & Coatings

• Various specialist tints and coatings.
• Examples: Cerium Colourimetry tints, Vistamesh/Honeycomb tints, Blue Blocker.

Blue Light

• Range of blue light (starts at 400nm).
• Various values depending on literature
• Concerns about harmful effects of blue light emitted by various sources.
• Blue-blocker treatments.

Identifying Lenses

• Methods to identify unknown lenses (material, power, edge finish, tints/coatings, type of frame).

Key Characteristics (Table format)

• Table of key lens characteristics: Material, Power, Edge finish, Cut/uncut, type of Lens (Simple Vision/Multifocal), Tints, and Coatings

Material - Glass or Plastic?

• Feeling the lens, drops test, tapping test.

Cut or Uncut?

• Identifying blanks (uncut/unmounted lens materials).
• Identifying cut lenses for a specific frame.

R or L Lens

• Identifying the R or L eye for which the lens is glazed.

Power - Negative or Positive?

• Determine the power (positive or negative) of a lens.

Power - Astigmatic?

• Checking if the lens has a simple spherical or astigmatic prescription.

Single Vision or Multifocal?

• Identifying single vision and multifocal lenses (e.g., bifocals or progressive PPLs).

Tints or Coatings

• Spotting tints and colors, and coatings based on light reflections.

Edge Finish

• Determining how the edge has been glazed, intended frame.

Test your knowledge on the differences between glass and plastic lenses, the properties of various lens materials, and UV radiation effects. This quiz includes questions on lens power, characteristics of astigmatism, and the impact of coatings on UV protection. Perfect for those studying optics or working in the eyewear industry.