Podcast
Questions and Answers
Which method is NOT used to differentiate between glass and plastic lenses?
Which method is NOT used to differentiate between glass and plastic lenses?
What is a characteristic of an uncut lens?
What is a characteristic of an uncut lens?
Which of these is a sign that a lens is astigmatic?
Which of these is a sign that a lens is astigmatic?
What does a high-pitched sound indicate when tapping a lens?
What does a high-pitched sound indicate when tapping a lens?
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When determining the power of a lens, what denotes a positive lens?
When determining the power of a lens, what denotes a positive lens?
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What type of UV light is primarily responsible for most skin cancers and can directly alter DNA?
What type of UV light is primarily responsible for most skin cancers and can directly alter DNA?
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Which lens material has the lowest approximate UV cut off level?
Which lens material has the lowest approximate UV cut off level?
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What is a potential outcome of using coatings that protect from a higher level of UV radiation?
What is a potential outcome of using coatings that protect from a higher level of UV radiation?
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Which type of UV radiation is absorbed by the ozone layer and atmosphere?
Which type of UV radiation is absorbed by the ozone layer and atmosphere?
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What wavelength range can UVA radiation cover to potentially reach the retina?
What wavelength range can UVA radiation cover to potentially reach the retina?
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Which lens material is described as 'bullet-proof' due to its strength?
Which lens material is described as 'bullet-proof' due to its strength?
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Which lens material is known to opacify when exposed to chemicals?
Which lens material is known to opacify when exposed to chemicals?
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What is the UV cut-off value for Trivex lenses?
What is the UV cut-off value for Trivex lenses?
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What is a notable disadvantage of Hi-Index lenses compared to Trivex?
What is a notable disadvantage of Hi-Index lenses compared to Trivex?
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If a patient requires the best impact resistance in their lenses, which option would be most suitable?
If a patient requires the best impact resistance in their lenses, which option would be most suitable?
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What is the primary concern for optometrists regarding solar radiation?
What is the primary concern for optometrists regarding solar radiation?
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What is a disadvantage of UVA radiation?
What is a disadvantage of UVA radiation?
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What purpose do tinted lenses serve?
What purpose do tinted lenses serve?
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Which radiation category is associated with the most potential for skin and ocular damage?
Which radiation category is associated with the most potential for skin and ocular damage?
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What is the range of the visible light spectrum?
What is the range of the visible light spectrum?
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What is one reason for adding coatings to lenses?
What is one reason for adding coatings to lenses?
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What is an important factor to consider when selecting the suitable form of optical correction for a patient?
What is an important factor to consider when selecting the suitable form of optical correction for a patient?
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What is the potential positive effect of tints in lenses related to learning difficulties?
What is the potential positive effect of tints in lenses related to learning difficulties?
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Which property describes an 'ideal lens' when considering patient concerns?
Which property describes an 'ideal lens' when considering patient concerns?
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Which of the following properties should be minimized to achieve a better lens performance?
Which of the following properties should be minimized to achieve a better lens performance?
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What is a key advantage of using lightweight lens materials?
What is a key advantage of using lightweight lens materials?
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Which undesirable effect can occur from the incorrect lens material choice?
Which undesirable effect can occur from the incorrect lens material choice?
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Why is scratch resistance an important property in lens materials?
Why is scratch resistance an important property in lens materials?
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Which factor does NOT typically influence the choice of lens coatings?
Which factor does NOT typically influence the choice of lens coatings?
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Which of the following aspects is considered when comparing lens materials?
Which of the following aspects is considered when comparing lens materials?
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What property is essential for lenses to efficiently protect against UV-light?
What property is essential for lenses to efficiently protect against UV-light?
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What is the Spectral Transmittance Factor (STF) used to measure?
What is the Spectral Transmittance Factor (STF) used to measure?
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Which statement about Luminance Transmittance Factor (LTF) is accurate?
Which statement about Luminance Transmittance Factor (LTF) is accurate?
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Which type of lens is described as providing a uniform color but may still cause uneven color in thicker prescriptions?
Which type of lens is described as providing a uniform color but may still cause uneven color in thicker prescriptions?
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What is the main disadvantage of using very dark tinted lenses at night?
What is the main disadvantage of using very dark tinted lenses at night?
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What is one of the primary functions of polarization in lenses?
What is one of the primary functions of polarization in lenses?
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In which scenario is it recommended not to use tinted lenses?
In which scenario is it recommended not to use tinted lenses?
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Which color of solid tint is known to improve contrast in hazy conditions?
Which color of solid tint is known to improve contrast in hazy conditions?
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Which property does a lens need to have to fall under filter category 4 according to the sun glare classification?
Which property does a lens need to have to fall under filter category 4 according to the sun glare classification?
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What is the primary mechanism that allows photochromic lenses to darken?
What is the primary mechanism that allows photochromic lenses to darken?
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The absorption percentage of a lens is represented as 20% Abs. What does this indicate?
The absorption percentage of a lens is represented as 20% Abs. What does this indicate?
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How does the color of the G15 tint affect its performance?
How does the color of the G15 tint affect its performance?
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What is a notable advantage of a coated tint compared to a solid tint?
What is a notable advantage of a coated tint compared to a solid tint?
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What is the purpose of the BS EN ISO 12312 standard?
What is the purpose of the BS EN ISO 12312 standard?
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Study Notes
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.
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Description
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.
