Week 3: Lens Material Properties/Coatings Flashcards
Explain refractive index
- An increase in refractive index will typically make the lens thinner
- It is different for different wavelengths of lights
What are the Implications of Refractive Wavelength on Refractive Index
- Europe will quote higher refractive indices for the same physical material
- Optical devices will measure power to a default reference wavelength. If the wrong reference wavelength is used, measurement error can occur
Benefits of Higher Refractive Index Materials
- If we increase refractive index of the lens (n’), then to achieve same dioptric power we increase radius of curvature (r) of the lens to make the lens flatter
- Making the lenses flatter decreases the amount that the lens bulges out from the frame by reducing the sag
Which groups of people who would wear high refractive index?
- High degrees of; myopia, hyperopia, astigmatism
- Pxs who want the best/lighter lens
What is abbe number?
- A measure of the chromatic aberrations produced by dispersion
- Abbe number is a measure of constringence
- Higher the Abbe number, lower the amount of chromatic aberration
Are lens transparent?
- No lens material is perfectly transparent – always some reflections and absorption
What are the implication of reflection?
- Light may be reflected from both front & back surfaces of the lens
- Can cause visual fatigue
- As we increase the refractive index of a lens, magnitude of reflections increase
What do patients experience with glaring of glasses?
- Blurred image of distance object
- Sharp view of their own eye and adnexa
- Moving image with head motion
How does photochromic glass lenses work?
- Colour comes from the tiny amounts of silver chloride dispersed throughout the glass
- When exposed to UVA light it activates making the lens gradually darker
- Transmittance increases as temperature increases for photochromatic glass
- Transmittance decreases as thickness increases for photochromic glass
How does polarising lenses work?
- Polarised lenses decreases the amount of reflected glare
- Light reflected off horizontal surfaces (e.g. road, water) vibrates in horizontal plane
- Filters set to only allow vertical vibrating light
What does the weight of spectacle lenses depends on?
- Shape and size
- Density of the material
- Specific Gravity = density of lens material compared with water = mass / volume
- Will give an indication of weight of finished spectacles
- Density of plastic materials about half that of glass
An increase in the scratch resistance of a lens will?
- Increase optical quality of the lens
- Improve the appearance of the lens on the patient
- Increase the longevity of the lens
Which groups of people who would wear greater abrasion resistance?
• Everyone
- Older people
- Children
- Workmen
Explain impact resistance?
- Ability of a material to withstand the impact of a hard body
Groups of people who would wear greater impact resistance?
- People who play sport
- Children
- People who work with moving parts (e.g. carpenters, builders)
- Police
- People with poor mobility
What are the 4 main categories of lens material on the optical market?
- Glass
- Plastic
- High-index plastic
- Polycarbonate/Trivex
What is the Refractive Index, Scratch Resistance & Impact Resistance for; Crown White Glass, Plastic, High-Index Plastics, Polycarbonate/Trivex
Crown White Glass: Low, High, Poor
Plastic: Low, Poor/Moderate, Moderate
High-Index Plastics: High, Poor/Moderate Moderate
Polycarbonate/Trivex: Moderate, Poor, High
Explain what are glass lenses
- Lenses are typically made of crown glass (n = 1.523)
- Glass currently makes up less than 20% of the Australasian lens market
What are the advantages and disadvantages of glass lenses?
Advantages:
- Scratch resistant
- Chemical resistant
- Very high index materials available
Disadvantages:
- Fragile – break on impact
- Unsafe – break into sharp shards of glass unless toughened
Explain what are plastic lenses
- A polymeric material know as resin
- CR-39 = most common plastic lens material
- Refractive index n = 1.498
- Primary advantage of plastic lenses over glass is that plastic lenses are much lighter
Explain what are high index plastic lenses
- To make lenses thinner and lighter by increasing the refractive index
- Refractive index n = 1.560 – 1.740
Explain what are polycarbonate lenses
- Important due to providing the best available protection for patient/legal action
What are the advantages and disadvantages of polycarbonate lenses?
Advantages:
- Lighter
- Greater impact resistance
- Thinner
- UV filter up to 380 nm
Disadvantages:
- Low scratch resistance
- Low Abbe number
- Increased reflections
- More difficult to manufacture
- Affected by solvents
Explain what are trivex lenses
Three good properties:
- Superior optics
- Impact resistance
- Ultra light weight (lightest material)
What are the advantages and disadvantages of trivex lenses?
Advantages:
- Lighter than polycarbonate
- Moderate Abbe number
- Thinner than CR39
- Good chemical resistance
- Similar impact resistance as polycarbonate
Disadvantages:
- Moderate scratch resistance
- Moderate Abbe number
- Normal refractive index
- Edging requires similar techniques to polycarbonate
What are the four different types of lens coatings?
- UV coat
- Anti-reflection coating
- Scratch resistant coating
- Multicoat
What are UV coating?
- Coating is used to filter uv radiation from entering the eye
- Polycarbonate provides almost total protection without a UV coat
- Glass lenses are poor absorbers of UV light, and require a tint
Attenuation of ultra violet radiation/infrared radiation:
- Plastic lenses with UV coating
- Polycarbonate
- Solid glass tints
List some conditions that are affected by UVA, UVB & UVC
UVA: acute effects, chronic effects, tanning of eyelid, bunescent cataract
UVB: acute effects, chronic effects, cataract, pterygium
UVC (no chronic effects):
- acute effects are very damaging
Explain what are anti-reflection coating
- AR coating increases the transmittance of a lens, and dramatically reduces the reflections off both surfaces of the lens
- AR reflects ultraviolet radiation
What are the two conditions must be met for the coating to be effective?
- Path condition
- AR layer must be ¼ of wavelength of light (to cancel reflected light) - Amplitude condition
- Amplitude of the light waves in the coat & lens are required to be equal for correct destructive interference to occur
Explain what are scratch resistance coating
- Acts to increase the hardness of the surface of the lens (applied to both front & back surface)
- Developed due to the tendency for uncoated plastic & polycarbonate lenses that scratch easily
