Week 3: Lens Material Properties/Coatings

Question 1

Explain refractive index

Answer 1

• An increase in refractive index will typically make the lens thinner
• It is different for different wavelengths of lights

Question 2

What are the Implications of Refractive Wavelength on Refractive Index

Answer 2

• 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

Question 3

Benefits of Higher Refractive Index Materials

Answer 3

• 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

Question 4

Which groups of people who would wear high refractive index?

Answer 4

• High degrees of; myopia, hyperopia, astigmatism
• Pxs who want the best/lighter lens

Question 5

What is abbe number?

Answer 5

• 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

Question 6

Are lens transparent?

Answer 6

• No lens material is perfectly transparent – always some reflections and absorption

Question 7

What are the implication of reflection?

Answer 7

• 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

Question 8

What do patients experience with glaring of glasses?

Answer 8

• Blurred image of distance object
• Sharp view of their own eye and adnexa
• Moving image with head motion

Question 9

How does photochromic glass lenses work?

Answer 9

• 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

Question 10

How does polarising lenses work?

Answer 10

• 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

Question 11

What does the weight of spectacle lenses depends on?

Answer 11

• 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

Question 12

An increase in the scratch resistance of a lens will?

Answer 12

• Increase optical quality of the lens
• Improve the appearance of the lens on the patient
• Increase the longevity of the lens

Question 13

Which groups of people who would wear greater abrasion resistance?

Answer 13

• Everyone
- Older people
- Children
- Workmen

Question 14

Explain impact resistance?

Answer 14

• Ability of a material to withstand the impact of a hard body

Question 15

Groups of people who would wear greater impact resistance?

Answer 15

• People who play sport
• Children
• People who work with moving parts (e.g. carpenters, builders)
• Police
• People with poor mobility

Question 16

What are the 4 main categories of lens material on the optical market?

Answer 16

1. Glass
2. Plastic
3. High-index plastic
4. Polycarbonate/Trivex

Question 17

What is the Refractive Index, Scratch Resistance & Impact Resistance for; Crown White Glass, Plastic, High-Index Plastics, Polycarbonate/Trivex

Answer 17

Crown White Glass: Low, High, Poor

Plastic: Low, Poor/Moderate, Moderate

High-Index Plastics: High, Poor/Moderate Moderate

Polycarbonate/Trivex: Moderate, Poor, High

Question 18

Explain what are glass lenses

Answer 18

• Lenses are typically made of crown glass (n = 1.523)
• Glass currently makes up less than 20% of the Australasian lens market

Question 19

What are the advantages and disadvantages of glass lenses?

Answer 19

Advantages:
- Scratch resistant
- Chemical resistant
- Very high index materials available

Disadvantages:
- Fragile – break on impact
- Unsafe – break into sharp shards of glass unless toughened

Question 20

Explain what are plastic lenses

Answer 20

• 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

Question 21

Explain what are high index plastic lenses

Answer 21

• To make lenses thinner and lighter by increasing the refractive index
• Refractive index n = 1.560 – 1.740

Question 22

Explain what are polycarbonate lenses

Answer 22

• Important due to providing the best available protection for patient/legal action

Question 23

What are the advantages and disadvantages of polycarbonate lenses?

Answer 23

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

Question 24

Explain what are trivex lenses

Answer 24

Three good properties:
- Superior optics
- Impact resistance
- Ultra light weight (lightest material)

Question 25

What are the advantages and disadvantages of trivex lenses?

Answer 25

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

Question 26

What are the four different types of lens coatings?

Answer 26

1. UV coat
2. Anti-reflection coating
3. Scratch resistant coating
4. Multicoat

Question 27

What are UV coating?

Answer 27

• 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

Question 28

Attenuation of ultra violet radiation/infrared radiation:

Answer 28

• Plastic lenses with UV coating
• Polycarbonate
• Solid glass tints

Question 29

List some conditions that are affected by UVA, UVB & UVC

Answer 29

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

Question 30

Explain what are anti-reflection coating

Answer 30

• AR coating increases the transmittance of a lens, and dramatically reduces the reflections off both surfaces of the lens
• AR reflects ultraviolet radiation

Question 31

What are the two conditions must be met for the coating to be effective?

Answer 31

1. Path condition
   - AR layer must be ¼ of wavelength of light (to cancel reflected light)
2. Amplitude condition
   - Amplitude of the light waves in the coat & lens are required to be equal for correct destructive interference to occur

Question 32

Explain what are scratch resistance coating

Answer 32

• 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