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10410 Clinical Dispensing > Week 5 Lens Materials and Coating > Flashcards

Week 5 Lens Materials and Coating Flashcards

1
Q

Which one of the following lens materials already has low ultraviolet radiation transmissibility?

A) CR39
B) High index glass
C) Crown glass
D) Polycarbonate

A

polycarbonate

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2
Q

Which one of the following is the lightest lens material?

A) CR39
B) Trivex
C) Crown glass
D) Polycarbonate

A

Trivex

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3
Q

What does An Abbe number of 32 indicate?

A

indicates poor constringence. The higher the number, the less chromatic aberration.

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4
Q

The least impact resistant material is:

A) CR39
B) Toughened glass
C) Crown glass
D) Polycarbonate

A

Crown glass

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5
Q

The most impact resistant lens material is:

A) TR90
B) Phoenix
C) Trivex
D) Polycarbonate

A

Polycarbonate

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6
Q

Reflection free coatings require the following to be true:

A) Reflected light from the lens and coating surfaces must be 180 degrees out of phase and undergo destructive interference

B) The thickness of the antireflective layer must be half the thickness of yellow light and the refractive index of the lens material must be the square root of the refractive index of the lens coating.

C) Antireflective layer must be equivalent in thickness to 1/4 the wavelength of yellow light and the refractive index of the coating must be equivalent to the square root of the refractive index of the lens material.

D) Reflected light from the lens and coatings must be 360 degrees out of phase and undergo addition.

A

A and C

Path condition – The AR layer must be ¼ of the wavelength of light (yellow is selected to represent white light). Because the light reflected back from the front surface of the lens has traveled through ½ the wavelength, it is 180o out of phase, and destructive interference will occur, hence canceling the reflected light.

Amplitude Condition – the amplitude of the light waves in the coat and lens are required to be equal for correct destructive interference to occur. This occurs when

nc = √n

where nc = refractive index of coat, and n = refractive index of lens material.

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7
Q

The most damaging of the ultraviolet radiations

A
  1. has wavelength of 320-400 nm
  2. has chronic effects such as brunescent cataract
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8
Q

How do you calculate absolute refractive index?

A

Speed of light in a vacuum = c

Speed of light in material =v

C/V = absolute refractive index

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9
Q

How do you calculate relative refractive index?

A

Speed of light in air = vair

Speed of light in material = vmaterial

V(air) / V (material) = n (refractive index)

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10
Q

What wavelength is used to represent light in Australia?

A

587.56nm (Australia)

546.07nm (Europe)

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11
Q

What is the formula for the speed of a wave?

A

v = wavelength x frequency

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12
Q

What is the formule for refractive index?

A

n = c/v

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13
Q

AUS VS Europe wavelength of light means

A

AUS - smaller n for the same material
EUR - higher n for the same material

  • caution when patients get glasses from overseas, and how the lens is manufacturered
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14
Q

What are the benefits of a higher refractive index lens materials?

A

The same dioptric power can be achieved with an increased radius of curvature, make the lens flatter.

*less lens bulging out of the frame

  • good for higher rx
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15
Q

What is the formula of refractive power (F) ?

A

F = n’ - n / 2

F = refracting power (D)
r = radius of curvature of surface in m
n = index of refraction of medium through which light passes before reaching the surface
n’ = index of refraction of the medium on the emergent side of the surface

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16
Q

What is constringence?

A

A measure of the chromatic aberrations produced by dispersion.

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17
Q

What is the Abbe number?

A

A measure of constringence, ability of a lens material to keep white light together (ie to prevent it from producing chromatic abberations)

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18
Q

What is the formula for Abbe value?

A

V (d) =( n (d) - 1 ) / ( n (f) - n (c)

n (d) = refractive index of yellow light for the material
n (f) = refractive index of blue light for the material
n (c) = refractive index of red light for the material

  • larger denominatior = smaller abbe number
  • higher abbe number = less chromatic abberation
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19
Q

Important information about Abbe number?

A
  • lower abbe number = more likely to reduce visual acuity in the periphery of high powered lens
  • px may complain vision is “less sharp”
  • low dispersion V = 45 <
  • medium dispersion V = >39 but <45
  • high dispersion = v = <39
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20
Q

What is the formalu for Longitudinal chromatic aberration?

A

Longitudinal chromatic aberration = F/Abbe value

F=lens power

21
Q

What is the formalu for Transverse chromatic aberration?

(depends on how after from OC)

A

Transverse chromatic aberration = dF/Abbe value

dF= prismatic effect (cm)

22
Q

What is transpency in clinical optics?

A

Transmission is of material is used to express the transpency of the lens.

A measure of the % of incident light which emerges from the lens

23
Q

Factors that effect transpency?

A
  • light lost to reflection
  • light transmitted
  • light transmitted (ghost image)
  • light may also be absorbed by the lens
24
Q

What is the relationship between surface reflections and refractive index?

A

Lower refractive index:
- lower % front surface reflection
- lower % back surface reflection
- higher % of light transmitted

25
What are in implications of reflections?
* may cause visual fatigue, blurred image distance object, sharp view of own eye, moving image with head motion * increase refractive index typically has magnitude of reflections increase * transmission may be changed by wavelength or tints/filters
26
Australian Standards Definition for clear lens
lens with no noticeable colour in transmission
27
Australian Standards Definition for tinted lens
lens having a noticeable colour (including gray) in transmission
28
Australian Standards Definition for uniformly tinted lens
lens made from material in which either the tint is incorporated throughout, or which, after manufacture, is surface-treated to provide a uniform tint
29
Australian Standards Definition for gradient - tinted lens
lens having a controlled variation in tint transmittance and/or colour over the whole or part of the surface
30
What are photochromic lens?
lens which reversibly changes its luminous transmittance characteristics depending upon the intensity and wavelength of the radiation falling upon it *lens is designed to react to wavelengths within 300 to 450 nm usually * transmission properties are usually affected by ambient temperatur
31
Polarising lens
lens showing differential luminous absorption” according to the plane of polarization of the incident light
32
UVA transmittance relationships with photochromatic lens:
* greater transmittance at higher temperatures * less transmittance in darkened state * trasmittance decreases as lens thickness increases * different materials have different spectral transmittance values
33
How do plastic photochromatic lenses work?
These mostly carbon molecules such as pyridobenzoxazines, naphthopyrans, and indenonaphthopyrans—react to UVA light by rearranging their chemical bonds into new species that can absorb and essentially block UV and visible light. In the unradiated state, all wavelengths above 425 nm are transmitted as absorbance is 0. In the irradiated stated, absorbance increases, so transmissionof all wavelengths decreases and the lens appears darker
34
What do polarised lens do?
Decrease the amount of reflected glare. Roads water - light vibrates in horizontal plane Filters will be allow vertical light to block this
35
What is lens density?
*weight in gram of 1cm3 of material *indication of weight of finished spectacles
36
What factors effect lens density / weight?
Shape and size of lens Density of material
37
What is specific gravity?
density of lens material compared with water = mass / volume (units = g/cm3) *higher refractive index may have higher specific gravity (glass has a linear relationship, plastic is less distinct)
38
What is abrasion resistance?
Ability to resist scratches. * leads to better optical quality, appearance of the lens and longevivy of the lens
39
What is the order of least to most scratch resistance based on Bayer abrasion test?
Polycarbonate Trivex Hi Index CR39
40
What is impact resistance?
ability of a material to withstand the impact of a hard body. Measured using speed of a projection for the material to break
41
“We have confirmed previous observations that polycarbonate lenses are more susceptible to penetration by sharp, high-speed missiles than blunt missiles What should we avoid?
the use of 2-mm center thickness and MAR-coated polycarbonate lenses should be discouraged for industrial eye protectors where sharp missile hazards are possible.
42
Relationship between UVR exposure and impact resistance?
extended UVR exposure causes a significant reduction in the impact resistance of the ophthalmic lens substrates commonly used for occupational eye protectors. Protective lenses that have been exposed to high levels of UVR for extended periods should be replaced regularly to maintain optimal impact protection, even if they do not show visible damage owing to wear and tear.
43
What is chemical resistance?
Resistance of a lens material to chemicals in both an industrial setting, and in everyday use. Chemical resistance can be classified according to the thermodyne test, which classifies materials from S1 (highest resistance) to S6 (lowest resistance. *time for damage to occur is typically long than expected life of spectacles
44
What is the make up of glass lens?
Typically crown glass - silica based compound (70%) - 15% soda - 12% lime *decreased production throughout the years
45
What is the process of glass lens manufacture?
* Materials heated to 1400-1500 degrees * Pressed to form round glass block 1-3 cm thick *Diamond grinding tool grinds correct front surface curvature * Polished for transparency
46
Pros and cons for prescibing glasses lens
Pros: * Scratch resistant * Chemical resistant (including chemicals and solvents) * Very high index materials available Cons: * Fragile – break on impact * Unsafe – break into sharp shards of unless toughened
47
Definition of plastic lens
‘a polymeric material (usually organic) of large molecular weight which can be shaped by flow’ AKA resin Columbia Resin batch 39 - CR39
48
What is the chemical name for CR39?
Allyl diglycol carbonate
49
what is the refractive index of plastic lens?
n = 1.498

10410 Clinical Dispensing (9 decks)

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