Week 5 Lens Materials and Coating Flashcards
Which one of the following lens materials already has low ultraviolet radiation transmissibility?
A) CR39
B) High index glass
C) Crown glass
D) Polycarbonate
polycarbonate
Which one of the following is the lightest lens material?
A) CR39
B) Trivex
C) Crown glass
D) Polycarbonate
Trivex
What does An Abbe number of 32 indicate?
indicates poor constringence. The higher the number, the less chromatic aberration.
The least impact resistant material is:
A) CR39
B) Toughened glass
C) Crown glass
D) Polycarbonate
Crown glass
The most impact resistant lens material is:
A) TR90
B) Phoenix
C) Trivex
D) Polycarbonate
Polycarbonate
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 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.
The most damaging of the ultraviolet radiations
- has wavelength of 320-400 nm
- has chronic effects such as brunescent cataract
How do you calculate absolute refractive index?
Speed of light in a vacuum = c
Speed of light in material =v
C/V = absolute refractive index
How do you calculate relative refractive index?
Speed of light in air = vair
Speed of light in material = vmaterial
V(air) / V (material) = n (refractive index)
What wavelength is used to represent light in Australia?
587.56nm (Australia)
546.07nm (Europe)
What is the formula for the speed of a wave?
v = wavelength x frequency
What is the formule for refractive index?
n = c/v
AUS VS Europe wavelength of light means
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
What are the benefits of a higher refractive index lens materials?
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
What is the formula of refractive power (F) ?
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
What is constringence?
A measure of the chromatic aberrations produced by dispersion.
What is the Abbe number?
A measure of constringence, ability of a lens material to keep white light together (ie to prevent it from producing chromatic abberations)
What is the formula for Abbe value?
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
Important information about Abbe number?
- 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
What is the formalu for Longitudinal chromatic aberration?
Longitudinal chromatic aberration = F/Abbe value
F=lens power
What is the formalu for Transverse chromatic aberration?
(depends on how after from OC)
Transverse chromatic aberration = dF/Abbe value
dF= prismatic effect (cm)
What is transpency in clinical optics?
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
Factors that effect transpency?
- light lost to reflection
- light transmitted
- light transmitted (ghost image)
- light may also be absorbed by the lens
What is the relationship between surface reflections and refractive index?
Lower refractive index:
- lower % front surface reflection
- lower % back surface reflection
- higher % of light transmitted
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
Australian Standards Definition for clear lens
lens with no noticeable colour in transmission
Australian Standards Definition for tinted lens
lens having a noticeable colour (including gray) in transmission
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
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
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
Polarising lens
lens showing differential luminous absorption”
according to the plane of polarization of the
incident light
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
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
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
What is lens density?
*weight in gram of 1cm3 of material
*indication of weight of finished spectacles
What factors effect lens density / weight?
Shape and size of lens
Density of material
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)
What is abrasion resistance?
Ability to resist scratches.
- leads to better optical quality, appearance of the lens and longevivy of the lens
What is the order of least to most scratch resistance based on Bayer abrasion test?
Polycarbonate
Trivex
Hi Index
CR39
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
“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.
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.
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
What is the make up of glass lens?
Typically crown glass
- silica based compound (70%)
- 15% soda
- 12% lime
*decreased production throughout the years
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
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
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
What is the chemical name for CR39?
Allyl diglycol carbonate
what is the refractive index of plastic lens?
n = 1.498
