Materials for Spectacle Lenses UG STUDENTS.pptx

Kausar Nahid
Student of Optometry, Vision Science,
Philosophy and Psychology

Requirements for spectacle lens materials ???
nahid.vision@gmail.com

3
Requirements for spectacle lens materials are given in:
1) BS EN ISO 8980:2004, “Uncut finished spectacle lenses”
2) BS EN ISO 14889:2003, “Fundamental requirements for uncut finished
lenses”
3) BS 7394: Part 2:1994, “Specification for complete spectacles”
When a new material is produced, certain items of physical
data are published either by the material supplier or by the
lens manufacturer. These includes:
1. Refractive Index
2. Curve variation factor (CVF)
3. Abbe Number
4. Density
5. Reflectance
6. UV cut-off point

4
Refractive index
1
Density
2
Curve variation factor (CVF)
3
Abbe number
4
Reflectance
5
UV Cut-off
6

6
Refractive index expresses the ratio of the velocity of
light of a given wavelength in air, to the velocity of light
of the same wavelength in the refracting medium.
At present, in the UK and the USA, refractive index is
measured on the helium d-line (wavelength 587.56nm)

7
Whereas in Continental Europe it is measured on the
mercury e-line (wavelength 546.07nm).

9
BS 7394: Part 2, “Specification for complete
spectacles”, classifies materials in terms of refractive
index as follows:
1)Normal index n ≥ 1.48 but < 1.54
2)Mid index n ≥ 1.54 but < 1.64
3)High index n ≥ 1.64 but < 1.74
4)Very high index n ≥ 1.74

10
Density tells us how
heavy the material
is, and a comparison
of densities can give
the likely change in
weight to be
expected by
using the material.

12
GLASS
PLASTIC

13
Densities of high refractive index glasses are seen to be greater
than that of Crown glass (about 2.5)
Refractive Index ά Density /Specific Gravity
In order to compare the weights of lenses made in
different materials it is also necessary to consider the
saving in volume.
Refractive Index is not the basis for less weight of High Index
Lenses

15
CVF (Curve Variation Factor) is a parameter of a spectacle lens
which tells us how thin the finished lens (for a particular lens
material) will be, compared to the basic material (For glass:
Crown Glass & For Plastic: CR-39).
It is useful to know the likely change in volume and thickness
which will be obtained when another material is compared
with a standard Crown glass (for glass) or CR-39 (for plastic).

CVF is simply the ratio of the refractivity of Crown glass to that of
the material:
CVF = 0.523/(nd -1)
Plastics materials are compared with CR39:
CVF = 0.498/(nd – 1)

Where,
nbase is the refractive index of the base material
nmat is the refractive index of the material being compared

Thinnest
Thickest
High Index Lenses are thin as CVF is Less

One of the most practical uses for the CVF is to convert the
power of the lens that is to be made into its Crown glass
equivalent.
This is done, simply, by multiplying the power of the lens by
the CVF for the material.
For example,
Suppose we wish to dispense a -8.00D lens in 1.705 index
material, the Crown glass equivalent is 0.75 (CVF) x -8.00
(Power) or -6.00.

RI POWER (D)
1.498 3.5
1.523 4.00
1.600 5.00
1.701 6
RI POWER (D)
1.498 5.50
1.523 6.00
1.600 7.00
1.701 8.00

22
The Abbe number informs us of the optical properties of the
material
The Abbe numbers for the helium d-line:
Vd = (nd - 1)/(nF - nC)
nC is the refractive index of the material for the wavelength, hydrogen red, C
(656.27nm) and nF is the index for the wavelength, hydrogen blue, F
(486.13nm)

23
587.56nm
656.27nm 486.13nm
Vd = (nd - 1)/(nF - nC)
Vd = (1.50 - 1)/(1.505- 1.496)

24
When light from a small white object is refracted by a prism, it is
dispersed into its monochromatic constituents, the blue
wavelengths being deviated more than the red
To an eye viewing through the prism, the image of the object appears fringed
with blue on the apex side of the prism.

25
Under conditions of low contrast, colour fringing may not be
noticed.
Instead, the effect of TCA is to cause a reduction in visual acuity
(off-axis blur).
This often presents in the complaint, “These lenses are fine when I look
through the centres but vision is blurred when I look through the edge”.

26
BS 7394: Part 2, “Specification for complete spectacles”3,
classifies materials in terms of their Abbe number as follows:
TYPES Abbe Number
Low dispersion V ≥ 45
Medium dispersion V ≥ 39 but < 45
High dispersion V < 39

27
To a good approximation, the magnitude of the TCA at any
given point on a lens can be found by calculating the prismatic
effect, P, at that point and dividing by the Abbe number of the
Material:
TCA = P/V
= cF/V
To minimize chromatic aberration Abbe Number should be high

28
JALI’S THRESHOLD: It is generally considered that the average
threshold value for TCA is 0.1Δ. TCA less than 0.1Δ is unlikely to
give rise to complaints
Abbe numbers for some normal index materials (e.g. Crown
glass and CR39) is about 60 and the prismatic effect at the
visual point would need to be about 6Δ before the typical
threshold is reached.
Using paraxial theory, this amount of prism would be
encountered, for example, at a point 15mm from the optical
centre of a +4.00D lens.

29
Materials whose Abbe numbers are in the region of 40 would
give rise to 0.1Δ of TCA at a point where the prismatic effect is
4Δ, i.e. 10mm away from the optical centre of a +4.00D lens.
10mm
15mm
V = 60 V = 40
TCA = P/V
= cF/V
It is for this reason that it is wise to select a material with the
highest possible Abbe number

30
It is useful to be able to determine the diameter of the zone
concentric with the optical centre of the lens within which the
visual point must remain before the threshold value of 0.1Δ is
reached. This is given by:
c = V / F 2C
where c is the radius of the
zone in mm, V is the Abbe
number and F is the power
of the lens

32
No
noticeable
TCA
at
15mm
No
loss
of
V/A
TCA
at
15mm

33
No
noticeable
TCA
at
30mm
No
loss
of
V/A
TCA
at
30mm

34
The reflectance of the lens surfaces is calculated from the
refractive index of a material.
Fresnel reflectance factor:
When light is incident normally on a lens surface in air, the
percentage of light reflected at each surface is given by:
X 100%

35
Assuming n = 1.50, the fraction
reflected is given by 100x (0.5 / 2.5)² =
4% per surface.
Thus, 4% of the incident light is lost
by reflection at the first surface and
96% enters the lens material; 4% of
this is lost by reflection at the second
surface (3.84%) so the amount of the
original intensity transmitted by the
lens is 92.16%.
100%
4%
3.84%
92.16%

nahid.vision@gm
ail.com
42
1. Which one of the following is true for a plastics
material type with nd = 1.635?
a. CVF = 0.80 reflectance = 5.6%
b. CVF = 0.78 reflectance = 5.8%
c. CVF = 0.76 reflectance = 6.0%
d. CVF = 0.74 reflectance = 6.4%

nahid.vision@gm
ail.com
43
2. How far from the optical centre of a -6.00D lens made in polycarbonate
material can the eye roam before it meets the threshold value for TCA,
0.1Δ?
a. 4mm
b. 5mm
c. 6mm
d. 7mm

nahid.vision@gm
ail.com
44
3. Which one of the following is true for a glass
type with nd = 1.747?
a. CVF = 0.65 reflectance = 7.2%
b. CVF = 0.70 reflectance = 7.2%
c. CVF = 0.70 reflectance = 7.4%
d. CVF = 0.71 reflectance = 7.4%

nahid.vision@gm
ail.com
45
4. At 70mm diameter, a -5.00D lens made in polycarbonate
material is thinner but heavier than a lens made to the same
power and diameter but in Trivex material. Why is this so?
a. It has a higher refractive index and higher density
b. It has a greater centre thickness and higher density
c. It has a greater centre thickness and higher refractive index
d. It has a higher refractive index and lower density

nahid.vision@gm
ail.com
46
5. A -6.00D lens is made in a 1.80 index material that has a
density of 3.3g/cm3. Which one of the following statements
best describes the weight of the lens when compared with the
weight of the same power lens made in Crown glass?
a. Much heavier
b. A little heavier
c. More or less the same
d. A little lighter

nahid.vision@gm
ail.com
47
6. Taking only losses by surface reflectance into account (i.e.
ignoring any loss of radiation by absorption), what would be
the transmittance of a plastics lens made in material of
refractive index 1.61 assuming no AR
coating is applied?
a. 94.5%
b. 90.4%
c. 89.4%
d. 88.4%

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