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Kausar Nahid
Student of Optometry, Vision Science,
Philosophy and Psychology
Requirements for spectacle lens materials ???
nahid.vision@gmail.com
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
nahid.vision@gmail.com
4
Refractive index
1
Density
2
Curve variation factor (CVF)
3
Abbe number
4
Reflectance
5
UV Cut-off
6
nahid.vision@gmail.com
5
nahid.vision@gmail.com
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)
nahid.vision@gmail.com
7
Whereas in Continental Europe it is measured on the
mercury e-line (wavelength 546.07nm).
nahid.vision@gmail.com
8
nahid.vision@gmail.com
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
nahid.vision@gmail.com
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.
nahid.vision@gmail.com
11
nahid.vision@gmail.com
12
GLASS
PLASTIC
nahid.vision@gmail.com
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
nahid.vision@gmail.com
14
nahid.vision@gmail.com
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)
nahid.vision@gmail.com
Where,
nbase is the refractive index of the base material
nmat is the refractive index of the material being compared
nahid.vision@gmail.com
Thinnest
Thickest
High Index Lenses are thin as CVF is Less
nahid.vision@gmail.com
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.
nahid.vision@gmail.com
nahid.vision@gmail.com
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
nahid.vision@gmail.com
nahid.vision@gmail.com
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)
nahid.vision@gmail.com
23
587.56nm
656.27nm 486.13nm
Vd = (nd - 1)/(nF - nC)
Vd = (1.50 - 1)/(1.505- 1.496)
nahid.vision@gmail.com
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.
nahid.vision@gmail.com
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”.
nahid.vision@gmail.com
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
nahid.vision@gmail.com
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
nahid.vision@gmail.com
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.
nahid.vision@gmail.com
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
nahid.vision@gmail.com
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
nahid.vision@gmail.com
31
nahid.vision@gmail.com
32
No
noticeable
TCA
at
15mm
No
loss
of
V/A
TCA
at
15mm
nahid.vision@gmail.com
33
No
noticeable
TCA
at
30mm
No
loss
of
V/A
TCA
at
30mm
nahid.vision@gmail.com
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%
nahid.vision@gmail.com
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@gmail.com
36
nahid.vision@gmail.com
37
nahid.vision@gmail.com
38
nahid.vision@gm
ail.com
39
nahid.vision@gm
ail.com
40
nahid.vision@gmail.com
41
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%
Thank You

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Materials for Spectacle Lenses​ UG STUDENTS.pptx

  • 1. Kausar Nahid Student of Optometry, Vision Science, Philosophy and Psychology
  • 2. Requirements for spectacle lens materials ??? nahid.vision@gmail.com
  • 3. 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. nahid.vision@gmail.com 4 Refractive index 1 Density 2 Curve variation factor (CVF) 3 Abbe number 4 Reflectance 5 UV Cut-off 6
  • 6. nahid.vision@gmail.com 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. nahid.vision@gmail.com 7 Whereas in Continental Europe it is measured on the mercury e-line (wavelength 546.07nm).
  • 9. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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.
  • 13. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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).
  • 16. 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) nahid.vision@gmail.com
  • 17. Where, nbase is the refractive index of the base material nmat is the refractive index of the material being compared nahid.vision@gmail.com
  • 18. Thinnest Thickest High Index Lenses are thin as CVF is Less nahid.vision@gmail.com
  • 19. 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. nahid.vision@gmail.com
  • 21. 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 nahid.vision@gmail.com
  • 22. nahid.vision@gmail.com 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. nahid.vision@gmail.com 23 587.56nm 656.27nm 486.13nm Vd = (nd - 1)/(nF - nC) Vd = (1.50 - 1)/(1.505- 1.496)
  • 24. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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
  • 34. nahid.vision@gmail.com 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. nahid.vision@gmail.com 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%
  • 42. 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%
  • 43. 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
  • 44. 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%
  • 45. 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
  • 46. 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
  • 47. 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%