3. Refractive Index (µ)
Depends upon the speed of white light inside the
material.
Hence varies according to the source of white light.
We follow the American standard light source of
Helium (d line)
Note: µ = c/ v where “ v” is the velocity of light in
lens material and “ c ” is the velocity of light in
vacuum.
07/16/19 3
4. Higher the refractive index ; thinner will be the
lens.( lesser will be the thickness. )
Example: Index of Crown glass is 1.523
Index of CR – 39(Plastic) material is 1.4985
Index range: 1.4 to 1.9
07/16/19 4
5. Index Classification
Normal index : 1.48 ≤ µ d< 1.54
Mid-index : 1.54 ≤ µ d < 1.64
High-index : 1.64 ≤ µ d < 1.74
Very high index : 1.74 ≤ µ d
where µ d is the µ or refractive index of the
material measured using Helium d line.
07/16/19 5
6. Density (kg/m3
)
Density of any material is defined as the mass per unit
volume.
The higher the density, the tighter (denser) the
particles are packed inside the substance.
Density is a physical property constant at a given
temperature and density can help to identify a
substance.
Density ? Related to sp.gravity
07/16/19 6
7. Specific gravity (unit less)
The Specific Gravity is a dimensionless unit, defined as the
ratio of density of the material to the density of pure water
at a specified temperature.
It gives an idea of the weight of one lens material on
comparison to another.
Since it is relative it lacks unit, similar to index of a
material.
Was in vogue in olden days which has been sidestepped
nowadays by density measures.
07/16/19 7
8. e.g.: if we say CR has a specific gravity of 2.5 what it
implies is that it is 2.5 times heavier than water.
e.g.: if specific gravity of CR-39 is 2.5 and that of glass
is 5 then, glass is twice as heavy as CR -39.
Note: Specific gravity is related to Refractive Index
( µ).
Usually as index increases; specific gravity also
increases.
07/16/19 8
9. Refractive Index Vs Density
1.9 (µ) glass : 4.0gm /cm3
1.8 glass : 3.7gm/cm3
1.74 high index plastics : 3.5 gm/cm3
1.7 glass : 3.2gm/cm3
1.6 plastics (Polycarbonate) :1.2 gm/cm3
CR-39 ( µ=1.49) : 1.32 gm/cm3
Crown glass (µ=1.523) : 2.54 gm/cm3
Trivex : 1.16gm/cm3
07/16/19 9
11. Abbe Number
It is a measure of the ability of lens material to
keep white light together.( that is without
producing chromatic aberration)
As the Abbe value increases chromatic
aberration decreases.
It is the reciprocal of the dispersive power of
the materials.
It is inversely proportional to refractive
index.
e.g.: Abbe Value of Poly carbonate 30 and CR-39--58.07/16/19 11
13. ULTRA VIOLET (UV) RADIATION (UVR)
07/16/19 13
UV CLASSIFICATION
ABBRE-
VIATION
RANGE NOTES
Near NUV 400 nm - 200 nm ENTERS EARTH
Long wave or black
light
UVA 400 nm - 320 nm
CAUSES OCULAR
DAMAGE
Medium wave UVB 320 nm - 280 nm
BLOCKED BY USUAL
LENS MATERIAL
Short wave or
germicidal
UVC Below 280 nm
BLOCKED BY USUAL
LENS MATERIAL
Far or vacuum FUV, VUV 200 nm - 10 nm BLOCKED BY OZONE
Extreme or deep EUV, XUV 31 nm - 1 nm BLOCKED BY OZONE
14. ULTRA VIOLET (UV) RADIATION (UVR)
Natural sources of UV
Sun emits UV-A , UV-B, and UV-C
Atmosphere's absorbs almost whole of UV- C and UV-
B through ozone layer.
Thus only UVA reaches the Earth's surface.
.
Ordinary crown glass is partially transparent to UVA
but is opaque to shorter wavelengths.( uv b and c)
While quartz or Silica glass, depending on quality, can
be transparent even to vacuum UV(FAR
UV)wavelengths.
07/16/19 14
15. Since radiation in UV-A range (320-400nm)
causes ocular damage, it is important for a lens
material to cut off this range of wave length.
The lens material can be arranged in the
following (increasing) order based on their
effectivety in absorbing UV-A.
Crown glass < CR-39 < Polycarbonate or Trivex
07/16/19 15
Note µ = c/ v where “ v “is the velocity of light in lens material and “ c ” is the velocity of light in vacuum.
nT = 1 + FT(nLM –1)/FLM
where FT = true power of lens read from
the focimeter;
FLM = sum of surface powers as
given by lens measure; and
nLM = refractive index for which the
lens measure has been scaled.
Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.
