2. Lens Properties
The main material properties of spectacle lenses fall into
several categories, including:
• Refractive index
• Specific gravity or density
• Abbe number or V-value*
• UV absorption
• Impact resistance
• Durability
• Reflectance
3. Introduction
The Abbe number is optical properties of the material
rather than of its mechanical characteristics.
The Abbe number is the reciprocal of the dispersive
power of the material and indicates the degree of
transverse chromatic aberration (TCA) which the wearer
will experience.
It also called as ‘nu’ value
ABBE value defined after the German Physicist Ernst
Abbe (1840-1905), who defined this useful
measurement of optical quality.
4. Key Notes
Lower Abbe value lenses will have the higher
Chromatic Aberration at periphery of the lens & vice-
versa.
ABBE value of the human eye is about 45 to 50
To reduce Chromatic aberration in high power
lenses one should prescribe lenses with high ABBE
value.
5. Cont....
Chromatic Aberration depends on the 2
properties:
1. Wavelengths of the light rays
2. Refractive Indices of the lens material
6. Chromatic Aberration of Lens
Light entering an optical medium will be
dispersed into different refractive effects
according to the variation in refractive index
with wavelength(4)
. This is known as chromatic
aberration.
7. Transverse Chromatic Aberration
It is the type of Chromatic aberration in which
there is the differences in image size of
chromatically formed images.
8. Refractive Index (R.I)
Defined as the ratio of the velocity of light in
vacuum to the velocity of light in another
medium is called refractive index.
9. Abbe value formula
Equation for calculating the ABBE Value(v):
Where, nD, nF & nC are the refractive indices of
the material at the wavelengths of the
Fraunhofer D- , F-, & C- Spectral lines
12. We can classifies materials in terms
of their Abbe number as follows:
Low dispersion V ≥ 45 (e.g- Crown=59)
Medium dispersion V ≥ 39 but < 45
High dispersion V < 39 (e.g- high density flint = 29)
Therefore, low dispersion materials almost never give rise
to complaints of coloured fringes or off-axis blur.
13. Often in optical practice we notice the
complain:
“These lenses are fine when I look through the
centres but vision is blurred when I look through the
edge”
14. Solution:
Off- axis blur is due to the TCA (Transverse Chromatic
Aberration)
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, that is,
TCA = P/V. (where, P= d.F)
Where, P= prismatic effect (Prentice’s rule) & V= ABBE value
15. Cont...
It is generally considered that the average
threshold value for TCA is 0.1∆.
TCA less than 0.1∆ is unlikely to give rise to
complaints.
18. Clinical Application:
For an example,
If distance prescription of -4.00 DS, and the
patient is looking through 15mm temporal to the
optical centre.
In this case, for CR-39(ABBE 58) lens the value
of TCA will be around 0.10 ∆.
19. Cont..
For Polycarbonate (ABBE 30) lens the value
of TCA will be around 0.20 ∆,
Therefore, we can say in polycarbonate lens
TCA will be more than CR-39 lens & thus off-
axis blur will be more.
20. Important dispensing factors
for lenses with low Abbe value
Use monocular interpupillary distances.
Use shorter vertex distances.
Use sufficient pantoscopic tilt angle, but not
more than 10 degree in case of high power
lenses.
Measure major reference point heights,
considering pantoscopic angle
21. References:
System for Opthalmic Dispensing, 2nd
Edition, Irvin M
Borish, O.D, D.O.S, L.L.D, D.Sc.
Clinical Optics, 2nd
Edition, Troy E. Fannin
www. pmoptics.com(Pricision Micro Optics)
Ot Article; CET Continuing Education & Traning, Course
code: C-34968 (Module 2), Course Code: C-12758 (Module 3)
