Several items of data are needed in order to assess a spectacle lens
material. This information is usually provided by the lens manufacturer
who is using the material:
1 REFRACTIVE INDEX
3 ABBE NUMBER
4 UV CUT-OFF POINT
From the refractive index we can deduce two other useful items of
information, the Curve Variation Factor CVF, and the reflectance of
the surface of the material, R.
Table 1 gives us a typical selection of lens materials and lists these
various properties. The significance of the data is discussed below.
The refractive index expresses the ratio of the velocity of light
of a given frequency in a given refracting medium.
In the UK and the USA refractive index is measured at present on the
helium d-line (wavelength 587.56nm)
whereas in continental Europe it is measured on the mercury
e-line (wavelength 546.07nm). Both indices, nd
and ne are given in table 1 to
facilitate identification of the material. Note that the value for
ne is given, the material appears to have a slightly higher refractive
index! CVF, V-value and reflectance,R,
are quoted for nd.
BS7394: Part 2, Specification for completed spectacles, classifies
materials in terms of refractive index as follows:
NORMAL INDEX n≥1.48 but <1.54
MID INDEX n≥1.54 but <1.64
HIGH INDEX n≥1.64 but <1.74
VERY HIGH INDEX n≥1.74
It is useful to know the likely difference in thickness when a given
lens material is compared with a standard crown glass. This information
can be obtained form the curve variation factor (CVF), which enables
a direct comparison of thickness to be obtained. For example, a 1.700
index material has a CVF of 0.75, which informs us that the reduction
in thickness will be about 25% if this material is substituted for
One of the most practical uses for the CVF is to convert the power
of the lens that is to be made into is crown glass equivalent. This
is done, simply, by multiplying thee power of the lens by the CVF
for the material. For example, suppose we wish to dispense a -10.00D
lens in 1.7 index material. The crown glass equivalent is 0.75*-10.00
or -7.50D. In other words, the use of a 1.700 index material would
result in a lens that has a power of -10.00D but, in all other respects,
looks like a -7.50 lens made in crown glass. A 1.600 index materials
has a CVF of 0.87, so that we may expect a 13% reduction in thickness,
and a -10.00D lens made in this material would look like a -8.75D
made in crown glass.
CVF is simply the ratio of the refractivity of crown glass to that
of the chosen material , 0.523/(nd-1),
and compares the actual curves which are obtained on crown glass and
the material in question for a given curvature of the surface. Plastic
materials are compared to CR39.
Density tells us how heavy a material is, and a comparison of densities
can inform upon the likely change in weight to be expected b using
a particular material.
The value given is the weight in grams of 1cm2 of the material. Densities
of high refractive index materials are seen to be greater than that
of crown glass (about 2.5), but in order to compare the weights of
lenses made in different materials it is also necessary to consider
the saving in volume. For example, if the density of a material is
quoted as 3.0, it means that the material is 20% heavier than crown
glass. As a guide, provided that the saving in volume which is obtained
(indicated by the CVF) is greater than the increase in density, the
final lens would be no heavier than if it had been made in crown glass.
The Abbe number informs us of the material's optical properties rather
than of its mechanical characteristics. The Abbe number is the reciprocal
of the dispersive power of the materials and indicates the degree
of transverse chromatic aberration (TCA) which the wearer will experience.
The values quoted in Table 1 are also Abbe numbers for the helium
d-line, Vd, where
Vd = (nd - 1)/(nf - nc)
nc is the refractive index of the
materials for the wavelength hydrogen
C (656.27nm), and nf is
the index for the wavelength hydrogen
The effects of chromatic aberration are well known. When light is
dispersed into its monochromatic constituents the blue wavelengths
being deviated more than the red (Figure 1). To an eye viewing through
the prism, the image of the object appears fringed with blue on the
apex side of the prism. 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).
BS 7394: Part 2 Specification for completed spectacles, classifies
materials in terms of their constringence as follows:
LOW DISPERSION V≥45
MEDIUM DISPERSION V≥39 but <45
HIGH DISPERSION V<39
Ordinary crown glass and plastic materials such as CR39 have V-values
in the region of 59. Experience has shown that these low dispersion
materials almost never give rise to complaints of coloured fringes
or off-axis blur.
The reflectance of the lens surfaces is calculated from the refractive
index of a material. When light hits a lens surface in air normally,
the percentage of light reflected at each surface is given by:
R = (n - 1)2/(n + 1)2 * 100%
Thus a material of refractive index 1.5, has a reflectance of
(0.5/2.5)2*100 = 4% per surface
An understanding of the significance of these qualities of lens materials
will enable opticians to assess the suitability of the materials for
the customer they have in mind, and thus help patients to get the
best solution for their visual needs.