OPTICAL CORRECTION
The physical data: What you need to know
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
- 2 DENSITY
- 3 ABBE NUMBER (constringence or V-value)
- 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, p. Table 1 gives a typical selection of lens materials and
lists these various properties. The significance of the physical data is
discussed below.
REFRACTIVE INDEX
The refractive index expresses the ratio of the velocity of light of a given
frequency in air, to the velocity of light of the same frequency in a given
refracting medium. In the UK and the USA, refractive index is measured at
present on the helium d-line (wavelength 587.56nrn) 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 n e is a little greater than for nd, so that when the
value of n e is given, the material appears to have a slightly higher refractive
index! CVF, V-value and the reflectance, p, are quoted for nd. BS 7394: Part 2,
Specification for complete 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
CURVE VARIATION FACTOR
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 from 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 (see Table 1) of 0.75, which informs us that the reduction in thickness
will be about 25% if this material is substituted for crown glass. 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 -10.00 D lens in 1.700 index material. The crown glass equivalent
is 0.75 x -10 or -7.50. In other words, the use of a 1.700 index material would
result in a lens that has a power of -10.00 D but, in all other respects, looks
like a -7.50 lens made in crown glass. A 1.600 index material has a CVF of 0.87,
so that we may expect a 13O/o reduction in thickness, and a -10.00 D lens made
in this material would look like a -8.75 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. Plastics materials
are compared with CR39.
DENSITY
Density tells us how heavy a material is, and a comparison of densities can
inform upon the likely change in weight to be expected by using a particular
material. The value given is the weight in grams of lcm = 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 200/0 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.
ABBE NUMBER
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 material and indicates the degree of transverse
chromatic aberration (TCA) which the wearer will experience. The values quoted
in Table 1 are the Abbe numbers for the helium d-line, Vo, where Vd, (nd-1) I (nF-nc)
nc is the refractive index of the material for the wavelength hydrogen C
(656.27nm), and nF is the index for the wavelength hydrogen F (486.13nm).
The effects of chromatic aberration are well known. 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 (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
complete spectacles, classifies materials in terms of their constringence as
follows:
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.
REFLECTANCE ρ
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:
ρ = (n-1)'/(n +1)' x 1000/o
Thus a material of refractive index 1.5, has a reflectance
of
(0.5/2.5)' x 100 = 4% per surface.
An understanding of the significance of these qualities of lens materials will
enable opticians to assess the suitability of the material for the customer they
have in mind, and thus help patients to get the best solution for their visual
needs.
20/20 02/2003
