Precision Optical Ltd

Physical Data
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
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.
 
Refractive Index
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
 
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 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 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 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
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.
 
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 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 F (486.13nm).
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.
 
Reflectance, R
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.
   
Fig 1: Dispersed light Fig 2: Reflectance
   
  nd ne CVF DENSITY UV
CUT-OFF
ABBE R (%)
Glasses              
White Crown 1.523 1.525 1.0 2.5 320 59 4.3
Light Flint 1.600 1.604 0.87 2.6 334 42 5.3
1.7 1.700 1.705 0.75 3.2 340 35 6.7
  1.701 1.706 0.75 3.2 320 42 6.7
1.8 1.802 1.807 0.65 3.7 332 35 8.2
  1.830 1.838 0.63 3.6 340 32 8.6
1.9 1.885 1.893 0.59 4.0 340 31 9.4
               
Plastics              
CR39 1.498 1.500 1.0 1.3 355 58 4.0
Trivex® 1.532 1.535 0.94 1.1 380 46 4.4
Sola Spectralite 1.537 1.540 0.89 1.2 370 38 4.8
AO Alphalite 1.582 1.585 0.90 1.3 380 34 5.1
Polycarbonate 1.586 1.589 0.86 1.2 385 30 5.2
Hoya Eyas 1.600 1.603 0.83 1.3 380 42 5.3
Polyurethanes 1.600 1.603 0.83 1.3 380 36 5.3
  1.609 1.612 0.82 1.4 380 32 5.4
  1.660 1.664 0.75 1.4 375 32 6.2
Stylis 1.670 1.674 0.74 1.4 375 32 6.3
Hoya Tesalid 1.710 1.715 0.70 1.4 380 32 6.9
Nikon 1.740 1.746 0.67 1.4 380 32 7.3

Table 1
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