OPTICAL CORRECTION

  The physical data: What you need to know

Normal-index materials:

"For really lightweight lenses, the more rigid plastics materials offer lenses almost half the weight of either CR39 or glass. As the lens diameter increases, the weight advantage of plastics versus glass becomes more and more apparent."

Normal-index materials are classified as those whose refractive index lies in the range n :> 1.48 but < 1.54, according to BS 7394: Part 2, Specification for complete spectacles. The physical data for normal-index white spectacle lens materials currently available are given in Part 1 of this series (20/20 Europe JanlFeb 2002 pp 46-51, Table 1).

 It should not be forgotten that these materials are also available in photochromic form. The mechanical characteristics of spectacle lenses made in these various materials are best understood from the information in Table 1, which compares edge thickness and weight of -5.00 D lenses which are circular in shape and of various diameters from 40 to 70ram. Study of the information given in Table 1 reveals some interesting facts about the variation in thickness and weight of finished spectacle lenses and provides several useful pointers to successful dispensing. For example, if the diameter of the lens is kept fairly small, there is very little to choose between CR39 and glass lenses as far as weight is concerned, owing to the increased centre thickness necessary for the plastics material. On the other hand, for really lightweight lenses, the more rigid plastics materials Trivex and Spectralite offer lenses almost half the weight of either CR39 or glass. As the lens diameter increases, the weight advantage of plastics versus glass becomes more and more apparent.

The influence of diameter upon the lens weight is also easy to see from the table. Considering spectacle crown glass (1.523/2.54), tabulated in the second row of the table, when the lens diameter is increased from 40 to 50 mm the edge thickness increases by 40 % and the weight of the lens doubles. Increasing the diameter again from 50 to 60ram causes the edge thickness to increase by another 40% and, once again, the weight of the lens doubles. Compared with a lens diameter of 40ram, at 60ram diameter, the edge thickness has virtually doubled and the weight has increased four times! Although the table considers only lenses of power -5.00, these tendencies are true for all lens powers.

TRANSVERSE CHROMATIC ABERRATION

Materials whose Abbe numbers are greater than 45 are classified as low-dispersion materials by BS 7394: Part 2, Specification for complete spectacles. The Abbe numbers of normal-index materials are all in excess of 45, so all these materials would be classified as having low dispersion. The dispersion produced by a lens (transverse chromatic aberration, or TCA is not normally troublesome until it exceeds about 0.15Δ. It is generally considered that the average threshold value for TCA is 0.1Δ. TCA less than 0.1Δ is unlikely to give rise to complaints. For a material whose Abbe number is 45, the value of 0.15Δ for TCA is reached when the prismatic effect at the point on the lens is 6.75Δ.
For a 2.00 D lens this amount of prism is not encountered until the eye is using a point some 34mm from the optical centre of the lens!
For a 6.00 D lens, the eye would encounter this magnitude of prism some 11mm from the optical centre. Ordinary crown glass and the hard resin plastics material, 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.

NORMAL-INDEX PHOTOCHROMIC AND TINTED MATERIALS

All the normal index materials listed in Table 1 are available in both photochromic and tinted form. Crown glass photochromic material is available as Corning's Photogray and Photobrown Extra, with most lens types being available in these two materials. Photochromic plastics lenses are available in each of the different materials listed in the Table, as either Transitions III or Transitions ® Next Generation designs, by Transitions Optical.

The photochromic property is absorbed in the form of a dye by the lens material, or by a special coating applied to the lens surface. It is also possible to obtain photochromic plastics materials where the photochromic property of the material exists throughout the entire mass of the lens material. Crown glass is available in several different fixed tint forms, from white with special UV-attenuating properties (UV400 glass), through grey, green, yellow, amethyst and brown shades. It is also possible to vacuum-coat glass lenses with metallic coatings which deflect unwanted radiation, these too being available in various colours and shades. Plastics materials can be dyed with photographic dyes to any colour and shade including graduated tints and rainbow effects. Plastics polarizing lenses are also available in various lens designs, which attenuate reflected glare, in addition to offering strong absorption in the visible spectrum.

COATINGS FOR NORMAL-INDEX MATERIALS

All normal-index materials can be coated in high vacuum to offer multilayer reflection-free surfaces. The transmission of white reflection-free lenses can be as high as 99 %, providing improved contrast and freedom from the ghost images often experienced by wearers of uncoated lenses. Plastics lenses can be supplied with hard anti- abrasion coatings in several different forms. The hard coating may be applied to the lens surfaces in the form of a polysiloxane lacquer, using a dipping or spinning process, or by an in-mould treatment during the casting process, or by applying quartz-like layers to the surfaces during the vacuum coating process. Most multilayer coatings are finished with a topcoat, which has antistatic and hydrophobic properties, helping to maintain the transparency of the surfaces and making them easier to clean.
20/20 03/2003