When purchasing a pair of spectacles online, you need to be careful of your frame and lens combination choice.
There are quite a few different lens materials available.
The most common plastic optical lens material is CR39 which has a refractive index of 1.498 (often rounded to 1.5).
CR39 has very good optical qualities so it is a very popular lens material. The Abbe number for CR39 is 58. This means that it produces a low amount of chromatic aberration. An Abbe number is a measure of the lens material’s dispersion of light in relation the the refractive index of the lens. A lens with a low Abbe number will be more dispersive. Chromatic aberrations will appear as image blur or colour fringing to the spectacle wearer so lenses with very low Abbe numbers should be avoided at all costs!
CR39 is suited to full rimmed frames but it is not recommended for semi rimless frames or 3 piece rimless frames. CR39 has quite a low tensile strength which means that it is quite brittle so it will chip easily. A semi rimless frame is a frame where the metal or plastic rim of the frame is partially replaced by a thin nylon cord. This means that the nylon cord is holding the lens in place rather than the lens being fitted into either a metal or a plastic “V” shaped bevel. A 3 piece rimless frame is a frame which has no metal or plastic rims. The lenses will be attached to the frame by drilling holes through the lenses.
For a semi rimless or a 3 piece rimless frame, I would recommend any of the following lens materials:
1.53 = Trivex is extremely impact resistant and is recommended for safety spectacles and non full rimmed frames. Trivex has an Abbe number of 43 so it provides much better vision than polycarbonate lenses. Polycarbonate (1.586) lenses have an Abbe number of only 30. Polycarbonate is the most dispersive lens material available and may still be recommended by lots of professionals in the optical industry even though it provides poor vision. Polycarbonate should not be used for anybody with a prescription higher than +1.50D or -1.50D as the higher the lens power, the more dispersive the material becomes.
1.6 = Many companies have a 1.6 lens material available. They are all quite similar but the lens material with the highest tensile strength is known as Eyas which is exclusive to Hoya. This material has an Abbe number of 42 so it will also provide very good vision as well as being light weight.
1.67 = Many companies have a 1.67 lens material available. Eynoa from Hoya has a high tensile strength and is also very thin and light weight.
1.7 = Not all 1.7 materials are suitable for rimless frames. Eyry from Hoya is a good option if your prescription is high and you want extremely thin lenses which also provide good vision.
For safety reasons, glass should never be used in a semi rimless or a 3 piece rimless frame as it is very brittle and will shatter easily.
Choosing the correct lens material to suit the frame
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1 October 2009
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