It is preferable to make prescription glasses as thin as possible for two reasons. Firstly they have a better cosmetic appearance and secondly they may be lighter on the nose. There are two ways of making prescription spectacles thinner.

The first way is to choose a lens with a higher refractive index. The higher the refractive index the thinner the final prescription lens will be. Typical refractive indices for plastic lenses are 1.5, 1.6, 1.67 and 1.74. The thinnest of these of course will be the 1.74 but it will also be the most expensive. Glass prescription glasses are generally thinner but heavier. Typical values for the refractive index of glass lenses are 1.7, 1.8 and 1.9, the 1.9 being the thinnest.

There is another way to achieve thin prescription glasses. That is to be careful with the spectacle frame selection. The smaller the frame, the thinner the lenses should be. In the case of minus lenses (concave) which correct myopia (short sightedness) they are thinner in the middle and thicker at the edge. Therefore if the lens is put into a smaller frame the glazing process grinds away the thicker edge leaving the thinner middle part and the myopic glasses end up with thinner lenses. Plus lenses (convex) correct hypermetropia (sometimes abbreviated to hyperopia). Hypermetropic prescription lenses are thicker in the middle and thinner at the edge so simply cutting them into a smaller ophthalmic frame does not make them any thinner. However, hyperopic lenses can be manufactured in different blank sizes. The blank size is the original diameter of the ophthalmic lens before it is cut down. Typical blank sizes might be 55mm, 60mm, 65mm and 70mm. A smaller blank size will always be thinner than a larger blank size for a given power of lens. Therefore if you choose a smaller frame it can be manufactured using smaller blanks and the finished glasses will be thinner.

Tags: blank size, concave, convex, Glasses, hypermetropia, hypermetrpic, hyperopia, hyperopic, myopia, myopic, ophthalmic, ophthalmic frame, prescription glasses, prescription lens, prescription spectacles, refractive index, refractive indices, short sightedness, spectacle frame

This entry was posted on Monday, December 22nd, 2008 at 9:37 pm and is filed under Eye Care. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.