All glasses need lenses - even spectacles worn as accessories have plano lenses, as the glasses themselves don’t correct any vision or sight errors. Modern spectacles have complex technology built into the lenses themselves - just because you have bought cheap glasses online, don’t assume that the quality or complexity of the glasses lens has been compromised in any way.
In the early days of glass lenses (or glasses lenses !), you could take a piece of glass with parallel surfaces and grind a spherical surface on one or both sides and presto, you had a lens and your eyesight improved. If you made the spherical surface bulging out in the middle you had a convex or plus lens and if you made it curve inwards in the middle then you had a concave or minus lens. The more scientific reader will observe that this terminology is a little “sloppy” but you get the picture, (to use an even more sloppy expression). This type of lens was later known as a “flat” lens.

Armed with such high technological expertise one could produce lenses to alleviate the problems of long and short sightedness or myopia and hypermetropia. But what about astigmatism? the more difficult reader may enquire. Aha says the erudite writer, in that case you take a similar piece of glass to the aforementioned and grind a cylindrical surface on to one side. If, in addition, a spherical power is required then that can be ground on the other side So you have the spherocylindrical lens. It makes one wonder what will be the next giant leap forward in lens technology. The answer is (almost) the toric lens. But first we must consider the reason for this development .

When light passes through a lens it is focussed according to the focal length of the lens. But this applies only to the paraxial rays of light. As you are well aware, these are the rays close to the principle axis. Less fortunate rays are likely to fall victim to a condition known as spherical aberration, or even worse, oblique astigmatism, conditions even worse than they sound. In order to alleviate, to a limited extent, the appalling consequences of these physical phenomena, the curved lens was invented. In the case of the lens with only a spherical power, this was known as a convex (or more accurately, concavo-convex) or meniscus lens, meniscus being , as you are aware, the surface of a column of water in a vertical glass tube which clings to the side of the tube higher at the periphery than at the middle, thus forming a concave spherical surface. Mercury, on the other hand, forms a convex surface in the same circumstances. The science graduates will doubtless explain to the lesser mortals, the reason for this.

This is all very well, but what about the sphrocylindrical lens. The answer is the toroidal lens, a.k.a. the toric lens or more commonly the”toric”. The classically educated readers of this column, (in other words the vast majority of you), will be well aware that the ancient Greeks, when building their Parthenons etc. placed the Ionic columns on a piece of stone known as a “Torus”. This piece of masonry was of a shape resembling a spherical balloon which has been sat upon, forming two flat surfaces, top and bottom and resulting in the major curved surface having a radius of curvature greater in one meridian than the other. One might reasonably refer to this type of surface as a “toric” surface. What a good idea for combining a spherical and cylindrical power in one curved surface! Thus, a prescription requiring 1.00D sph with 0.50D cyl could be made as 1.00D in one meridian and 1.50D in the other. Exactly how this is to be made in practice can easily be explained by your local surfacing shop foreman.

A more thorough study of this subject may be reported in “Base curves and Best Form Lenses”, watch this space!

The word “toric” passed into our language as meaning any curved lenses, as opposed to “flat”, lenses even where there was no cylindrical power. “Torics” meant curved lenses and for example “Toric Fused” the first choice bifocal lenses, indicated curved fused bifocal lenses.

When next ordering your glasses online or in the high street (with glass lenses of course), please state clearly whether flats or torics are required.

Tags: astigmatism, hypermetropia, meniscus, myopia, spherocylindrical, toric, toroidal

This entry was posted on Friday, March 6th, 2009 at 4:46 pm and is filed under Glasses. 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.