When people spend significant sums of money on a new pair of glasses they want to know (naturally) how thin and light their prescription lenses are going to be.
They also want to know that if they spend even money on thinner, extra thin or ultra thin lenses, how much thickness will be shaved off "standard thickness" lenses.
Although a simple question, the answer is often not quite as straightforward.
The edge thickness of any prescription lens depends on a number of factors.
Firstly the higher the prescription the thicker the lens.
Negative spherical power lenses are thicker on the edge than in the centre, whereas positive power lenses are thicker in the middle than at the edge.
The higher the negative power, the greater the edge thickness.
The higher the positive power, the greater the centre thickness.
Secondly the density of the lens material also affects lens thickness.
 Lenses made from denser materials (i.
e.
lenses with a higher refractive index), bend the light more.
So you can make higher index lenses thinner than lower index lenses for the same degree of light bending.
Standard index lenses have a refractive index of 1.
5 (i.
e.
light travels 1.
5 faster in a vacuum than in the standard index material).
Higher index plastic materials used for prescription lenses are 1.
6, 1.
67 and 1.
74.
Glass lenses can have an index of up to 1.
9.
It is important to note that a 1.
74 index lens will not guarantee a 1.
74mm edge thickness! The design of a lens also affects edge thickness.
Spherical lenses (with front faces curved like part of a sphere) are commonly used, yet aspheric lenses (flatter front lens faces) and bi-aspheric lenses provide designs which can reduce lens edge thickness.
Other factors which determine lens edge thickness include the eyesize of the frame (the horizontal distance between a lens in a frame), the bridge size of the frame, and the Pupillary Distance (PD) of the wearer (the distance between the centres of the pupils of the wearer).
For negative spherical prescriptions larger eyesizes will result in lenses with thicker edges.
To ensure that the thickness of a lens at the nasal side is equal to the thickness at the temple side, the sum of the bridge and eyesize should equal the PD of the wearer.
If the PD is either greater or less that the sum of the bridge and eyesize measurements each lens will have different nasal and temple thicknesses.
We have developed a program which estimates the edge thickness of a lens for a given prescription, eyesize, PD, bridge size and index of material.
This has helped customers decide on which lens option to choose.
To conclude, glasses wearers are seeking help on whether to choose thinner lenses when purchasing a new pair of glasses.
As there are several factors governing lens thickness, straightforward answers are not always forthcoming.
However, armed with a little experience and some simple technical know how, optical professionals should be in a position to provide glasses wearers with helpful estimates on how thick their customers' lenses will be.