Optical properties of lens materials

In the third in our short series on dispensing matters,
Dr Keziah Petre outlines the optical properties of lens materials and their influence on use in spectacles. CET module C1379

When dispensing a patient with a pair of glasses there are many considerations to take into account. Will the lens type be single vision or multifocal? Will the form be spherical or aspheric? Will the lenses be tinted? What is the best lens material for this patient? The purpose of this article is to address the last of these questions by reviewing the main properties of a spectacle lens that are affected by the choice of lens material. These will be compared across the three basic categories of material: glass, plastic and polycarbonate.

REFRACTIVE INDEX
Let's first go back to basics for a reminder of the maths behind the most fundamental attribute of an ophthalmic lens, its refractive index. The mean refractive index (n) of a lens is defined as the velocity of light in a vacuum divided by the velocity of light in the lens material (which is always slower). In practice, the refractive index is measured in air, rather than a vacuum, but this difference is insignificant for spectacle lenses.

Refractive index is measured for a specific wavelength in the middle of the visible spectrum. The reason for this is that any material will refract different wavelengths by different amounts, and this will be considered more carefully in the section on chromatic aberration. Traditionally, the reference wavelength used in the UK has been the helium 'd' line, at 587.56nm, giving a refractive index of nd. Others have used the mercury 'e' line, a slightly shorter wavelength of 546.07nm giving slightly higher values of refractive index, described as ne. Table 1 shows the difference between the refractive indices of standard lens materials when they are measured with the two common reference wavelengths.

When light hits a lens surface obliquely it is slowed, and undergoes a change in direction. The higher the refractive index, the more a lens material will 'bend' the light hitting its surface. At a flat surface, the amount of bending, or refraction, is given by Snell's law: