Modern spectacle lens design.

Ophthalmic lens design concerns the control of spectacle lens aberrations which occur when the eye rotates away from the optical centre of the lens. The most significant aberrations are oblique astigmatism and mean oblique error (power error). A brief review of these aberrations is given, explaining how the lens designer can control them using just the bending of the lens, and what results can be achieved using simple spherical and toroidal surfaces. Before 1985, aspherical surfaces were used only for post-cataract spectacle lenses and high-power magnifiers. Today, aspherical surfaces are used by all major lens manufacturers to produce thinner, lighter and more attractive best-form lenses in the normal power range. Aspherical surfaces are employed because the surface itself is astigmatic and the surface astigmatism is used to combat aberrational astigmatism due to oblique incidence. The various types of aspherical surface and how the surface astigmatism arises is described, before considering how this feature is used to produce flatter, thinner lenses. In the case of astigmatic prescriptions, the surface requires different asphericities along its principal meridians and the geometry of these atoroidal surfaces is also described. The advent of free-form manufacturing techniques requires the lens designer to convert the surface description to the (x,y,z) co-ordinates needed to generate the surface. Examples of how these co-ordinates can be obtained from the equation to the surface are given for toroidal and aspherical surfaces. In the case of free-form progressive surfaces, the pre-determined z-co-ordinates must be added to the z-co-ordinates of the prescription surface to obtain the final free-form surface. In the case of optimised prescription surfaces, on-board software will analyse the result by ray tracing to obtain the final z-co-ordinates.