How important is backside and digital surfacing in modern lens manufacturing? .
asks Consultant Editor Richard Chaffin (article printed at MAFO/Optical Labs&Industry)
Lenses have a front and back. They are more properly referred to as the convex and concave. The outsid or front side of -the lens is convex. The concave is the inside of the lens, or backside. Historically, both sides of the lens are important and the curves that are ground on them have moved from front to back and vice versa.
At first, lenses were any piece of glass that was thick in the middle and thin on the edge. Soon, however, lenses were ground to different powers. This was the first instance where the backside made a difference. All the correction plus or minus was essentially on the concave or backside that may or may not have been concave (convex/convex). The front side on these lenses was a lmost flat or one and a half dioptres convex. These lenses were ground to simple spherical curves.
As lenses became factory mass-produced, the front side of the lens became the important side. It was ground with the compound curves. There were semi-finished cylinder lenses that were mass-produced. The laboratory could finish these lenses to prescription by grinding the concave side with a spherical curve.
Corrective curve lenses
In the 1930's Dr. Tillyer at American Optical, and Zeiss in Germany, promoted the idea of corrected curve lenses. The idea was that certain prescriptions should be ground on a series of front or base curves. Again, the front or convex surface of factory finished lenses had one or two curves on the front of the lens.
The backside had only to be finished with a single spherical curve. However, as fused bifocal lenses became popular, the laboratory was required to put the astigmatic and spherical curve on the backside. Laboratories were equipped with tools for grinding the backside. In addition, lenses for examining eyesight, trial sets, were made with socalled minus cylinder lenses to go in the phoropter.
What curves are ground on which side of the lens has gone back and forth with the evolution of lens manufacturing and technology. The question of where to place the curves on the lens front or back may have some optical significance: as light travels through the lens, how it is influenced by the vertex power and index of refraction. All of these factors may be trumped by economics and practicality.
Semi-finished spherical blanks are the cheapest lens raw material for the laboratory. If the laboratory is equipped to put a progressive surface and cylinder on the concave (backside) there is a greater profit margin.
During the 1960s when American Optical was the leading lens manufacturer in the world and glass lenses were 90 per cent of the lens market, minus cylinders were sug gested. John K. Davis, a well-known scientist at American Optical, advocated grinding the cylindrical correction on the concave side of the lens as better optics. From a manufac turing perspective factory mass-produced glass lenses had been made as plus cylinder (cylinder on the convex) for 50 years or more.
Growth of plastic lenses
New processes and machinery were required for factories to make minus cylinder lenses. However, the laboratory was already equipped and grinding minus cylinders on bifocals and single vision prescriptions. A further impetus for the backside was the growth of plastic lenss. Plastic lenses were cast from glass moulds that had the cylinder on the convex surface, resulting in a minus cylinder plastic lens. Plastic lenses having taken over a significant part of the lens market has promoted minus cylinders.
Progressive lenses are another factor. They have been made with the progressive surface on the convex side dictating all the finishing grinding being done on the concave. Mass produced progressive lenses are manufactured using two basic processes. One is the Essilor process that grinds and polishes the progressive on a glass surface and the other an American Optical method, a slumping process.
In the case of the slumping process, a simple spherical surface is ground on to the convex surface of a semi-finished blank. The semifinished spher.e. is placed in an oven on a ceramic form that has the progressive design. Plastic progressive lenses a re cast from glass moulds that have the progressive design on the concave side. Until now the progressive surface has a lways been on the convex side of the lens, whether glass or plastic.
Recently a change has come about. The laboratory can now put a progressive surface on the backside of a lens. The computer and advances in generating and polishing machinery for surfacing in the 1aboratory have made it possible. Now any laboratory that can invest in machinery and a computer program can make its own progressive lens on either the convex or concave side.
The backside progressive lens may be optically advantageous. The advantage may depend on whether or not the wearer is myopic or hyperopic and the design princi pals of the progressive. A sophisticated computer program can theoretica lly accom modate the grinding of either surface for optimum optical results. Combining the progressive design and a ll the other curves required is not possible for a progressive lens with the progressive surface from the mass manufacturer on the convex side.
If the backside is the better side, why not have the progressive companies provide the progressive surface on the concave and the laboratory finish the prescription on the convex side. That could be done. The laboratory with the latest digital capa bility can grind either side of the lens equally well.
The question may then come down to the fact the laboratory's least expensive source for raw material is a semi-finished sphere that only needs to be ground on one side. For the present that one side would be the backside. Therefore the backside is important.
What then is the future for mass-produced progressive lenses? Digital surfacing in the laboratory means any laboratory can produce a progressive lens on either surface. The proper equipment and a computer program is all that is needed. The laboratory of the future will invest in equipment and a computer program, not progressive blanks. Wearers will have individually designed progressives with the best possible optics. A great new optical world!