Understanding Free-Form
What you need to know about this state-of-the-art technology and how it's changing the way spectacle lenses are made
By Joseph L. Bruneni
Photography by Peter Baker

Most progressive lenses are made of plastic, with the most critical components in a progressive found on the front surface. When casting plastic progressive semi-finished blanks, it is the front-side glass mold that creates the smooth, constantly changing progressive curves that change add power in a progressive channel. The most complex operation in manufacturing PALs is creating those front-surface glass molds. A special type of generator had to be developed to cut the complex computer-generated curves required to produce the ever-changing progressive curves. This new generation process was called "free-form."

Traditional generators use ring-like diamond wheels to form the ophthalmic surfaces found in conventional lenses. Free-form generators, however, use a precise diamond tool that cuts curves with a single point, much like those found in contact lens lathes. With the right kind of computer software, a single-point generator can reproduce virtually any complex multi-curve surface.

Free-form generators made it possible to produce the front-side glass molds required for today's complex progressive designs. Polishing these surfaces presented an equally complex problem for lens manufacturers, but "point" polishers were developed and used in tandem with free-form generators.

Manufacturers have been using free-form generating to produce glass molds for their progressive lenses for some time, but it was always a slow, tedious process and totally unsuited for producing custom progressives in a laboratory. During the past five years, however, there have been important advancements in free-form technology.

A basic problem in generating with a diamond wheel tool (as current lens generators do) is that the final curves are not precisely true because of something called "elliptical error." One of the primary functions of the traditional fining operation before polishing is to remove that small, unwanted amount of elliptical error.

When the lens surface is cut with a "point" tool, however, no elliptical error is produced. For this reason, lenses surfaced with free-form equipment require no laps or fining following creation of the new surface.

Free-form generation is a very different way of producing prescription lenses. There has been a steady succession of better technology for making lenses during the past 50 years, dating back to the development of lens generators which replaced the crude hand pan surfacing that had been used for centuries.

What makes the development of free-form totally different is this: Many previous improvements simply made lenses a little faster or made the curves more accurate or reduced spoilage for the lab--but, essentially, they were producing the same basic lenses. Free-form equipment, however, has the capacity to produce lenses with accuracy down to .05 diopters. This is a considerable improvement over current technology used in labs, which is accurate to +/- 0.12 diopters.

Free-form offers another exciting difference. It enables the lab or manufacturer to produce lens forms that were never possible in the past.

Following are examples of what is now possible.

Improvement No. 1: During recent years, several major progressive lens manufacturers have introduced advanced progressives that use the patient's personal data to create a personalized progressive designed specifically for each patient.

Personalized PALs are expensive, but they have proven to provide enhanced benefits to the wearer. The only problem has been that the lenses could only be produced in the manufacturers' factory labs, usually located in countries outside the United States.

What makes it possible for factory labs to produce personalized PALs is that they are equipped with free-form generators and polishers--equipment no U.S. laboratory has had before now. The equipment is expensive and requires sophisticated software programs only available to the lens manufacturer.

Improvement No. 2: Recently, several progressive manufacturers have introduced another major innovation in progressives. These lenses differ from traditional PALs in that the progressive curves are on the backside of the lens. The curves in the distance portion of the lens are atoric. Moving progressive curves to the backside is claimed by the manufacturers who utilize this approach to broaden the field of view for distance, near, and intermediate zones, as well as other subtle improvements.

Improvement No. 3: A few years ago, several lens manufacturers introduced single vision stock lenses that featured atoric curves on the backside of the blanks. An atoric lens offers reported advantages over aspheric lenses, but requires free-form generation to produce. Labs do all surfacing on the backside of a lens and have not had free-form equipment. As a result, these advanced atoric lenses were only available in factory-produced stock lens form. Backside atoric surfaces can, however, be readily produced with free-form equipment.

Improvement No. 4: One of the most recent advancements in progressive design features some of the progressive curves on the front side of the lens and the balance of the progressive curves positioned on the backside. Dividing progressive curves in this way is reported to offer distinct advantages. Only free-form generation makes such a lens possible (more on this later).

ASPHERICS TO ATORICS

Let's consider aspheric lenses--those modern lenses that were introduced some 15 years ago. Aspheric lenses marked visual improvements over conventional lenses. They also enhanced the cosmetics of lenses with their thinner and flatter shapes that were also lighter in weight. Their introduction added a marked improvement for ophthalmic lenses, and many new lens lines introduced in the last few years have been all-aspheric.

Aspheric designs help to improve optics by reducing the oblique astigmatism that is created as wearers look away from the optical center of a lens. Replacing the normally spherical front surface with an aspheric surface also made the lens flatter, producing thinner lens edges for minus powers.

Even though aspherics were a major improvement, these lenses still represented somewhat of a compromise, particularly when patients wore cylinder corrections. The following explanation will make this clearer.

Assume an Rx calls for a two-diopter cylinder and requires a back surface that is -4.00D by -6.00D. The aspheric curve on the front side will be optimized for a curve somewhere between four and six diopters. Even when the Rx is a sphere, the aspheric curves selected on the front of a semi-finished blank must be averaged for the wide range of curves that the lab may grind on the backside. The lens has improved optics, but they are not as good as they could be.

What was the answer? Atorics. These were first introduced as stock lenses in a 1.66 index and designed primarily for high-minus corrections. Atoric stock lenses were advertised primarily on how they thinned the edges on high-minus lenses. Moving aspheric surfaces to the back does create a remarkable thinning of the edges of high-minus lenses, but there is also a decided improvement in acuity of the lenses.

Let's consider the factors that make that possible.

Front-side aspheric curves. When aspheric curves are on the front side, the aspheric curve is averaged for a wide range of curves that could be used on the back surface. With a cylinder Rx, one meridian might be reasonably improved, while the other one would be less so. Though better than conventional lenses, aspheric lenses do represent a compromise.

Backside aspheric curves. When the aspheric curves are moved to the backside of the lens, each meridian of the cylinder is totally aspherized. In the case of the earlier example, the 4.00D curve and 6.00D curve are each aspherized specifically to the curve used on the front surface of the lens. As a result, atoric lenses produce improved visual acuity.

With cylinder lenses, each meridian is specifically aspherized for the power of that meridian. Even spheres benefit because the back surface has been aspherized specifically to the base curve on the front. Two companies currently produce atoric stock lenses that can be ordered in 1.66 index and polycarbonate (minus powers only). Until now, labs have not had the means to surface or polish aspheric or atoric surfaces.

THE ULTIMATE LENS

In terms of acuity, it's safe to say that atoric lenses produce the most accurate vision over the widest area of the lens. This is why the sophisticated new personalized progressives are atoric, with each meridian of the back surface aspherized to match the base curve (front surface). Moving aspheric curves to the back surface enhances the optics of any lens.

The progressive lens market has become intensely competitive. And, lens manufacturers recognize that aspherizing back curves can produce a noticeable improvement in acuity.

Some manufacturers take things one step further and also move the progressive curves (which are made up of aspheric curves) to the backside as well.

The only atoric lenses labs have been able to offer their accounts were single vision for patients whose correction fell within a given stock lens range. The range of minus powers available in atoric lenses is extensive. Factory-produced stock lenses work well for single vision, but what about multifocals?

Atoric lenses may soon come into their own now that it is possible for labs to generate and polish atoric surfaces. Over time, this is expected to have a major impact on ophthalmic lenses.

Several U.S. labs have installed and are using free-form equipment, and it looks like the genie is out of the bottle. A smaller footprint than earlier free-form equipment, it enables labs to produce atoric progressive lenses as well as single vision atorics. This will open up a new lens market for laboratories and eyecare professionals.

FREE-FORM EQUIPMENT

Generators capable of cutting aspheric, atoric, or progressive surfaces (the new term is "free-form" generating) are expensive. The generator is totally computer-operated, as cutting the complicated curves required to change power in a progressive involves complicated calculations only a computer can produce.

The problem is that the computer has to have instructions on how to do the calculations for each lens being produced. This brings up another term the industry is learning: Point files.

At least one major lens manufacturer currently sells highly complex point files that instruct and guide free-form generators in surfacing a brand name progressive lens. Point files are only used once since each file must be
programmed to the patient's precise prescription. Labs purchase point files in batches, and the purchase gives the lab the right to sell the lens as a brand name customized progressive. This all sounds futuristic, but it is happening right now.

Other progressive manufacturers are expected ultimately to offer point files to labs to produce their brand name progressive designs, and at least one has stated it plans to do just that.

Free-form equipment represents a major investment for laboratories, but the possibility of creating state-of-the-art lenses producing the ultimate in visual acuity will provide eyecare professionals and their patients some remarkably advanced lenses.

Yes, the lenses produced with this equipment will be expensive, but the optical industry has learned one thing during recent years: There are many consumers who want the "best," whether it's clothing, automobiles, or eyeglasses. They look for value, but will pay for the best once they understand what the best will do for them.

Backside atorics produce discernable benefits to the patient and certainly fall into that important category.