Direct Surfacing... Is It Really Better?

By Steele Young
So we’re a couple of years into this revolutionary new way of processing lenses and the marketing side of our industry has managed to confuse a lot of folks about what it is, what it means, and what it’s worth. Potentially, the biggest concern with the confusion in the marketplace is “who” has become confused. Your customers, the ECPs are confused. But can you blame them? Just consider the many different terms being thrown around…. Freeform, Direct Surfacing, High-Definition, Digital Surfacing, Digitally Enhanced, cut-to-polish, and All-Format are just a few terms (most are even trade marked) that attempt to describe the newest lens processing technology that is being delivered to the industry. Compound the confusion with the marketing from the lens designers and manufacturers and you end up with a bunch of blank stares from customers who don’t know what it means to them.

This article will simply explain the new processing method (direst surfacing) and compare it to the way we have been making lenses (conventional processing). We will avoid the hype, steer away from buzz words, and cut to the chase. We will describe the differences between the new way, and the old way, and show objective measurements of both. This is not an article about free-form. It’s about surfacing lenses.

Fundamentally, in order to produce optics of any kind, we must first machine the geometry of the optic and then polish the surfaces so that the majority of incident light transmits and refracts through the lens. The primary difference in 2008 processing technology is that we can now build machines to process these lenses better because of the unbelievable speed of today’s CNC controllers, the phenomenal tolerances of new encoders, and unlimited calculation power in today’s computers.

But let’s be more specific and break down the differences between “Conventional Processing” (the old way), and “Direct Surfacing” (the new way).

Conventional Processing Steps

1. Generate the rough Geometry using a milling technique.

2. Fine Grinding the surface to final “Form” and ready the surface roughness “Ra” for polishing

3. Polishing the surface

Direct Surfacing Steps

1. Generate the rough Geometry using a milling technique.

2. Turning (known more commonly as lathing) the surface to final “Form” and ready the surface roughness “Ra” for polish

3. Polishing the surface

The three steps accomplish the same objective in both methods, but it is the second step of Conventional Processing that has been limiting us since Benjamin Franklin ground his first pair of lenses. Until now, we had to rely on the physics associated with sand paper stuck on one of 5000 or so pre-shaped 3” round molds (lovingly known has the “lapping tool”) which hopefully was a spherical, or toric representation of the patient’s power requirement. We simply could not build a machine to do what the sacred lapping tools did. Not a machine that was capable, affordable, and practical. Welcome to the 21st Century!

Now that we do... how does it really compare?

For arguments sake, let’s make the statement that Generating (Step 1) is done exactly the same way in each of our two processing methods. This step has progressively improved in form accuracy, surface quality, and productivity year after year and has been delivered into our industry on a continuous basis. Let’s begin our direct comparison at Step 2.

Conventional Process = Fining

Stock Removal – 200 - 300µ

Surface Geometry shaped by grinding on a pre-shaped mold (lapping tool)

Direct Surfacing = Turning

Stock Removal – 100µ

Surface Geometry CNC Controlled Results of Surface Structure or “Form”

Conventional Process = Fining

Form Deviation +/- 5µm from desired (red line) Range = 10µm

Direct Surfacing = Turning

Form Deviation less than 1µm

Summary

When it comes to lens form or curvature after conventional fining, we know that the lens takes on the general shape of the lapping tool with a measurable range of deviation of 10µm across the surface. This deviation is usually the cause for those occasional grey areas, off-axis, and power errors after conventional polishing.

What’s not shown here is the statistical deviation that occurs from lens-to-lens during the 90 seconds of grinding. We know that in a well controlled lab, stock removal varies from lens to lens from 0.1mm to 0.3mm. This deviation is related to many things such as surface curvature, diameter, and lens material. We are controlling the final outcome by estimating the correct time and pressure on a cylinder machine. This is a non-deterministic process based on historical experience. In the end, this process induces a minimum of a 300 percent deviation of results into your production process. Make no mistake about it - fining is the “red-headed step-child” responsible for the majority of your surfacing errors.

Results of Surface Roughness or “Ra”

Conventional Process = Fining

Surface Roughness = 0.4µm

Direct Surfacing = Turning

Surface Roughness = 0.05µm

These graphs are set to different scales, otherwise the “lack-of-roughness” of the turned surface from a natural diamond turning tip would not show up as well. We can now machine a lens smoother than we can mechanically fine it with P1200 sand paper and water; and smoother by a factor of 40 employing single tip turning.

Conventional Process = Polishing

Stock Removal = 30µm

Cycle Time = 3 – 5 min

Direct Surfacing = Soft Tool Polishing

Stock Removal = 12-15µm

Cycle Time = 1–3 min

Description of the Polishing Process Differences

In conventional processing we commit a “hard” polish where we use a fair amount of force to achieve higher overall stock removal during polishing. This is required to overcome residual form deviation and surface roughness from the fining operation. Essentially, we are still machining the lens to some extent during conventional polishing.

In Direct Surfacing, we polish only to improve final surface smoothness. There is no altering of a near geometrically perfect surface.

Here are two very relevant bottom line questions:

1. Does direct surfacing produce a better lens?

2. Is direct surfacing a better method of surfacing a lens?

Quite simply put, “Absolutely yes.” It’s not just a little better; it is substantially better. It’s measurably better. We can now make a finished lens to tolerances never before seen. Ophthalmic lenses can now be processed at tolerance levels that are closer to Precision Optics than they are to ANSI standards. The days of chasing a 0.25D power error should be history considering the lens can be accurately cut down to 0.01D. A thickness tolerance of 0.4mm should never even be approached with a process that holds a thickness to within 0.01mm. Consider the departure date of your “Red-Headed Step-Child.” Our future lies in direct surfacing!

Freeform High-Definition Digitally Enhanced and Optimized Spectacle Lenses So all of this discussion is irrelevant to the free-form hype. The big future financial benefit will come from the fact that you’re capable of machining an exact shape or design on the back surface of the lens, in your lab. This is where the lens designers, lens manufacturers, and marketing gurus get involved, as they should. This is where our future ability to make a couple of bucks on a pair of lenses will come from. But that’s a different article. The point of this article…whatever you end up calling your super duper high definition digital product, the manufacturing process that you will use to make that lens is measurably and remarkably superior to the conventional method.


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Labtalk September/October 2018