Does anyone have trouble understanding all the new terms used to describe progressive lenses? I am referring to terms like; free-form, direct-to-surface, high definition, digitally computed, digital processing, digitally enhanced, and so on. I will try to blow away the clouds of confusion to give you a better understanding of what going digital is all about.
When you read about a lens that is made by some kind of digital processing, don’t assume that the lens is somehow better. It may or may not be. Digital suggests a higher level of precision. I would argue that all lenses created since the late eighties have been made using some form of digital processing. I think it is important not to get caught up in the digital hype. Digital lenses are often generically called free-form lenses.
The term free-form is often misunderstood. Many optical professionals think of free-form as the lenses they receive from the laboratory. They think that the lenses are superior because digital production has been used. A twenty five year old progressive design lens can be made using digital or free-form equipment. Does that make that older design lens better? Confused? Yes? So are a lot of other ECPs!
I want you to think of free-form as a manufacturing process, not as a lens. Think of free-form as direct surfacing. What that really means is that this technology allows lens designers great freedom for lens designs because they are not confined to using the traditional semi-finished lens blanks. The optical design of these new lenses can be customized and optimized to the fitting and prescription requirements of the patient. Direct surfacing allows the manufacturer to produce a lens that you can grind the front or back surface of to produce a highly accurate and customized finished product. The remainder of my discussion of lenses will be geared towards progressive lenses.
Traditional lens manufacturing involved using a semi-finished lens blank that was molded using glass molds. The progressive design, base curve, and add power were molded on the front surface of the lens using these molds. The laboratory would then grind the patient’s prescription into the back surface of the blank using a generator. The lens would then go to surface finers and polishers to complete the grinding process. This process did not allow for any customization of the progressive designs. Traditional surfacing has many limitations.
Free-form manufacturing can produce a highly customized and highly accurate finished lens. The starting point for free-form manufacturing starts with the highly sophisticated software. This software allows the input of prescription, frame fitting details, and position of wear to create a truly customized lens that is not possible with traditional lenses. This software sends data to the free-form generators in the form of point files. These point files are mathematical data files that guide the free-form generator. Lens manufacturers sell these point files to the laboratories. The point files are usually sold in batches, which allow the laboratories to produce the branded progressive lens of the manufacturer. The point files can only be used once since each is customized to the patients individual prescription needs.
The generating creates the prescription, add power, base curve, and corridor position of the finished lens. The generator uses a single point cutter to produce the lenses. This cutting can be done on both the front and back of a lens. Most free-form generators will have a second or even a third cutter that produces a very smooth surface. The lens is then finished on a specialized lens polisher that uses conformable tools. These conformable tools, or soft laps, have been developed to buff the surface of the lens without destroying the sophisticated lens surface. The polishing of these lenses has been a weak point in the free-form production chain. Equipment manufacturers have made big strides in this area. Again, think of free-form as a manufacturing process involving the design and fabrication of lenses, not the actual lens itself.
Let’s take another look at the word digital. The word digital means processing, operating on, storing, transmitting, representing, or displaying data in the form of numeric digits. There is no mention of optical lenses in that definition! We now know that lenses have been made using digital equipment for a long time. We also know that free-form is a process, not a lens. Let us examine the different ways a progressive lens can be fabricated using digital surfacing, or free-form processing.
Progressive lenses can be made with:
- Traditional front molded designs that have been made using molds that have been digitally manufactured.
- Traditional front molded design with digital backside surfacing.
- Front side spherical curves with the progressive design on the back of the lens.
- Progressive front surface with atoric/aspheric back surface.
- Add power distributed on the front and back surface
The word digital can be used to describe the way the mold was made, the way the lens was designed, the process used to generate the curves, and any number of real or imaginary ways. Remember that just because a lens is made using digital surfacing, or it was digitally surfaced, or was made using free-form technology, that it does not automatically mean that the lens is somehow better.
I think that the world of free-form manufacturing has great potential. The accuracy and customization capabilities are amazing. The finished product is only as good as the manufacturing process. The process cannot improve a poorly designed lens. It is our job to study the available technology and make an informed decision. There are some very good lens designs that maximize the benefits of digital surfacing. Contact the lens manufacturers and your laboratories to discuss free-form technology. Educate yourself so you can educate your patients. Your patients deserve the best. Give it to them!
Bob Fesmire, ABOC