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Understanding
Digital Lenses |
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:
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Traditional front molded designs that have been made
using molds that have been digitally manufactured.
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Traditional front molded design with digital backside
surfacing.
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Front side spherical curves with the progressive
design on the back of the lens.
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Progressive front surface with atoric/aspheric back
surface.
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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!
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