Essentials of Ophthalmic Lens Finishing

476 Pages·2003·14.991 MB·English

by  Clifford W. Brooks, OD (Auth.)

Acknowledgments
Iwould like to thank Ginger Long, Antonio Turner, each chapter. When he discovered deficiencies, we
and Glenn Herringshaw, who have helped consid- worked together to figure out how to overcome them.
erably in the preparation of the second edition A number of procedures outlined in the text were a
of this book. They also have been a valuable part of result of his suggestions. Glenn helped in photography
both the operation of and education at the optical and also carefully examined each drawing and pointed
laboratory at Indiana University. out needed improvements. When something did
Ginger Long reviewed the manuscript while it was not seem right, Glenn was the first one I would call to
used in the course on lense finishing. She was helpful discuss the issue. He is optically knowledgeable and
in pointing out mistakes and ambiguities in the equally skilled in the practicalities of optical lens
questions at the back of each chapter. Students were finishing.
assigned problems and Ginger graded them. (They did Dan Torgersen, OLA (Optical Laboratories Associa-
not have the answers now found in the back of the tion) Technical Director, was a valuable resource for
book!) Ginger also helped with many of the photos and questions on safety and impact resistance issues. His
made suggestions on how to convey certain ideas. responses were always thorough and well thought out.
Thanks to Antonio Turner for the help he provided I also appreciate Joe Bruneni’s help in answering a
with the photographs in Chapter 13, Drilled, Slotted, spectrum of questions on a number of topics. Joe is a
and Notched Mountings. He supplied a number of valuable resource for the whole optical industry and
beneficial ideas and his hands are the hands seen in a much appreciated by all.
majority of the photos in that chapter. And again, as with the first edition, I would like
I am especially grateful to Glenn Herringshaw, to thank my wife, Vickie, and my children, Debbie,
Indiana University’s Optical Laboratory Manager, for Cliff, Abigail, and Kenneth, for their support and
the valuable help he has given. Glenn carefully reviewed encouragement.
vii
To Him who by wisdom founded the earth;
And by understanding established the heavens.
Proverbs 3:19
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ESSENTIALS OF OPHTHALMIC LENS FINISHING 0-7506-7213-7
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An Overview of
11
the Fabrication
Process
The Optical Laboratory
When someone needs glasses, the first requirement is
an eye examination in the doctor’s office to determine
the correct lens prescription. The next step takes place
in the optical dispensary, where a frame is chosen. The
frame should be one that is cosmetically pleasing and
appropriate for the type and power of lenses needed. In
the dispensary, measurements are taken to ensure the
lens will be correctly placed for the location of the eyes,
the style of lens chosen, and occupation or avocation of
the wearer. Once these are completed, the frame and
lens requirements are sent to the optical laboratory.
The optical laboratory may be located in close
proximity to the dispensary or halfway across the
county. All operations of the optical laboratory may be
carried out in one facility, or they may be divided
among laboratories. Traditionally the optical laboratory
consists of two main areas, a surfacing laboratory and a
finishing laboratory.
SURFACING AND FINISHING
LABORATORIES
As stated previously, an optical laboratory may consist
of two separate areas. One area creates the needed lens
power, usually by a process called lens surfacing, which
is performed at a facility referred to as a surfacing
laboratory.
The second area takes the correctly powered lens
and finishes it. Finishing is accomplished through
optical positioning of the lens and grinding of the
edges so that the lens fits the shape of the chosen
1
2 CHAPTER 1 AN OVERVIEW OF THE FABRICATION PROCESS
frame. The area where this occurs is known as the second surface to the required power. A lens with only
finishing laboratory. A finishing laboratory is also one of the two surfaces finished is called a semifinished
referred to as an edging laboratory because here the lens because it is only half finished. The prefix semi-
lenses are “edged” to the proper shape to fit the means half (Figure 1-2, B).
spectacle frame. Finished uncut and semifinished lenses have not
A great deal more happens in the lens finishing been edged. Before a lens has been edged it is called a
process than just lens edging. This text focuses lens blank.
primarily on the finishing aspect of lens fabrication.
An edging laboratory does not require a surfacing
SEGMENTED MULTIFOCAL LENSES
laboratory to function. Most facilities that have a
surfacing laboratory also have a finishing laboratory. Segmented multifocal lenses have more than one power.
However, the finishing laboratory has versatility. It may Each power is located in a distinct area of the lens
be associated closely with a surfacing laboratory, or it bordered clearly by a visible demarcation line. When
may function independently. Figure 1-1 is an overview two different areas exist, the lens is called a bifocal
of how lenses are processed in the optical laboratory. (Figure 1-3, A). When three areas exist, the lens is
called a trifocal (Figure 1-3, B).
Multifocal lenses may be made ready for the
finishing laboratory in one of several ways:
Finished and Semifinished Lens
• Multifocals may be individually ground and polished
Terminology
to power by a surfacing laboratory from a semi-
finished lens blank.
Ophthalmic lenses may be divided into the following
• Multifocals may be individually cast molded to the
three broad categories:
prescribed power, instead of being surfaced from a
• Single vision lenses semifinished lens blank. Cast molding creates the
• Segmented multifocal lenses lens from a liquid resin material. It is the same
• Progressive addition lenses process used to make both plastic semifinished lenses
and stock single vision plastic lenses. Cast molding
multifocal lenses to power skips the semifinished
SINGLE VISION LENSES lens stage. Cast molding to power may be done by a
larger wholesale facility or, if equipment is available,
Single vision lensesare the most basic type of lens. These
in conjunction with a finishing laboratory (Figure 1-4).
lenses have the same power over the entire surface of
• Multifocals may be made by laminating front and
the lens. Single vision lenses are used when the same
back lens halves together.In simple terms, the front
optical power is needed for both distance and near
half of the lens contains the multifocal segment and
vision. They also are used when a person requires no
the back half contains the power. The two are glued
prescription for distance but needs reading glasses.
together to make one complete lens. This lamina-
Whenever possible, single vision lenses are edged from
tion process also skips the semifinished lens stage. It
lenses kept in stock at the finishing laboratory. Because
does not require a lot of equipment and, like cast
these lenses are finished optically to the correct power
molding, may be carried out at the finishing
on both the front and back surfaces, they are called
laboratory (Figure 1-5).
finished lenses. Finished lenses are also referred to as
• Multifocals may be kept as stock, finished bifocalsin
uncuts because they have not yet been “cut” to the
the finishing laboratory. This option is possible only
correct shape and size (Figure 1-2, A). When single
if the lens power is spherical. At present, stock,
vision lenses are in uncut form and do not require
finished bifocals are not often used.1
surfacing, they are called stock single vision lenses.
The finishing laboratory personnel would much
prefer to use a stock single vision uncut lens because it 1Instead of being custom made, stock, finished bifocals may be mass
is less expensive than a custom surfaced lens. However, produced with both inside and outside surfaces already ground and
if the stock lens is too small for the frame, then a stock polished. Stock, finished bifocals are normally used only when both
left and right eyes are spherical in power. Attempting to match one
single vision lens will not work. Instead the lens must
spherocylinder lens from a surfacing laboratory with a prefinished
be produced in the surfacing laboratory. The surfacing bifocal from another source may create unnecessary problems and
laboratory starts with a lens having only one surface mismatches. To prevent optical errors, the exact vertical position of
the optical center of the stock, finished bifocal in relationship to the
that is ready to use, or “finished.” This is usually the
near segment must be known. This position must be duplicated
front surface. The laboratory must grind and polish the accurately in the custom-surfaced, paired spherocylinder lens.
CHAPTER 1 AN OVERVIEW OF THE FABRICATION PROCESS 3
LENS PROCESSING SEQUENCE
ORDER ENTRY
Tray up order
Determine required blank
size
Pull lens blanks from stock or
order lens blanks
Semifinished Finished Blanks
Blanks
SURFACING EDGING
Determine correct base curve Spot lenses
Determine needed center or Trace the frame for patternless
edge thickness edging, or, for patterned
edging, pull pattern from
Determine lap tools required pattern stock or make a
pattern
Lay out lenses for surfacing
Perform centration for blocking
Block for surfacing
Block the lenses
Crib lenses—i.e., reduce
blank diameter (optional) Edge the lens
Perform the following: Hand-edge (unless already
done in the edger)
Surface generating
Fining For CR-39,
Polycarbonate, or High-
Polishing For Glass Lenses Index Plastic Lenses
Hardening Edge grooving for nylon
Deblocking cord mounting, or
AR coating, color, or drilling for rimless
Inspection for correct power, mirror coating
surface quality, and other (optional) Tinting (optional)
imperfections
Impact-resistance AR coating, color, or
testing mirror coating (optional)
Engraving
Lens insertion
Standard alignment
Verification and inspection
Cleanup
FIGURE 1-1 The processes listed on the right-hand side in the main column sequence
may be performed in the finishing laboratory. The processes in the left-hand “loop” are
functions of the surfacing laboratory.
4 CHAPTER 1 AN OVERVIEW OF THE FABRICATION PROCESS
Finished Finished Not yet
surfaces surface finished
Finished lens Semifinished
A (an “uncut”) blank B
FIGURE 1-2 A, A finished lens is also referred to as an uncut. Most single vision lenses
are premanufactured to power as finished lenses and are also referred to as stock single
vision lenses. B, Most any type of lens of any material may be made beginning with a
semifinished lens. (From Brooks CW: Understanding lens surfacing, Boston, 1992, Butterworth-
Heinemann, p 17.)
A B
FIGURE 1-3 When a lens has a different power for near vision than distance vision, the
lens area is divided between distance and near powers. A,A segment area for near vision
is placed within the distance power lens. A lens with two different powers is a bifocal lens.
B,Two segment areas are included: one for intermediate viewing and one for near viewing.
This type of lens is a trifocal lens. Both lenses are flat-top–style multifocals.
PROGRESSIVE ADDITION LENSES Overview of the Lens Finishing
Progressive addition lenses are used as an alternative to a Process
segmented multifocal lens. They have distance power in
the upper half of the lens. Lens power gradually increases Edging often is used to denote the entire lens finishing
as the wearer looks down and inward to view near objects. process. In actuality, many steps come before and after
Progressive addition lenses are prepared for the the actual edging process. These are outlined in the
finishing laboratory in the same way as segmented following section and described in more detail in later
lenses. These are listed in the previous section. chapters.
CHAPTER 1 AN OVERVIEW OF THE FABRICATION PROCESS 5
FIGURE 1-4 Cast molding of lenses is done with a front and back mold placed in a
gasket to hold the surface molds. The front mold is shown on the left and the back mold
on the right. The gasket is at the top. For multifocal lenses, the back mold is rotated so that
the prescribed cylinder axis will be correct. Once assembled, liquid resin is poured into the
molds to form the lens. The front surface mold may contain a near add power area. This
area of different surface curvature turns the lens into a multifocal such as a bifocal or
progressive addition lens. The back surface includes cylinder power for astigmatism, when
needed.
SELECTION OF THE MOST APPROPRIATE
LENS BLANK
Within limitations set by the lens order, a laboratory has
the responsibility of choosing a lens blank that gives
the best cosmetic and optical results. This is especially
important for plus lenses. An inappropriately large plus
lens blank creates unneeded lens center and edge
thickness. This is explained in more detail in Chapter 2.
SPOTTING THE LENS
A lens-measuring device is needed to determine lens
power, optical center location, and other optical
characteristics of a lens. It may be referred to as a
lensometer, lensmeter, focimeter, vertometer, or lens analyzer,
depending upon the manufacturer. In this text the FIGURE 1-5 Another alternative to the lens surfacing
author uses the more generic term lensmeter. The process is to create the needed power using a front and a
lensmeter precisely determines lens power and exactly back half. The front half contains the near add power, when
locates optical points within the lens. The process of needed. The back half completes the distance power and
contains any needed cylinder power. The two halves are glued
finding these optical points, orienting the lens properly
together to create the lens. (From Brooks CW: Understanding
to meet the needs of the prescription, and then placing
lens surfacing, Boston, 1992, Butterworth-Heinemann,
dots on the lens is referred to as spotting. It takes its
p299.)
name from the three horizontally aligned ink “spots”
6 CHAPTER 1 AN OVERVIEW OF THE FABRICATION PROCESS
placed on the lens surface by the lensmeter. See the frame, several other possible procedures may be
Chapter 2 for a complete discussion of spotting. carried out. If the lens is ordered with a specific color
and is plastic, it may be tinted. Tinting may be
performed with hot dyes or by use of a vacuum coating
CENTRATION AND BLOCKING OF THE
process. Depending on ever-changing fashion, a lens
LENS
could be decorated with an engraving.
Because the pupil of the eye is seldom found to be If the lens is glass, it must be treated to increase
directly on line with the middle of the frame’s lens impact resistance. This hardening process may be done
opening, the lens must be moved to correspond to the by heat treating or chemical tempering the lens. Both glass
location of the eye. Because the lens is to be centered and plastic lenses may be antireflection (AR) coated to
in front of the eye, this next process is called centration. reduce surface reflections.
Centration is done by use of the three reference dots
that were placed on the lens during spotting. Once the
lens has been positioned, a small block is secured to
LENS INSERTION OR MOUNTING
the lens so that it may be edged. Securing the block
to the lens is called blocking. The edge of the lens may be angled to a point to allow
Taken together, spotting and centration make up insertion into a frame with a groove. Lens edges that are
lens layout. The instrument used to center and block the angled to a point are called beveled lenses. The process
lens is called a layout blockeror simply a blocker. of putting beveled lenses in a frame is lensinsertion.
Alternatively a lens may be held in the frame with
a nylon cord, with screws, or by methods closely
DETERMINATION OF LENS SHAPE
related to these. If a nylon cord is used, the edge of the
Before a lens may be cut to the proper shape for the lens must be grooved to accept the nylon cord. Some
frame, the needed shape must be quantified. Two types edgers groove the lens during the edging process. If
of edgers exist—one that uses an actual plastic pattern the lens is not grooved in the edger, the edge is made
to guide it in shaping the lens and another that uses an flat. Then a groove is cut into the edge on a separate
electronic method to supply lens shape. Following are lens groover.
more detailed explanations of both types: If a lens is to be held in place with screws, the edge
is made flat without a bevel. Holes then are drilled in
• The first is called a patterned edger. A patterned edger
the lens to accept the screws. When lenses are grooved
operates using a small, flat piece of plastic that
or drilled, the process of placing them in the frame is
matches the lens shape needed for the frame. This
called mounting. (Frames that secure the lenses in place
pattern itself is either supplied by the frame manu-
in this manner often are called mountings instead of
facturer or is made by the laboratory using a pattern
frames.)
maker.
• The second type of edger is called a patternless edger. It
does not use a physical pattern but uses an electronic
shape generated from the frame itself by way of a STANDARD ALIGNMENT
frame tracer. The tracer makes an electronic version
After the lenses have been placed in the frame, the
of the shape needed and downloads it to the edger.
glasses may not be aligned properly. The process by
Some edgers operate in both patterned and which the glasses are bent or readjusted to conform to
patternless modes. a proper alignment is known as standard alignment or
truing.
EDGING THE LENS
The blocked lens is now placed in the edger and the
VERIFICATION AND CLEANUP
lens is edged to shape.
Before a prescription is released to the dispensary, it
needs to be verified for accuracy. This is done using the
DEBLOCKING, TINTING, COATING,
same instrument as was used during lens spotting—the
ENGRAVING, AND HARDENING
lensmeter. The prescription must optically conform to
Once the lens is edged to the proper shape and size for accepted standards of tolerance. Once judged accept-
the frame, it is taken off the block. The process of block able, the frames and lenses are cleaned and passed on
removal is called deblocking. Before the lens is placed in to the dispensary.
CHAPTER 1 AN OVERVIEW OF THE FABRICATION PROCESS 7
Proficiency Test Questions
1. True or False? Lenses are surfaced in a finishing 6. A “frame tracer” is often used in conjunction with
laboratory. which of the following?
2. Which of the following lens types has the same a. Lensmeter
power over the entire lens? b. Lens blocker
c. Lens edger
a. A single vision lens
b. A segmented multifocal lens 7. Of the following steps in lens fabrication, which
c. A progressive addition lens process occurs last?
3. Which of the following terms is a synonym for a a. Blocking
“finished lens”? b. Grooving
c. Edging
a. Single vision lens d. Spotting
b. Semifinished lens
c. Uncut lens 8. Arrange the steps in the edging process in their
d. Progressive addition lens correct order.
e. Multifocal lens
1. blocking
4. True or False? It is possible to individually cast mold 2. centration
both segmented multifocal lenses and progressive 3. edging
addition lenses to the prescribed power without 4. finding lens axis and MRP location
surfacing the lens.
a. 2, 3, 1, 4
5. True or False? Some edgers operate in both b. 2, 4, 1, 3
patterned and patternless modes. c. 1, 2, 3, 4
d. 4, 2, 1, 3
e. 4, 3, 2, 1