Optical Lab Knowledge for the Non-Lab Optician.

Optical Lab Knowledge for the Non-Lab Optician.
Optical Lab Knowledge for the
Non-Lab Optician.
Dispensing opticians and ophthalmic lab technicians unfortunately rarely cross paths. For both,
this is unfortunate. Dispensing opticians who know what goes on behind the lab door not only
communicate with patients with more authority, it also helps you ask the right questions when calling
your lab for assistance. This helps prevents problems from arising both in the lab and on your
dispensing floor.
This chapter will discuss the three key processes a dispensing optician should to know about
what goes on in the lab. 1) Lens fabrication 2) Troubleshooting 3) Lens tinting tips. The purpose of this
is not to make you an expert lab technician, but rather to help you become better versed in
understanding what goes on behind the lab door.
Lens Fabrication
The lens fabrication process can be divided into two major steps, surfacing and finishing.
Understanding what takes place in each of these steps will help you to understand why certain jobs can
be done “in house” and some sent off site. It will also help you foresee challenges lab technicians may
face and help you communicate with them regarding these issues. Lab technicians are a wonderful
source for information and are typically eager to speak with you about an order. They would much
rather you speak with them at the time of placing an order than to have to call you after the fact which
will delay the job and threaten customer satisfaction. The following infographic will provide you with a
visual map of the lens fabrication process. This chapter will examine each of these steps in further
detail.
Surfacing
Surfacing is the process by which a prescription is placed onto a lens and preparing its surface
prior to being cut for the frame. Although large commercial ophthalmic labs do their own surfacing, this
is not a process typically done at the small in-office lab. There are 4 steps involved in surfacing. These
are 1) Blank selection 2) Generation 3) Polishing and Fining and 4) Hard coating.
Blank selection
A “blank” is a hockey puck size lens. There is no prescription within the “blank”. The
parameters preinstalled in the blank are only the front curvature (base curve), lens material such as
polycarbonate, and the overall diameter.
To know which blank to use for a given job, the lab technician inputs the patient’s prescription
and frame information into a computer program or uses a nomogram as seen below. Based on the
nomogram, the lab technician is provided information about the lens blank to be used. This will include
such things as the base curve and blank size needed. The base curve does not affect the final
prescription; however, it can affect the lens’ ability to fit well within a frame. For example, an extremely
wrapped frame will need a higher base curve in order to have the lens curve better match the frame
curve. Similarly, a very flat frame will require a lower base curve lens.
The base curve may also affect off-axis clarity depending on the prescription ground into it. For
these reasons, proper base curve selection is important both to ensure the lens will fit well in the fame
and to ensure the best possible optical outcome.
Simplified graphical example of base curve selection criteria. Notice how as prescription becomes more
plus (+) in power the necessary base curve increases.
Generating
Generation is the process by which the patient’s prescription is ground into the back surface of
the blank. The generator is a grinding instrument that etches the necessary curves into the back surface
of the blank to achieve the desired prescription outcome.
In order to prepare the blank for prescription generation, there must be something fused to the
blank to hold it in place in the generator. This is called a “block”, and the process of adding the block to
the blank is called “blocking”.
In order to add the “block”, the optical center of the blank must first be determined. The blank
is placed in a lensometer and the location of the optical center is found. This is necessary in order to
insure the lens is centered properly when the prescription is ground onto the lens. Tape is applied to
the front surface of the lens to insure it does not become scratched during the blocking and generation
process. The front surface of the blank (base curve) is not modified during the generation process.
Lens blank with block applied and ready for generator
The blank is placed in a blocker which uses heated wax to fuse the block to the blank. Once the
block is applied to the blank, it is placed in the generator which grinds the patient’s prescription into the
blank. Prescriptions are ground onto the back surface of the blank.
Fining and Polishing
When the lens is removed from the generator the back surface is rough, with a frosted
appearance, from the process of grinding the prescription onto the blank. This multistep process is
called “Fining and polishing”. The process involves placing the lenses into a Finer which uses a polishing
block called a lap. The first step in the finer involves the lens being polished by the lap in a circular
motion against an abrasive fining pad similar to sandpaper to polish out the roughest parts created by
the generator.
Typical fining pad
The second step uses a softer fining pad to remove further roughness. Step three polishes the
lens again with an even softer fining pad and a polishing solution made of Aluminum oxide to create the
crystal clear surface expected in the final product.
The block is detached and the protective tape applied prior to the process is removed by hand.
When detaching the block, a small hammer is sometimes used, but if it can be done by twisting the lens,
instead of a peeling motion of the block, there will be less chance of causing a lens defect.
Hard Coating
Once generated, scratch protection is applied to the lens. This process is called Hard Coating.
The process of generating and polishing the prescription onto the back surface of the lens blank will
leave the back surface of the lens soft and vulnerable to scratching. For this reason, a hard coat is
applied to back side of the lens at this time.
A hard coat is a scratch protectant layer to prevent the newly generated lens from becoming
scratched. The raw hard coat material is rather toxic, so must be handled with caution. Once applied to
the lens, the hard coat solution is cured on to the surface of the lens using ultraviolet light to turn the
liquid solution into a permanent hard scratch coating. Once cured, the coating is no longer toxic. And
the lens can he safely handled.
Finishing
Finishing is the process of edging and mounting the surfaced lens blank for the selected frame.
As opposed to surfacing, finishing is a process often done at in-office labs. The in-office lab will typically
order a pre surfaced lens from a large commercial lab and finish it themselves. Finish includes 1) Edging
2) Mounting 3) Inspection and 4) Dispensing.
Edging
Now that the blank has the patient’s prescription ground into its back surface and has been
polished, it is time to cut the blank to fit the selected frame using an Edger. An edger is a lathe, using a
diamond or ceramic grinding wheel, which cuts the blank to the shape of the frame. This process is
called “Edging”.
The lab technician will use either a pattern (if provided by the optician or frame manufacturer)
or a tracer to trace the frame shape directly. A pattern is a plastic disc the exact size and shape the lens
needs to be in order to fit the frame. This is usually provided by the frame manufacturer to the optical
shop that purchases the frame. If a pattern is not available, the lab technician places the frame into a
tracer, a machine that traces the shape of the frame. The tracer then electronically sends the shape
information to the edger to cut the lens into the appropriate shape.
Frame tracer
Once the shape of lens needed is determined, the surfaced blank is prepped to be cut by the
edger. Similar to the blocking step in lens generation, in edging the lens also must be blocked to give
the edger a way to hold the lens securely while being cut. However, unlike the hot wax needed to hold
the lens in a generator, the edging blocks are held in place by lightweight adhesive pads called blocking
pads.
Roll of adhesive edging blocking pads.
Illustration of edger blocking pad and block mounting to lens.
Once mounted, the edger proceeds to cut the lens into the desired shape and cuts a groove or
bevel in the edge depending on the frame design. Water is frequently pumped onto the lens by the
edger to keep the lens from becoming to hot in the process. A hot lens is more prone to warpage, so a
flow of water onto the lens is a standard feature of most edgers. Once complete, if the block can be
removed using a gentle twisting motion of the lens instead of a peeling motion there will be less chance
of creating a lens abrasion.
Typical in-office edger
Lens mounted in edger.
The lens can be ground or polished by a hand edger if needed for a perfect fit into the frame.
Example of manual hand edger.
Edging lenses can create toxic waste from the lens shavings and polishing compounds. These
waste materials are funneled by the edger into bins stationed under the edger table. Edging high index
materials will create a characteristically and strong odor, so your waste bin will need to be emptied
frequently. Investing in a ventilation system for the lab is often a good idea for odor control.
Mounting
Mounting is the process of manually inserting the newly cut lens into the frame. Having a hand
edger in the lab is useful for times when small modifications of the lens shape is needed to get a good fit
in the frame.
Inspection
Once mounted, the completed job is inspected. An inspection looks to insure the optical center
is in the proper placement, checking for scratches or other defects, and insuring there are no gaps
between the frame and lens. An inspection checklist can be found on the following page.
Dispensing
The moment of dispense is the only moment the patient sees. All the hard work in the lab is for
naught if a bad first impression is given at the moment the patient picks up their new glasses. The
following pages give a checklist for dispensing a new pair of glasses.
Inspection & Dispensing Checklist
Inspection:
The following checks must be made on all glasses just before they are given to the patient:
_________ Power check
_________ Prism check
_________ Tint check
_________ Lens inspection for defects
_________ Frame inspection for defects
_________ Optical centers marked in lensometer
_________ Adjust frame
_________ Ensure optical center marks align with patient’s PD (if dispensing PAL,
must be checked on patient with template on lenses)
_________ Ensure bifocal line is appropriate height.
_________ Clean glasses
Dispensing:
Proper Eyeglass presentation and dispensing instruction:
_____ Proper eyeglass packaging.
Are they on a nice presentation mat with a quality eyeglass case?
_____ Exceed Expectations.
Include a cleaning spray bottle, cleaning cloth, or any other product or service.
_____ Provide clean and new-looking reading material for the patient’s first look.
Is your reading material dog-eared and worn, or does it look crisp, new, and bright?
_____ Review all benefits of the glasses and premium upgrades.
By the time the patient picks up her glasses, she may have forgotten many of the benefits
of the premium upgrades she purchased, such as an antireflective coating or a titanium
frame. It is important to review these with her so when she goes home and hears a
commercial for two pairs of glasses for $99, she does not feel buyer’s remorse.
_____ Adjust the glasses, even if no adjustment is necessary.
Make the patient feel she is getting that customized fit.
_____ Compliment the patient on her new glasses.
Have you ever bought a nice piece of jewelry and the salesperson did not tell you how
nice you looked in it? Probably not.
_____ Instruct patients on proper eyeglass care and handling.
1. Always store them in their case.
2. Never clean them with a paper product (Kleenex, paper towels, etc.), which can
scratch the lenses. Use only a lint-free cleaning cloth.
3. Show proper cleaning with an eyeglass cleaner and silk cloth and recommend
stores that sell them.
Troubleshooting problems at lens inspection
1. Lens haze:
Many times caused by the lens getting to hot. Plastic materials expand when heated,
but antireflective coatings do not. So if the lens gets to hot during the edging process the stress
caused to the AR coating by the expanding lens may cause the lens to appear hazy. If this is the
case, ensure the edger is being properly cooled. It will likely have a water cooling system.
2. Linear scratches:
Scratches can occur at nearly any phase of lens creation. Protecting the lens is the best
protection by the use of tape on the surface. Most lab supply stores sell surface protectant
tape. By making sure the front surface (base curve) of a lens is taped while the back surface is
being generated or making sure both sides are taped when being mounted in the frame or not
being worked on, the lab can minimize the chance of scratches happening.
3. Circular scratches in center of lens:
Many times caused the lens being held to tightly in the edger. Modern edgers adjust
the pressure based on the material being edged. Excessive pressure and a permanent abrasion
such as this is created, to little pressure and the lens slips and is edged incorrectly. Some
materials are more prone to scratching, such as polycarbonate, so the edger will loosen its grip
slightly. However, if the machine is told it is edging CR39 when it is really edging polycarbonate
scratches may occur.
For softer materials, such as polycarbonate, some lab technicians will use two edging
pads instead of one for added protection. If the lab technician ensures they have the edger set
for the proper material and are using two pads, but the central scratches continue, the edger
may need to be recalibrated by the edger’s manufacturer technician.
4. Gaps between frame and lens:
This indicates the lens was edges a little too small. This could arise from a poor job of
the tracer determining the size of the frame or possibly from the lens having to be manually
edged prior to mounting to remove edge flaws from the automatic edger.
5. Rainbow appearance of the Antireflective coating:
This can be an indication that the lens is to large for the frame. Having to tighten the
frame around a lens edged to large will cause the lens to warp. The aberrations caused by this
warpage will appear as a rainbow or an oil slick on the antireflective coating. The lens may
need to be removed from the frame and manually edges to slightly reduce its overall size.
6. Diffuse pinpoint scratches:
This is typically due to dust on the lens prior to application of the antireflective coating.
Antireflective coatings are thinner than a grain of dust, so the dust will leave a button-hole
within the coating. This will leave small areas on the lens without antireflective coating. These
small areas, looking like a pinprick to the naked eye, is actually a spot on lens where a speck of
dust prevented the antireflective coating from adhering.
7. Geographic blotching:
This is typically the antireflective coating not adhering to the lens. Although sometimes
due to poor quality antireflective coatings, many times it is from the lens containing surface oils
such as from fingerprints prior to the antireflective coating being applied.
Tinting tips and tricks
Different lens materials will absorb tints with variable rates of success. The material’s polymer
dictates how well or poorly a tint will penetrate. The following will help guide you towards
both matching the best tint with lens material as well as be able to better educate your patients
on how a given tint can be expected to perform on a given lens material.
Polycarbonate:
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-
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The most difficult material to properly tint.
Most tints use a carbon molecule that binds to a carbon molecule in plastic (CR-39)
for its adhesion and stability. Polycarbonate chemical structure makes it resistant to
this carbon binding.
The best way to tint polycarbonate is to tint its scratch coating, however historically
the best scratch coats have been resistant to tinting.
To overcome these limitations, manufacturing labs have recently developed scratch
coats better able to absorb tints. However, in general, the best hard coat scratch
protectors still make tinting polycarbonate more challenging than other materials.
Always be sure the lens pairs being tinted are from the same manufacturer and
same batch to insure consistent tinting.
Cr-39:
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Wider range of tinting color options and darkness levels than Polycarbonate due to
its chemical make-up binding to tints more easily.
Uncoated CR39 is the most porous of lens materials, making it easier to get a darker
tint than other materials.
Hard coated CR39 is still easier to tint than polycarbonate since the hard coat does
not need to be as dense. This is due to CR39 being naturally less prone to scratching
than Poly.
Trivex:
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Trivex is easy to tint, but since its material is not very porous the tint will not
penetrate very deep. Therefore, harsh cleaners such as alcohol will fade the tint
over time.
Using a tintable hard coat may be the most stable method for tinting Trivex.
High Index:
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Able to be tinted well, however prepare for the process to take on the order of
hours to complete. A tint that CR39 requires 10 minutes to complete may take a
high index lens 3-4 hours to achieve the same outcome.
Rules for any material:
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Regardless of the material, lenses from different manufactures may tint differently
depending on their processing and curing techniques. Even if from the same
manufacturer, different batches of the same material may even tint slightly
differently. So if replacing just one sunglass lens for a patient, be sure to educate
them that the tint may not be a perfect match with the fellow lens. Replacing both
lenses is typically the safer option to achieve the best cosmetic effect.
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Be sure to completely clean a lens using lens soap prior to tinting. Oils, dust, or
other impurities will affect the uniformity of the tint.
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UV coating should be applied prior to any color tinting. This will prevent the tint
color from fading.
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For lenses which you know have been previously hard or scratch coated, you will
want to check with the lens manufacturer for their recommended tinting method.
Some manufactures hard coatings will accept color and others will not, some
manufacture coatings require a lower tint bath temperature or longer tint times. It
is always our recommendation to check with the manufacturer first to avoid a tinting
misadventure.
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All lens treatments such as tinting, UV treatments, and edge polishing must be done
before an AR coat is applied. Tints should be made about 10% darker than intended
prior to AR coating applications since the AR coating process will fade the tint by
about 10%.
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For gradient tints, if you are experiencing a sharp demarcation between the lighter
and darker areas instead of a smooth transition, your tint bath is likely to hot.
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In general, the hotter the tint bath the faster the lens will tint, however there are
limits. Get the tint to hot and the color will decompose. Be sure to read the
package insert for the color being used for recommended temperatures.
Tinting procedure:
Most in-office finishing labs will have a tint bath with 4-6 wells.
A typical tint bath with 6 wells.
Each tint bath well will contain a liquid dye created by mixing a Heat Transfer Fluid
(usually not water) with a dye concentrate (or pill) and heated to 205° F. Using a Heat Transfer
Fluid instead of water ensures the fluid within the bath heats quickly and is of a uniform
temperature.
Although color concentrates or dye pills are available for nearly any color under the sun,
your lab likely will not want to empty a tint bath to make room for a new color every time a
new color is needed. Therefore, the tint bath wells will likely each contain one of the three
primary colors… Blue, red, and yellow and maybe one for black given its popularity for tint
color. From these colors alone almost any other color can be created using the following
combinations.
Blue + Yellow = Green
Blue + Yellow + Red = Black to Grey depending on concentration of colors
Black + extra Red = Yellowish Brown
Yellowish Brown + extra red = Natural Brown
For more info on creating tints using only primary colors, visit
http://optochemicals.com/tinting_colorguide.htm
Although tinting can be done manually, most labs use an electronic mechanical lever
connected to a timer. The lever is mounted to the tint bath. It holds the lens and automatically
dips the lens into the tint bath and raises it out of the bath based upon the timer setting.
Darker tints will require longer tint times.
Typical mechanical lens holder
Copyright 2016 Optician U
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