Planos, Prism,.....is it obsessive?

[uncut]

I fabricated a pair of sunglasses. I paid special attention to the custom measured vertical and horizontal OC placements. I rejected 3 possible intact Planos, before settling on lenses that place the OC in the right place.

I was wondering if anyone else did the same thing? Is it a lab background thing? Is it a wrap/rap influence?


Is it a possible medical/medication problem?

[harry888]

When you have an itch, you've just got to scratch it.

Harry

[chip anderson]

How did you find the optical centers in Planos?

[uncut]

How did you find the optical centers in Planos?

Punch line: V e r y c a r e f u l l y !

I use a digital projection lensometer, which allows me to be finicky with centering the image, is the non-punch line answer.

[MikeAurelius]

A "true" plano will have neither power nor prism.

If you are going to be OCD about it, you should have rejected them all.

However, in the real world that we live in, power and prism within ANSI (or your lab's internal requirements) should be sufficient. I qualified the "lab's internal requirements" because some labs do manufacture to tighter than ANSI specs, especially on planos.

[Barry Santini]

The above discussion does not apply to wrap eyewear.

B

[Wes]

The above discussion does not apply to wrap eyewear.

B

Most quality wrap eyewear that I've checked out have between .25 and .50 prism diopters base in OU to compensate for the induced prismatic effect caused by the tilted/wrapped lens. The greater the tilt/wrap, the greater the compensation.

[Robert Martellaro]

The OC may be off the lens entirely, and still be acceptable!

http://www.dickwhitney.net/FSVPrismS...1957AOMemo.pdf

[Wes]

I fabricated a pair of sunglasses. I paid special attention to the custom measured vertical and horizontal OC placements. I rejected 3 possible intact Planos, before settling on lenses that place the OC in the right place.

I was wondering if anyone else did the same thing? Is it a lab background thing? Is it a wrap/rap influence?


Is it a possible medical/medication problem?

I thought I was the only one... No one I work with understands why I spot plano lenses. Explanations are useless.

[uncut]

Wes: Should we start a support group?

[uncut]

Perhaps the prism sensitivity stems from the old "surface lenses from scratch" environment. There, when creating, and surfacing a new base curve, the prism(edge thickness) was paid attention to.

[MikeAurelius]

I thought I was the only one... No one I work with understands why I spot plano lenses. Explanations are useless.

Me too.

[Darryl Meister]

A "true" plano will have neither power nor prism.

A meniscus plano lens will actually produce a small (though typically negligible) amount of prism away from the "center" of the lens when measured in a focimeter, since the instrument measures the lens with the aperture stop perpendicular to the back surface. The only lens that will not produce prism in a focimeter is a concentric lens, which will generally have a small amount of minus power.

Best regards,
Darryl

[MikeAurelius]

A meniscus plano lens will actually produce a small (though typically negligible) amount of prism away from the "center" of the lens when measured in a focimeter, since the instrument measures the lens with the aperture stop perpendicular to the back surface. The only lens that will not produce prism in a focimeter is a concentric lens, which will generally have a small amount of minus power.

Best regards,
Darryl

I'm glad you qualified your statement with "meniscus" and "concentric".

Plano flats, traceable to National Standards, have zero prism (to 6 places) and zero power (to 6 places). I've made them, so has my father. Unfortunately, the only way they can be tested is with a telescope rig and a National Standards focus chart.

Using a digital projection lensometer instead of an analog lensometer will show you that it is indeed possible to make a zero prism, zero power lens. It takes know-how and very accurate machining.

And at the end of the day, it all depends on your definition of "negligible". If by that you mean .02 or less power and .02 or less prism, then for all intents and purposes. you have a zero power, zero prism lens.

Back in 1990, we made plano power laser protective spectacles for the A-10 aircrews for Desert Shield/Desert Storm. The highest allowed dioptric power was +- 0.04 diopters. Zero prism was allowed in the center 35 mm of the lens. We achieved that with zero defects over 1350 pairs of lenses. The lenses were tested with a digital lensometer and the telescope apparatus mentioned above, and QC'd by a government inspector.

It CAN be done.

[Darryl Meister]

Using a digital projection lensometer instead of an analog lensometer will show you that it is indeed possible to make a zero prism, zero power lens. It takes know-how and very accurate machining.

You can certainly fabricate a Plano lens with no prism at the geometric center, just as you can any other lens power. However, if measured with the lens surface "normal" to the instrument axis, some prism is unavoidable away from the center of a properly manufactured Plano lens, since the surfaces are actually at an angle with respect to each other in the periphery of the lens.

There is actually a pretty informative AO article on plano prism in the always-useful OptiBoard File Archives, which discusses these topics at length.

Plano flats, traceable to National Standards, have zero prism (to 6 places) and zero power (to 6 places).

Just keep in mind that it is not really reasonable to compare the prism in a glass master to the prism in a mass-produced, molded plastic lens blank as a measure of quality. Even small errors in the centration of the glass molds or height of the gasket can introduce a small amount of unwanted prism at the geometric center of the lens blank.

Of course, it is possible to produce a perfect plastic lens blank, free from any optical errors, but not necessarily at a cost that eyecare professionals would be willing to pay. The same goes for surfacing the lens blank. This is why we have manufacturing tolerances and the ANSI standards.

Best regards,
Darryl

[uncut]

You can certainly fabricate a Plano lens with no prism at the geometric center, just as you can any other lens power. However, if measured with the lens surface "normal" to the instrument axis, some prism is unavoidable away from the center of a properly manufactured Plano lens, since the surfaces are actually at an angle with respect to each other in the periphery of the lens.

There is actually a pretty informative AO article on plano prism in the always-useful OptiBoard File Archives, which discusses these topics at length.




Just keep in mind that it is not really reasonable to compare the prism in a glass master to the prism in a mass-produced, molded plastic lens blank as a measure of quality. Even small errors in the centration of the glass molds or height of the gasket can introduce a small amount of unwanted prism at the geometric center of the lens blank.

Of course, it is possible to produce a perfect plastic lens blank, free from any optical errors, but not necessarily at a cost that eyecare professionals would be willing to pay. The same goes for surfacing the lens blank. This is why we have manufacturing tolerances and the ANSI standards.

Best regards,
Darryl

Thank you, Darryl for your contribution, and for the link to the Archive file. It is a very informative article.

[Wes]

Someone had a website up called handcrankers that had that paper and dozens more up. I think it was Dick Baker's site, but I'm not sure.

[nicksims]

I thought I was the only one... No one I work with understands why I spot plano lenses. Explanations are useless.

Ditto.

[MikeAurelius]

You can certainly fabricate a Plano lens with no prism at the geometric center, just as you can any other lens power. However, if measured with the lens surface "normal" to the instrument axis, some prism is unavoidable away from the center of a properly manufactured Plano lens, since the surfaces are actually at an angle with respect to each other in the periphery of the lens.

Quibble point: the prism at the periphery is meaningless. Consider: for the most part, the view through a lens is in the center 35 mm of any given lens, with perhaps sole exception being vertical for reading, and even then 17.5 down/17.5 up is a huge amount of room for the normal eye to see.

There is actually a pretty informative AO article on plano prism in the always-useful OptiBoard File Archives, which discusses these topics at length.

Thanks for that link. I had it stored in the back of my head, but it's always good to be able to read it through again.

Just keep in mind that it is not really reasonable to compare the prism in a glass master to the prism in a mass-produced, molded plastic lens blank as a measure of quality. Even small errors in the centration of the glass molds or height of the gasket can introduce a small amount of unwanted prism at the geometric center of the lens blank.

Of course, it is possible to produce a perfect plastic lens blank, free from any optical errors, but not necessarily at a cost that eyecare professionals would be willing to pay. The same goes for surfacing the lens blank. This is why we have manufacturing tolerances and the ANSI standards.

Best regards,
Darryl

Well, remember Darryl, I come from and still do work in a glass-only shop. We mass-produce glass planos every day with the tolerances I mentioned above, as 95% of our sales are plano filters in a variety of index materials (1.523, 1.570, 1.701, and 1.806). Our competitor doesn't pay attention to power/prism and he loses customers to that lack of "vision" of the quality of his products.

That is really the reason I could never work in a plastics environment. The lack of quality of the optics would drive me insane.

About the manufacturing tolerances and ANSI: yes, you are absolutely correct. But remember that they are nothing more than uneforceable guidelines. Well, ok, the doctor/dispenser can be the enforcer. The lab can set its own tolerances much tighter than ANSI if they choose to, especially in this day and age with modern computer controlled machinery. Quite honestly, the computer issue alone brings a far greater control over tolerances than we've ever had before.

Consider this: the phoropter uses precision ground optics (made of glass!!!) to test the patient's vision. Yes, it is a subjective test, but based on the readings coming off the device, a lens can be made to match those powers exactly. There should really be no reason for loosening the tolerances as has occured over the past 20 years. If anything, they should be tighter due to the increase in computer-based manufacturing.

A "perfect plastic lens blank, free from any optical error" can be made at a reasonable cost by merely instituting a set of manufacturing controls in the lab. That you say the cost would be unreasonable is somewhat troubling.

[Darryl Meister]

Quibble point: the prism at the periphery is meaningless.

We are talking about two different effects. I was not referring to prism induced while looking off-axis through the lens, which as you point out is typically of little consequence, especially in lower Rx powers.

I referred to prism in the "periphery" of the lens to indicate that a properly made Plano lens still has an optical center, so some (albeit negligible) prism may indeed be induced if the lens is not properly centered, as is the case with any other lens with power.

Although, admittedly, this entire topic is somewhat academic. And, in any event, any prism inadvertently molded into the lens would be more difficult to eliminate compared to the decentered prism effects that I was describing.

Well, remember Darryl, I come from and still do work in a glass-only shop.

I understand. And I do not necessarily disagree with you that you can achieve greater or more consistent accuracy with glass than with plastics. But you are probably aware that over 98% of the lenses currently sold in the US are not made from glass.

From experience, I know that manufacturing glass lenses is certainly not without its challenges. Although most lens manufacturers obviously work primarily with plastics today, plenty of us have mass manufactured everything from glass single vision to fused bifocal to glass progressive lenses.

Mass manufacturing glass lenses still involves manufacturing errors and tolerances. Although conceivably we could reject any lens blank that was not exactly perfect, at least within measurement limits, this would have obviously driven up the cost of goods associated with excessive breakage or rejected parts.

You can see this for yourself by simply comparing the cost of those perfectly ground glass masters and standard lenses ($$$) that you referred to earlier to the cost of mass produced spectacle lens blanks.

But this may just be an argument over semantics. Earlier, for instance, you referred to achieving "zero defects" within military specifications of +/-0.04 D. However, this is not the same as achieving "zero power errors." You still worked within a tolerance, however tight.

The lab can set its own tolerances much tighter than ANSI if they choose to... A "perfect plastic lens blank, free from any optical error" can be made at a reasonable cost by merely instituting a set of manufacturing controls in the lab. That you say the cost would be unreasonable is somewhat troubling

I'd have to disagree with you over these points.

Quality investigations among Rx optical labs over the past 50 years by the OLA and VCA have inevitably revealed that no laboratory consistently delivers spectacle lenses that are free from optical errors, in either glass or plastic. This is why the ANSI standard for spectacle lenses was developed in the first place.

So, either these laboratories have never bothered to implement "manufacturing controls" or, more likely, there is just more to it than that. Certainly, surfacing equipment and software have improved, but other factors have also come into play, such as a greater variety of lens materials, faster production requirements, lower skill levels, et cetera.

Even modern lens surfacing is subject to unavoidable errors due to the manufacturing variance of the front curve and to rounding errors with hard lap tools, in addition to the usual random and systematic errors associated with the surfacing process as a result of operator error, machine calibration, consumable quality, et cetera.

I am not aware of any mass production process that relies on a 100% inspection plan with zero manufacturing tolerance. Even a well-controlled manufacturing process usually seeks only to maintain manufacturing errors in a sample population that fall within a few standard deviations of a reasonable quality limit.

And, even if you did manage to produce "perfect" lenses, the final assessment of accuracy by you and your customers is still limited by the variance in and limitations of measurement technologies, such as differences in the reference wavelength or optical image alignment (i.e., "IOA" versus "FOA") of your focimeter.

I would bet you that no Rx optical laboratory in the world is consistently delivering 100% accurate lenses. In fact, if it really were possible to do this cost effectively, there would really be no need for ANSI/BSI/DIN/AUS/ETC. optical standards in the first place.

Best regards,
Darryl

[MikeAurelius]

My brain is too tired right now (I've been repairing an old wood floor circa 1920). Let me ponder on this and we can discuss further in the a.m.

And many thanks for the delightful and thoughtful discussion!!

[Darryl Meister]

(I've been repairing an old wood floor circa 1920)

You may be at that for a while, if you're planning to plane it to a tolerance of 0.0001"!

And many thanks for the delightful and thoughtful discussion!!

Mike, Regardless of whether we agree or disagree, I always find our debates a refreshing change of pace from writing memos and staring at contour plots all day!

Best regards,
Darryl

[normuck]

In response to the question asked is it to obsessive to reject plano / prism lenses 3 times because of a "mathematically" incorrect O.C................HECK yeah its to obsessive! lol. man do something else with your time.... reconcile some bills, try to find money lost in an insurance claim, wonder what your having for dinner....daydream about the hot blonde working at the front desk.....anything else is more productive IMO lol.... jeez and you brought out the 4 or 5 other guys in the world who wanted to talk about this with ya :)

[Darryl Meister]

do something else with your time.... daydream about the hot blonde working at the front desk..... and you brought out the 4 or 5 other guys in the world who wanted to talk about this with ya

In my defense, there are no hot blondes at my front desk.

Best regards,
Darryl

[normuck]

Hmm hot bruenette? if not ill come work for ya, im a hot balding burnette lol jk ;)