# Optical Forums > Ophthalmic Optics >  ANSI Z80.1 Prism inspection - question

## thomasz

Hi everyone,

I have a question regarding the prism inspection as described by the ANSI Z80.1 (2010) standard.
Indeed, it seems to me that there is a contradiction between two paragraphs and I would like to know your opinion.

Here are the two excerpts :_5.1.4 Prism Reference Point Location and Prismatic Power__The prism reference point shall not be more than 1.0 mm away from its specified__position in any direction. In addition, the prismatic power measured at the prism__reference point shall not exceed 0.33cm/m. [...]__ Measurement shall be done using the method in 8.4.
__8.4 Prismatic Power Measurement__1. Position the lens on the focimeter stage with the prism reference point centered in__front of the lens stop of the focimeter. [...]
__2. Measure the prism at the PRP. Proceed to Step 3 only if the prism error exceeds__0.33cm/m at the prism reference point.__3. Locate and mark the position at which the prismatic requirements of the__prescription are met. In the absence of prescribed prism this point corresponds__with the optical center. This point shall be within 1.0 mm of the prism reference__point as specified in 5.1.4._


In other words, for a lens without a prescribed prism, 
paragraph 5.1.4 says : "the prism at the PRP should be lower than 0.33cm/m *AND* the PRP should be at a maximum distance of 1mm regarding the optical center.paragaph 8.4 says : "the prism at the PRP should be lower than 0.33cm/m *OR* the PRP should be at a maximum distance of 1mm regarding the the optical center

The "AND"/"OR" confusion makes a big difference.
For example, if we consider the case of a spherical lens and translate the tolerance on the decentration on the prism value based on the Prentice rule, we can get a single value for the tolerance on the prismatic power. Using the "AND" or "OR" leads to a very different tolerance as illustrated here below : 



I believe paragraph 5.1.4 is wrong since the tolerance it implies would be impossible to apply (for a plano lens, the tolerance is equal to zero!, and for a +10D, a shift of 0.3mm would make the lens fail).

Do you agree with me?

What rule do you use to inspect the prism based on the ANSI standard?

Thanks in advance!

Thomas

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## MikeAurelius

I read the two paragraphs as identical. 8.4.3 gives the method of determining the amount of error and states that it "SHALL BE" within 1.0 mm.

I believe that there are exceptions for plano power prismatic lenses elsewhere in the standard.

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## Uilleann

Sounds like an ABO test writer had a hand writing the ANSI standards as well.   :Rolleyes:

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## lensgrinder

5.1.4 and 8.4 are general tolerances.  They are to be used for unmounted lenses.  Once you mount the lenses this will change the PRP locations.  This is why the tolerances are different for mounted pairs.
In regards to plano, 3.21.4 states "Unless otherwise specified, the prism reference point is assumed to be at thefollowing geometric locations on the lens. For uncut single vision lenses, this point is the geometric center of the lens."  So if you have 0.33 D at the geometric center of a plano then it will pass.

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## MikeAurelius

^^+1000

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## thomasz

@ lensgrinder
Thanks for your reply.
The distinction between uncut and mounted is not relevant to me since 5.1.4 and 8.4 are general paragraphs.
I also believe that a plano with a prism < 0.33cm/m at the PRP would need to pass. 
But I also believe that this is in contradiction with §5.1.4. Please read it again.

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## thomasz

@MikeAurelius.
Please read them again then.
To, me the logic is clearly different.
If A and B are two conditions, (A OR B) is different from (A AND B)!

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## MikeAurelius

I believe you are overthinking it. As lensgrinder states, for uncut lenses SV it doesn't matter, as long as the finished lens cuts out correctly with the prescribed prism.

From 5.1.4: 




> The prism reference point shall not be more than 1.0 mm away from its specified position in any direction. *In addition*, the prismatic power measured at the prism
> reference point shall not exceed 0.33cm/m


This is clearly (A *AND* B), note the "In Addition" phrase.

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## MakeOptics

> Hi everyone,
> 
> I have a question regarding the prism inspection as described by the ANSI Z80.1 (2010) standard.
> Indeed, it seems to me that there is a contradiction between two paragraphs and I would like to know your opinion.
> 
> Here are the two excerpts :_5.1.4 Prism Reference Point Location and Prismatic Power__The prism reference point shall not be more than 1.0 mm away from its specified__position in any direction. In addition, the prismatic power measured at the prism__reference point shall not exceed 0.33cm/m. [...]__ Measurement shall be done using the method in 8.4.
> __8.4 Prismatic Power Measurement__1. Position the lens on the focimeter stage with the prism reference point centered in__front of the lens stop of the focimeter. [...]
> __2. Measure the prism at the PRP. Proceed to Step 3 only if the prism error exceeds__0.33cm/m at the prism reference point.__3. Locate and mark the position at which the prismatic requirements of the__prescription are met. In the absence of prescribed prism this point corresponds__with the optical center. This point shall be within 1.0 mm of the prism reference__point as specified in 5.1.4._
> 
> ...


Looking at your graph you forgot a very important part of the equation, the domain.

0.33PD is only applied to prescription power 0 to <= 3.37 and the 1mm or less is applied to the >3.37 powers.

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## thomasz

@MikeAurelius.
In my situation, it is very important to understand exactly what the standard means. 
I agree with you regarding §5.1.4 but, if you read §8.4 carefully, you will realize it is a "OR" in that case.
Since a "AND" would make no sense in practice, I believe §8.4 is right and §5.1.4 is wrong. 
What do you think?

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## thomasz

@MakeOptics
Sorry if I wasn't clear. Actually, to be able to compare the two conditions under the same graphs, I consider the case of a spherical lens.
For such a lens, the relation between the prism and the decentration is given by the Prentice rule : PRISM[cm/m] = 0.1*POWER[D]*DECENTRATION[mm].
So I didn't forget the domain actually ;-)
In that case, you can see how the two conditions combined with a "OR" or "AND" and translated into a prism tolerance can lead to a totally different inspection result.

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## MikeAurelius

> @MikeAurelius.
> In my situation, it is very important to understand exactly what the standard means. 
> I agree with you regarding §5.1.4 but, if you read §8.4 carefully, you will realize it is a "OR" in that case.
> Since a "AND" would make no sense in practice, I believe §8.4 is right and §5.1.4 is wrong. 
> What do you think?


I think the mistake you've made is in thinking that both paragraphs are somehow equivalent, when they are not. When working with standards of any kind there are usually at least two parts, the description of the specification, IE: what it is you are measuring and what the allowable tolerances are, paragraph 5.1.4 gives you that. The other part is the method by which you test the item to see if it meets the specification of the standard, and that's whats in 8.4.

5.1.4 is the SPECIFICATION, 8.4 is the METHOD.

Two parts, one result, pass or fail.

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## thomasz

MikeAurelius, I agree with you on the two parts, one giving the concept and one giving the procedure to apply this concept.
However, I really don't agree with you in that case since the concept is clearly different between the two parts.
You probably understand since you actually changed your mind. 5.1.4 stated (A and B), which you believed was right... 8.4 says (A or B), which you seem to believe now... 
So you see how confusing it is :-)?

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## MikeAurelius

> MikeAurelius, I agree with you on the two parts, one giving the concept and one giving the procedure to apply this concept.
> However, I really don't agree with you in that case since the concept is clearly different between the two parts.
> You probably understand since you actually changed your mind. 5.1.4 stated (A and B), which you believed was right... 8.4 says (A or B), which you seem to believe now... 
> So you see how confusing it is :-)?


I haven't changed my mind. The wording is slightly different, however the intent is very clear to me (btw, I read specifications all the time, not just in the optical industry).

8.4 says to do this:

1. Locate lens on the lensometer at the PRP. (assumes you've already marked the PRP)
2. Read the prismatic power. If the difference between Rx and read power is greater than .33 prism diopters, proceed to step 3.
3. Move the lens in the lensometer until it matches Rx. Mark the lens. Remove lens and measure distance between PRP and actual prismatic location. If the measurement is 1 mm or less, the lens is good.

Plus or minus 1.00 mm in positioning error is the tolerance.

There's nothing to confuse here, its very straight forward.

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## thomasz

That's a bit funny Mike :-)

Let's say that I agree with you, it is (A or B)!
Either the prism is lower than 0.33D or the distance from the PRP to the point where the prescribed prism is met is lower than 1mm.
Just not 0.33D and distance lower than 1mm.

If you consider a non prismatic lens with a very low power, you can have a prism of 0.25cm/m at the PRP and an optical center that is 10mm from there.

So I guess we agree 8.4 is right :-)

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## MikeAurelius

In a lens with prism, there is no OC, just PRP.

Erg...that came out wrong...

S/B: When edging a lens with prism, the only thing (especially in SV) that matters is the PRP.

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## lensgrinder

> @ lensgrinder
> Thanks for your reply.
> The distinction between uncut and mounted is not relevant to me since 5.1.4 and 8.4 are general paragraphs.
> I also believe that a plano with a prism < 0.33cm/m at the PRP would need to pass. 
> But I also believe that this is in contradiction with §5.1.4. Please read it again.


It is relevant to ANSI. This is why they have general tolerances for uncut and edged versus mounted.  Also note that the general tolerances do not reference power in regards to prism, which is another reason they are general.
*
8.4
*If you have an edged lens that is 50 mm round with 5 mm of decentration and no prescribed prism then your PRP should be located at 30 mm from the temporal edge.  If, at this point you have 0.33  then it is OK.  If you notice more prism than 0.33 , you will move the lens until it has no prism.  If this movement is greater than 1 mm then it fails.

*5.1.4* 
You need to fall within 1mm of the specified PRP(30 mm in the above case) at the PRP you can have 0.33

The way to measure this is to place the lens in the focimeter at the requested PRP and if it has prism greater than 0.33  then move it and if it is not within 1mm of the desired PRP then it fails.

They are saying the same thing.

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## ml43

> Hi everyone,
> 
> Here are the two excerpts :_5.1.4 Prism Reference Point Location and Prismatic Power__The prism reference point shall not be more than 1.0 mm away from its specified__position in any direction. In addition, the prismatic power measured at the prism__reference point shall not exceed 0.33cm/m. [...]__ Measurement shall be done using the method in 8.4.
> __8.4 Prismatic Power Measurement__1. Position the lens on the focimeter stage with the prism reference point centered in__front of the lens stop of the focimeter. [...]
> __2. Measure the prism at the PRP. Proceed to Step 3 only if the prism error exceeds__0.33cm/m at the prism reference point.__3. Locate and mark the position at which the prismatic requirements of the__prescription are met. In the absence of prescribed prism this point corresponds__with the optical center. This point shall be within 1.0 mm of the prism reference__point as specified in 5.1.4._
> 
> 
> In other words, for a lens without a prescribed prism, 
> paragraph 5.1.4 says : "the prism at the PRP should be lower than 0.33cm/m *AND* the PRP should be at a maximum distance of 1mm regarding the optical center.paragaph 8.4 says : "the prism at the PRP should be lower than 0.33cm/m *OR* the PRP should be at a maximum distance of 1mm regarding the the optical center


Don't let your English skills hinder your logic.

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## thomasz

Hi lensgrinder and ml43,

Thanks for taking the time to respond!
I'm sorry but I don't follow you guys :-)

I understand the discussion for decentered SV can be more complex.
However, the discussion is also valid for a progressive lens with a PRP at the center of the engravings.

Let's turn this in mathematical terms and let's consider the case of a progressive lens where the prescribed prism equal to zero.
Let's define PRP = center of the engravings and OC = point where the prism equal zero.

A : PRISM(PRP)<0.33cm/m
B : distance(PRP,OC) < 1mm

To me, 5.1.4 says lens passes if (A and B).
And 8.4 says : (A or((not A) and B)) which, as you can verify, is exactly the same logic as (A or B).

I'm sorry to insist.
Maybe I'm the only one to believe there is a problem :-)

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## thomasz

@MikeAurelius
Sorry if I wasn't clear. I'm considering a centered SV lens without any prescribed prism but with a power close to zero.
Let's say SPH=0.25D, PRVM=0.

During the inspection, the measured prism at the PRP=geometrical center could be 
INSPRVM=0.25cm/m. -> this prism is due to manufacturing incertitude.

It passes the A condition (lower than 0.33cm/m).
However, the optical center where the prism is equal to zero is at a distance of 10mm (prentice rule).
So B condition will fail.

Based on 8.4, the lens should pass. But 5.1.4 tends to say the opposite.

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## ml43

> @MikeAurelius
> Sorry if I wasn't clear. I'm considering a centered SV lens without any prescribed prism but with a power close to zero.
> Let's say SPH=0.25D, PRVM=0.
> 
> During the inspection, the measured prism at the PRP=geometrical center could be 
> INSPRVM=0.25cm/m. -> this prism is due to manufacturing incertitude.
> 
> It passes the A condition (lower than 0.33cm/m).
> However, the optical center where the prism is equal to zero is at a distance of 10mm (prentice rule).
> ...


Your logic is being corrupted by your reading comprehension and order of operations.

5.1.4 and 8.4 are complementary to each other, and are worded quite clever.
If you read carefully, what it's saying is: 

Unmounted Prism and PRP
 ≥ 0.00 D, ≤ ±3.37 D
> ±3.37 D
0.33 Δ
1.0 mm






In this case the lens would pass both 5.1.4 and 8.4
remember, in a SV SPH 0.25D lens, the PRP is the GC, not the OC.

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## MikeAurelius

Please do not take offense at this question, but is English your first language or a subsequent language?

This standard has stood *as-is* for the better part of 30 years and probably even longer. *IF* there had been an error in logic, it would have been caught by the hundreds of thousands of people who use it every single day as it is written.

If you want to believe it's a mistake, then write to ANSI and file a brief with them detailing what you believe the error to be. Send a copy to the VCA (Vision Council of America), as they are the ones most likely to respond.

We've explained to you that it's your logic that is in error, but you can't seem to accept that.

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## thomasz

English is not my first language as you probably noticed ;-)
I got your point. Maybe it is a matter of language...
I will address my concern to the Vision Council.
Thank you!

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## lensgrinder

> Let's turn this in mathematical terms and let's consider the case of a progressive lens where the prescribed prism equal to zero.
> Let's define PRP = center of the engravings and OC = point where the prism equal zero.
> 
> A : PRISM(PRP)<0.33cm/m
> B : distance(PRP,OC) < 1mm
> 
> To me, 5.1.4 says lens passes if (A and B).
> And 8.4 says : (A or((not A) and B)) which, as you can verify, is exactly the same logic as (A or B).


If a lens has zero prism diopters then it is still referred to as the PRP, you would be referencing zero prism diopters.  There is no OC.
In a progressive you have a known PRP, so if you check it at that location and it is <0.33 then it will pass > 0.33 it fails.  There is not within 1 mm because you know where the PRP is.




> I'm considering a centered SV lens without any prescribed prism but with a power close to zero.
> Let's say SPH=0.25D, PRVM=0.
> 
> During the inspection, the measured prism at the PRP=geometrical center could be 
> INSPRVM=0.25cm/m. -> this prism is due to manufacturing incertitude.
> 
> It passes the A condition (lower than 0.33cm/m).
> However, the optical center where the prism is equal to zero is at a distance of 10mm (prentice rule).
> So B condition will fail.
> ...


If the prism is 0.25 at the PRP then it will pass.

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## thomasz

Hi lensgrinder,

I don't agree with you on both points : 

- For uncut SV lenses without a prescribed prism, the PRP is the geometrical center and can be different from the OC where the prism is equal to zero, since the manufacturing is not perfect. There is an OC and there is a PRP and those points can be different.

- For a progressive lens, based on §8.4, the same procedure applies. Not understanding that is a serious flaw in your understanding of the optical principles behind the procedure. If you have a +15.00D lens, an error of positioning of the PRP at the focimeter of 0.2mm (which is low and can happen all the time with manual positioning) would lead to an error on the prism of 0.30cm/m. Therefore, the tolerance on the prism will easily fails, while the tolerance on the distance between the PRP and the OC (or point where the prescription prism is met) can pass, so the lens will pass.

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## thomasz

I guess we just don't agree and won't. I might be wrong, but I just don't understand your arguments. Thanks a lot for the time you spent responding to this thread. I will contact the standard experts to get their opinion.

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## MakeOptics

> @MakeOptics
> Sorry if I wasn't clear. Actually, to be able to compare the two conditions under the same graphs, I consider the case of a spherical lens.
> For such a lens, the relation between the prism and the decentration is given by the Prentice rule : PRISM[cm/m] = 0.1*POWER[D]*DECENTRATION[mm].
> So I didn't forget the domain actually ;-)
> In that case, you can see how the two conditions combined with a "OR" or "AND" and translated into a prism tolerance can lead to a totally different inspection result.


Prentices Rule is a small angle approximation so the prism will not hold true for low powers.  




> *5.1.4* Prism Reference Point Location and Prismatic PowerThe prism reference point shall not be more than 1.0 mm away from its specified
> position in any direction. In addition, the prismatic power measured at the prism
> reference point shall not exceed 0.33. This tolerance applies to lenses both with
> and without prescribed prismatic power. When prismatic thinning is used, it is
> treated as prescribed prism. Measurement shall be done using the method in 8.4.


If we look at the threshold for the tolerance 3.37D, using Prentices Rule the decntration value to reach 0.33PD:

0.33PD = 3.37D * dec /10 <= divide by 10 to get into mm
dec = 3.37 / 3.3
dec = 1.2 mm which is roughly equivalent to the 1.00mm toloerance

Looking at the 1/8th diopter above and below the threshold:

dec = 3.25 / 3.3
dec = 0.98mm

dec = 3.50 / 3.3
dec = 1.0mm

So the standard measures roughly true for all measures 3.50 and below, if anything I think the cut off could be higher however keep in mind the tolerance on power is +/-0.12D so that might account for the discrepancy.




> *8.4* Prismatic Power Measurement1. Position the lens on the focimeter stage with the prism reference point centered in
> front of the lens stop of the focimeter.
> 2. Measure the prism at the PRP. Proceed to Step 3 only if the prism error exceeds
> 1/3PD at the prism reference point.
> 3. Locate and mark the position at which the prismatic requirements of the
> prescription are met. In the absence of prescribed prism this point corresponds
> with the optical center. This point shall be within 1.0 mm of the prism reference
> point as specified in 5.1.4.


So first we check to see if the prism power is less than or equal to 0.33, if it is not then we check to see if the point on the lens that actually matches our prism requirements is within 1.0mm of the prism reference point.  If it is then you check to see if the lens power is above the 3.37D power threshold, if it is then the lens passes.  The method provided allows for quicker job verification as the logic does not apply until the lens is questionable which should be the minority.

The reason why the 0.33PD and 1.0mm, tolerances have a discrepancy is for manufacturing repeatability.  The number of rejects would sky rocket if the tolerance didn't switch from 0.33PD to 1.0mm in higher powers as the tolerance you pointed to in your earlier post of a 10D lens would be so tight the cost and time to produce the lens would be unrealistic.  I like to explain it as we have the technology to put a rocket engine in a car, but it's not realistic so that's the norm or expectation.

English is a second language to me, I often have to explain to others that when I spend significant time with my family my verbage gets jumbled a bit.  In other languages the order of a sentence can be interpreted as backwards to english speaking natives.  Math being a universal language offers a bridge.

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## Barry Santini

Always remember that the precision or accuracy of the prescribed prism is never a given. Discretion rules in prism prescribing.

B

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## MakeOptics

Attached is a simple truth table basically showing the ANSI prism standard.  Which can be summarized as: powers under 3.37D the optical tolerance takes precedence over manufacturing tolerance, above a 3.37 the manufacturing tolerance takes precedence over the optical tolerance.

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## MikeAurelius

> Attached is a simple truth table basically showing the ANSI prism standard.  Which can be summarized as: powers under 3.37D the optical tolerance takes precedence over manufacturing tolerance, above a 3.37 the optical tolerance takes precedence over the manufacturing tolerance.


Care to rephrase? ;-)

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## MakeOptics

> Care to rephrase? ;-)


Nice catch.

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