Ansi Horizontal Prism Revisted

[Aarlan]

Hello all,

I had started a thread a bit back regarding horizontal prism according to ANSI, and whether it was 0.66 total or .33 each eye. Now I have another question.

I have been arguing with a colleague about the horizontal imbalance for higher power lenses (I was and am still of the understanding that we are allowed 2.5 mm for any lens over 2.75D). He showed me this link and is arguing that we are only allowed .66 total FOR ALL GLASSES (I cannot imagine most labs getting a -15 w/in .66 every time).

http://www.odfileexchange.com/cms/in...&limitstart=20

As Opposed to http://www.opticampus.com/tools/ansi.php , which gives us the 2.5 mm


Does anyone have the actual TEXT version of ANSI Z.80 for just the horizontal prism portion so I can figure out who is correct?


Thanks

AA

[Aarlan]

The PDF is almost towards the bottom of the page and is titled:
OUTLINE - ANSI Z80.1 2005 Rx Ophthalmic Standards.pdf

[HarryChiling]

Vertical Imbalance = less than 0.33 or less than 1mm difference. (8.4 step 2 - 3)
Horizontal Imbalance = less than 2.5mm or less than 0.33 per eye. (8.5.2 steps 1 - 5)

Believe me when I tell you 0.33 per eye is the rule to use, if you follow the steps used to measure horizontal imbalance you would fail a pair that had one lens with 0.67 and the other with no prism.

[Crickett13]

5.2.1.1 Single-vision and Multifocal Lenses
Vertical prismatic imbalance of mounted pairs of single-vision and multifocal lenses
with refractive power from 0.00 D to ±3.375 D in the vertical meridian shall not exceed
0.33Δ. Pairs with refractive power greater than ±3.375 D in the vertical meridian shall
not have more than 1.0 mm difference in the height of the two lenses prism reference
points. Measurement shall be done using the method specified in 8.5.1.
Horizontal prismatic imbalance between mounted single-vision and multifocal lenses
with refractive power from 0.00 D to ±2.75 D in the horizontal meridian shall not
exceed 0.67. The horizontal distance between the prism reference points of singlevision
and multifocal lenses with refractive power greater than ±2.75 D in the
horizontal meridian shall not differ from the specified distance interpupillary distance
by more than 2.5 mm. Measurement shall be done using the method specified in

8.5.2.

[HarryChiling]

Matt,

Check out the methods, the methods compared to the tolerances are almost contradicting.

For instance a lens with power -2.75 with 2.5mm of decentration would exhibit 0.6875 diopters of prism, that would be outside of the prism tolerance but on tolerance for the 2.5mm rule. Since the method describes measureing the PRP for pupilary distance specified the pair would pass before you would even have a need for steps 4 and 5 in which you would check the prism. The tolerance would be 0.00 to 2.75 should be 0.67 or less, and powers over should be within 2.5mm but the method is a bit ambiguous. Anyway I think the best way to do it is to fail anything 2.5mm off on the PD. It would mean that you would fail some lenses that were of lower power due to the PD being off when the prism wasn't but if your lab is missing by more than 2.5mm you have bigger issues on your hands.

[Crickett13]

Matt,

Check out the methods, the methods compared to the tolerances are almost contradicting.

For instance a lens with power -2.75 with 2.5mm of decentration would exhibit 0.6875 diopters of prism, that would be outside of the prism tolerance but on tolerance for the 2.5mm rule. Since the method describes measureing the PRP for pupilary distance specified the pair would pass before you would even have a need for steps 4 and 5 in which you would check the prism. The tolerance would be 0.00 to 2.75 should be 0.67 or less, and powers over should be within 2.5mm but the method is a bit ambiguous. Anyway I think the best way to do it is to fail anything 2.5mm off on the PD. It would mean that you would fail some lenses that were of lower power due to the PD being off when the prism wasn't but if your lab is missing by more than 2.5mm you have bigger issues on your hands.

I agree about the methods but not with the 2.5 on any power. Pair of +0.25 Spheres 1 mans 3 is anothers 2 just because of the size of the dots from some lensometers. While guys like you and I understand that many people dont. I have had lenses with powers of +1.00 -0.75 X 90 returned for PD off 3 mm. Heck we have had powers of +2.00 -2.00 X 90 returned for PD off.

My preffered method was always just dot up the lenses on low powers and calculate the prism induced but that is not for everybody. For beginners we just teach them to dot the centers and if it is "off" by more than 2.5 re dot with 1/3 diopter of prism in each eye in the direction the PD needs to go. If the prescribed PD falls between the 2 measurements then the job passes.

[HarryChiling]

I agree about the methods but not with the 2.5 on any power. Pair of +0.25 Spheres 1 mans 3 is anothers 2 just because of the size of the dots from some lensometers. While guys like you and I understand that many people dont. I have had lenses with powers of +1.00 -0.75 X 90 returned for PD off 3 mm. Heck we have had powers of +2.00 -2.00 X 90 returned for PD off.

My preffered method was always just dot up the lenses on low powers and calculate the prism induced but that is not for everybody. For beginners we just teach them to dot the centers and if it is "off" by more than 2.5 re dot with 1/3 diopter of prism in each eye in the direction the PD needs to go. If the prescribed PD falls between the 2 measurements then the job passes.

That's the method described in the ANSI, it's a little vague but your right it's difficult to use the 2.5mm method, so when questionable it's a good idea to dot up 0.33 in and out and then make sure the PD's fall within those measures.

I once dispensed a +0.25 -0.25 x 090 that was 10mm off to a patient. How irresponsible of me, the funny ones are the +1.00 -2.00 x 045, they are not as easily evident that you have a plano running the 180.:hammer:

[OpticianVlad]

The following is the prism tolerance for New Jersey:

Vertical + or - 0.25 prism for each lens or a total of 1/3 of specified optical center prism imbalance.
Horizontal + or - 0.25 prism for each lens or a total of 0.50 prism diopter imbalance; if prism exceeds .50 prism diopter, the optical centers must be within 2 mm. If prism is less than .50 prism diopter, the optical
centers must be within 4 mm.

[Darryl Meister]

As Opposed to http://www.opticampus.com/tools/ansi.php , which gives us the 2.5 mm

My summary is based upon the actual Z80.1-2005 standard. (I should also add that I served on the Z80 subcommittee that wrote this standard, if that helps allay any doubts or concerns...;))

[Aarlan]

Darryl,

No offense intended.

Thanks for your help.

AA

[Darryl Meister]

No offense intended. Thanks for your help.

And none taken. ;) Glad to help.

[Crickett13]

That's the method described in the ANSI, it's a little vague but your right it's difficult to use the 2.5mm method, so when questionable it's a good idea to dot up 0.33 in and out and then make sure the PD's fall within those measures.

I once dispensed a +0.25 -0.25 x 090 that was 10mm off to a patient. How irresponsible of me, the funny ones are the +1.00 -2.00 x 045, they are not as easily evident that you have a plano running the 180.:hammer:

Those ones with the oblique axis are a lot of fun when you are training. You get the most amazing puzzled looks from people! Still not as much fun as finding a Zyl frame with only a partial groove and watching the new guy or gal try and figure out wht the lens won't stay in:D

[HarryChiling]

Those ones with the oblique axis are a lot of fun when you are training. You get the most amazing puzzled looks from people! Still not as much fun as finding a Zyl frame with only a partial groove and watching the new guy or gal try and figure out wht the lens won't stay in:D

Matt,

Your evil man. That's a good one.

[Fezz]

Matt,

Your evil man. That's a good one.

He sure is!

Heck, I bet that he then makes the poor GreenHorns go around the lab asking everybody for the lens stretcher to make the lens fit!

;):cheers::D:cheers:;)

[HarryChiling]

He sure is!

Heck, I bet that he then makes the poor GreenHorns go around the lab asking everybody for the lens stretcher to make the lens fit!

;):cheers::D:cheers:;)

I believe I may have a lens stretcher laying around the lab here somewhere.

[Against the Rule]

Horizontal prism rarely matters anyway except in extreme cases. I honestly can't recall the last time I saw horizontal prism (whether caused by inaccurate PD or inaccurate lab work) cause a problem. Vertical prism is what you need to worry about. If I owned my own optical, I'd be perfect on both, but the truth is that horizontal prism isn't a big deal in most cases.

[FullCircle]

Horizontal prism rarely matters anyway except in extreme cases. I honestly can't recall the last time I saw horizontal prism (whether caused by inaccurate PD or inaccurate lab work) cause a problem. Vertical prism is what you need to worry about. If I owned my own optical, I'd be perfect on both, but the truth is that horizontal prism isn't a big deal in most cases.

Really? My Rx isn't extreme (+1.75) and I'm sensitive to it. I'm not a big fan of that pulling sensation

[Darryl Meister]

Unwanted base out prism is considerably more problematic than unwanted base in prism, since the eyes have much smaller negative fusional vergence reserves (or the capacity to diverge).