Questions on ANSI Standards

[Roy R. Ferguson]

Hi Guys:



I’m doing a bit of informal test validation and would like to solicit your assistance. According to ANSI standards for progressive addition lenses, “the fitting cross and the distance interpupillary distance will be used, and each individual lens will vary by no more than 1 mm from the monocular specification.” My questions to this group of assembled experts are:

1. Given a pair of unmarked progressive lenses mounted in a frame, should an experienced, licensed/certified optician be expected to determine the monocular fitting cross height and monocular decentration within 1mm?

2. If not, what would be a reasonable standard? Please explain.

3. Given a pair of unmarked progressive lenses mounted in a frame, should a candidate taking a state licensing examination be expected to determine the monocular fitting cross height and monocular decentration within 1mm?

4. If not, what would be a reasonable standard? Please explain.



When answering, please include your total number of years in the field and professional certifications. Thanks a great deal for your thoughts.



Roy R. Ferguson

[mrba]

Answers:

1. Yes, given that they have previous knowledge of the desighn, and the proper layout chart. They will need to mark it if it is unmarked. This should be a standard daily used skill of any dispensor.

2. Answer was Yes

3. If in the exam you are provided the proper tools, ie: pen, pd stick, layout chart, then yes it is extreamly fair and an excellent Idea to test for that!

4. It may be reasonable for the examination board to give a tiny bit of breathing room say .5mm since sometimes people's mm are not always the same! Opticians are human.

ABOC, BA
15yrs exp.

[Darryl Meister]

Hi Roy,

Just to clarify, you mention "unmarked" progressives. In order to verify a progressive lens, whether in practice or as part of a licensing exam, the lens must be remarked using the semi-visible etchings of the lens and the manufacturer's verification template, if the markings have indeed been removed. Once this has been done, verifying the position of the fitting point is no more difficult (and probably easier) than verifying the position of a lined multifocal segment.

Also, some of your questions might seem a bit ambiguous or confusing. The tolerance provided by the Z80.1 Standard is 1 mm, and this is independent of measurement precision and accuracy. For instance, if a job calls for a fitting height of 17 mm, and the lab produces an actual fitting height of 17.2 mm, but I measure it as roughly 19 mm (an inaccurate measurement), I would simply apply my tolerance of 1 mm to my measurement of 19 mm. In this case, my tolerance was 1 mm, my precision was less than 1 mm (using a PD ruler), and my accuracy was way off -- even after taking into consideration my precision. Consequently, we should keep separate the notions of measurement tolerance, precision, and accuracy.

That said, the development of tolerances is certainly influenced by what we expect opticians to be capable of in terms of measurement precision and accuracy. It would make no sense to provide a geometric tolerance of 0.25 mm if opticians are not capable of discerning that measurement with the tools typically at their disposal. However, we know that opticians measure to within at least 1 mm of precision since all measuring devices currently on the market have at least that much precision. Even a PD ruler is precise to at least 0.5 mm (one-half the increment of measurement). This fact is reflected by the measurements sent to processing labs, as well.

Consequently, the tolerance is not limited so much by the measurement capability of opticians in this case, but rather the process capability of the finishing laboratory -- as well as the demands of the actual wearer. In the case of progressive lenses, a 2.5-mm error in the monocular PD, which previous Z80.1 Standards allowed, would translate the intermediate corridor of some lenses entirely out of the primary line of sight, in addition to reducing severely the binocular fields of view through all viewing zones.

Best regards,
Darryl Meister
(14 years, ABOM & OAAR, and a Z80.1 Co-Editor since 1997)

[harry a saake]

Roy, salutations and greetings, certainly good to see you back on the board. In my own opinion, i have always thought that far too much emphasis has been put on the accuracy of PD,s. I do not believe in most cases they are that critical. I noted that once in a post i wrote and was pleased to see that David Wilson, Darryl Meisters Australian counterpart, had much the same thoughts, and i consider David as one of thee authourities on optics.
.....Alluding to your comment on the heights, because of so many factors, probably the main one being the parallax factor, opticians as a rule are probably not going to be that accurate. As a supposition if the height were within 2mm, would the patient see well, probably so. I think it may be more of a question of how far can you be off before a problem is created.
.....The test should probably be if the progessive has discernable markings, and the optician has the right layout chart, one would expect the opticain should be able to mark the lens with reasonable accuracy, keeping in mind that even when laying out on a chart, if your not careful paralllax can creep into the equation. None of this should be constreud by anyone to think i am advocating not being accurate.
.....As to your last question 40 years, licensed in 6 states, abo,, ncle, currently licensed in 2.

[Roy R. Ferguson]

Hi Again:



Let me attempt to clarify my first clumsy question.

Given a pair progressive lenses with semi-visible etchings of the lens and the manufacturer's verification template, should an experienced, licensed/certified optician or candidate taking a state licensing examination be expected to determine the monocular fitting cross height and monocular decentration within 1mm after the lenses are mounted in a frame? A Pilot Fineliner Marker, Wet-Erase Fine Point Marker, and PAL Identifier would be provided. The individual would provide his or her own millimeter ruler.



In other words, Ms. Jones walks into your dispensary with a pair of progressive lenses purchased elsewhere and asks that only one lens be replaced. The assigned task is to mark the lenses up and determine the monocular fitting cross height and monocular decentration for each lens. At this point, I’m not interested in any other parameters.



Again, please include your total number of years in the field and professional certifications.



Roy R. Ferguson

[Darryl Meister]

Then, yes, in my opinion a minimally trained optician should have no problems measuring these parameters to within 1 mm for most jobs. Of course, factors such as thick rims, exotic frame shapes (e.g., aviators), and steep base curves may make accurate measurements more difficult. However, I feel that a thoroughly trained optician should understand how to ensure accurate measurements even with these additional factors at work.

Best regards,
Darryl

[harry a saake]

Roy, while i agree with what Darryl has stated, i don,t think your going to see it happen. My estimate would be 60% will be off more then that, from what i have seen in the past. Roy, i would also think that wherever the fitting cross is , would only be subject to whomever the authourity at the test said it was.
...MY second thought was would this question be better answered by you taking this to 10 random licensed opticians and see what they determine.
..... Again while i agree with what Darryl alluded to, and you notice he stated MOST, there is a subjective element here that may come into the equation, regards

[Blake]

I don't see any reason why what you are proposing shouldn't be possible. Marking up a progressive lens isn't exactly advanced stuff. Heck, I showed someone how to do it yesterday in about 2 minutes. Of course I am assuming the pair of glasses provided would be a fairly straightforward job - nothing too fancy like what Darryl mentioned.


Blake Wigfield
6 1/2 yrs.
ABOC

[David Wilson]

Hi Roy,
Nice to hear from you. We actually met many years ago when you were down here in Sydney. I agree with Darryl's comments, and that is essentially how our standards are framed too. I am on the Aussie equivalent of the Z80 committee (our version of Z80 is AS2228). I also agree with his comments on the distinction between accuracy of measurements and the tolerances. I seem to recall making a comment on an earlier post (much earlier) that tolerances 'assume' that measurements have been made accurately, though the committees are aware of the issues of measurement accuracy and realistic precision in manufacture. I think that this may be the comment you were referring to, Harry. [Since writing this, Harry, I've had a second thought about what you may be referring to (hence the edit). I've long argued that aspheric single vision lenses, when used for readers, should be set at distance PD, for reasons of improved optics. While this creates unwanted differential prismatic effect it generally falls well within the range of fusional reserves and matches the DPE created by bifocals or progressives when reading. Perhaps this is what I was referring to, Harry.] By the way, Harry. thanks for the comments but any comparison with Darryl is best described as flattering; not accurate! As for PD, generally, my views on PDs and their measurement are best summed up in a post a couple of years back in which both Darryl and I declared our support for the pupillometer.

Regards
David

[Texas Ranger]

Roy, first, this determination is quite elementary. and I don't think you'd even require a layout chart. I don't know of any pal designs that are different from the reference dots being 34 mm apart, and most mrps are 4 mm above the plane of the reference dots, a couple designs are 3mm. occatioanlly, the nasal dot may be cut off in the finishing process on very narrow pds. but a good thing to do would be to place an ink dot 17mm nasally from the temporal reference dot, measure from there to the frames lowest margin, then add 4mm, voila, height, now measure from the 17mm in dot to the center of the bridege and you have the monocular pd, no need of a layout...38 years exp. ABOC, non-licensed Texan.

[Pete Hanlin]

Given a pair progressive lenses with semi-visible etchings of the lens and the manufacturer's verification template, should an experienced, licensed/certified optician or candidate taking a state licensing examination be expected to determine the monocular fitting cross height and monocular decentration within 1mm after the lenses are mounted in a frame? A Pilot Fineliner Marker, Wet-Erase Fine Point Marker, and PAL Identifier would be provided. The individual would provide his or her own millimeter ruler.
Yes, an experienced optician should be capable of determining the monocular fitting height and pupillary distances of a pair of mounted PAL lenses within 1mm of each direction if s/he is provided with the following equipment: 1.) a pen capable of accurately marking the lens (i.e., capable of producing a mark which is not repelled by the surface of the lens), and 2.) a fitting verification chart appropriate to the particular design of the PAL.

If your intention in providing only a PAL Identifier (e.g., the OLA publication) is to require the candidate to discern and recognize the geometrical layout of the PAL, I would suggest the provision of two generic verification charts- one with a LRP 4mm above the PRP and one with the LRP 2mm above the PRP (which covers most PAL designs). In my opinion, a PD ruler by itself would be inadequate to measure the location of a LRP within 1mm in all directions (although it may be adequate for actual practice, for testing purposes it introduces an unnecessary variable).

Pete Hanlin (ABOM, LDO [Florida] - 15 years)

[Lee Prewitt]

Roy,


Given all the data that you mentioned, all the tools are there to accurately lay out the lens and take the measurement. This exercise is truly no more difficult as marking a lens' MRP and measuring a PD. Texas Ranger's solution is accurate too. I have on many occassions only been able to find the temporal marking but knowing that the nasal marking was 34mm distance was able to complete markup. I had forgotten that. I am always amazed how much of the "little" stuff that I commonly forget. :hammer:

21 years
ABOC-AC
WA licensed

PS. What test are you preparing for?

[Pete Hanlin]

While I agree a PD ruler could be used to accurately measure the location of the layout reference point, it could also be argued that a PD ruler can be used to measure pupillary distance.

However, I would expect the experienced optician would be familiar with the tools designed to do these tasks with the greatest degree of accuracy (in this case, a fitting/layout chart for the PAL and a pupilometer for measuring pupillary distance).

I think a measure of competence (I'm assuming you are developing a test) should be based upon the equipment that is both optimal and readily available to the practitioner- not upon how adeptly the practitioner can utilize a less optimal device. The same argument applies to the use of devices like calculators. The candidate's capacity to properly analyze and answer the question variables is being tested- not the raw ability to perform mathematical calculations.

[Darryl Meister]

While I agree a PD ruler could be used to accurately measure the location of the layout reference point, it could also be argued that a PD ruler can be used to measure pupillary distance...

Is that really the same comparison though? A PAL layout is little more than a PD ruler (millimetric scale) with some fixed reference lines to keep everything square. While this is certainly more convenient than trying to measure the vertical most distance to the plane tangent to the bottom of the lens with a PD ruler, that's also the extent of its improvement in accuracy. And it's even less of an issue nowadays since most frames are relatively "boxy" in shape.

A pupilometer, on the other hand, is nothing like a PD ruler. It measures to a greater degree of precision than a PD ruler, ensures no parallax error, locates the visual axis of the eye, holds the patient's head still, takes monocular measurements relative to how the frame will sit on the bridge, and even allows you to occlude one eye at a time.

Best regards,
Darryl