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.
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.
Prentices Rule is a small angle approximation so the prism will not hold true for low powers.
If we look at the threshold for the tolerance 3.37D, using Prentices Rule the decntration value to reach 0.33PD: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.
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.
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.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.
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.
Last edited by MakeOptics; 04-28-2015 at 08:27 AM.
http://www.opticians.cc
Creator of the industries 1st HTML5 Browser based tracer software.
Creator of the industries 1st Mac tracer software.
Creator of the industries 1st Linux tracer software.
Always remember that the precision or accuracy of the prescribed prism is never a given. Discretion rules in prism prescribing.
B
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.
Last edited by MakeOptics; 05-01-2015 at 08:42 AM.
http://www.opticians.cc
Creator of the industries 1st HTML5 Browser based tracer software.
Creator of the industries 1st Mac tracer software.
Creator of the industries 1st Linux tracer software.
http://www.opticians.cc
Creator of the industries 1st HTML5 Browser based tracer software.
Creator of the industries 1st Mac tracer software.
Creator of the industries 1st Linux tracer software.
There are currently 1 users browsing this thread. (0 members and 1 guests)
Bookmarks