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, as illustrated 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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