I am studing to take my board exam. They are going to make us neutralize a few pairs of glasses. How do you measure for prism without knowing the pd? What are the steps?? I have an auto lensometer and have to re-learn how to use a manual. Any help would be greatly appreciated!!!
Originally Posted by cr8smfun
If it's single vision, you can't. If it's a flattop, you can assume-which-means-guess that the seg inset is 1.5 OU and use the seg location to generate a pd number. If it's a PAL, you've got it made; there is only one point on a PAL where you check for prism. It's not the MRP, by the way.
The only good news here if the lens has little spherical power measuring at a near assumed center for prism will be fine. Or in other words a -1.00 will have to be displace 10 mm to result in 1 diopter of prism, so if you come within a few mm of a "normal" PD say 56-62 mm, you can assmume you are reading the prism accurately. Less powers would even be more reliable. Now if the lens has a good bit of sphere you are probably shot down.
Possibly by measureing both lenses at assumed distance center you might get a close guess.
Chip
Thank you for your help!!!
When I was on the NY State board we would give a pd and have them figure if it had prism or not. Chip gave good advise as well as the others.
You cannot accuratly measure prism w/o pd if the prism is induced by decentering the lens. However if its ground prism, you can get a close guess by measuring prism at different points and averaging.
Now Sharpstick you done re-entered muddy ground. There is no difference between ground in prism and decentered prism.
The only difference is that on low powers one cannot decenter enough to get much prism.
Chip
Oh yeah? Can you decenter a plano? Yet you can have prism in a plano! How many angels can there be on the head of a pin?
Just stirring the pot!
Proving Chip's point. You can decenter for 5 prism diopters with a 0.25 diopter lens; you just need a blank big enough to allow 200mm of decentration. Having found such a blank, and having so decentered, you would have prism just as though you had blocked it in. His second sentence answers your real question. The metaphysics I leave to the eye of the beholder.Oh yeah? Can you decenter a plano? Yet you can have prism in a plano! How many angels can there be on the head of a pin?
Just stirring the pot!
You are correct in the view of patients perscpective, but if I put 2 lenses in a lensomter they would look very different. My context is in reference to neutralizing lenses and trying to assess prism.
How so? I can't perceive any difference between ground and decentered prism, either with a B&L 70 or with automated focimeters.
Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA
“As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein
I'm curious also.
Me too. I believe that if this statement is true, you can also tell whether a -4.00 has been surfaced, or was pulled from stock.
Originally Posted by chip anderson
Now Sharpstick you done re-entered muddy ground. There is no difference between ground in prism and decentered prism. The only difference is that on low powers one cannot decenter enough to get much prism.
Chip
If you re-read the underlined portion of Chips quote you will note that there is only one exception to the rule, otherwise there is no difference.
Mike
A decentered prismatic lens will neutralize in a lensometer just a like a regular lens would, if you don't know the PD you may never know there is prism. It will center in the reticule lines at some point. Prism is obtained only away from that point.
A lens with ground prism will almost never appear perfectly centered on the reticule lines in a lensometer (unless the cyl is at 90 degrees and matches the prism). The prism will extend across the entire lens.
This statement is untrue. If the optical center is ground off the effective lens blank then this statement holds true as in the example given above of a -0.25 decentered 200mm, but if the lens were 400+mm in diameter then you would eventually find the optical center of the glasses.
Yes you can decenter a plano and still get prism if the lens blank were big enough, the exception to the rule is a lens with front and back curves that remain parralel to one another (concentric) which is always assumed a plano lens however is not. A plano lens is actually similar in curvature to a minus lens since the front surface is flatter than the back surface.Can you decenter a plano? Yet you can have prism in a plano!
Another way of looking is that if we grind prism into a lens, are adding prismatic power to the effective corrected power.
If we are dencentering a lens, we are simulating prism through using only the power that is already there, we are not really adding anything. That is why low powers with prism have to be ground, there is not enough power to simulate prism through decentering.
I can put both in the lensometer and tell if the prism is ground or induced through decentration. They will look the same at PD but will appear differently off PD.
Prism is not power, prism is afocal where as a diopter in reference to power is focal. Grinding prism is only done when their are set front surface optics in the case of aspherics, multifocals, and progressive addition lenses. For spherical lenses decentration is equal to the "EXACT" same thing as ground in prism. The only reason to grind prism into a spherical lens is that the blank is not big enough to produce the intended prismatic effect.
My knowledge on the subject has been exhausted but I do want to wish you a happy holiday.
With spherical lenses, there is no difference in curvature, thickness, or ray point deviation between decenter or ground prisms (all things equal; material, edge/center thickness, ect). Once fabricated into the frame, you can't tell the difference. The only lens type to tell if the prism was induced or ground would be an aspheric or atoric, and at that you would need to determine the center of asphericity to calculate the amount of prism.
All lenses (that aren't plano) are a set of prisms in concentric form. Observing any portion of the lens (except the OC) means observing prism. Whether the front and back curves are non-parallel because of angling one surface, or simply the natural deviation that comes from lens power, makes no difference. If a lens has base out prism, it will show the same prism at any point on the 180, plus any prism resulting from shifting your point of observation. All this is true however you produce the prism in the first place.
Last edited by finefocus; 12-28-2011 at 02:19 PM. Reason: I didn't allow for aspheric/atoric in the above post.
I have to say that I love and hate these conversations.
On the love side, we are exploring and exchanging ideas and opinions - they vary, and it's all in the spirit of learning.
On the hate side - everyone who has taken the time to join here, read, and try to help others learn represents the top 1% of opticians to me; but even among us, we don't have a consistent base of knowledge. The question of differences between the two ways to create prism is an optical question with only one correct answer - yet our answers have range where there shouldn't be range. Can someone run the experiment and take pics of video to show the final answer once and for all?
To the original poster, I would expect the neutralizing test to go one of two ways. For SV, if they do not designate a PD, you will be asked to assume that there is no prism, and will just neutralize with the OC lined up in the center of the reticle. I think more likely they will give you a PD, and you'll read the lenses at that PD, notating any prism. (I think we did the latter at my boards).
Good luck!
William Walker
Associates in Science in Opticianry
Associates in Science in Optical Business Management
Licensed Dispensing Optician
Board Certified
Certified Paraoptometric Assistant
American Board of Opticianry Advanced Certified
National Contact Lens Examiners Certified
Next Goal: ABOM
Optician with Lenscrafters in Jacksonville, FL
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