Aspheric surfacing question

[opticalslave]

this may be a stupid question, and I feel stupid for having to ask it, but here goes...

My lab techs were having some problens surfacing an Airwear aspheric transition lens. What was supposed to be a -0.50 sphere came out with about a half diopter of cylinder. They ran it twice. I love puzzles and have about ten plus years experience surfacing myself, so I looked at the work ticket and verified that rx was entered into rxcalc correctly, correct base curve was pulled, etc. So I aked if they laid out the lens on the cross. They told me that they never have and they just put a dot on the center of the cross engraved on the lens and disregard the actual "cross". I had no training documentation to pull for them regarding the use or reason for layout of the cross. I was always trained that the cross is there for a reason and to layout the lens with the cross at 0,180, and 90,270. you know like a cross. These guys insist that it makes no difference, yet when I ran the lens for them myself (using the blasted cross) the lens came out right.

So the question is, Am I right that the cross is there as a layout guide, and if so, where can I find documention to explain/prove the point to these techs?

[HarryChiling]

It shouldn't matter how you layout the lenses as long as the blank is rotationaly symetrical. If you look at the work ticket and the tool used is spherical then it whould not make a difference how the cross on the lens is lined up as long as it is centered.

Ok, now that we got that out of the way. It does matter that they are ignoring the layout crosses. They are not just there, because essilor thought it would be fun to put crosss on all their blanks, no they are their because that is a common way for technicians in the lab to layout lenses. They should be laying them out correctly for consistentcy if nothing else.

[opticalslave]

Thank you. They are finally paying attention to the cross and we are having less breakage on these lenses. I still could not find any technical data to explain the reason for the cross on these lenses (techincal people have to have technical reasons), but I am happy that we are not remaking so many lenses.

[braheem24]

The sag on the center of the lens is different from the sag on the peripheries, therefore you want the center of the lens to sit in front of the patient's pupil and the lens to be surfaced with the same concept in mind.

[Angel of Grace]

I also agree that the crew should be using the markings provided by the mfgr to line the lenses up for surfacing. This is just common sense. Have you been making sure that the surfacing equipment is calibrated on a regular basis and kept clean and free of any debris?? Just a thought for imput. :bbg:

[shrimper~dan]

I would venture to say that if an ashperic lens is not blocked on it's geometric center the portion of the lens resting on the block would differ rotaionally, thus creating your prism. Given that an aspheric lens is by definition not shperical across the front of the lens, you would have different curves coming in contact with the blocking suface on either side of the 180 line if the lens is not perfectly centered.

[gemstone]

this may be a stupid question, and I feel stupid for having to ask it, but here goes...

My lab techs were having some problens surfacing an Airwear aspheric transition lens. What was supposed to be a -0.50 sphere came out with about a half diopter of cylinder. They ran it twice. I love puzzles and have about ten plus years experience surfacing myself, so I looked at the work ticket and verified that rx was entered into rxcalc correctly, correct base curve was pulled, etc. So I aked if they laid out the lens on the cross. They told me that they never have and they just put a dot on the center of the cross engraved on the lens and disregard the actual "cross". I had no training documentation to pull for them regarding the use or reason for layout of the cross. I was always trained that the cross is there for a reason and to layout the lens with the cross at 0,180, and 90,270. you know like a cross. These guys insist that it makes no difference, yet when I ran the lens for them myself (using the blasted cross) the lens came out right.

So the question is, Am I right that the cross is there as a layout guide, and if so, where can I find documention to explain/prove the point to these techs?

Do not grind prism for decentration. If you do you move the OC into the aspheric section for the lens creating all kinds of problems.

[wabmorgan]

Do not grind prism for decentration. If you do you move the OC into the aspheric section for the lens creating all kinds of problems.

This aswer hit the NAIL ON THE HEAD. Your can't grind for decentration on aspheric lenses. Even if the Rx come out correct... the aspheric section will not line up correctly with the patients pupils.... Thus the patient will not be looking through the aspheric section of the lens correctly.