I have a question about free form technical.
The semi-finished lenses for free form, is there any special requirment or just whatever semi-fninished lenses are acceptable for the free form machine ?
I have a question about free form technical.
The semi-finished lenses for free form, is there any special requirment or just whatever semi-fninished lenses are acceptable for the free form machine ?
Usually they are sperical SV and are made in one diopter steps. Nothing really terribly special about them. :D
Some of the lens design systems require that you use lenses from an approved range; some don't.
From a fabrication standpoint, some labs use "pre-engraved" semi-finished single vision blanks, while others have equipment to apply the reference marks after the fact.
I suppose you could depend on the PRP to find the other reference points - a freeform PAL is less likely to have unwanted prism. I wouldn't use the axis at the PRP for rotational orientation; the axis at the DRP is probably OK... Still, I'd rather see reference marks, if barely.
Of course we have all had the occasional patient who absolutely rejected lenses because they could "see" the reference marks. In truth, laser marks are a form of aberration as is any deviation in a lens.
We should have instruments that we can absolutely positivily trust that give us this information.
Chip
Have a question here, is it means that a free-form multifocal lens is come from a spherical SV blank?
Most do. Some have tried to use aspheric SV, but I don't think successfuly. There are some progressives that have part of the progression on the plus side and the rest freeformed on the minus. A few labs are trying to use moulded segmented multifocals with the minus side surfaced using the same software as the freeform SV.
With the advent of freeform technology the possibilities have become mind boggleing.
:cheers:
Last edited by Jacqui; 07-13-2009 at 08:01 AM. Reason: Added, Deleted, Multiplied
This page may also help show different manufactures blanks.
http://www.thelensguru.com/digitalChart.php
Hope it helps.:)
thanks Jacqui, it is really a new idea to me..
I'm not a lab person so maybe they can add to my statements regarding your questions but in reference to question 1, yes- any lens blank can have freeform powers generated on the back.
Questiopn 2 quite honestly makes me wonder just how much optical experience you have. Do you know how a lens blank is created from a mold?
Before digitally compensated lens creation technology came along each company had a design that they feel makes a progressive with the best optics. Optical engineers determined that by polishing spherical molds with various hyperbolas, parabolas, ellipses and other curves they could create a corridor where the add power was controlled and no line showed. These curves left distortions that can never be eliminated only adjusted. Companies patent these designs.
Now along comes software that allows a much more refined technique of putting the add power onto the mold using what looks like a spindal able to rotate on multiple axis to create the design. If this is done on the front surface of the mold we have a digitally created mold. Any mold with add power on its front (convex) surface is aspherical. A back (concave) spherical mold can now be manufactured. Whatever resin is being used can now be injected (for poly under very high pressure) to create the blank. The semi finished blank can now have the power of the distance rx put into the concave unfinished surface by either conventional methods or freeform.
So the ways of applying the freeform to a blank can be either on a mold or directly onto a lens. The add can be on the front, back or both! If the add is only on the back any spherical front surfaced blank can be used however Seiko and Zeiss in the United States own patents that limits many companies from duplicating back surface only designs.
This is my understanding in brief terms how many progressives are created. As I said in the beginning I am the first to admit I am not a lens manufacturer or engineer so I'll welcome any correction they may offer to my simplistic understanding of how it works.
This course by Daryl Meister may also help-
http://www.opticampus.com/cecourse.p...essive_lenses/
Last edited by Uncle Fester; 07-15-2009 at 08:15 AM. Reason: Incorporate Guru Shanbaum's corrections...
C'mon, Fester, he said he was an Optometrist!
Only two corrections: while polycarb is injection molded at high pressure, CR-39 and its brethren are not; the assembled molds are simply filled. I don't really know into which category Trivex falls in that regard.
Seiko and Zeiss (quite remarkably) both have patents on back-side progressives (at least where the other corrective powers are combined with the add) in the U.S. but not in Europe, where the patent system is relatively more sane.
hi Fester really appreciate your post.. i really learn a lot here.. anyhow i'm just have 2 years experience in optical beside the 4 years of study.
i really fall in love in this board..
Yeap
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