Ray Tracing of Free-form Progressive Lenses
I have been told for years that Ray Tracing Free-form lenses was impossible. I think I found a way that will enable us to Ray Trace Free-form Progressives effectively, although imperfectly.
Other Posts in this forum explain Ray Tracing better, but to make it simple Ray Tracing is basically a lensometer that measures multiple points of a Progressive lens at the same time, and then graphs them in a drawing that looks like a colored Topo map. Ray Tracing traditionally will provide 2 sets of data, variation in spherical power and variation in astigmatism. Since we use samples without astigmatism, its presence is one sign of distortion. Light is moving through the areas of higher astigmatism without a clear focal point. Spherical power is easier to interpret as it shows us where the lens fails to provide the intended correction, just like your lensometer does but at multiple points.
The difficulty with traditional ground progressives is not getting the Ray Tracing data, but interpreting it. For someone to get better picture we have to overlay both sets of data which no software that I have seen actually does. The other problems with Ray Tracing are that there is no currently ability to gauge binocular symmetry or judge a corridor in reference to Listing Laws, having a wide corridor is one thing, placing it before the path of the eye another. Although these could be resolve with advanced software, I lack the resources to explore those options fully. So inherently Ray Tracing is an imperfect, but valuable tool to compare one progressive design to another. Again, this won’t solve the limitations inherent in Ray Tracing, but it will give new life to this great tool. I hope that someone can take this information and use it effectively to the benefit of the Optical community.
Some of the newer Ray Tracers are doing work of lensometers after surfacing. Instead of just checking a lens at one point, they verify both the design and powers of a Free-form lens are as intended across its entire surface. Ray Tracing is the only way to do this. They work by Ray Tracing the finished progressive and comparing it to its own theoretical point file. The point file is the design as the manufacturer intended. It then resolves the two data, and ideally the picture should be blank. If it isn’t it reveals the variations between the actual lens and the intended, giving the ability screen a poor lens. An example of this Ray Tracer is at http://www.rotlex.com/ffv.asp. By using this technology, I believe we can effectively Ray Trace Free-form lenses.
We would use comparative Ray Tracing but instead of comparing a Progressive to its own point file, we compare it to a SV Free-form lens by the same manufacturer. Although traditional Ray Tracing works best with Plano Distance Progressives samples, for this we would have to move into using powered lenses instead as they would be better representatives of the Aspheric (and other) compensations involved. Free-form lenses need some room to run so to speak, and we need to give it to them. For example, my sample lenses would be Brand A Progressive and Brand A Single Vision, with the same distance power (a -2.00 sphere would work) with a +2.00 Add on the Progressive (as its average). We would Ray Trace both, but then use the single vision point file as the comparison. Since they are built from the same algorithms, the Single Vision Lens would effectively neutralize everything but the actual progressive design. We would be left with an effective Ray Trace that would reveal only the Progressive attributes of that lens. In effect, we would have the same Ray Trace we have now with Grinders.
This could be done with Free-form progressives that did not have a Single Vision equivalent as well, but with less accuracy. Most of the algorithms used to compensate for face form, panto etc, are fairly standard. In this case would we take a standard -2.00 Aspheric SV, Ray Trace it, create a point file, and then modify that file based on the standard algorithms and matching the power at OC. Although this would not be perfect, this would neutralized out most (but not all) of the fudge factors. It would still reveal the basic flavor of that lens.
In the future, we would need to combine Comparative Ray Tracing with advance software that could analyze the lens against multiple criteria to form a better single picture. This is all possible to current technology; it’s just a matter of their being not enough market to justify the costs involved. I don't have access to the new Ray Tracers, but if someone does and wants to test this out, it would be terrific.
Van Y. Rue -- 2/2/2012
Single Vision Express – Director of Business Development
Optical Consulting Group - Consultant
Highline Community College - Instructor - Business and Optics