> Hi
> I found on Google a scene exemplifying the use of one fresnel lens. I have tried
> to change the scene to put another fresnel lens in front of the converged light
> coming from the fresnel lens. The problem is that light does not pass across
> this second fresnel lens and just "stops" there with no effect. I would be
> grateful if somebody pointed me out why is this simple addition not working or
> at least point to the concept which i am misunderstanding.
>
> Below is the scene file.
> Thanks
> Paulo Neves
>
>
> The newsgroup link is here:
>
http://news.povray.org/povray.binaries.images/message/%3Cweb.439fd98c9020f8e92b1529d00%40news.povray.org%3E/#%3Cweb.439
> fd98c9020f8e92b1529d00%40news.povray.org%3E
>
>
> //-------------------------------------------------------------
>
> // Persistence Of Vision raytracer version 3.5 sample file.
> // File: optics.pov
> // Author: Christopher J. Huff
> //
> // Modified by PM 2Ring. Show convergence of a Fresnel lens.
> //
> // -w320 -h240
> // -w800 -h600 +a0.3
>
> #version 3.5;
>
> global_settings {
>      assumed_gamma 1
>      //max_trace_level 10

Did you try uncommenting this line?

My diagnostic follow:
Trace level 1 at the media box. (should not count if you use version 3.7)
Trace level 2 at the front face of the first lense.
Trace level 3 at the back face of the first lense.
Trace level 4 at the front face of the second lense.
Trace level 5 at the back face of the second lense: tracing STOP here 
before exiting that lense.

max_trace_level default to 5.
You need at least max_trace_level 6 for your case, some more if there is 
any other transparent or reflectife surfaces to concider.

Another solution would be to extend the media box to include the 
light_source.

Whatever the version, that requirement will not change. In version 3.7, 
the trace level is not increased ONLY if there is no reflection NOR 
refraction. This means NO ior in the interior, and no reflection term in 
the finish.
Not sure how it work if the object contains any media.



Alain

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