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I've been messing the with fresnel macro for a while, trying to create a round
fresnel lens (rather than a flat one) and have had some problems, mostly in
getting the lens to do what I want... I'm comming to the conclusion that
either:
a) my math is wrong
or
b) this has something to do with how POV handles light....
which brings me to this post.
I'm assuming at this point that the reason there is a photon patch for POV is
that POV doesn't track light, but rather it's effect on other objects (i.e.
reflection).... if this is the case, could that be the reason why when I make
what should be a working fresnel lens and place a light source where I think
it should go the whole lens doesn't light up.
Or could it be that I a) don't have the light in the correct position, b) made
a silly asumption about the focal length, c) don't have a clue what I'm doing,
or d) that I'm trying to have POV do something it just can't do.
I can fake my way around the lens I think, that shouldn't be too big a
problem.... but this whole thing is beginning to fustrate me.
PHIL
---------------------------------------------------
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Philip Bartol wrote:
>
> b) this has something to do with how POV handles light....
>
> which brings me to this post.
>
> I'm assuming at this point that the reason there is a photon patch for POV is
> that POV doesn't track light, but rather it's effect on other objects (i.e.
> reflection).... if this is the case, could that be the reason why when I make
This is indeed the case. Ray tracers trace rays of light _backwards_ from
the camera into the scene and determine the amount of light at a point by
using 'shadow rays' which always go straight from the point in question to
the various light sources in the scene. Shadow rays are _not_ bent by
lenses, bounced by mirrors, etc. You will need to use photon mapping
(available at http://nathan.kopp.com/patched.htm) if you want to focus
light with a lens or reflect light off of a mirror.
-Nathan
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Philip;
Your attempt to model the physical behavior of a fresnel lens using POV-Ray
is probably doomed to failure, even with the "photon patch", because a
fresnel lens, unlike an ordinary glass lens, does not "bend" light by virtue
of slowing down light differentially along the radial axis, but by wave
interference and the mutual interaction of an infinite number of spherical
wavelets eminating from the disturbance, in this case, concentric circular
grooves on a transparent substrate. Since the photon patch probably relies
on a simple particle model and not a wave model of light, it is doubtful
that you will be able to reproduce this effect. To realistically model light
in all its glory, one must incorporate both particle and wave behavior. The
dominant property observed depends on the particular phenomenon being
studied. Specular reflection could be considered "most particle like", and
Young's two-slit experiment "most wave like".
People in the oil business, also perform "ray tracing" but their software
does in fact treat the energy, in this case a mechanical disturbance, as
waves, with frequency, amplitude, phase, mode, and polarization. Also,
seismic simulations are notorious for being computationally intensive.
Gregg
------------------
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I think you may be confusing a Fresnel lens with a diffraction grating.
Fresnel is simply a compression of the relevant portions of a standard lens.
...or maybe I am wrong. But I have used the Fresnel measurements to make a
solar reflector that worked just like the parabolic dish reflector I modeled
it on.
gregg <gp### [at] nmt edu> wrote in message news:37c5fb73@news.povray.org...
> Philip;
> Your attempt to model the physical behavior of a fresnel lens using
POV-Ray
> is probably doomed to failure, even with the "photon patch", because a
> fresnel lens, unlike an ordinary glass lens, does not "bend" light by
virtue
> of slowing down light differentially along the radial axis, but by wave
> interference and the mutual interaction of an infinite number of spherical
> wavelets eminating from the disturbance, in this case, concentric circular
> grooves on a transparent substrate. Since the photon patch probably relies
> on a simple particle model and not a wave model of light, it is doubtful
> that you will be able to reproduce this effect. To realistically model
light
> in all its glory, one must incorporate both particle and wave behavior.
The
> dominant property observed depends on the particular phenomenon being
> studied. Specular reflection could be considered "most particle like", and
> Young's two-slit experiment "most wave like".
>
> People in the oil business, also perform "ray tracing" but their software
> does in fact treat the energy, in this case a mechanical disturbance, as
> waves, with frequency, amplitude, phase, mode, and polarization. Also,
> seismic simulations are notorious for being computationally intensive.
>
> Gregg
> ------------------
>
>
>
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gregg wrote:
> that you will be able to reproduce this effect. To realistically model light
> in all its glory, one must incorporate both particle and wave behavior. The
If you model the wave behaviour that will be enough. There is an early
paper
in the siggraph journals about realistic wave modelling of a scene. I
have tried
to hack up a wave modeller, but it is slightly out of my reach
computationally.
I do think that a serious effort should be made in this direction.
--
Tore Nordstrand | nfy### [at] rasmusuibno
| http://www.uib.no/People/nfytn/
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In article <37c60299@news.povray.org>, "Bill DeWitt" <the### [at] earthlinknet> wrote:
>I think you may be confusing a Fresnel lens with a diffraction grating.
>Fresnel is simply a compression of the relevant portions of a standard lens.
>....or maybe I am wrong. But I have used the Fresnel measurements to make a
>solar reflector that worked just like the parabolic dish reflector I modeled
>it on.
Fresnel lenses have been used for years and years in lightouses to magnify the
light... not to long ago they showed how this is used on "How 2" a BBC
television show running on the "Wam" network (which I tape and watch on
Sundays on my local cable network). I tend to agree that I'm wasting my time
at this point, I think the best thing for me to do is go with a visual thing
and use a cylinder light source to represent the beam (this is the basic
principle of using a combination of a fresnel lens with a parabolic
reflector).
PHIL
---------------------------------------------------
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Actually, the correct technical name for the device I speak of is a zone
plate.
Gregg
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gregg wrote in message <37c5fb73@news.povray.org>...
>Philip;
>Your attempt to model the physical behavior of a fresnel lens using POV-Ray
>is probably doomed to failure, even with the "photon patch", because a
>fresnel lens, unlike an ordinary glass lens, does not "bend" light by
virtue
>of slowing down light differentially along the radial axis, but by wave
>interference and the mutual interaction of an infinite number of spherical
>wavelets eminating from the disturbance, in this case, concentric circular
>grooves on a transparent substrate.
The Fresnel lenses I have seen have components that are so much bigger that
a light wavelength that this cannot be the effect causing the bending of
light.
>Since the photon patch probably relies
>on a simple particle model and not a wave model of light, it is doubtful
>that you will be able to reproduce this effect.
True, the photon patch uses the particle model of light, but as a Fresnel
lens works by bending light the same way that a normal lens bends light, and
the photon patch works just fine with normal lenses, the photon patch will
work with Fresnel lenses.
Mark
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I've seen a description of Fresnel's invention a short time ago, how it was
designed for lighthouses. I agree, it's a normal lens with the "stair
steps" taken out.
I've seen the scratch thing called Fresnel as well, but I don't know if
that's correct, or if the name is now applied to any kind of flat lens.
--John
Bill DeWitt <the### [at] earthlinknet> wrote in message
news:37c60299@news.povray.org...
> I think you may be confusing a Fresnel lens with a diffraction grating.
> Fresnel is simply a compression of the relevant portions of a standard
lens.
> ...or maybe I am wrong. But I have used the Fresnel measurements to make a
> solar reflector that worked just like the parabolic dish reflector I
modeled
> it on.
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John M. Dlugosz wrote in message <37cc8b6c@news.povray.org>...
>I've seen a description of Fresnel's invention a short time ago, how it was
>designed for lighthouses. I agree, it's a normal lens with the "stair
>steps" taken out.
>
>I've seen the scratch thing called Fresnel as well, but I don't know if
>that's correct, or if the name is now applied to any kind of flat lens.
It's still a Fresnel lens, just even more compressed than the larger lenses.
The nice thing about Fresnel lenses is that you can make them quite flat and
the lens will still work.
Mark
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