Brian White nous illumina en ce 2009-01-11 16:11 -->
> In September I made a "compound" parabolic solar cooker. I took half a parabola,
> twisted it round its focus by 22.5 degrees, made a form to the curve and then
> used this to make a dish mold. I folded and cut corrugated plastic sheet over
> the mold to make the dishes. It worked really well. The theory is that it
> concentrates the light (even when not pointed right at the sun) to an area
> behind the focal point for up to 3 hours. (Which is where I place the cooking
> pot)  This is very useful for solar cooking. Parabolic dishes on the other
> hand, produce a point of concentrated light which rapidly becomes a spread out
> streak of light as the sun moves. (So the dish has to be constantly re
> adjusted).
> So, it worked really well, BUT. I only did the math in 2 dimensions. I do not
> know what happens in 3 d in real life.  Especially supposing the sun's path  is
>  a bit above or below where the dish is pointed. What happens?
> This is important if someone in a poor country makes a stationary large cheap
> solar cooker with mud backing the reflector.  Will the light all still focus on
> the cooking pot for almost 3 hours a day? Or will a large amount of light miss
> the pot completely?  A 30 degree twist to a half parabola will give a 4 hour
> collector if rotated round the axis.  This might be more suitable in that
> situation.  I will be modeling the situation soon with a little clay model and
> a laser pointer to represent the sun. (I will have to move the sun a lot to
> model it!) Anyone want to try with povray?
> Another useful task is modeling a parabolic dish NOT pointing directly at the
> sun.
>  People make parabolic dishes all the time but then find that there is no
> tracking system accurate enough to make them useful!  I have video of the
> compound parabolic dome mold on utube and there is some info at the link below
> too. Thank you for your time.
>  Brian White
> 
> 
> http://www.instructables.com/id/Compound_parabolic_dish_making_a_template_with_te/
> 
> 
Prety easy in POV-Ray using photons and a parallel light source.
Use some scattering media to show the light path, or put a target object at the 
focal point to show the light pattern at that location.

-- 
Alain
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God doesn't play dice. -- Albert Einstein

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