Re: ATM LED Fou Cult Tester: still not right

[Jean Marc Becker <Jean-Marc.Becker@wanadoo.fr>]

Nils Olof Carlin wrote:
> 
> Richard,
> 
> > I have cut the lens off of a radio shack jumbo 12000 mcd
> > orange LED, and polished the exit approximately flat.  It seems
> > to put out light in all directions.  When I shine this light on a
> > white screen, there are no hot spots or shadows.
> >
> > BUT...  when I put a pinhole in front of it, there is a pinhole
> > camera effect, and an image of the light source is formed.  This
> > is a ring shaped area with a dark center.  The goal of a uniformly
> > illuminated pinhole is VERY elusive!


If the pinhole is great enough to give a camera effect, that means he is 
too wide. In this case, it is the size of the source who determines the 
actual dimension of the hole. When the hole is of good size, the 
diffraction effect dominates, and the edges of the hole are the actual 
source of light. Information from the size or shape of the lamp behind 
are lost. A well sized hole is too tiny to admit enough light from a 
LED. A slit, 5mm in height is far more efficient. His width must be 
adjustable, so that, looking through, we see the interference fringes 
meanning that the lips of the slit are the actual source of light.
In my own Foucault tester, I use a 12 volts halogene, used in cars 
lighthing, with a 6 volts alimentation. I grinded the face of the bulb 
to unpolish it, as to not be diturbed by the spiral filament. 

Jean Marc BECKER