Didn't see a mention of working with the size of these.
I don't know any formula to it or anything, but the diameter is trickier
than regular conical spotlights from what I've seen.
The radius needs to be pretty high if you want coverage of an area where
the cylinder points to. Tightness makes a big difference too.
No matter what though the two types do not equal their illumination of
area. Spotlight can vary intensity from full to fading over any area and
cylinder lights remain constrained to a central point which varies
outward in size yet doesn't reach full intensity over the entire radius.

Following is a comparison:

//BEGIN
// approx. 4 X 3 window at 0*z
camera
{
  location  <0.0 , 0.0 ,-9.0>
  direction 3*z
  look_at   <0.0 , 0.0 , 0.0>
}

// create a point "spotlight" (conical/directed) light source
light_source
{
  -10*z
  color rgb 1
  spotlight
  point_at <0, 0, 0>
  radius 1
  tightness 10
  falloff 10
  translate -1.5*x
}

// create an area "spotlight" (cylindrical/directed) light source
light_source
{
  -10*z
  color rgb 1
  spotlight
  cylinder
  point_at <0, 0, 0>
  radius 50     // larger numbers required to equal area spot
  tightness 50 // smaller numbers here to expand, larger to shrink
  falloff 50  // notice radius and falloff never merge
  translate 1.5*x
}

plane {z,0 pigment {rgb 1}}
//END


Spider wrote:
> 
> Hello again.
> Time to end the html flame, at least in my opininon..
> So I thought I'd get RTFM'd a bit..
> Can someone explain the _uses_ of a cylinder light ???
> 
> //Spider

-- 
 omniVERSE: beyond the universe
  http://members.aol.com/inversez/POVring.htm
=Bob

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