Le 2019-12-22 à 19:01, AndreyG a écrit :
> Thank you everybody for the comments and encouragement. Ok. I accept that: I can
> not see the light only it's reflection I can see.
> I create a mirror (ambient 0 diffuse 0 reflection 1 -perfect mirror?) point
> spotlight to the mirror at 45deg and look at the place where it reflected with
> camera (at -45deg).
> 
> I do indeed see bright circular spot but ...
> I defined spot light with sharp cut-off: radius=falloff= 30(deg)
> With light source being 283(mm) from mirror I expect circle to be 327mm dia
> spot. And spot I see is much narrower.
> What's more, when I change beamwidth of spotlight from 30 to 15 deg, diameter of
> spot depicted by camera does not change.
> https://imgur.com/2ofeH7W
> ?
> I create 2d mirror to reflect the beam of light one more time. I expect to see
> spot twice as wide with intensity diminished 4 times
> Instead i see spot of smaller diameter.
> https://imgur.com/hnjxNTU
> ??
> 
> 

If you want to see the intensity of the light hitting a surface, that 
surface can't be a mirror, or at least, not a good mirror.

What you see in a shiny surface or a mirror is the specular highlight.
The extent of the specular highlight is controlled by the roughness 
parameter. It is never sharp even with ridiculously small values for the 
roughness.

What you want to see is the diffuse illumination of the surface.
Those are two totally different things controlled by different settings.

Use the following texture and you'll see the area illuminated by your 
spotlight clearly (totally replace your current texture with this one) :

texture{
  pigment{rgb 1} // A plain white pigment

  finish{
   diffuse 0.5 // smaller than standard diffuse
   ambient 0   // no ambient
   reflection { 1 }
  // Braces not need here but take the habit of using them.
   specular 1  roughness 0.000001
  // SHARP specular highlight
  }
}
This show that the highlight is different from the illumination.

Note that, as it is, the light coming from the light will not get 
reflected to illuminate something else in your scene.

For that, you need to use photons :
global_settings{
  photons{spacing 0.1} // OR photons{count 1e6} Don't mix.
}

And add this to your mirror :
photons{target reflection on}

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