Hello Bob,

thank you for your help! Indeed, the missing photon block in the scattering box
was the problem. I did not expect this to be necessary if the refracting glass
block contained a photon block, but obviously I was wrong.

But:

If I use the following source for rendering, I get a rather strange image:
After passing the glass block and being refracted/reflected/absorbed correctly,
the laser beam changes into a dotted line, that looks very much like some
interference pattern. As far as I understand (I mean, almost 99.99% sure!)
POV-ray is of course not capable of calculating phase interactions of light, so
any idea what could have happened here?

#include "colors.inc"
#include "textures.inc"

global_settings {
 assumed_gamma 1.0
 max_trace_level 10
 photons {
  spacing 0.02
  autostop 0
  media 50,5
 }
}

camera {
  location  <0,20,0>
  look_at   <0,0,0>
}

// scatterbox

box {
 <-10,0,-40>, <10,5,40>
 hollow
 pigment {
  color transmit 1
 }
 interior {
  media {
   scattering {
    1, 1 extinction 0
   }
  samples 10,30
  }
 }
 photons {
  target on
  refraction on
  reflection on
  collect off
  pass_through
 }
}

// interacting box

box {
 <2,4,-10>, 0
 translate <0,0,4>
 rotate <0, 45, 0>
 hollow
 pigment {
  color transmit 0.9
 }
 interior {
 ior 1.5
 }
 finish {
  diffuse 0.3
 }
 photons {
  target
  refraction on
  reflection on
 }
}

// laser

light_source {
 <0,1,-50>
 color red 1.0
 cylinder
 point_at <0,1,100>
 radius 0.2
 falloff 0.2
 tightness 0.1
 photons {
   refraction on
   reflection on
 }
}

// ambient light

light_source {
 <100,100,150>
 color rgb 1.3
 media_interaction off
 photons {
   refraction off
   reflection off
 }
}

// floor

plane {
 y, 0
 pigment {
  checker color White color White
 }
}

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