In POV-Ray 3.7, when a cylindrical beam of light is passed through a 
refracting object in a media environment with photons enabled, sometimes 
the beam is split into two parts, one of which is refracted and the 
other not.  This did not happen in POV-Ray 3.6.

(If photons are not enabled, the beam does not show any refraction at 
all in the media, whether in 3.7 or 3.6.)

The following code illustrates the problem:
____________________________________________________________________

#version 3.6; //[sic]

global_settings
{ assumed_gamma 1
   max_trace_level 15
   photons { spacing 0.005 autostop 0 media 200 }
}

camera
{ orthographic
   location -28 * z
   right 28 * x
   up 21 * y
}

// Medium to reveal the path of the light beam:
box
{ -0.5, 0.5 hollow
   scale <28, 21, 0.1005>
   pigment { rgbt 1 }
   finish { specular 0 reflection 0 }
   interior
   { media
     { scattering { 1, 1 extinction 0 }
       samples 3, 3
       aa_level 2
     }
   }
   photons { pass_through collect off }
}

// The light beam:
light_source
{ <-10, -10, 0>, rgb 10
   cylinder radius 0.025 falloff 0.05 point_at 0
   parallel point_at 0
}

// Refracting block:
box
{ -1, 1 scale <5, 10, 0.05>
   hollow
   pigment { rgbf 1 }
   finish { reflection { 0 1 fresnel } conserve_energy specular 0.2 }
   interior { ior 1.5 }
   photons { target refraction on reflection on collect off }
}

// Environmental light to make the refracting block visible:
light_source
{ <-1, 1, -1> * 28000, rgb 1
   parallel point_at 0
   media_interaction off
   photons { reflection off refraction off  }
}
____________________________________________________________________

The images split_beam_37bug-36run.png and split_beam_37bug-37run.png 
show this file rendered in POV-Ray 3.6.1 and 3.7, respectively.

Oddly enough, the test scene for BeamTest (in the Object Collection) 
renders perfectly, but anything else I try--even slight modifications to 
the BeamTest test scene--reveals the problem.

POV-Ray version: POV-Ray 3.7.0
Operating system: openSUSE 13.1 GNU/Linux
Hardware: Dell Inspiron 17R, Intel Core i7

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