The problem here is that POV-Ray can't easily search "over the horizon"
for a light source that "should be" illuminating the surface. (i.e. if
the highlight gets cut by the horizon line.)  This is well covered in
the docs.

But I'd like to bring up a completely different and similarly unphysical
issue with the phong highlight.  Since the amount of phong reflection is
proportional to 

	(R.L)^n
	where R is the Reflected ray
	and L is the ray to the Light

it is fairly easy to construct a scene where the Phong lighting model
*will not* illuminate an entire hemisphere because the term (R.L) is
negative for parts of the surface that are actually being illuminated by
the light source.

For example look at the following source.  By playing with the specular,
diffuse, and phong values you can see that the phong highlight is
unnaturally cut off before its actual horizon!

Just my $0.02

//=============================================================
//=============================================================

#version 3.5;

global_settings {
  assumed_gamma 1.0
  ambient_light 1
}

// ----------------------------------------

camera {
  location  <0.0, 0, -4.0>
  direction 1.5*z
  right     x*image_width/image_height
  look_at   <0.0, 0.0,  0.0>
}

light_source {
  <0, 0, 0>            // light's position (translated below)
  color rgb <1, 1, 1>  // light's color
  looks_like{ sphere{ <0,0,0>, 1 pigment{color<1,1,1>} finish{ ambient 1
} }  }
  translate <0, 30, 0>
  rotate<-40,0,0>
}

sphere {
  0.0, 1
  texture {
    pigment { color <1,0,0>    }
    finish{
      ambient 0
      
      specular .25
      roughness 1000

      diffuse 0.0
      brilliance 1000

      phong 0.25
      phong_size .001000

      reflection 1
    }
  }
}

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