> This is all really helpful, thank you. I think I'm making progress. I still have
> a few issues.
>
> This is what I now have:
>
> global_settings{photons{spacing 0.1}}
>
> #declare my_texture =
>   texture {
>    finish{
>       reflection{
>        0.3, 1
>        fresnel
>      metallic
>     }
>     specular 1
>     roughness 0.1
>    }
>   }
>
> background { color Blue }
> camera {
>      location <0, 10, -50>
>      look_at  <0, 0,  20>
>
>    }
>
> difference{
>    box{< -10,  0,  0>,<10,  20,   20>}//outside of the box
>    box{<-9.6, 0.4, 0.4>,<9.6, 19.6, 19.6>}//inside of the box
> // the walls now have a thickness of 0.4 unit
>    box{<-9,1, 0>,<9, 10, 0.4>}// the opening
>    texture{
>     object{
>      box{<-9,1,0.0001>,<9,10,1 >}
>      my_texture,
>      pigment {rgbt 1}
>    }
>    }
>
>
> photons{target reflection on}
>
>   rotate y*20
> }
>
> light_source { <5, 20, -30>
>   color White
> }
>
> I'm getting an error on the
>
> texture{
>     object{
>      box{<-9,1,0.0001>,<9,10,1 >}
>      my_texture,
>      pigment {rgbt 1}
>    }
>    }
>
> code, saying that there is a missing }, but I don't see it.
>
> Also, did I put the photons{target reflection on} in the right place, or do I
> need to create an object that includes the box and the photons?
>
> Rhonda
>
>
>

Using the box with the difference, you don't need to use the object 
pattern. It's a way of simulating an opening if you use a transparent 
pigment. Remove:
    object{
     box{<-9,1,0.0001>,<9,10,1 >}
     my_texture,
     pigment {rgbt 1}
   }

and only use:
texture{ my_texture }

Your photons block is correctly placed.


Alain

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