global_settings {
     max_trace_level 64
     max_intersections 1000
 }
 
 // "Disp" variable: Amount of dispersion: Looks reasonable 
 // from 2 (less) thru 6 (more dispersion), including both:
 #declare Disp=6;
 
 light_source {<-100,50,-30> color rgb .3}
 light_source {<-150,120,-100> color rgb .3}
 
 camera {
     location <-4,5,-9>
     look_at <3,-2,0>    
     angle 60    
     }
 
 #local L1a = tan(Disp/180*pi);
 #local L1b = 1.05+L1a*1.13;
 
 #local L2a = -tan(Disp/180*pi);
 #local L2b = .739+L2a*1.308;
 
 #local SPLx = (L2b-L1b)/(L1a-L2a);  // Center of radial rainbow pattern in the part of
 #local SPLy = L1a*SPLx+L1b;         // the light beam that's inside the prism.
 
 #local TOPx = (3-L1b)/(L1a+1.73205087569);
 #local TOPy = L1a*TOPx+L1b;
 
 #local BOTx = (3-L2b)/(L2a+1.73205087569);
 #local BOTy = L2a*BOTx+L2b;
 
 #local TOPa = tan(radians(2*Disp-30));
 #local TOPb = TOPy-TOPa*TOPx;

 #local BOTa = tan(radians(-2*Disp-30));
 #local BOTb = BOTy-BOTa*BOTx;

 #local OUTx = (BOTb-TOPb)/(TOPa-BOTa);
 #local OUTy = BOTa*OUTx+BOTb;
 
 #local txtOff=texture {pigment {color rgb 1}} // Texture for unlit walls
 
 #local txtFLT=texture {pigment {  // Radial pattern to fake the "rainbow-
         radial                    // illuminated" part of the wall and floor.
         color_map {
             [0.00 color rgb <.7,(1-Disp/10),1>]           // Colors vary from all white
             [0.20 color rgb <(1-Disp/10),(1-Disp/10),1>]  // to the rainbow of saturated
             [0.40 color rgb <(1-Disp/10),1,1>]            // colors, depending on "Disp"
             [0.60 color rgb <(1-Disp/10),1,(1-Disp/10)>]  // setting.
             [0.80 color rgb <1,1,(1-Disp/10)>]
             [1.00 color rgb <1,(1-Disp/10),(1-Disp/10)>]
             }                                                  
         frequency int(90/Disp+.5) // Agular width - causes discrete intervals in width!
     } finish {ambient .4}}
 
 #local txtSTR=texture { // This texture encloses the above, "ranbow-illuminated"
     radial              // texture with zones at the red and purple ends where the
     texture_map {       // rainbow "fades out" into the invisible, and thus appa-
         [0.00 txtOff]   // rently unlit parts of the floor and walls.
         [0.01 txtFLT]
         [Disp/90-.01 txtFLT]
         [Disp/90 txtOff]
         [1.00 txtOff]
         }
     rotate <90,0,-30-2*Disp>
     translate <OUTx,OUTy,0>
     }
 
 union {   // The floor and walls in the scenery:
     plane {<0,1,0>,-3.001}  // Floor
     plane {<-1,0,0>,-10}    // Wall struck by colored light
     plane {<0,0,-1>,-15}    // Unlit wall in the background
     texture {               // This texture is the rainbow band in txtSTR inside a
         gradient z          // small strip along the triangular surfaces of the
         texture_map {       // prism, surrounded by zones where the light fades out
             [0.000 txtOff]  // into the dark gray, "unlit" texture.
             [0.004 txtSTR]
             [0.016 txtSTR]
             [0.020 txtOff]
             [1.000 txtOff]
             }
         translate <0,0,-.01>
         scale <1,1,200>
         }
     }    
 
 difference {  // The prism itself
     box {<-10,-3,-2>,<10,3,2>}              // Shape made from a box, "cut" into
     plane {<0,-1,0>,-1.5 rotate <0,0,60>}   // shape using two planes at 60* angles.
     plane {<0,-1,0>,-1.5 rotate <0,0,-60>}
     hollow on                                 // Media stuff: The prism has an index
     pigment {color rgbt <.8,1,.9,.7>}         // of refraction (ior) of 1.66, and
     finish {phong 1 reflection .1 ambient .3} // disperses colors according to the
     interior {ior 1.66 dispersion 1+Disp/100} // "Disp" variable.
     }
 
 difference {  // The narrow "sheet" of white light entering the prism from the left:
     box {<-10,-.1,-1.5>,<1,.1,1.5> rotate <0,0,26.099> translate <-1,1,0>}
     plane {<0,1,0>,1.5001 rotate <0,0,60>}  // Box "cut" into shape with a plane.
     hollow on               // Media stuff: The beam shape itself is invisible, but
     pigment {color rgbt 1}  // contains an emitting media, giving the beam the
     interior {              // "fuzzy", translucent look.
         media {
             intervals 10  samples 1,1  confidence 0.9  variance 1.0/128
             ratio 0.9  emission rgb .6  method 3
             }
         }
     }
 
 difference {    // Part of the light beam inside the prism:
     box {<-2,-2,-1.5>,<5,2,1.5>}                                    // Box cut to shape
     plane {<0,-1,0>,0 rotate <0,0,Disp> translate <-1.13,1.05,0>}   // with planes, ac-
     plane {<0,1,0>,0 rotate <0,0,-Disp> translate <-1.308,0.739,0>} // cording to the
     plane {<0,-1,0>,-1.499 rotate <0,0,60>}                         // "Disp" variable
     plane {<0,-1,0>,-1.499 rotate <0,0,-60>}
     hollow on                       // Media stuff: The shape itself is invisible inside
     pigment {color rgbt <1,1,1,1>}  // the prism since it has the same ior; 1.66
     finish {ambient .7}
     interior {
         ior 1.66 dispersion 1+Disp/100
         media {
             intervals 10        // number of intervals used for sampling [10]
             samples 1,1         // minimum and maximum number of samples taken per interval [1,1]
             confidence 0.9      // statistic parameter higher->better quality [0.9]
             variance 1.0/128    // statistic parameter lower->better quality [1.0/128]
             ratio 0.9           // distribution between lit and unlit areas [0.9]
             emission rgb<1,.95,.85>*.4
             method 3               // adaptive sampling
             density {
                 radial
                 color_map { // Color spectrum in the beam inside the prism:
                     [0.00 color rgb <1,(1-Disp/10),(1-Disp/10)>*(1.5-.05*Disp)]
                     [0.20 color rgb <1,1,(1-Disp/10)>          *(1.5-.05*Disp)]
                     [0.40 color rgb <(1-Disp/10),1,(1-Disp/10)>*(1.5-.05*Disp)]
                     [0.60 color rgb <(1-Disp/10),1,1>          *(1.5-.05*Disp)]
                     [0.80 color rgb <(1-Disp/10),(1-Disp/10),1>*(1.5-.05*Disp)]
                     [1.00 color rgb <1,(1-Disp/10),1>          *(1.5-.05*Disp)]
                     }
                 frequency int(180/Disp+.5)
                 rotate <-90,0,-Disp>
                 translate <SPLx,SPLy,0>
                 }
             }
         }
     }
 
 difference {  // Part of the light beam exiting at the right-hand side of the prism:
     box {<-5,-500,-1.5>,<500,5,1.5>}                                  // Box cut to sha-
     plane {<0,1,0>,1.5001 rotate <0,0,-60>}                           // pe using three
     plane {<0,-1,0>,0 rotate <0,0,2*Disp-30> translate <TOPx,TOPy,0>} // planes.
     plane {<0,1,0>,0 rotate <0,0,-2*Disp-30> translate <BOTx,BOTy,0>}
     hollow on                       // Media stuff: Like the entering beam, this one's
     pigment {color rgbt <1,1,1,1>}  // shape is invisible, but contains emitting media
     finish {ambient .7}             // showing the rainbow "band" of colors
     interior {        
         media {
             intervals 10  samples 1,1  confidence 0.9  variance 1.0/128
             ratio 0.9  emission rgb<1,.95,.85>*.4  method 3
             density {
                 radial
                 color_map { // Color spectrum in the beam leaving the prism:
                     [0.00 color rgb <1,(1-Disp/10),(1-Disp/10)>*(.4-.01*Disp)]
                     [0.20 color rgb <1,1,(1-Disp/10)>          *(.4-.01*Disp)]
                     [0.40 color rgb <(1-Disp/10),1,(1-Disp/10)>*(.4-.01*Disp)]
                     [0.60 color rgb <(1-Disp/10),1,1>          *(.4-.01*Disp)]
                     [0.80 color rgb <(1-Disp/10),(1-Disp/10),1>*(.4-.01*Disp)]
                     [1.00 color rgb <.7,(1-Disp/10),1>          *(.4-.01*Disp)]
                     }                                                  
                 frequency int(90/Disp+.5)
                 rotate <-90,0,-30-2*Disp>
                 translate <OUTx,OUTy,0>
                 }
             }
         }
     }