// filter and transparency base ------------------------------- start rainbow arc
#declare Filter = 0.5; // <= ~0.80;
#declare T_Base = 0.3; // <= ~0.70; (0~1 less/more intensive)
// the rainbow colors -----------------------------------------------------------
#declare Rainbow_Color_1  = color rgbf<1.0, 0.5, 1.0, Filter+0.15>; // Violet_End
#declare Rainbow_Color_2 = color rgbf<1.0, 0.5, 1.0, Filter>;  // Violet_Start
#declare Rainbow_Color_3 = color rgbf<0.5, 0.5, 1.0, Filter>;  // Violet_Blue
#declare Rainbow_Color_4 = color rgbf<0.2, 0.2, 1.0, Filter>; // Blue
#declare Rainbow_Color_5 = color rgbf<0.2, 1.0, 1.0, Filter>; // Cyan
#declare Rainbow_Color_6 = color rgbf<0.2, 1.0, 0.2, Filter>; // Green
#declare Rainbow_Color_7 = color rgbf<1.0, 1.0, 0.2, Filter>; // Yellow
#declare Rainbow_Color_8 = color rgbf<1.0, 0.5, 0.2, Filter>; // Orange
#declare Rainbow_Color_9 = color rgbf<1.0, 0.2, 0.2, Filter>;   // Red_Start
#declare Rainbow_Color_10= color rgbf<1.0, 0.2, 0.2, Filter+0.15>; // Red_End

// the rainbow arc //------------------------------------------------------------
rainbow {
  angle 40         // 30~50 apparture against center in degrees
  width  6         // width in degrees
  distance 1500     // 
  direction < 0.05,-0.27,1> //direction to the center of the rainbow arc   
  arc_angle 150    // length of arc in degrees
  falloff_angle 10 // in degrees, must be smaller than arc_angle 
  // up <0.6,1,0>  // 
  // jitter 0.01   // don't use this with animations

  color_map {
    [0.000  color Rainbow_Color_1  transmit T_Base+0.08]
    [0.100  color Rainbow_Color_2  transmit T_Base+0.06]
    [0.214  color Rainbow_Color_3  transmit T_Base+0.04]
    [0.328  color Rainbow_Color_4  transmit T_Base+0.02]
    [0.442  color Rainbow_Color_5  transmit T_Base+0.00]
    [0.556  color Rainbow_Color_6  transmit T_Base+0.02]
    [0.670  color Rainbow_Color_7  transmit T_Base+0.04]
    [0.784  color Rainbow_Color_8  transmit T_Base+0.06]
    [0.900  color Rainbow_Color_9  transmit T_Base+0.08]
    [1.000  color Rainbow_Color_10 transmit T_Base+0.09]
  }// end of color_map
}// -------------------------------------------- end of rainbow arc  -----------