No, this has nothing to do with spectral rendering.  Sorry to disappoint.
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#include "CIE.inc"

#declare SHORTEST = 378;
#declare LONGEST = 826;
#declare STEP = 2;

#declare Max = 0;
#declare c_Values = array [(LONGEST - SHORTEST) / STEP + 1]
#local I = 0;
#for (Wl, SHORTEST, LONGEST, STEP)
  #declare c_Values[I] = MapGamut (Wavelength2RGB (Wl));
  #declare Max = max (Max, c_Values[I].x, c_Values[I].y, c_Values[I].z);
  #local I = I + 1;
#end

#declare cm_Spectrum = color_map
{ #local I = 0;
  #for (Wl, SHORTEST, LONGEST, STEP)
    [Wl/1000 rgb c_Values[I] / Max]
    #local I = I + 1;
  #end
}

#declare p_Spectrum = pigment { gradient x color_map { cm_Spectrum } }
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P.S.  Macro MapGamut() has the same pass-by-reference issue that CH2RGB() used
to have.  (See "CH2RGB somehow causes infinite loop" in p.general.)

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