The leftmost sphere in each image has no dispersion.  The remaining  
spheres have dispersion_samples set to the number above each.  The first  
image uses faked caustics and the second uses photons.

It seems that the smaller the dispersion_samples, the darker the glass.   
Could this be the 2 missing dispersion samples making their absence felt?
_____________________________________________________________________

#include "consts.inc"
#include "transforms.inc"

#declare Test = sphere
{  y, 1
    pigment { rgbf 1 }
    finish
    {  reflection { 0 1 fresnel } conserve_energy
       specular 1 roughness 0.001
    }
    photons { target reflection on refraction on collect off }
    interior
    {  ior Crown_Glass_Ior dispersion Quartz_Glass_Dispersion
       #if (!PH) caustics 1 #end //PH is defined elsewhere
    }
}

union
{  object { Test interior { dispersion 1 } translate -3.6 * x }
    #declare T = 4;
    #while (T <= 8)
       #declare Samps = text
       {  ttf "arial.ttf" str(T,0,0) 0.01, 0 scale 0.6
          pigment { rgb x }
          no_shadow no_reflection
       }
       union
       {  object { Test interior { dispersion_samples T } }
          object { Samps Center_Trans (Samps, x) translate 2.25 * y }
          translate (T * 1.2 - 6) * x
       }
       #declare T = T + 2;
    #end
}

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Download 'dispersion_light.jpg' (51 KB) Download 'dispersion_light-ph.jpg' (52 KB)

Preview of image 'dispersion_light.jpg'

Preview of image 'dispersion_light-ph.jpg'