If one has a closer look at Iridescence, the results are devastating.

Take the scene below, for instance:
- We have two spheres, one with "irid" and one without.
- The non-iridescent sphere is set to be 100% white, the brightest you 
can realistically have. The iridescent one is set to pitch black except 
for the iridescence contribution.
- We have two very dim light sources, one of which is a spotlight with 
smooth falloff.

What's wrong?
- The iridescent sphere is brighter than the white one!
- The iridescence effect breaks the smooth spotlight.

If you toy around more with lighting, you'll notice that the iridescence 
effect perfectly ignores how bright the light of any particular source 
is - as long as it isn't zero.

What else?
- Try setting the "thickness" value to something extremely low; 
theoretically, this should kill off any interference at visible 
wavelengths; yet the iridescence effect adds some plain grey.
- If you toy around with camera perspective, you may notice that it has 
no influence on the iridescence effect; theoretically, it should be 
influenced by both the light source *and* observer position.

To make a long story short: Iridescence is badly broken (and has always 
been, by the way). I suggest to give it a major overhaul - even at the 
cost of breaking compatibility with 3.6.

----------------------------
camera {
   location  <0.0, 0.5, -4.0>
   direction 1.5*z
   right     x*image_width/image_height
   look_at   <0,0,0>
}

light_source { <0,30,-30> color rgb 0.2
   spotlight point_at 0.0 radius 0.5 falloff 1.0
}

light_source { <-30,30,-30> color rgb 0.1 }

default {
   finish { ambient 0 diffuse 1 specular 0 }
}

plane { y, -1
   pigment { checker color rgb 1 color rgb 0.5 rotate y*30 }
}

sphere { -x, 1
   pigment { color rgb 1 }
}

sphere { x, 1
   pigment { color rgb 0 }
   finish { irid { 0.5 thickness 1 turbulence 0.5 } }
}
----------------------------

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