Is there a way to use Mantra to quantify the illuminance (not luminance) on surfaces in my scene?

I basically want to calculate the total luminous flux incident on surfaces and to then to create a false colour image that shows illumnance levels.

e.g.
http://www.uncg.edu/iar/elight/learn/qualitative/la_sub/false.html [uncg.edu]

Hmmm, is this helpy:
http://vimeo.com/6017969 [vimeo.com]
and then use a gradient in a comp :-?
Maybe I've totally misunderstood your needs…

I suppose the light buffers are luminance not illuminance.
For very simple cases you should be able to build your own shader with an illuminance loop [sidefx.com] and output the raw incoming values, but it will most probably become unwieldy when multiple bounces and such come into play.
For a more modern solution I could imagine a custom bsdf [sidefx.com] that would always bounce all light back to camera, but that's not trivial. Dunno if you could sidestep the multiple bounces issue with labels [sidefx.com]..?

All in all, I don't see a simple solution - but maybe someone has a clever trick up their sleeves

Can someone give me a few more details on how to use the Illuminance Loop for this.

It seems promising, but I can't figure out how to do it. I have not done much with VOPs before.