[Radiance-general] mist

[matan]

hallo to u all!!!
im a graduate student dealing with agricultural eng. and i found radiance
very usefull in determining optimal tree prunning.
my problem is such. i found from field experiments extinction of light into
the canopy (avocado in my case) and i was told using the "mist" material i
could model it on radiance.
does any one have experince or know how to turn an extinction formula
 ( i.e. I=Io*exp(-0.5x)    x being the distance) into mist parameters?

thx
matan