Dimensionless quantities
When dimensionless units are used, light propagation is dependent upon
three parameters: the albedo, the optical depth or thickness, and the phase
function. The albedo a is a dimensionless parameter defined as the ratio of
scattering to the sum of scattering and absorption. The albedo varies between
zero and one: a value of zero indicates the absence of scattering and a value of
one indicates that the tissue does not absorb light. The optical depth 
is the
product of the tissue thickness and the sum of the scattering and absorption
coefficient. An optical depth of one indicates that there is a probability
of e-1=37% that light will travel at least that distance without scattering or absorbing.
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