> My recollection is that wavelengths in the range of 550 to 660 are used
> to measure "scattering" or turbidity. They are appropriate for
> microorganisms due to the refraction of light of those wavelengths
> around the objects of about 1 um in size. One problem with this
> technique is that it gives a representation proportional to the number
> of "objects" in solution, i.e. it does not differenitate between live
> and dead bacteria. To discriminate live from dead organisms, titration,
> growth and colony counting is required.
It seems that 550 to 650 nm measure turbidity but it's not proportional
to the number of "objects" in solution. It's proportional to organic
mater in solution.