CAT scans for plants?

Oliver Sparrow ohgs at
Tue Feb 22 04:09:44 EST 1994

I cannot offer a reference, but I recall someone in Australia
who exploited the fact that the roots of young plants are effective
light pipes. What he did was to build a grid of conventional light
fibres through the growing medium, irradiate the growing plant 
with a frequency modulated IR signal and then interrogate the fibres 
with a photodiode and a phase locked loop tuned to the modulating signal.
As a further tweak, the PLL error signal was fed to an averaging box car
system which gave an overall signal to noise of phenomenal proportion.
This allowed field measurements to be taken to a 1 cm precision to a depth
of about 1.5 meters. The plant was undisturbed but the soil was, of
couse, far from its conventional structure when a 1 cm cube grid had been
lain through it!

Roots - and in particular, old roots - are not very good light pipes and
the irradiation worked best at night, when the huge gains achievable
with a box-car PLL filter could be employed free of background noise.

  Oliver Sparrow
  ohgs at

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