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I'd like to monitor the growth of a yeast population by shining light through it and monitoring the change in light that gets through as it grows. I'm not sure if I should use IR, or any particular colour of light.

Are there any charts that show the light absorption of brewers yeast to different spectra of light?

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    $\begingroup$ Usually the growth of microorganisms is monitored at 600nm. $\endgroup$ – Chris Mar 14 '15 at 8:52
  • $\begingroup$ Why not just run a spectrum of a yeast suspension and zero it to blank growth media? $\endgroup$ – March Ho Mar 14 '15 at 9:03
  • $\begingroup$ You can also use a plate reader and 96 well plates it its feasible for your experiment. $\endgroup$ – Nandor Poka Mar 14 '15 at 16:08
  • $\begingroup$ Thanks for the comments. I am going to use Chris's information and try with a 600nm LED. $\endgroup$ – Michael Mar 17 '15 at 0:13
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Most of the loss of light being transmitted through individual cells is not absorbed, rather it is scattered (redirected from its original direction to a new one without loss of energy). There is actually not very much in cells that can absorb visible light, and virtually nothing at all that can absorb many red or near-infrared wavelengths.

Scattering does depend on wavelength, but the relationship is very complex. For scattering from small cellular components (mitochondria for example), Rayleigh scattering dominates, which will decrease with the 4th power of wavelength, so that green light will scatter much more strongly than red. For larger components that are comparable or larger than a wavelength (nuclei or whole cells), scattering will increase much more slowly. The relationship between the direction of scattering and the size/wavelength is even more complex, but is very useful in applications like label-free cytometry where cells can be sorted based on scattered light.

600-800 nm light is a reasonable choice because it will not damage the cells unless the power level is very high, although using shorter wavelengths may give you more sensitive measurements.

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