May arise from molecular organisation of the material (form birefringence.), alignment of molecules due to tension (stress birefringence.) or alignment of rod like particles in flow (flow birefringence).
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... little more compared to the background, but I suspect that will be a fleeting thing and hard to catch and maybe even hard to distinguish from the birefringence of the cells--but you're welcome to try. And at what wavelength are you choosing to do the colorimetry? Your reference samples should ...
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