This paper proposes digital holographic microscopy applied to the study of the production of lipids in Chlorella vulgaris microalgae. We propose to use several wavelengths to establish a chromatic phase contrast being the signature of the presence, or absence, of lipids in a set of microalga culture. In order to achieve this goal, the 3-color microscope has to be perfectly calibrated so that a point-to-point chromatic contrast can be evaluated without any distortion due to aberrations. The analysis of chromatic phase contrasts leads to measure the experimental probability density functions of the chromatic phase contrast distribution. Statistical analysis of the phase distribution in stressed cells and for not stressed cells reveals a difference in the probability densities. These experimental curves are fitted with two different theoretical modeling, one for the stressed algae (production of lipids), and one other for not-stressed algae. Fitting to experimental data demonstrates that the proposed approach opens the way for an in-situ and non-destructive experimental method to evaluate the production rate of lipids from Chlorella vulgaris algae.
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