A method for measuring coccolith thickness in polarizing microscopy independent from light intensity
Résumé
Birefringence characteristics of coccolith in polarized microscopy has been used to estimate their mass with linear polarizers (Beaufort 2005), rotating polarizers (Beaufort et al. 2014) and circular polarizers (Bollmann 2014, Fuertes et al. 2014). This method is rapid and precise. Camera sensors produce reliable measurements of light that can be converted into thickness. This general methodology has been validated by recent independent measurements made by X-ray tomography at ESRF synchrotron (Beuvier, et al. 2019). One of its limitations is that it requires a precise calibration of the lightness of the microscope. The light intensity, the diaphragm opening, the position of the condenser, and the exposure time of the camera, have to be strictly identical during the calibration and the analysis of the calcite crystal. Slight change on one of those parameters have important consequence on the results. Another limitation is that the measured light intensity is not linearly proportional to thickness but follow a sigmoid (Beaufort, et al. 2014, Bollmann 2014) making difficult to estimate thickness precisely at the two ends of the calibration. The use of standard polychromatic light induces small imprecision related to temperature of the light used. There is a theoretical limit of the thickness estimation around 1.56 μm when using a black and white camera. Here we propose a new method that solve those problems. The estimations are independent from any calibration nor precise tuning of the microscope and light. The calcite thickness results from a simple equation that can be applied to crystal as thick as 2.0 μm. It is based on the alternative use of one left circular polarizer and one right circular polarizer with a monochromatic light source.
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