Abstract. Coccoliths are major contributors to the particulate inorganic carbon in the
ocean that is a key part of the carbon cycle. The coccoliths are a few micrometres
in length and weigh a few picogrammes. Their birefringence characteristics in
polarized optical microscopy have been used to estimate their mass. This
method is rapid and precise because camera sensors produce excellent
measurements of light. However, the current method is limited because it
requires a precise and replicable set-up and calibration of the light in the
optical equipment. More precisely, 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
calcite crystal. Here we present a new method that is universal in the sense
that the thickness estimations are independent from a calibration but
result from a simple equation. It can be used with different cameras and
microscope brands. Moreover, the light intensity used in the microscope does
not have to be strictly and precisely controlled. This method permits the
measurement of crystal thickness up to 1.7 µm. It is based on the use of one
left circular polarizer and one right circular polarizer with a
monochromatic light source using the following equation: d=λπΔnarctanILRILL, where d is the thickness, λ the wavelength of the light used,
Δn the birefringence, and ILR and ILL the light intensity
measured with a right and a left circular polarizer. Because of the
alternative and rotational motion of the quarter-wave plate of the circular
polarizer, we coined the name of this method “bidirectional circular
polarization” (BCP).
Reviewer comments on Beaufort et al. Technical Note: A universal method for measuring the thickness of microscopic calcite crystals, based on Bidirectional Circular Polarisation
