Four-Polarisation Camera for Anisotropy Mapping at Three Orientations: Micro-Grain of Olivine

Abstract

A four-polarisation camera was used to map the absorbance of olivine micro-grains before and after high-temperature annealing (HTA). It is shown that HTA of olivine xenoliths at above 1200 °C in O2 flow makes them magnetised. Different modes of operation of the polariscope with polarisation control before and after the sample in transmission and reflection modes were used. The reflection type was assembled for non-transparent samples of olivine after HTA. The sample for optical observation in transmission was placed on an achromatic, plastic, quarter-wavelength waveplate as a sample holder. Inspection of the sample’s birefringence (retardance), as well as absorbance, was undertaken. The best fit for the transmitted intensity or transmittance T (hence, absorbance A=−log10T) is obtainable using a simple best fit with only three orientations (from the four orientations measured by the camera). When the intensity of transmitted light at one of the orientations is very low due to a cross-polarised condition (polariser–analyser arrangement), the three-point fit can be used. The three-point fit in transmission and reflection modes was validated for T(θ)=Amp×cos(2θ−2θshift)+offset, where the amplitude Amp, offset offset, and orientation azimuth θshift were extracted for each pixel via the best fit.

Teeth Microcracks Research: Towards Multi-Modal Imaging

“…An example of natural birefringent material that also has a linear dichroism at THz wavelengths is bamboo [68]. The four-polarization analysis of transmittance spectra [69] (also applicable to reflectance [70]) provides access to structural anisotropies in absorption and birefringence and also reveals them even when spatial resolution is low [71]. The four-polarization technique is promising for the investigation of structural changes to enamel and dentin caused by ablation with ultra-short laser pulses at a high repetition rate, which is a promising technique for dental repairs and is approaching material removal rates of ∼30 mm 3 /min and could replace currently used mechanical tools for drilling [72].…”