Characterization of a petrographic microscope for potential use with image processing techniques

Abstract

Petrographic and other microscopy analyses which require the use of polarised microscope, is an essential tool for the geologist and materials scientists, and thus, there is an effort to advance it technologically by introducing digital automation and digital image acquisition and processing. Studies with the polarising microscope now become faster, easier and quantitative. However, still the combination of the microscope’s optical components, such as lenses, filters, polarisers, mirrors, and often light sources, can introduce chromatic aberrations, light intensity variations, different degrees of polarisation, which can significantly affect digital image acquisition and processing, especially when sensitive birefringence colours are acquired. In this paper, we describe a method to characterise the full polarising microscope setup, by registering the parameters that affect image acquisition. Image corrections can then be dynamically applied during the operation of the microscope, increasing the precision of digital image processing techniques. The suggested procedure involves the use of a spectrometer to quantify the applied corrections. Different modes in transmitted light conditions, such as under plane polarised, crossed polarised, and circularly polarised light conditions are studied and the Correlated Colour Temperatures (CCT) are computed, also during rotation of the specimen under the polarising light. We demonstrate this on petrographic thin rocks sections, however, the same method can also be used when inspecting technological materials that inhibit birefringence, such as liquid crystals, polymeric fluids, thin films, plastics, optical fibres, and biological samples such as collagen and proteins.