Contrast techniques in microscopic observation

When the observed specimen is a thin slide with polished parallel faces of a known thickness, you can describe it more in detail. Indeed, all homogeneous anisotropic material slides of parallel faces, whatever their uniaxial or biaxial natures and their main axis orientations are, act as retardation slides, of which characteristics (neutral lines orientations and optical path differences between both linearly polarized plane waves to every neutral lines) depend on material birefringences and orientations (see [ Born 1999^[1]]). With orthoscopic illumination, polarized light microscopes are used as polariscopes and all standard methods of polarization analysis can be performed. So you can use a de Senarmont compensator (which is constituted by a \(\lambda /4\) slide and a polarizer), a Soleil compensator, a Babinet compensator, etc. The measurement obtained can be very useful to analyze or even to identify the specimen's nature. However, quantitative polarization microscopy is very delicate work and requires highly specialized equipment (middle or top-of-the-range microscopes with suitable optional accessories). Above all, to make good quality measurements, it is necessary to work with particular objectives, called “strain free”objectives, whose lenses, made by specific methods, are mounted on particular opto-mechanical barrels which avoid stress, so as not to have inferred birefringence which would give false results

Among these methods, the one known as «  Newton shades » is the easiest to perform and it is actually used very often in geology to identify rocks by means of a polished section of known thickness. This method will be described more in details in the Case study section of this unit M11G2.