One of the fundamental problems with image processing of petrographic
thin sections is that the appearance (colour I intensity) of a mineral grain will vary
with the orientation of the crystal lattice to the preferred direction of the polarizing
filters on a petrographic microscope. This makes it very difficult to determine
grain boundaries, grain orientation and mineral species from a single captured
image. To overcome this problem, the Rotating Polarizer Stage was used to
replace the fixed polarizer and analyzer on a standard petrographic microscope.
The Rotating Polarizer Stage rotates the polarizers while the thin section remains
stationary, allowing for better data gathering possibilities.
Instead of capturing a single image of a thin section, six composite data
sets are created by rotating the polarizers through 900 (or 1800 if quartz c-axes
measurements need to be taken) in both plane and cross polarized light. The
composite data sets can be viewed as separate images and consist of the
average intensity image, the maximum intensity image, the minimum intensity
image, the maximum position image, the minimum position image and the
gradient image.
The overall strategy used by the image processing system is to gather the
composite data sets, determine the grain boundaries using the gradient image,
classify the different mineral species present using the minimum and maximum
intensity images and then perform measurements of grain shape and, where
possible, partial crystallographic orientation using the maximum intensity and
maximum position images