Petrographic microscopes are often used by geologists to identify minerals and estimate grain size. In these microscopes, light is projected though a polarizing filter and a glass slide that holds a thin section of mineral or rock. A second filter, with a polarizing direction of 90° to that of the first, is mounted between the section of rock and the eyepiece. If no specimen is placed between the polarizers, the image appears dark. But, because most mineral samples will rotate the direction of polarized light, an interference pattern can show the grain sizes and shapes of minerals in the rock section. Rotating the microscope stage with respect to both polarizers will obtain different levels of contrast in the image. "The human brain and the vision system have no problems in keeping track of individual grains as they are rotated around the field of view," says Frank Fueten (firstname.lastname@example.org. BrockU.CA), an associate professor in the department of earth sciences at Brock University (St. Catharines, Ontario, Canada).
"Unfortunately, though, this procedure is a major problem to automate as the computer has to track the behavior of a point within a grain in color space and the motion of that point as the thin sections of rock are rotated." To overcome this problem, Fueten has designed a fully automated polarizing stage for a petrographic microscope that allows a thin section of rock to remain fixed while the polarizers are rotated. As a result, any point within a grain is registered to the same pixel at all positions of the polarizers, simplifying the computational requirements. The polarizing stage is used in conjunction with an off-the-shelf computer, camera, and video capture board. By selectively obtaining data from images with different polarizer positions, the polarizing stage greatly enhances the potential for petrographic image processing.
A single-CCD XC-999 color camera from Sony (Park Ridge, NJ) is attached to the petrographic microscope to capture images of mineral samples. If the primary use of the system is to measure individual mineral phases, any color camera can be used, according to Fueten. But to perform crystallographic measurements, other polarizing effects need to be minimized and single-CCD cameras are more appropriate since prisms in three-CCD cameras cause added polarizing effects.
To capture images into a 1.3-GHz Pentium 4 PC, a Meteor II board from Matrox (Dorval, Quebec, Canada) is used. Once digitized, images are grabbed using Matrox Mil-Lite software library and then analyzed using Geovision, a stand-alone Windows program developed in Visual C++. To control the polarizer stages, stepper motors spin the filters by means of notched belts that can be incremented in 200 steps for a 180° rotation. These stepper motors are controlled by a proprietary controller designed and built at Brock University. The controller is connected to the computer through a COM port. The Geovision package automatically controls the complete operation.
According to Fueten, full sampling of 200 frames under crosspolarized light and constructing the final data set data takes less than 30 seconds on the Pentium 4-based host. The design of the polarizing stage allows the system to identify mineral types, measure grain size and shape, and provide crystallographic orientation data. Color images of 640 x 480 x 24-bit resolution require approximately 1 Mbyte of memory; a complete set of images for a 180° rotation of the polarizers under both plane and crosspolarized light requires approximately 400 Mbytes. The constructed data set requires less than 6 Mbytes of storage. Combining data from different images into a composite image is also possible since each pixel remains registered to the same point within a grain for all orientation of the polarizers.