A computer-controlled rotating polarizer stage for the petrographic microscope

The computer-controlled polarizing stage replaces the polarizer and analyzer of the standard petrographic microscope for image-processing applications. The stage allows a thin section to remain fixed while the polarizing filters are rotated by stepper motors. This has the advantage that a point within a grain in the field-of-view remains registered to the same pixel at all positions of the polarizers, greatly simplifying the computational requirements. The independent rotations of the polarizer and analyzer are controlled by two stepper motors. The analyzer is mounted on an upper assembly which can be rotated in and out of the light path by a third stepper motor. The upper assembly also contains a neutral density filter for plane-polarized light observation and a fixed polarizing filter, used to set the rotating polarizing filters to a known orientation. All stepper motors, and hence all stage functions, are controlled by a simple command language through a microcontroller that communicates via a serial port to a computer with a video-capture board. The ability to keep the sample stationary while changing the polarization direction of the polarizing filters permits the collection of data which could not be obtained from any single image. The stage greatly improves the potential uses of the petrographic microscope in geological image-processing applications. (C) 1997 Elsevier Science Ltd.