The GFP 1200c sensors incorporates a rotating polarizer between the lens and camera to measure the dimensions of the ellipse. As the polarizer lines up with the long axis of a particular ellipse the pixel gets brighter, as the polarizer lines up with the short axis of the ellipse the pixel gets darker. The magnitude of flashing determines the stress and the phase of the flashing determines the direction of the stress. The flashing light is normalized by the average light to desensitize the measurement to quality of illumination.
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