Grey-Field Polariscopes (GFP) illuminate the glass with achromatic circularly polarized light. If the glass is under stress, the circularly polarized light becomes elliptically polarized. The amount of ellipticity measured for Red, Green, and Blue light determines the stress level in the glass and the tilt of the ellipse indicates the direction of the stress.
The stress measured is equivalent to
where sx is the normal stress in a given direction and sy is the orthganal normal stress. For measurements near the edge of glass sy = 0 when sx is chosen parallel to the edge. A direct measurement of edge stress is made.
All GFP systems can be used directly in black frit band.