This pair of polaroid filters is mounted inside a rotating scale which can be supported on an optical bench. A pointer indicates around a circular scale and points to 'zero' where the filters are aligned so that transmission of light is at a maximum. The rotation in degrees on either side of zero is easily measured as the filters reduce the transmitted light to zero at 90 degrees. The diameter of the filter at the centre is 50mm

The transmission is 38% when polaroids are aligned. When misaligned at 90 degrees, the transmission reduces to less than 0.2% (almost complete blackout). By holding the front ring, the pointer can be moved to +/- 90 degrees from alignment which rotates one of the filters relative to the other filter to change the transmission from maximum to minimum.