This device readily indicates a 1% or 2% misalignment at a 1m focal-film distance (FFD), but it may be used at any FFD. It consists of a flat 20 x 25 cm plate with a 14 x 18 cm pattern etched on its surface. It can also be used to check fluoroscopy alignment and collimation.
A steel ball is mounted in the center of a disc at each end of the 15 cm high clear plastic cylinder. When the balls are positioned over one another and at a right angle to the film, their images will appear as one if the central ray is truly perpendicular to the film. The approximate degree of improper angulation can also be determined.