Rigid, L-shaped piece of laminated plastic 1 mm thick millimeter markings accurate to 0.1 mm
or better overall size is 5 cm x 5 cm
grey areas have a reflectance value of approximately 18% alternating black and white bars (1 cm) to permit measurements from grossly over and underexposed photographs
The incorporation of three circles is useful in helping to compensate for distortion resulting from
oblique camera angles.
The graduations are all metric, except for the width of the legs which are 1-inch wide, the 1-cm
division lines are traced and extended across the photographic print to grid an image.
Measurements within the image are then made relative to these inscribed 1-cm grid lines to compensate for any
distortion resulting from non-parallelism between the film and the object planes.