Abstract: A device for calibrating an apparatus for measuring the formation index of a sheet of paper in which an emitted light beam is directed through the sheet of paper and detected and electronically processed into a modulated output signal. The device has a frosted glass for diffusing the measuring beam of light and an opaque rod mounted for rotation in and out of the beam of light so as to provide a modulation of the detected light of a known amount which can then be used to calibrate the measuring apparatus by adjusting the various parameters of the apparatus to yield the desired known output modulation.