The unit is performed as a basement on height adjustable supports. A vertical rod and an optical sensor are installed on the basement. A rotatable frame is suspended on the upper bracket by means of a steel thread. The central cross-arm of the rotatable frame can freely move along the guideways changing the area necessary for the fixation of the tested pattern. The tested pattern is fixed between two cones of the rotatable frame. The thread tension can be adjusted by changing the position of the frame upper cross-arm.
An electronic stopwatch is used for fixing the time of the pendulum oscillations.