Abstract: The invention concerns an instrument (50) and a method for measuring an essentially periodic motion, which has a first average frequency, of an object (10). An image of the object (10) is produced by means of an optical window (30), which comprises liquid crystals (31) to be electrically controllable between different states of transmission. A periodic transmission change, corresponding to the periodic motion, of the optical window (30) between a state with relatively low transmission and a state with relatively high transmission is accomplished, such that the motion of the image (B) obtains a second average frequency, which is lower than the first average frequency. By means of an electronic image-analysing means (20), the image (B) of the periodic motion of the object (10) is received via the optical window (30), and an indication is made to show when the second average frequency falls below a predetermined value.

Abstract: A detector for sensing or measuring objects passing a measurement path (5) has a detection element (6), the output signal of which is a function of objects discovered in the measurement path. The output signal is fed to an amplifier and filter circuit (7-9), the output of which is connected to two parallel integration cirucits (11, 12). The integration circuits have time constant (.tau..sub.SH, .tau..sub.L) of different magnitudes, and their output signals are fed to a comparator (13) for obtaining an operative signal indicating the presence of an object in the measurement path, this signal being unresponsive to long-duration fluctuations in the detector's operating conditions, e.g., dirtying of the detection element. The detector includes a dirtiness alarm with the threshold response adjustable to a long-time-constant integrator output.