A photoelectronic instrument for measuring the length of an object by means of a slim scanning beam of light which, when not interrupted by the object, activates a photoelectric transducer, wherein, for the purpose of determining trigger points when half the light beam is obscured at the beginning and reemerges at the end of the period of cut-off, irrespective of the absolute magnitude of the light flux of the unobscured light beam, a circuit is provided for differentiating, with respect to time, a signal obtained from the photoelectric transducer and proportional to the magnitude of the light flux (light flux signal U.sub.1), a signal proportional to the differential quotient (dU.sub.1 /dt) thus obtained being rectified, multiplied by a preset factor (p.T), and the product of the multiplication and the light flux signal (U.sub.1) being applied to the inputs of a comparator functioning as a coincidence circuit for generating an output signal (U.sub.4) which, subject to an appropriate selection of the factor (p.T), abruptly changes when ##EQU1## WHERE U.sub.o is the light flux signal of the unobscured scanning light beam.

A photoelectric detection system which has two sequential photoelectric receivers to determine the precise moment at which the boundary between two contrasting fields passes beneath the detectors irrespective of the relative contrast between the two fields. The signals from the two photoelectric detectors which change as the boundary progressively obscures their respective fields of view are firstly subtracted to produce a difference signal. The difference signal is then passed to a gated maximum value store which is operative during each difference pulse to store the maximum value of the difference occurring during that pulse. A fraction of this maximum value is then taken and compared with the instantaneous value of the difference as this value reduces, when these two values are equal an output pulse is generated and this output pulse always occurs at the same position of the boundary relative to the photoelectric receivers irrespective of the respective levels of contrast on either side of the boundary. In a modification especially useful for detecting the two boundaries of a register mark two sequential pulses are formed indicative of the movement of the two respective boundaries past the photoelectric receivers.

The method of measuring the optical scattering co-efficient of the ocean by means of a laser beam (15) from a platform (16) above the ocean surface (3) in which an ocean penetrating beam (15) is swept across the direct path of travel to the surface of the ocean (3) and through it to the ocean bottom (4) to be back-reflected to the ocean surface (3) and to a receiver on the platform (16) characterized by changing the field of view to alternately use a small and a large field of view and calculating from the larger field of view an estimate of the absorption co-efficient and from the smaller field of view an estimate of beam attenuation co-efficient.

A ratiometric edge detector system for use in controlling the position of the edge of a web or other material. At least two zones on the web are illuminated with light. The first zone is of greater area than the second zone. The second zone has a border at the web edge. Light reflected from the two zones is optically sensed. A control signal is generated proportional to the ratio of the light reflected from the two zones. The ratio indicates deviation of the web edge from a preselected position.