Abstract: An apparatus for remote detection of selected trace constituents in a fluid, for example flue gases from a stack, has a laser for generating a laser beam. The laser beam is transmitted through the fluid and the returned laser beam is detected after transmission through the fluid. A detector receives the returned laser beam, to detect the presence of any of the selected trace constituents, by comparison of the transmitted and returned beams. An optical fiber connection means, which can comprise either a single optical fiber or a pair of optical fibers, provides a connection between at least one of (i) the laser and a transmitter for the laser beam, and (ii) a receiver for the laser beam and the detector. This enables the detector and the laser to be located remotely and away from hostile environments.

Abstract: An apparatus for use in Differential Optical Absorption Spectroscopy (DOAS) or in other optical apparatus provides a moveable or oscillating aperture. It has a sensor for detecting the position of the aperture, and a drive for oscillating the aperture; the drive and sensor can comprise coils forming a linear variable differential transformer, and the aperture can be spring biased to a neutral position. A control circuit is connected between the coil or other sensor and the drive coils, so that the motion of the aperture follows a desired profile. This enables significant parts of the apertures motion to be constrained to be linear. The aperture can be provided in a shuttle mounted on extension arms, whose ends are connected to resilient biassing means which serve both to support the shuttle and to bias it. More preferably, extension arms extend through pole pieces of magnets, which provide a magnetic field for drive coils.