Abstract: The invention relates to a method of and apparatus for detecting unwanted material in the form of string or woven fabric in a flow of wanted fiber flocks or in a wanted fiber fleece. To this end, the flock flow is guided through a wave field which detects the presence of unwanted material in the flock flow so that the unwanted material can be separated out in a separator facility. The wave field can be an optical or acoustic wave field. The result obtained can, in addition to the separation of the unwanted material, be displayed in display means and/or recorded.

Abstract: The apparatus for detecting gases comprises a photoacoustic gas detector 35, in which the intensity-modulated absorbed test light beam 32' is converted into noise and detected by means of microphone 36. The photoacoustic gas detector 35 is acoustically decoupled from the gas collecting point 54 by a gas-permeable diaphragm 52 or a rigid, porous material 52'. The gas chamber 35' of photoacoustic detector 35 can be scavenged by means of supply lines 38, 38' and valves 56, 56'. It is advantageous to use a thermal source 32 intensity-modulated by phase lag. The signal is processed at microphone 36 by means of a N-path filter comprising cyclic switch 63 and R-elements 62, 65, 65'. The apparatus can be used for detecting CO.sub.2 and CO.

Abstract: The device for collecting the radiation of a light source (20) within a concave mirror comprises a cylindrical or rotationally symmetrical parabolic or elliptical reflector (10'), which is covered with a curved or planar retroreflector (40) having an opening (42). The retroreflector (40) returns part of the light into the vicinity of the source (20) and brings about, due to the partial transparency of this area, that an intense, quasi-parallel light beam can be coupled out through opening (42). After passing through a monochromator element (31), this light can e.g. be supplied to a gas measuring cell (68). The optical efficiency of the device is increased by the concave mirror (57) in a shape supplementing reflector (10', 10"). The measuring signal can be determined by means of a light detector or a microphone (69). In the latter case, an acoustically decoupling gas exchange device is required, which can e.g. be realized by means of capillaries filled with the liquid (75, 75').

Abstract: An optical detection system for selectively detecting gases comprises a light source emitting light thermally or mechanically modulated and supplied to a measuring cell. The light source includes an emitter enclosed in an ellipsoid reflector. The ellipsoid reflector can receive measuring gas directly, or is evacuated or filled with an inert gas which does not absorb in the measuring gas absorption range. With measuring gas supplied directly to the reflector, the light emanating from the reflector is supplied to a wide-band detector, combined with a monochromator, e.g. an optical narrow-band filter. With an evacuated or inert gas filled reflector, after traversing a monochromator, the light is irradiated into a photoacoustic detection cell containing the measuring gas. The photoacoustic cell is completely or approximately closed during measurement.