“Absorption” generally refers to the absorption of a wave (electromagnetic waves, sound waves, as well as light) in an absorbent material. During absorption, transmission is dampened by the material. Absorption measurement in process automation is, for example, used to determine nitrate or to measure the spectral absorption coefficient (SAC), e.g., in order to determine the organic load/charge in sewage treatment plants or in drinking water.
FIG. 1 shows the basic measurement principle according to the prior art. The light of a light source 1, e.g., a pulsed flash lamp, irradiates the measurement section 5. Medium 5 is located in gap 6, wherein the measurement light radiated into said medium is absorbed by the determining process variable. The measurement section comprises optical windows 2, which, where applicable, also comprise lenses. A beam splitter 7 finally guides the light onto the two detectors 3 for measurement light or 3.ref for reference light. A filter 4, which allows only light of the measurement wavelength or only light of the reference wavelength to pass through in the case of the detector 3, 3.ref, is respectively mounted, where applicable, in front of the two detectors 3, 3.ref. The beam splitter 7 may also be arranged after the gap 6.
The above shall be explained below by way of example on the basis of a nitrate measurement. Nitrations absorb UV light in the range of approximately 190 nm to 230 nm. Nitritions have a similar absorption in the same range. In the gap 6, the nitrations and nitritions absorb UV light in the range of the measurement wavelength 214 nm in proportion to their concentration.
If signs of aging or temperature influences of a light source do not have any influence on the measured value of an optical probe, the light source must be monitored. In many applications, this is achieved by a second detector system (photodiode or spectrometer) directly at the light source (see FIG. 1). Any existing measurement and reference filters are in this case designed to be identical (same wavelength). If, however, this method cannot be used, either because there is no space for a second detector system in the probe or because a second detector system is too expensive, and, if a regular calibration in a zero solution also cannot be performed, the light of the light source must be guided around the measurement cuvette onto the measurement detector or the measurement spectrometer.
For process sensors that are to remain in the process for a long period of time, and thus also during the necessary reference measurements of the light source, the light of the light source therefore must be guided within the sensor along an optical path that is different from the measurement beam path past the measurement section onto the detector system. Since this reference beam cannot be located on the same optical axis of the measurement beam, various optical free-beam components are required in order to guide the reference beam past the measurement section and, on the receiving side, once again onto the detector system. These optical free-beam components can, for example, include lenses, beam splitters, and mirrors (see, for example, DE 100 84 057 B4). In order to generate a stable reference signal, it is also necessary to align these components correctly and robustly in relation to each other.
The necessary optical free-beam components occupy a lot of space, which may be an obstacle in process sensors that become smaller and smaller. Both the optical free-beam components and the production costs for their alignment are costly. Optical free-beam components can, moreover, be soiled, e.g., by humidity or by accumulation of evaporated materials.