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Using light as a tool for measurements – many advantages are evident for this approach. The technique can be contact-free, non-destructive and fast. Light from a variety of well-known light sources can be transported in free-space or in optical fibres and optically formed or processed in many ways by optical components. It can be measured by various detectors or arrays of detectors taking advantage oft the photoelectric effect like e.g. photodiodes, phototransistors or camera sensors, as well as UV VIS NIR measuring systems based on spectrometers. When light interacts with matter, its changes can be easily monitored by optical sensors.
Fibre sensing is used to detect mechanical parameters like deformation or stress by applying optical fibres to e.g. buildings. Periodic changes in the refractive index of these fibres cause reflections at specific wavelengths, the behavior changes by the mechanical deformation. This type of optical sensor uses the optical fibre as a light guide probe. The light detection may be a filter photometer or a spectrometer.
If the optical measurement determines the composition of light, we are talking about spectroscopic sensing. This is achieved e.g. by illuminating a sample by light with a known composition to determine the changes to the light composition after the sample-light interaction. The decomposition of light into its individual components is performed by so-called monochromators or polychromators. Depending on the desired measurement accuracy and resolution, polychromators may end up in high complexity and large dimensions. Optical wedges, filters or gratings are frequently used as a basis for the mono- or polychromator.
Several suppliers have miniaturized the polychromators to so-called spectral sensors in order to make them suitable for compact instruments. They frequently use fiber optics at their input, flat or imaging gratings and photodetector arrays for detection.
The detection in classical photometers is based on a photodetector or phototransistor. The incident light can be processed or selected by an optical filter, a fabry-perot filter or other optical elements. In fibre sensing, specially prepared fiber optics is applied to the measurement object. Photometers allow very high dynamic range and very high dynamic range by the use of quality photodiodes and powerful electronics.
Ruggedized containers designed for the requirements of the industrial application, capability to operate 24/7 in the intended industrial environment and the desired industrial interfaces make optical sensors suitable for an industrial application. Especially, the optical interfaces, e.g. windows, have to be protected from degradation due to dirt, dust or wear. Alternatively, windows may be self-cleaning or tolerate some degradation due to referencing.
In many cases, no moving parts and contactless, non-destructive operation make the optical sensors the first choice for many industrial inspection and quality measurement tasks in the process environment.