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Prototyping of automated systems
Published in Fuewen Frank Liou, Rapid Prototyping and Engineering Applications, 2019
Photosensors are used for the detection of light. The commonly seen photosensors are photocells used to detect the presence of light radiating naturally from some object in the process. For example, the device sold in the supermarket to turn a light on at dusk and off at dawn is a photosensor. Many of the industrial photosensors use an artificial light source instead of the sun. Photosensors are on/off switches that change state as a result of light radiation. They are an alternative to proximity sensors when the sensing distance is longer or when the item to be detected is nonmetallic. On the basis of reflecting surfaces, reflectors are of three types, namely, diffusive reflectors, specular reflectors, and retroreflectors, as shown in Figure 8.17. Different photosensors will have different reflector requirements. A diffusive surface can be any flat surface as long as it can reflect light. A specular reflector is a very smooth surface such as a mirror, and thus, only when the light is aiming from a particular direction, can the sensor detect it. The light source and receiver are placed at a particular angle. This can selectively activate the sensor for the light source from a specific direction. A retroreflector is able to reflect the light in the same direction from which it comes. Because of this characteristic, it is always used to make safety signs such as the rear lights of bicycles and vehicles and the safety uniforms worn by people working at night. Figure 8.18 shows an example of using a retroreflector.
Length Measurement Techniques
Published in Rajpal S. Sirohi, Mahendra P. Kothiyal, Optical Components, Systems, and Measurement Techniques, 2017
Rajpal S. Sirohi, Mahendra P. Kothiyal
When a plane-mirror Twyman-Green interferometer is used for the measurement of displacement, the mirror should move parallel to itself within a very close tolerance, because any tilt will result in a change in fringe spacing and give a false signal that will be recorded as a displacement. This puts very stringent conditions on the manufacture of the guides on which the moving member is guided. The use of retroreflectors overcomes this difficulty. The retroreflector is an optical component that sends back an incident ray parallel to itself irrespective of the orientation of the reflector, as against the case of a plane mirror, where a change in orientation by an angle α results in a 2α change in the direction of the returning beam. Cube corner retroreflectors are generally used.
Measurement of gases and particles
Published in Abhishek Tiwary, Jeremy Colls, Air Pollution, 2017
Light detection and ranging (LIDAR) describes a family of active remote sensing methods. The most basic technique is long-path absorption, in which a beam of laser light is reflected from a distant retroreflector and returned to a detector which is co-located with the source. A retroreflector is a corner cube which works in the same way as a ‘cat’s eye’ road marker as used in the UK, and returns the incident beam in exactly the opposite direction that it came from, no matter what that direction. The wavelength of the radiation is chosen so that it coincides with an absorption line of the gas of interest. The concentration of that gas is found by applying the Beer–Lambert law to the reduction in beam flux density over the path length. No information is obtained on the variation in density of the gas along the path length; if the gas is present at twice the average concentration along half the path, and zero along the other half, then the same signal will be received as for uniform distribution at the average concentration.
Determination of methane during anaerobic digestion by tunable diode laser absorption spectroscopy (TDLAS)
Published in Instrumentation Science & Technology, 2023
Haibin Cui, Fei Wang, Shengyu Hu, Wenyuan Wang, Jinhui Fan
Between the wedged window and photodetector is a cavity that partially covers the optical path; therefore, it is vital that there is no methane to be measured in the cavity. The optical window allows the isolation of the gas medium inside the tank reactor from the atmosphere and transmits the laser beam to the retroreflector (HRS1015-M01, reflectance > 96%, Thorlabs, United States) mounted at the distal end of the optical probe. A standard adjustment mount was used for collimator adjustment to ensure that the laser beam reflected by the protected gold mirror and retroreflector fell on the active area of the photodetector (PDA50B-EC, Thorlabs, United States), yielding two passes inside the optical probe. This configuration implies that the absorption path is doubled, thereby doubling the precision of the system. Consequently, reasonable detection limits were achieved, even in small spaces, which significantly improves the measurement of the gas concentration with the optical probe.
LIDAR systems operating in a non-Kolmogorov turbulent atmosphere
Published in Waves in Random and Complex Media, 2019
I. Toselli, F. Wang, O. Korotkova
We would like to highlight that our treatment of target classification is intentionally brief; we are focusing only on target size relative to the illumination beam size and on general target characteristics rather than on specific details. A target of interest may have both specular (glint) and diffuse reflection components. Specular targets are usually considered to be smooth surfaces such as the surface of a lens or retro-reflector and this is the case in this paper. The limiting case of a small specular target leads to the notion of a ‘point target’. Targets are further classified as resolved or unresolved. An unresolved target is the one that lies entirely within the spot size of the illumination beam, and a resolved target (also called an extended target) is one larger than the illumination beam spot size. The size of an unresolved target is the target size itself. The same is also true if the target is an extended smooth Gaussian reflector (mirror or retroreflector). Also, in this paper the size of an extended target is generally taken to be the size of the illumination beam [2].