Photoelectric sensing

The principle and use of an optical non-contact extensometer available commercially has been described in some detail77. Two photoelectric sensing devices automatically follow, by means of a servo mechanism, contrastingly coloured gauge marks on the test piece. The separation of the auto followers... 

Carotenuto, G., Longo, A., Repetto, R, Perlo, R, and Ambrosio, L., New polymer additives for photoelectric sensing. Sens. Actuators B, 125, 202-206 (2007). 

In the simplest form, light from a fixed wavelength UV lamp passes through a cell, through which the column eluent flows, and the fluorescent light that is emitted by a solute is sensed by a photoelectric cell positioned normal to the direction of exciting UV light. 

The photoelectric effect is the basis for many light-sensing devices, such as automatic door openers and camera exposure... 

C07-0115. The photoelectric effect for magnesium metal has a threshold frequency of 8.95 X 10 s". Can Mg be used in photoelectric devices that sense visible light Do a calculation in support of your answer. 

Smoke detector Senses invisible and/or visible products of combustion. The two principal types of smoke detector are photoelectric and ionization detectors. The major differences between these devices are described below Photoelectric smoke detectors react to visible particles of smoke. These detectors are more sensitive to the cooler smoke with large smoke particles that is typical of smoldering fires. Ionization smoke detectors are sensitive to the presence of ions produced by the chemical reactions that take place with few smoke particles, such as those typically produced by fast-burning/flaming fi res. 

Multi-sensor detector Is a combination of photoelectric and thermal detectors. The photoelectric sensor serves to detect smoldering fires, while the thermal detector senses the heat given off from fast-burning/flaming fires. 

Einstein connected Planck s idea of limited, discontinuous energy levels to the problem of the photoelectric effect. Light shone on certain metals knocks electrons free the effect is applied today in the solar panels that power spacecraft. But the energy of the electrons knocked free of the metal does not depend, as common sense would suggest, on the brightness of the light. It depends instead on the color of the light—on its frequency. 

Advantages of diffuse photoelectric sensors are low cost, wide sensing field, and ease of installation (no need to install a separate receiver or reflector). The main disadvantage of these sensors is their inability to detect objects near a background. They must be installed so that they look off into space, so to speak. Otherwi.se, the background will interfere with the sensing field. Also, remember not to install these sensors so that they are facing each other. 

Figure 13-12. Example of a photoelectric presence sensing device on a machine. (From OSHA 3067.)...

In some types of photoelectric detectors, a vacuum pump draws air liom one or more locations to a central sensing device. Any of the sources can trigger a response from the sensor. 

The majority of methods of observation involve photons which are used to illuminate the object. After bouncing off the object the photons are transmitted to a recording device, which may be a photographic plate, a photoelectric cell or the eye. Such methods are satisfactory for the majority of normal objects. The photons which are used do not in any measurable way affect the object. The situation is different with microscopic objects such as atoms and electrons. Electrons cannot be seen in the normal sense of that word. They are too small. To observe such small objects we should have to use a microscope with a resolving power greater than any actual microscope now in existence. 

Quantum computers using photonic circuits are a possible future development in the field of optics. A quantum computer that takes advantage of the photoelectric effect may be able to increase the capacity of computation over conventional computers. Op>-tics and photonics may also be applied to chemical sensing, imaging through adverse atmospheric conditions, and solid-state lighting. 

The radicfrequency (capacitance) presence-sensing device uses a radio beam as part of the machine control circuit. When the capacitance field is broken, the machine stops or will not activate. Like the photoelectric device, this device is only to be used on machines that can be stopped before the worker can reach the danger area. This requires the machine to have a friction clutch or other reliable means for stopping. This device allows freer movement for the operator but does not protect against mechanical failure. In addition, antennae sensitivity must be properly adjusted this adjustment must be maintained properly. 

In a closely analogous manner radiation is found to have both a particle and a wave aspect, the former being shown, for example, in the photoelectric effect. In short, the things of the senses, matter and radiation, do not correspond in a simple one-to-one relationship with the mental images of the particle and wave respectively. 

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