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Component Options
Fiber optics for temperature measurements as well as for communications depends on minimizing losses in the light or infrared radiation being transmitted. Basics of light conduction (Figure 5-4) is a central glass fiber which has been carefully produced to have nearly zero absorption losses at the wavelengths of interest. A cladding material with a much lower index of refraction reflects all non-axial light rays back into the central fiber core so that most of the conducted radiation actually bounces down the length of the cable. Various metal, PFA or plastics are used for outer protective jackets.

Figure 5-4: Fiber Optic Cable Construction

  The difference in refractive indices of the core and cladding also identify an acceptance cone angle for radiation to enter the fiber and be transmitted. However, lenses are often used to better couple the fiber with a target surface.
  For relatively short run temperature sensing, losses in the fiber optic link are generally negligible. Losses in connectors, splices and couplers predominate and deserve appropriate engineering attention. Along with the fiber optic cable, a temperature measuring system will include an array of components such as probes, sensors or receivers, terminals, lenses, couplers, connectors, etc. Supplemental items like blackbody calibrators and backlighter units which illuminate actual field of view are often needed to ensure reliable operation.

  References and Further Reading
  Handbook of Temperature Measurement & Control, Omega Press, 1997.
  "Fiber Optic PLC Links," Kenneth Ball, Programmable Controls, Nov/Dec 1988.
  Fiber Optic Sensors, Eric Udd, John Wiley & Sons, 1991
  Handbook of Intelligent Sensors for Industrial Automation, Nello Zuech, Addison-Wesley Publishing Company, 1992.
  "Infrared Optical Fibers", M.G. Drexhage and C.T. Moynihan, Scientific American, November 1988.
  Measurements for Competitiveness in Electronics, NIST Electronics and Electrical Engineering Laboratory, 1993.
  "Multichannel Fiber-Optic Temperature Monitor," L. Jeffers, Babcock & Wilcox Report; B&W R&D Division; Alliance, Ohio.
  Optical Fiber Sensors: Systems and Applications, Vol 2, B. Culshaw and J. Dakin, Artech House; 1989.
  Process Measurement and Analysis; Instrument Engineers' Handbook, Third Edition, B. Liptak, Chilton Book Company, 1995.
  "Radiation Thermometers/Pyrometers," C. Warren, Measurements & Control, February, 1995.
  Sensors and Control Systems in Manufacturing, S. Soloman, McGraw-Hill, 1994.

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