Optical Systems All of the types of automatic pyrometers, listed in Section and shown in Figure 8 .2 are considered in this chapter. To reach a sufficiently high measurement precision, the radiation emitted by the body under measurement is concentrated on the radiation detector by lenses, light-guides or mirrors . Thus, they also reduce the pyrometer viewing angle and consequently the necessary object diameter . It its also essential that the pyrometer optical system should be able to aim properly at the target . . | Temperature Measurement Second Edition L. Michalski K. Eckersdorf J. Kucharski J. McGhee Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-86779-9 Hardback 0-470-84613-5 Electronic 10 Automatic Pyrometers Optical Systems All of the types of automatic pyrometers listed in Section and shown in Figure are considered in this chapter. To reach a sufficiently high measurement precision the radiation emitted by the body under measurement is concentrated on the radiation detector by lenses light-guides or mirrors. Thus they also reduce the pyrometer viewing angle and consequently the necessary object diameter. It is also essential that the pyrometer optical system should be able to aim properly at the target. Lenses Lenses should be made of materials characterised by high transmission factor over a wide wavelength range high mechanical strength possibly high working temperature good resistance to atmospheric and chemical influences good resistance to abrasion good resistance to rapid temperature variations. As it passes through the lens as illustrated in Figure incident thermal radiation is attenuated by absorption and reflection at both lens surfaces. The same effects occur at the sighting window. It is normally enough to take only one internal reflection into account. Hackforth 1960 points out that coated lenses are used to reduce the surface reflection factor. He also notes that the overall lens transmission may be even doubled by correctly choosing the lens coating and its thickness. Materials such as SiO ZnS CeO2 MgF2 and so on each with a thickness equal to one quarter of the wavelength of the incident radiation are suitable. The application range of different optical materials depends upon their transmission factors as a function of the wavelength and on the thickness of the lens or window. In pyrometry the upper cut-off wavelength of incident infrared radiation caused by the lens material is extremely important. Following Wien s displacement .
