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10 Automatic Pyrometers 10 .1  Optical Systems All of the types of automatic pyrometers, listed in Section 8 .1 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 . 10 .1 .1 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 8 .2, 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, Ce0 2 , MgF 2 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 law, given in equation (8 .11), this long wavelength transmission limit determines the lowest temperature which the pyrometer can measure . Figure 10 .1 gives some of the transmission limits of the more popular materials used for lenses and sighting windows of radiation pyrometers . 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) 178  AUTOMATIC PYROMETERS KRS S (42%Ti8r, 58% Tl I) Go Asz S3 FLUORITE ICaF ) Li F IRTRAN (M9F Z ) SYNTHETIC SAPPHIRE (A1 2 O 3 ) QUARTZ PYREX GLASS 1  2  3 4 5 6 8 10  20 30 40 WAVELENGTH X jim Figure 10 .1 Transmission limits of some materials used for pyrometer lenses Different plates of known thickness, made of materials used for pyrometer lenses, have their relative spectral transmission, zA , displayed as a function of the wavelength in Figure 10 .2 (Warnke, 1972, Baker et al . 1953, Hackforth, 1960, Harrison 1960) . Commonly used lens materials are now described . Pyrex glass, transparent from 0 .3 to 3 pm, is used when high mechanical and chemical resistance is necessary . Quartz (Si02), transparent from 0 .2 to 4 pm, can withstand temperatures higher than those of glass, has high mechanical and chemical resistance and may also withstand rapid temperature variations . Synthetic sapphire (A1 2 0 3 ), transparent from 0 .3 to 5 pm, is hard and abrasion resistant . As it can be applied up to about 1000 °C it is also used for light guides . Unfortunately it is easily broken and cannot stand rapid temperature variations . Fluorite (calcium fluoride, CaF 2 ), transparent from 0 .1 to 9 .5 [um, can be used for measuring temperatures as low as +50 °C . Its applications are limited by low mechanical strength, 100 v Asp 5 3 aR 80 E  1  1 i \ z 60  +  r o `^ SC 1 z t <c 30  KBr  Si  Go! OjARTZ  `A .3 n .  20 4mm  1mm 1mm  ;S10,)  i 5mm w 20  mro s  jG :F,  KR5S  +  IRTRAN AS . 'a 1mm  'ntn  1,7smm 1 \  t 0,1  0,2 0 .3 0,5 0,7 1  2 3 5 7 10  20  40 WAVELENGTH  "k , PM Figure 10 .2 Spectral transmission, ra, of plates of given thickness used for pyrometer lenses OPTICAL SYSTEMS  179 softness and poor workability . KRS-5 (42 % TlBr, 58 % T1I), transparent from 0 .5 to 36 Nrrt is now the most commonly used material for the lenses of low temperature pyrometers, starting from -50 °C, where its mechanical strength is adequate . Silicon, transparent from 8 to 14 pm, sometimes replaces KRS-5, for low temperature pyrometers . The Ardometer pyrometer from Siemens AG (Germany) uses this material . Hackforth (1960) gives more detailed information on lens and window materials . Although most automatic pyrometers are equipped with a constant focus lens, focusable optical systems are more rarely used . Some pyrometers, such as Cyclops 300 AF, by Land Infrared Ltd, are also equipped with autofocus systems . With each pyrometer, producers supply a diagram of target diameter, d, versus target distance, l, similar to that shown in Figure 10 .3 for a MiniView pyrometer of the Cyclops Series by Land Infrared Ltd UK . An approximate distance ratio, l/d, which is sometimes given, is very useful in the comparison of different pyrometers . The value of the distance ratio also enables a rapid estimation of maximum necessary target diameter, d, for a given target distance, l . In modern pyrometers the optical system is often equipped with a laser aiming device . This permits that part of the target, whose temperature is to be measured, to be determined correctly . It may be either a point at the centre of the target area or a circular light with a centre spot . 10 .1 .2 Light guides When the objects, whose temperature is to be measured, are too small or not easily accessible, as well as in those cases when the pyrometer would be endangered by excessive temperatures, light guides (optical fibres) may successfully replace lenses . The operating principle of optical fibres is given in Section 6 .1 . Figure 10 .4 illustrates the working principle of a fibreoptic pyrometer . The end of the light guide is placed near the object which emits thermal radiation . This radiation arrives at the radiation detector after multiple internal reflections from the inner polished rod surface . Owing to absorption along the rod, imperfect reflection from the rod walls and reflection losses at the entrance and exit ends of the rod, some of the transmitted energy is lost . The efficiency of the energy transmission depends on the radiation entrance angle, and so, on the distances between the object and rod as well as between the rod and the detector . It is also affected by the length and design of the light guide . Light guides are made of artificial sapphire (A 1 203) or quartz (Si0 2 ) as a solid rod . or as a flexible stranded ftbreoptic cable of thin fibres, up to 2 m long . As described in Section 6 .1, light guides can be bent, provided m 1 3  2,5  2,1  2  1,5 12111  0  0 45 30 30  30 74 70 , 95 6V" 120 Figure 10 .3 Example of target diameter, d, versus target distance, l, for the optical system of a pyrometer 180  AUTOMATICPYROMETERS FURNACE OBJECT ~  DETECTOR LIGHT GUIDE  p a PYROMETER Figure 10 .4 Working principle of fibreoptic pyrometer that the angle of incidence at the side wall is always greater than the critical angle . The end of a light guide facing the body under measurement is usually equipped with an optical head having a small diameter, concentrating lens . 10 .1 .3 Mirrors At the lowest measured temperatures, where no lenses may be applied, mirrors can be used . As described in Section 8 .2, they are made of metals with good electrical conductivity, which are characterised by a high reflection factor at low temperatures and long wavelengths . Although mirrors absorb less infrared radiation than lenses, this advantage is partially cancelled by the need to use a protecting window . They are made mainly of polished gold, silver or aluminium of high reflectivity . Gold has good resistance to atmospheric and chemical influences, while the other metals have to be covered by a protective coating, which is transparent to infrared . This coating cannot be too thin, otherwise it will not act as a reflection reducing layer . Figure 10 .5 gives the specific spectral reflectivities, pa, of different metals as a function of radiation wavelengths as reported by Harrison (1960) . Mirror type pyrometers, which are no longer popular, are only used in some exceptional cases . 1,0 I . W 0,6 I . ALUMINIUM 0,4  SILVER ,4 I .  - ._ GOLD COPPER a  I I  - STEEL 0 .2 I 0 0  1  2  3  4  5 WAVELENGTH  R , pm Figure 10 .5 Specific spectral reflectivities, pa, of metals for pyrometer mirrors RADIATION DETECTORS  181 10 .2  Radiation Detectors Thermal radiation detectors are used in automatic total radiation pyrometers . Photoelectric detectors are used in automatic photoelectric, two-wavelength and multi-wavelength pyrometers . 10 .2 .1 Thermal detectors Total radiation pyrometers use thermal radiation detectors, which are heated by incident radiation . These detectors should have the following properties : "  high sensitivity, defined as the ratio of output signal to the incident radiation power, "  time stable properties, "  high resistance to shocks and vibrations, "  low thermal inertia, " output signal independent of the pyrometer position, "  high output signal-to-noise ratio, " high emissivity, " sensitivity independent of wavelength . Thermopiles, which are the most commonly used thermoelectric detectors, possess all these properties, together with an easily measurableor transformable output signal . These detectors are miniaturised elements in which the measuring junctions of a number of series connected thermocouples are exposed to the incident radiation from the object, whose temperature is to be measured . The reference junctions of the detector are kept at the pyrometer housing temperature . Lieneweg (1975) asserts that a good solution to the problem is the enclosure of the thermopile in an air evacuated glass bulb . As well as increasing the detector sensitivity this eliminates convective heat exchange, making the pyrometer output signal totally independent of the pyrometer position . Thermopile types are now described . Wire thermopiles made of thin thermocouple wires of diameter 0 .1 to 0 .15 mm with thin blackened plates, are used as radiation receivers . Ribbon thermopiles consist of thin thermocouple ribbons which are 0.025 mm thick and 0 .5 mm wide . They are soldered or welded together with one surface blackened to form the radiation receiver . Thin film thermopiles are deposited on a non-metallic plate, which is the radiation receiver . These types of thermopile have extremely low thermal inertia, having a time constant even as low as 15 ms . Hair pin thermopiles are similar to wire thermopiles but have much larger cross-sections, thus preventing brittle effect breakage . They are made of R-tellur-bismuth with a really high sensitivity , of 600 pV/K . The larger cross-sections are possible due to the low thermal conductivity of both metals . Thermistor and metal bolometerN which are also used, are constructed in thin film technology, with a resistance of 1 to 5 MO . In most cases they are used in ac bridge circuits to allow easy amplification of the output signals . Sometimes a do bridge circuit, with modulationof incident radiation, is used . Baker et al . (1953) report that the time constants of bolometers are from 1 to 16 ms . 182  AUTOMATIC PYROMETERS Pyroelectric detectors are also used in low temperature radiation pyrometers (Lang, 1972) . They are based on the phenomenon that the dipole moments of the charges in pyroelectric crystals, such as triglicine sulphate (TGS) change their orientation as a function of temperature . As the temperature varies, a temporary imbalance of charges appears so that an easily amplified alternating voltage which is modulated by the incident radiation flux, is generated . Despite their very high sensitivity, pyroelectric detectors are rarely used, because of the complicated construction ofpyrometers with modulationof incident radiation . 10 .2 .2 Photoelectric detectors Photoconductors, photodiodes, photovoltaic cells and vacuum photocells all belong to this group of detectors . Photoconductors (also called photoresistors) are built from glass plates with thin film coatings of thickness 1 pm, from the materials PbS, CdS, PbSe or PbTe . When the incident radiation has the same wavelength as the materials are able to absorb, the captured incident photons free photoelectrons, which are then able to form a conducting electric current . As the resistance of a photoconductor, which decreases with increasing radiation intensity, also depends on its temperature, this phenomenon has to be considered in the construction of a pyrometer . If not irradiated the `dark' resistance of a photoconductor is from 10 ¢ to 10 , S2 . Considering that the photoconductor sensitivity depends on the radiation wavelength, the concept of the operating wavelength band, can be introduced . Since the sensitivity and spectral response of photoconductors undergo some changes with ambient temperature and time, they are applied in most cases as a null detector . This may be achieved by comparing, for instance, two radiation intensities, falling alternatively upon its surface . In most cases the surface of a photoconductor has to be protected against atmospheric influences by covering it with a protective varnish layer of materials like polystyrene . Photodiodes, in germanium or silicon, are operated under a reverse bias voltage . Their conductivity as well as their reverse saturation current, under the influence of incident radiation, is proportional to the intensity of the radiation within the spectral response band from 0 .4 to 1 .7 hum for Ge and 0 .6 to 1 .1 pm for Si . The high sensitivity of photodiodes permits the construction of pyrometerswith high distance ratios . To compensate for the dark current, which occurs in the non-irradiated state, a second identical diode, protected from radiation, is used . Photovoltaic cells, which generate a voltage depending upon incident radiation, are constructed with a thin semiconductor film deposited on a metal plate . Under no-load conditions, this generated voltage is a logarithmic function of the incident radiation intensity . They are simple and robust in construction . As photovoltaic cells generate strong output signals that can be utilised without any further amplification, there is no need to apply any external voltage . However, because their sensitivity is low in the infrared range, they can only be used for higher temperatures . Materials used for photovoltaic detectors are selenium, silicon, indium antimonide (InSb) andindium arsenide (InAs) . Vacuum photocells operate on the principle that the incident infrared radiation causes the emission of electrons from a metallic photocathode, which is placed in a vacuum glass bulb with an anode . Ata given do voltage between cathode and anode, the electric current is a I ~  1 . " . MEN m III I 184  AUTOMATIC PYROMETERS Table 10 .1 Commonly used photoelectric radiation detectors (Wamke, 1972) . Wavelength, A Corresponding to  Maximum maximum detector  operating value, sensitivity  ANBX (Pm)  Resistance  Time constant, Detector (pm)  (S2) (lIs) Ge  1 .2  1 .8  1 Si  0 .9  1 .2  10 7 =1 PbS 2-2 .4  2 .3-3 .1 10 6 -10 7 150-500 InAs 3 .4  3 .7  20  2 InSb 6 .0  7 .0  4 .5-9 <1 the monochromatic radiation wavelength and at the applied frequency, f o , of the optical modulation up to about 1 kHz . 10 .3  Total Radiation Pyrometers 10 .3 .1 General information In total radiation pyrometers the temperature of a body is determined by the thermal radiation, which it emits over a large range of wavelengths . This radiation is concentrated onto a thermal radiation detector by a lightguide as shown previously in Figure 10 .4 or by a lens or mirror system as shown in Figure 10 .7 . Heating of the thermal detector by the concentrated incident thermal radiation gives a detector output signal, which is proportional to its temperature and thus at the same time to the value of the measured temperature . A total radiation pyrometer using a lens, was first constructed by Fery (1902) who later (Fery, 1908) also used a concave mirror in 1904 . Pyrometers may have an optical system with fixed or adjustable focal length . The former type is now more popular . TARGET  LENS  THERMAL DETECTOR OBSERVER /EYE I I I ______ - _ J MEASURING OKULAR INSTRUMENT LENS SYSTEM  °C M Im  PROTECTING WINDOW  MIRROR I  I r I MIRRORSYSTEM M Figure 10 .7 Basic diagrams of total radiation pyrometers TOTAL RADIATION PYROMETERS  185 10 .3 .2 Scale defining equation for black bodies Consider a pyrometer shown in a simplified way in Figure 10 .8 . The thermal radiation, emitted by a black body, 1, whose temperature, T t , is to be measured, passes through a window before falling onto a thermal detector plate, 2 . The window side of this plate is blackened to give as highan emissivity as possible while its other side should have as low an emissivity as possible . The incident thermal radiation heats the plate up to a certain temperature, Tp, which is measured by a thermocouple or a thermopile . A reference junction temperature for this thermopile is provided by the temperature, T H , of the pyrometer housing . Although there is no concentrating optical system in the form of a lens or mirror in the vastly simplified Figure 10 .8, neither the working principle nor the sensitivity of the pyrometer are altered . This is apparent as the existence of a concentrating optical system only reduces the necessary area of the radiating body . On the surface of the detector plate, the heat flux density of the flux, emitted by the body and absorbed by the plate, is given by : Rl ) 2 -_ 6,E2 K t sin g tp(T t 4 - T p )  (10 .2) where 6 o is the radiation constant from equation (8 .16), E2 is the total emissivity of the blackened side of the detector plate, K i is a coefficient depending on the construction of the pyrometer and the absorption of the optical system, T t is the true, measured temperature of the blackbody, T p is the plate temperature and (p is the viewing angle given in Figure 10 .8 . Instead of the viewing angle some producers give the ratio of working distance, l, to the minimum target diameter, d, (Figure 10 .8) or simply the distance ratio . An increasing number of producers now supply more precise and more convenient diagramsof the target diameter versus working distance . Figure 10 .3 is a typical example of such diagrams . In practice, when the detector plate has very , small dimensions, its viewing angle, (fi , is the same all over its surface . Thus the thermal flux, or heating power absorbed by the plate is given by : (D1 -) 2 ° CV 2K, A p sin e !p(T 4 - TP)  (10 .3) where A p is the one side plate area and the other symbols are as in equation (10 .2) . THERMAL DETECTOR PLATE HOUSING WINDOW T 2 T H T P BLACK BODY THERMOPILE  I Figure 10 .8 Total radiation pyrometer - simplified design 186  AUTOMATIC PYROMETERS As the area of the plate, A P , is much smaller than the inner area of the pyrometer housing, and neglecting the radiant heat exchange at the unblackened back side of the plate, the total heat flux transmitted from the plate to the pyrometer housing is expressed as : 'D2 I x =6oE2KtAP(Tp-TH)+K2(Tp-TH)  (10 .4) where K 2 is the heat transfer coefficient by convection and conduction from the plate to the housing and the other symbols are as in equation (10 .3) . When the plate is in the thermal steady-state, the received radiant heat flux (D I , 2 equals the heat flux (D2 , H transferred to the pyrometer housing so that : 4  4 6o E2 K,Apsin2cp(Tt4-Tp ) =6 .E2KIAp(Tp -TH)+K2(Tp-TH)  (10 .5) The output signal of the pyrometer, which is the thermal emf, E, of the thermocouple or thermopile of Figure 10 .8 is a nearly linear function of the temperature difference between the plate temperature, T p , and that of the housing, T H , is thus given by : E= Ke(TP-TH)  (10 .6) where K e is the thermocouple gain, mV/K . The gain of a thermopile, composed of n thermocouples is, nK e . Calculation of the characteristic T p - TH = f(Tt)  (10 .7) of a pyrometer is based on the solution of equation (10 .5), whose complicated form as well as the temperature dependence of E 2 , K t and K 2 , excludes the possibility for a practical analytical solution . In practice, the pyrometer characteristic, which is always determined experimentally, has the approximate form (Ribaud et al . 1959) : E = K(T b - Tp)  (10 .8) in which the exponent, b, with a value between 3 .5 and 4 .5, and the constant, K, depend on the construction of the pyrometer . Equation (10 .8) concerns thermocouple and thermopile detectors . For resistance and semiconductor bolometers other formulae are used . 10 .3 .3 Temperature measurement of non-black bodies Total radiation pyrometers are calibrated under the assumption that the measuring target is a black body . From equation (10 .3), the radiant heat flux emitted by the target at the temperature T t and absorbed by the detector plate is given by :