GB2113384A - Contactless measurement of temperature - Google Patents

Abstract

A method of, and an apparatus for, performing contactless measurement of temperature, independent of the emissivity, on the basis of radiation pyrometry aims at enabling measurement both in the high-temperature range and in the low-temperature range, with a minimum of preconditions and preliminary information being required. The true temperature of a test object is to be attainable even with inaccurate knowledge and/or variable emissivity. The essence of the invention resides in successively recording as input variables the band signals of radiation of a test object by means of a pyrometric arrangement responsive in a plurality of spectral regions with and without auxiliary radiation of known spectral composition which is reflected from the surface of the test object, the band signals being fed to a computer. The computer combines the input variables such that the unknown variables such as the true temperature of the object and spectral emissivities are ascertained by iterative solution of a non-linear set of equations.

Description

SPECIFICATION A method of, and an apparatus for, contactles measurement of temperature The invention relates to a method, and an apparatus for performing the method, for the contactless measurement of temperature, independent of the emissivity, on the basis of radiation pyrometry.

Whilst scarcely any problems are involved in the detection of relative changes in temperature when using pyrometric measuring apparata, determination of the absolute temperature with purely pyrometric means still involes considerable difficulties which are caused particularly by the fact that no information, or only inaccurate information, exists concerning the emissivity of the test objects under actual test conditions. While total pyrometers, band pyrometers and spectro pyrometers can determine the true temperature by corresponding correction of the measured value only when the emissivity is known and is constant with respect to time, ratio pyrometers or colour pyrometers can detect the true temperature even on surfaces of test objects having variable emissivity, provided that a "grey body" is involved.However, this only applies in a few exceptional cases (Lieneweg: Handbuch, Technische Temperaturmessung, Vieweg-Verlag 1976, pages 314-318).

Furthermore, pyrometric measuring apparata are known which determine the true temperature by means of front projection by an auxiliary radiator by measuring the reflectivity.

This requires the existance of regular reflection and a specific fixed geometrical arrangement of the pyrometric measuring apparatus, auxiliary radiator and surface of the test object. However, this only occurs in a few special cases (Tingwaldt, C.: Ein einfaches optisches Verfaren zur direkten Ermittlung wahrer Temperaturen glühender Metalle, Z.

Metallkunde 51 (1960) 2, page 116-119; Kellsall, D.: An automatic emissivity compensated radiation pyrometer J. Sci. lnstr. 40 (1963), pages 1-4; preisleben, R. Pyrometrische Messungen an Glühkatoden mit porö- ser Oberfläche, Int. Kolloquim der TH Ilmenau (1965), Vortragsreihe Messtechnik H. 9, pages 17-23); Svet (Offenlegungsschrift 1648233, 1972, 42 i, 8/60) proposes the use of various temperature-invariant quantities for compensating for the effect of emissivity and transmittance, a plurality of spectural regions being used.

The method proposed by Svet is only suitable for the high-temperature range when the Wien approximation applies and when sufficient preliminary information concerning the characteristic of the emissivity in dependence upon wavelength exists. The preliminary information must be established on the object, otherwise incorrect measurements occur which cannot be recognised as such (Svet: IMEKO Vill pages 13-5 to 13-11, Die Probleme der Strahlungs-Pyrometrie und einige neue Mdglichkeiten inhrer Ldsung, 1979).

The basic aim of the present invention is to avoid the above-mentioned disadvantages of the known technical solutions.

More specifically, the object of the invention is to enable determination of true temperature of a test object in the high-temperature range and in the low-temperature range with inaccurate knowledge and/or variable emissivity, with a monotonic characteristic of the spectral emissivity, and without a fixed geometrical correlation between the object, pyrometric measuring apparatus and auxiliary radiator.

In accordance with the invention, the ambient radiation of the test object or a quantity characterising it, such as the ambient temperature, as well as the band signals of the surface of the test object, are used as the input signals for a computer by use of a pyrometric arrangement which is responsive in a plurality of spectral regions. If the ambient radiation is negligible compared with the radiation of the test object, an auxiliary radiator of known spectral composition is used which, like the ambient radiation, is reflected into the pyrometric measuring apparatus by way of the test object.

The invention includes an apparatus for performing the method which comprises a pyrometric arrangment which is responsive in a plurality of spectral regions and which includes a device for selecting the spectral regions, which device comprises, for example, filters or prisms or a monochromatic grating, the apparatus having a radiation detector of single- or multi-channel construction or a radiation detector which is selective in a plurality of spectral regions, such as an Hg Cd Te detector comprising a plurality of layers, the apparatus being connected to a computer, and an auxiliary radiator being disposed such that, in addition to detecting the radiation from the test object, the pyrometric arrangement also detects the radiation of the auxiliary radiator reflected from the test object.

In contrast to the known methods which use auxiliary radiators, only the ratios of the reflectivity of the individual spectral regions are determined so that a fixed geometrical arrangement and regular reflection are not required.

In multi-spectral measurements of radiation, a new unknown of the spectral or band emissivity appears with each spectral or band measured value. Information is gained only when data concerning the combination of the emissivity and the individual spectral regions exists or when the wavelength dependency is known. A special case for this is the simple ratio pyrometer in which Er = e2.

The principle of ratio pyrometry can be construed as a solution of a non-linear set of equations of the form U1 = f (, A1, T,, Tu) U2 = f (EZ' h2r to, Tu) E1 = E2 = E and the unknown temperature To of the object and the emissivity e (two equations with two unknowns) can be determined from the known variables U (band signal), (effective wavelength) and Tu (ambient temperature).

Conventional ratio formation is not successful in the low-temperature range, since the ratio signal cannot be calibrated in accordance with the temperature of the object independently of the emissivity and the ambient temperature. Therefore, a non-linear set of equations has to be solved.

The principle of ratio pyrometry can be expanded to M spectral measurements or band measurements. N is then less than or equal to M (N < M). Unknowns ascertainable: the temperature of the object and a wavelength-dependency of the emissivity (for example E = a + b A+c N2 + . x AN-2) which can be described with (N-1) parameters. The prepared test space can be construed as a front projection (Aufprojektion) of a known radiation. Hence, during measurements in three spectral regions, it is possible to state whether the object is radiating "grey" in these regions and, in the case of four spectral regions, whether a linear emissivity characteristic exists.

The reflectivity ratio v = Q 2 can be ascertained when measurements are made with a front-projected auxiliary radiation in two spectral regions. The temperature of the object can be ascertained therefrom by the measured values without auxiliary radiation (3 equations with 3 unknowns). The condition imposed on the geometrical arrangement and the reflectivity is only that a sufficiently accurately measurable proportion of reflected auxiliary radiation must be detectable by the pyrometric measuring apparatus.

Reflectivity ratio measurements in 3 spectral regions render it possible to admit the ambient temperature as an unknown or to check the preparation of the test space.

Signal processing (solving the non-linear set of equations) is effected by the iteration method and requires the use of efficient microcomputer technology, and it must be possible to store the band signal characteristics ascertained from a "black body". Furthermore, the manual input of spectral or band emissivities and emissivity ratios is advantageous.

The true temperature of the object and the emissivity are ascertained by means of the iteration method by combining the measured values of the band signals with the stored values of the band signal characteristics.

An advantageous effect when using the invention resides particularly in the fact that it can be used in both high-temperature and low-temperature ranges. Furthermore, the various conditions which have to be met when using the known methods and apparata, are inapplicable. Thus, for example, regular-reflection and a fixed geometrical arrangement between the pyrometric measuring apparatus, auxiliary radiator and surface of the test object are not required. Furthermore the preliminary information concerning the characteristic of the spectral emissivity does not have to be established on the object in order to obtain well-defined measurement results. Furthermore, use outside the range of validity of the Wien approximation is possible, so that the method in accordance with the invention can be considered to be the most universally usable method now known.

One particular embodiment of the invention is now described.

The pyrometric arrangement has three fixed spectral regions. The detector used is a multispectral detector made from, for example, Hg Cd Te layers, which is responsive in three spectral regions, so that the spectral regions are split up. A lens objective lens or mirror is used for focussing the infrared radiation. The data are processed by means of a microcomputer system. It is also possible to indicate and correct the individual band signals by appropriate switch-over. It is possible to use only two spectral regions when "grey bodies" are present. The characteristic of the spectral emissivity may be unknown when the spectral regions are used. The existence of a "grey body" can be established by forming two ratios. If the basic characteristic of the spectral emissivity is known, the true temperature of the object can be ascertained by solving the non-linear set of equations.

Known variables, such as emissivities, emission ratios and emission characteristics can be fed into a suitable store. Furthermore, emissivities which have been ascertained can be automatically stored.

Furthermore, the computer is used to compensate for the effect of emissivity.

In a further embodiment, the pyrometric arrangement has four fixed spectral regions.

This permits a linear emissivity characteristic, which is confirmed by the fourth spectral region by correspondance of the object temperatures ascertained.

Furthermore, it is possible for the pyrometric arrangement to have three fixed spectral regions, and additionally to use an auxiliary radiator. The ambient temperature is thereby admitted as an unknown. The ambient temperature must be known when using only two spectral regions. Automatic recalibration is also possible by means of the auxiliary radiator.

Claims (7)

1. A method for the contactless measure ment of temperature independent of the emissivity, wherein the band signals of the radiation of a test object are successively recorded as input variables by a pyrometric arrangement responsive in a plurality of spectral regions, with and without auxiliary radiation of known spectral composition which is reflected from the surface of the test object, and are fed to a computer in which they are combined such that the spectral reflectivity ratios of the surface of the test object are calculated and, together with all the band signals and the stored band signal characteristics of the individual spectral regions which are ascertained by a black radiator, are used as known variables of a non-linear set of equations, the set of equations being solved by means of an iteration method, whereby the unknown variables, namely the true temperature of the object, and the spectral emissivity are ascertained

2. A method as claimed in claim 1, wherein the radiation from the environment of the test object is used instead of separate auxiliary radiation, and a variable characterising it, such as the ambient temperature, is used as an input for the computer, and the unknown variables, namely the true temperature of the object and the spectral emissivity, are ascertained in the computer by the same combination alogrithm.

3. A method as claimed in claim 1, wherein known vairables, such as the emissivity, emissivity ratios and emissivity characteristics are fed into the computer.

4. A method as claimed in claim 1, wherein the combination is effected by the computer such that the intensity ratios of each band signal or of two band signals are ascertained and compared, the presence of a "grey body" and the true temperature of the object being deduced when the temperatures of the object which have been ascertained are equal.

5. A method as claimed in claim 1, wherein the computer of the pyrometric arrangement is additionally used to compensate for the effect of the temperature of the housing and, in conjunction with the auxiliary radiator used for determining the temperature of the object, is used for automatic recalibration of the pyrometric arrangement, and permits the storage of known and measured emissivities.

6. An apparatus for performing the method as claimed in claim 1, comprising a pyrometric arrangement which is responsive in a plurality of spectral regions and which includes a device for selecting the spectral regions, which device comprises, for example filters or prisms or a monochromatic grating, the apparatus having a radiation detector of single- or multi-channel construction or a radiation detector which is selective in a plurality of spectral regions, such as an Hg Cd Te detector comprising a plurality of layers, the apparatus being connected to a computer, and an auxiliary radiator being disposed such that, in addition to detecting the radiation from the test object, the pyrometric arrangement also detects the radiation of the auxiliary radiator reflected from the test object.

7. An apparatus as claimed in claim 6, wherein the pyrometric arrangement also selectively and switchably includes indication of the individual band signals or the radiation temperatures corresponding thereto, and effects correction of the band signals by means of emissivity adjustment.