Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N25/00—Investigating or analyzing materials by the use of thermal means
  • G01N25/72—Investigating presence of flaws

Definitions

• This invention relates to a method and an apparatus for the characterization and control of substances and materials as well as factors of physical and chemical nature being associated therewith.
• J_ __ W( ⁇ )d ⁇ - E - ( ⁇ kT ⁇ - i) d ⁇ , ( 1 ) ⁇ s
• T the temperature of the body
• h Planck's con ⁇ stant
• k Boltzmann's constant
• c the speed of light
• W( ⁇ ) is termed the spectral radiantexcitationof the body.
• T - ⁇ __ 2897,9 K ⁇ m, (2) max which can be derived from (1) .
• equation (4) usually may not be expressed analytically.
• both W( ⁇ ; ⁇ _) and ⁇ ( ⁇ ; ⁇ 2 ) may be measured by means of radiation detectors being sensitive within the spec ⁇ tral range concerned. This range is usually selected so as to comprise also the wavelength for maximum emission, ⁇ , but specific advantages and effects may be obtained by selecting other spectral ranges for the detection. Possible changes measured in W( ⁇ ; ⁇ 2 ) from an object then will be directly re ⁇ lated to inherent variations in temperature and/or emissivity of the object. This is utilized today in standard measurement techniques, and commercial equipment has been available for a long time. The method is contact free and therefore does not interfere with the measurement object.
• the object is photothermally transparent.
• the illumination at wavelength ⁇ penetrates deeper into the material than one diffusion length, and it is found that the temperature at the outer surface is
• the object is photothermally opaque. All incident radiation is then absorbed in a surface layer being thinner than the thermal diffusion length, and it is found that the surface temperature is
• ⁇ T _I (2 ⁇ fkpC) ⁇ *. (6)
• the induced temperature variations will then be independent of the absorption coefficient of the object; and measurement of radiant exitance at frequency f will give information about the parameters k,p, C and/or ⁇ . This also applies when the changes in k, p or C take place within the object and less than one thermal diffusion length from the surface. Internal structures in the object not being visible at the surface will then give changes both in the amplitude and the phase of ⁇ W (Ref. 5) . In other words this makes it possible to "look" into the object to a certain depth ⁇ .
• a particularly inte ⁇ resting case is when the temperature variations are measured at the back sideof the object, as a result of illumination of the front side thereof.
• the induced temperature oscillations may be very small, the detection limit being ', iinn tthhee rraannggee ooff 1100 "" KK..
• SUBSTITUTESHEET be able to measure ⁇ with a good signal-to-noise ratio it is therefore necessary to correlate the measured thermal emission with the illumination, so that only those thermal oscillations may be identified, which are synchroneous with the pulsations of the illumination. This is performed as a matter of routine by means of electronic lock-in amplifiers which essentially filter away all signals outside a narrow bandwidth ⁇ f around the pulse frequency f; ⁇ f being most often smaller than 1 Hz. For obtaining representative measure ⁇ ments the time for each measurement point must be at least equal to the inverse of the bandwidth, so that steady-state conditions apply. The measurements will then be slow and time consuming; and by sweeping across objects typical velo ⁇ cities are often lower than 1 mm/s (Refs.
• the last term given by integral expression is a transient thermal disturbance in the object, due to the start of the temperature oscillations on the surface. This transient dies away when t —> ⁇ , and there only remains the steady- state time variable solution given by the first term.
• This term describes the temperature oscillations being utilized in steady-state photothermal radiometry as described above.
• the solution has the form of damped waves the amplitude of which is reduced by a factor e over a distance equal to the thermal diffusion length ⁇ .
• thermal waves are strongly dis ⁇ persive; waves of higher frequency propagating more quickly.
• high frequency thermal waves are more strongly attenuated with x because of the factor e " ⁇ ⁇ .
• Equation (11) is employed in a standard pulse method for determining thediffusivity ⁇ in solid substances (Ref. 9) : By measuring t, on the basis of the temperature variation at the back side of an object having a thickness I upon heating of the front side with pulses of duration ⁇ t, it is possible to determine K from
• the pulse method has been developed further to a pulse type photothermal radiometry (Ref. 10) , wherein by recording the exact curve shape of the temperature variation at the front or back side of an object upon pulse heating of the front side, it is possible to de ⁇ termine absorption coefficients, thermal diffusivity and the thickness of both the object itself and of possible sur ⁇ face layers.
• an individual pulse is suffi ⁇ cient for determining all these parameters.
• the pulse method is faster than the steady-state techniques discussed above, and this by a factor being approximately corresponding to the number of periods f/ ⁇ f required for each steady-state measurement at frequency f and bandwidth ⁇ f.
• the pulse method for the characterization of for example a complete object, a material stream or the like, one would in such case have to record correspondingly detailed temperature variations or curves at all points of interest, and then extract the relevant data by an individual analysis of each individual time curve. This recording and the subsequent signal processing thereby will take so long time that also the pulse method will be too slow for real time measurements. No such use of the pulse method has been demonstrated. The reason is that the pulse method acquires too many data, most of which is without significance for the measurements to be made. This will be explained more- closely in connection with the following presentation of the invention.
• the present invention takes as a basis the fact that as a rule one is not interested in the complete temperature curve or history at each individual point of the object. Most often it is sufficient to record possible variations of the tempera ⁇ ture from point to point at the surface of the object, thereby to survey and identify possible dissimilarities in chemical and physical conditions at and beneath the surface.
• the invention consists in a particular utilization of the transient thermal waves in the object, making it possible continuously and in real time to exclusively record the thermal radiation from a series of object points after a time delay corresponding to thermal diffusion over a selectable length or depth d in the object.
• the invention provides for giving the object a relative movement in relation to a source for continuous (and possibly constant) thermalexcitation, whereby the rela ⁇ tive velocity between object and source, v. , is higher than the effective thermal diffusion velocity u(d) for the distance concerned in the object.
• This combination of continuous thermal exitation and physical movement may be compared to a consecutive series of independent, instantaneous thermal sources being displaced over the object.
• the thermal diffu ⁇ sion conditions for object points at a larger distance than d along the path of movement, will be independent of each other because the source is displaced faster than the transient
• Figure 1 schematically shows an arrangement for thermal exci ⁇ tation of the front side (upper side) of the object.
• Figure 2 schematically shows detection of thermal radiation from the front side (fully drawn lines) or from the back side (dotted lines) of the object.
• Figure 3 schematically shows examples of systematic and characteristic structures and patterns which may have been included in materials and objects.
• SUBSTITUTE SHEET Figure 4 schematically shows thermal excitation and detec ⁇ tion at a distance L from each other
• Figure 5 schematically shows a space filter in connection with the detection system.
• FIGS 6, 7 and 8 illustrate alternative forms of thermal excitation of the object
• Figures 9 - 12 illustrate some interesting configurations of excitation devices and detectors shown schematically in relation to a part of an elongate object seen from above.
• FIG. 1 there is shown a thermal excitation from a source 2 onto an object 1 being moved at a velocity v, with respect to the thermal excitation.
• the excitation has been illustrated here in an idealized manner as point shaped, whereas in practice it will have a certain extension a in the direction of movement.
• the thermal excitation can take place by a slip contact with a body having good thermal con ⁇ ductivity, possibly with a "heat pipe” in communication with _. heated or cooled thermal reservoir, but it may also take place without any mechanical contact by means of cold or hot gas, electromagnetic radiation, electron or other particle radiation or also by acoustic waves. Gas and contact excita ⁇ tion will position the initial heating or cooling at the sur ⁇ face of the object.
• electromag ⁇ netic radiation will penetrate into the object to a typical distance of ⁇ ( ⁇ )- ⁇ equal to the inverse of the absorption co ⁇ efficient for radiation at the wavelength concerned.
• ⁇ the thermal excitation can to some degree be adjusted to the situation of interest, and then particu ⁇ larly in connection with the detection of substances being exposed to the surface.
• Particle radiation penetrates in a corresponding manner into the object depending upon the energy of the particles as well as the material properties and the com ⁇ position of elements in the object, and this may also be utilized for adapting the excitation in the volume of the object.
• Acoustic waves will penetrate into and propagate in the object depending upon the modulus of elasticity E there ⁇ of, and this may be able to heat selectively internal struc ⁇ tures having an E causing the waves to be absorbed. This gives altogether an arsenal of different thermal excitation modes, from which there may be selected a thermal excitation being particularly suited for each individual use of the invention.
• the object may be heated or cooled re ⁇ spectively, along the same path or along two parallel paths, with separate detection for each of these.
• the temperature excursions will then be directed mutually opposite, which gives complementary signals in the detection system.
• This can be used for example for distinguishing variations in emissivity from the thermally related signals. Detection along a neutral path, i.e. without thermal excitation, will in addition give a reference signal from a thermally undisturbed object.
• Figure 2 shows detection of the resulting thermal radi ⁇ ation both from the same side (the front side) (a) as the thermal excitation, and from the opposite side (back side) (b) .
• the field of view ⁇ of the detection system has a velocity v_ in relation to the object 1.
• the detection system consists of a device 3 which collects the thermal radiation from the object, and in the
• SUBSTITUTE SHEET figure has been symbolized as a lens.
• Other devices of inte ⁇ rest for collecting thermal radiation may be mirror arrange ⁇ ments, optical fibers, optical waveguides and others.
• the device 3 collects the thermal radiation towards a detector 4 which converts the radiation into electrical signals.
• detectors are often made of semiconductor materials and may be adapted for different spectral ranges of the thermal radi ⁇ ation, depending inter alia upon the temperature of the objec but there are also thermal detectors having a more flat spectral characteristic. These are usually not so fast actin and sensitive as the semiconductor detectors.
• the signals from the detector are amplified in an amplifier 5 and can further be subjected to a more specific analysis in a unit 6 which comprises an electronic signal processing system.
• SUBSTITUTE SHEET advantageous when thin objects and materials are concerned, whereby there may be the question of identifying internal structures or possibly variations in thickness and other object parameters being characteristic to the whole volume of the object.
• Investigation of the surface layer of objects in which the structures of interest are exposed to the surface and of internal structures in objects being so thick that thermal diffusion to the back side takes un ⁇ reasonably long time and results in a corresponding reduction in the resolution, on the contrary is done best by detection from the front side.
• Figure 3 shows examples of objects and materials contai ⁇ ning systematic and characteristic structures and patterns either at the surface (3a) , within the volume (3b) or as variations (for example in thickness as shown) of parameters which affect the whole cross-section of the object (3c) .
• a suitable form of thermal excitation as well as detection from the front or back side determined inter alia by the thickness and other properties of the object.
• this may thus be utilized for characterizing each individual object in relation to a recording of such structures and patterns, and thereby also for checking that the object contains structures having a predetermined pattern.
• the dimension or extent d of the structures and patterns in the object is the important parameter in this connection, and
• SUBSTITUTESHEET determines specifically the limits to the respective relative velocities of movement of the object as stated in claim 1.
• the invention can also be employed for investigating and characterizing objects having random patterns or structures at the surface, in the volume thereof or through the whole cross-section of the object. This may serve many purposes, for example to identify objects in which the variation of such parameters are outside certain limits. This may also be utilized for establishing feed-back in industrial processes, so that these may be controlled in a manner which brings the parameters of interest to be kept within relevant limiting values.
• d satisfies
• FIG. 4 An arrangement for effecting movement of the object 1 is shown in figure 4 in the form of roller pairs 11 and 12 which by their rotation gives the object the velocity v.
• Another embodiment may for example consist of one therma excitation with two or more detectors behind each other at distances from the excitation corresponding to different diffu sion times ⁇ .
• Figure 5 shows shielding of the detector and the detec ⁇ tion system by means of a spatial filter 9.
• This can often only be in the form of a shield which prevents the direct influence on the detector from the thermal excitation of the object.
• excitation by means of electromagnetic radiation can result in scattering into the detection system, and since this radiation will be orders of magnitude stronger than the resulting thermal radiation, even little scattering may disturb and mask the actual signals.
• excitation of for example hot or cold gas which may lea into the area between the object and the detector and influ ⁇ ence the detection.
• the space filter is often -made as a housing completely enclosing the units 3 and 4, having only a small opening towards the object through which thermal radiation can enter.
• FIG. 6 illustrates alternative forms of thermal excitation to the form shown in Fig. 1.
• a slip contact form of excitation is shown in Fig. 6, in which a moving object 61 is contacted by a heat conducting foot ⁇ like member 63 being supplied with heat from a heat source 62 and being pressed to engagement with the surface of object 61 by means of a spring 64.
• the contact portion of member 63 will engage the object surface at a small area the dimension of which is indicated with a in Fig. 6.
• Fig. 7 shows another form of mechanical contact for thermal excitation.
• a rod shaped member is adapted to con ⁇ duct heat from a heat source 72 to a rounded end or head portion 73 engaging the surface 71 of the object.
• a spring 74 is provided in order to secure a safe contact.
• Fig. 8 there is shown a form of thermal excitation
• SUBSTITUTESHEET based upon a flame 83 obtained by the combustion of a suit ⁇ able gas supplied from a source 82 through a tube and nozzle 84.
• the heated or excited area is indicated by dimension a on the surface 81 of the object.
• the following figures 9, 10, 11 and 12 include an elongate or continuous object moving in a direction towards the left of this sheet of drawings, as indicated with arrow V in Fig. 9 (and in Fig. 6) .
• an object 91 is moved (arrow V) in relation to a point or area of exci ⁇ tation 92 and two detectors 94 and 95 respectively, so as to establish a path of relative movement indicated at 90.
• the signal recorded by means of detector 95 is a result of the excitation at 92.
• Fig. 10 illustrates an arrangement in which an object has two parallel paths of relative movement with respect to excitation point 102A with an accompanying detector 104A and excitation point 102B with detector 104B respectively.
• Excitations at 102A and 102B can be by means of separate electromagnetic beams at two different wavelengths and de ⁇ tectors 104A and 104B respectively, being adapted to respond to the respective wavelengths.
• excitation at 102A may be some form of heat excitation whereas excitation at 102B may be in the form of cooling, as briefly suggested in connection with Fig. 1 above.
• the dual arrangement of excitation and detectors can be related to one and the same path as illustrated in Fig. 11.
• i-n Fig. 11 one path 110 is common to two excitation points 112 and 116 as well as two detectors 114 and 118.
• Excitation at 112 is detected at 114 and excitation at 116
• SUBSTITUTESHEET is detected at 118.
• Excitation at 112 can be cooling and excitation at 116 can be heating or both may be heating for example by electromagnetic radiation at different wave ⁇ lengths.
• Fig. 12 illustrates a configuration in which an excitation point 122 and a corresponding detector 124A are provided along one path 120A, whereas only a detector 124B (without preceding excitation) is provided along path 120B.
• the latter detector serves to detect thermal radiation from this object along a path being adjacent and parallel to the excited path 120A, so that the signal from detector 124B can be used as a reference signal.
• V Fifthly it will be possible just because of the relative movement between the object and the field of view in the detection system, to adjust the relative velocity v_ such that all the structural details to be investigated, give signal variations which in frequency fall within the electrical bandwidth of the detection system. Then these electrical signals can be amplified and processed in a conventional manner.
• the relative movement means that the thermal excitationand the detection take place atphysically separated locations, which in a simple way separates the detection from the thermalexcitation so that theexcitation is effected out ⁇ side the field of view of the detection system. This involves inter alia that the same electromagnetic wavelength can be employed both for the thermal excitation and for the detection without interference (mutual disturbance) .
• the geometric dimen ⁇ sion or extent ⁇ of the field of view on the object must satisfy ⁇ _ d. This requires that the relative velocity v_ between the object and the field of view of the detection system satisfies
• the effective diffusion length in this case is equal to ⁇ , and the effective diffusion velocity is
• a main point of the invention is to implement a high resolution transversally to the direction of movement of the object. Assume that structures of interest having a dimension d transversally to the direction of movement shall be identified with a resolution ⁇ d ⁇ d. ( This may for exampl apply to the determination of thickness variations ⁇ d of the object or of a surface layer having an average thickness d.) For propagating this additional distance the thermal wave front needs a time ⁇ which follows from differentiating (12)
• the field of view with an extent ⁇ will then be passed through by each individual point on the object during time ⁇ ⁇ , and the detection can still be considered as instantaneous in relation to the thermal diffusion processes. Even with a longitudinal resolution restricted by the field of view ⁇ of the detection system and not by the diffusion processes, it will in other words still be possible to obtain the same transversal resolution, determined by the thermal waves, but then at higher relative velocities.
• the highest frequencies in the signal will be. given by v_/g, assuming a diffusion limited longitudinal re ⁇ solution g equal to the dimension of the smallest structure in the object to be identified.
• the relative velocity be ⁇ tween the object and the field of view in the detection sys ⁇ tem must therefore be chosen so that all frequencies within the range
• v.. and v- in the principle are unequal and this is also often the case in practice. Nor do they have to be directed along the same path in relation to the object in order that the invention shall be useful, as long as the conditions stated above are satisfied.
• v 1 and v- can be directed along the same path as the object, while for example v- is constant and v, varies periodically about an average value.
• the distance between the thermal excitationand the detection given by the diffu ⁇ sion time ⁇ will then also vary periodically, so that there is essentially effected a scanning of the object over a range of diffusion lengths d corresponding to the variety of different diffusion times ⁇ . This may be of interest in cases where the relevant distance d is not known or varies, possibly also when investigating a number of structures each having its own characteristic length d, by traversing a spectrum of diffusion lengths.
• Protective coatings on surfaces is another important field of use of the invention. Assuming a coating of thick-
• the velocity of the object in relation to the detection system in this case will be the dimension determining the choice of the relative movements, c) .
• Measurement of thickness of for example aluminium products and identification of possible flaws in the material is another relevant example.
• Chemical substances exposed to the surface of the object can be identified by means of heating with electromag ⁇ netic radiation at the absorption line characteristic to the substances.
• the objects can include characteristic structures of different materials being for example included at a certain depth in the object (as in semi-conductor ma ⁇ terials) .
• the method described above it will then be possible to bring about temperature variations at the surface which reflect these internal patterns, and then by means of the signal processing system it can be checked that they have the correct form.
• the object contains systematic and characteristic mass changes not being visible at the surface, for example holes, inclusions and the like, systematic variations in thickness, ror example watermarks in paper, characteristic patterns and structures of chemical nature, for example dye stuffs having pronounced spectral absorption lines arranged as text, pictures and the like.
• the invention can be utilized for recog ⁇ nizing, controlling and/or characterizing objects through the systematic and characteristic structures discussed, by thermal excitation at one side of the object and detection of resulting thermal radiation from the same or the opposite side of the object.

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• Health & Medical Sciences (AREA)
• General Physics & Mathematics (AREA)
• Physics & Mathematics (AREA)
• Pathology (AREA)
• Investigating Or Analyzing Materials Using Thermal Means (AREA)
• Photometry And Measurement Of Optical Pulse Characteristics (AREA)
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Citations (3)

• 1985
  • 1985-07-15 NO NO852833A patent/NO164133C/no not_active IP Right Cessation
• 1986
  • 1986-07-14 EP EP86904424A patent/EP0229816A1/en not_active Withdrawn
  • 1986-07-14 WO PCT/NO1986/000052 patent/WO1987000632A1/en not_active Application Discontinuation
  • 1986-07-14 AU AU61357/86A patent/AU6135786A/en not_active Withdrawn
  • 1986-07-14 JP JP61503843A patent/JPS63500336A/ja active Pending
  • 1986-07-14 BR BR8606794A patent/BR8606794A/pt unknown
  • 1986-07-15 CN CN198686105818A patent/CN86105818A/zh not_active Withdrawn
• 1987
  • 1987-03-13 FI FI871117A patent/FI871117A/fi not_active Application Discontinuation
  • 1987-03-13 DK DK130087A patent/DK130087A/da not_active IP Right Cessation

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