NO164133B - PROCEDURAL TEA AND APPARATUS FOR CHARACTERIZATION AND CONVENTION MATERIALS, MATERIALS AND OBJECTS. - Google Patents

Description

This invention relates to a method and an apparatus for the characterization and control of substances, materials and objects as well as factors of a physical and chemical nature which are included in these. By exciting transient thermal waves in an object and measuring the resulting changes in thermal radiation from the object, the invention will thus in its broadest aspect enable a completely contact-free characterization and control as mentioned, with significant improvements in relation to comparable existing techniques.

All bodies and objects that have a certain temperature emit

thermal, electromagnetic radiation. For ideal black bodies, the emitted power per unit area within a wavelength interval d\

given by Planck's radiation law,

where T is the temperature of the body, h is Planck's constant, k is Boltzmann's constant and a is the speed of light; W(\) is called that of the body

spectral radiant excitation. For bodies at room temperature (T = 300 K) (1) gives an emission spectrum with a maximum at approx. 10 ym wavelength in the mid-infrared spectral range. If the temperature is increased, the spectral distribution will change according to (1), and the top point ^max in the spectrum will shift towards lower wavelengths; for T > 4000 K

X falls near or within the visible area. This displacement

max

is described with a good approximation by Vienna's displacement law,

which can be derived from (1).

Bodies that are not ideally black can with a good approximation

is described by multiplying given in (1) by an effective emissivity z( T,\) £ 1. Emitted power per unit area from a body that is not black, within the wavelength interval Ai SX to X2 is then given by

For a small temperature change of 6T, the change in radiated power per area unit then be

Because z( T,\) is generally a very complex function, which also depends

of the geometry of the body, (A) will usually not be expressed analytically.

However, both W(\ i;\ i) and 6W(Ax;\ 2) can be measured using radiation detectors which are sensitive in the relevant spectral range.

This range is usually chosen so that it also includes the wavelength

for maximum emission, A , but special benefits and effects can

etc. x

is achieved by choosing other spectral ranges for the detection. Any changes measured in W(\ i;\ 2) from an object will then be directly linked to inherent variations in temperature and/or emissivity in the object. This is used today in standard measuring techniques, and commercial equipment has long been available. The technique is non-contact and therefore does not disturb the measurement object. But because it is passive, it is limited to measuring the contrasts in temperature and emissivity that naturally occur in the object; this limits the information that the technique can obtain from the measurement object. The sensitivity is also relatively low, typical limits for observable temperature contrasts are in the range 10 -10 K.

In recent years, an active measurement technique called photothermal radiometry (PTR) has been developed^. In case of external thermal pressure, e.g. through illumination, temperature changes are then induced in the object, and the magnitude, phase or time course of the resulting thermal radiation from the illuminated point on the object is measured. This was originally used to directly measure the temperature rise in the illuminated point under the influence of strong radiation (<2>), but the measurement technical possibilities have now been significantly expanded. In all known variants of PTR, the lighting has always been pulse-shaped, most often in shape

of a continuous pulse train. For the sake of the analysis, one usually assumes that the intensity of the lighting varies harmonically, 1= iXqC1 + ^n^"*) , where / is the frequency, t the time and I is the amplitude. It is then also

common to define a thermal diffu°sj. length u - ( k/ vpfC) h = de/ nf) h, where k is thermal conductivity, p density, C specific heat and k is the diffusivity of the object material. u is the length in the object that the heat has time to propagate during a pulse period. One also defines

an optical absorption length a(A) ^, where o(A) is the spectral absorption coefficient of the object material at the wavelength A.

Based on this, the induced temperature change 6T can then be calculated

(3)

in the object for different types of objects and lighting. As an example, we can consider the case where a(X) ^ << the thickness of the object; the object is then optically opaque. One then has two current situations given by u > ct(A) and p < ci(X) respectively

For u < ct(A) ^ the object is photothermally transparent. The lighting

at the wavelength X penetrates deeper into the material than one diffusion length, and one finds that the temperature at the outermost surface is

The temperature fluctuations at the frequency / in step with the illumination are then proportional to the object material's absorption coefficient a(X); measurement of the thermal radiation at the pulse frequency f according to (4) then gives the possibility of spectral characterization of the object's surface by (4) vary the spectral content of the lighting, and has resulted in a new spectroscopic technique. For u > a(X) ^ the object is photothermally impermeable. All incident radiation is then absorbed in a surface layer that is thinner than the thermal diffusion length, and one finds that the surface temperature is

The induced temperature variations are then independent of the object's absorption coefficient; radiant excitation at the frequency f will then, when measured, provide information about the parameters k, p, C and/or e. This also applies if the changes in k, p or C are located inside the object and less than one thermal diffusion wavelength from the surface; internal structures in the object that are not visible on the surface will then produce changes in both amplitude and phase to 6W ^ . In other words, this makes it possible to "see" into the object a certain depth u. A particularly interesting case occurs when measuring the temperature variations on the back of the object as a result of illumination on the front. Structures inside the object with deviating k, p and/or C will then affect the heat propagation through the object and cause changes in amplitude and phase of the temperature fluctuations on the back. This makes it possible to examine the interior of opaque objects that are significantly thicker than one thermal diffusion length^ .

The induced temperature fluctuations can be very small, the detection limit is in the range of 10 ^ K. In order to be able to measure SW with a good signal-to-noise ratio, it is therefore necessary to correlate the measured thermal emission with the illumination, so that one can identify only the the thermal fluctuations which are synchronous with the pulsations in the lighting. This is routinely done using electronic lock-in amplifiers, which essentially filter out all signals outside a narrow bandwidth & f around the pulse frequency /; A/ is usually less than 1 Hz. To achieve representative measurements, the time for each measurement point must be at least equal to the inverse of the bandwidth, so that stationary conditions apply. The measurements then become slow and time-consuming; when sweeping over objects, typical speeds are often less than 1 mm/s^'^. Although such a sweeping technique has been demonstrated for use in scientific analyzes of surfaces and internal structures in objects, the technique has fundamental limitations that in practice make it unsuitable for real-time measurements. The technique is thus unsuitable for a number of tasks such as e.g. may be associated with ongoing characterization and control of various industrial products and processes (chemical substances and compounds, semiconductor components, surface coatings, film and layer-shaped materials, curing processes etc); the same applies to the ability to quickly form corresponding two-dimensional (and possibly also three-dimensional) photothermal images of objects with repeated sweeps. In all such situations, each measurement point should be able to be effected in less than a millisecond; the stationary photothermal radiometry technique is therefore more than three orders of magnitude too slow to be used for the aforementioned purposes.

Heat propagation occurs by diffusion. This is a chaotic process, which in the one-dimensional case is described by the equation

where x is the coordinate, t the time and T(x, t) is the temperature in the object at point x at time t. The complete solution of (7) can be found in each individual case depending on the initial and boundary conditions. For a harmonically varying surface temperature T( 0, t) = T^ e1' 2^ impressed on an object with

(7)

extent 0 S x i <* > is then found

The last term given by the integral expression is a transient thermal disturbance in the object, which is written from the start of the temperature fluctuations on the surface. This transient dies away when t •*■ °>, and only the stationary time-varying solution given by the first term remains. This section describes the temperature fluctuations that are used in stationary photothermal radiometry as described above. The solutions take the form of damped waves, whose amplitude is reduced by a factor e' over a distance equal to the thermal diffusion length u. The wave speed s = dx/ dt is found by requiring that the phase term 2- nft - x/\ i is constant, which gives

From this one can see that the thermal waves are highly dispersive; higher frequency waves propagate faster. At the same time, the high-frequency thermal waves are attenuated more strongly with x o due to the factor e — CC I u. The wavelength of a thermal wave with frequency / is according to (9) A = 2ttu.

If it is to be possible to carry out faster measurements than can be achieved by the previously described utilization of the stationary thermal waves, one must consequently make use of the transient solutions to (8). This requires a more explicit calculation of the heat propagation in the object. We then consider a single pulse that instantly heats the surface of an object to the temperature Tq. At depth x, the object will experience a transient temperature swing that rises to a maximum and then falls towards a stationary value. The course of this temperature front is given to a good approximation by

where A is a constant that depends on k, C and the thermal pressure on the (8) surface. When calculating this function, it is found that the temperature at point x has reached half of its maximum value at the time

To a good approximation, this also applies to heat propagation in three dimensions; after heating the surface with a point-shaped instantaneous source, the transient heat wave at a point on the surface at a distance x from the excitation point will thus also have reached half of its maximum fluctuation approximately after time t^.

From this we can define a thermal time constant

which corresponds to the time it takes before the temperature at a point at a distance d from an instantaneous excitation point has reached approx. 1/3 of its maximum swing. It follows that the effective thermal diffusion rate u( d) = d/ x for diffusion over the distance d is

In other words, the effective thermal diffusion rate over a certain length is inversely proportional to the length, which means that the diffusion time to a point increases squarely with the distance from the excitation point. This is related to the dispersion of the stationary thermal waves given by (9). If we set d = A - 2iru, we find that the effective diffusion speed of a transient thermal wave over a distance d equal to the wavelength of a stationary thermal wave of frequency / is u( 2v\ i) = tnc/2u s s, where s is the corresponding stationary wave velocity given by (9).

Equation (11) is used in a standard pulse technique for determining the diffusivity k in solids (9): When measuring from the temperature course on the back of an object with thickness i. after heating on the front with pulses of duration << t^, can k is determined from

The pulse technique has been further developed over the past two to three years

to a type of pulsed photothermal radiometry^1<^ , where by recording the exact curve form of the temperature course on the front or back of an object after pulse heating on the front, absorption coefficients, thermal diffusivity and thickness both of the object itself can be determined

as well as of any surface layers. A single pulse is in principle

sufficient to determine all these parameters. The pulse technique is therefore faster than the stationary technique discussed earlier, and by a factor that roughly corresponds to the number of periods, /"/A/, required for each stationary measurement at frequency / and bandwidth A/. However, if one wishes to use the pulse technique to characterize, for example, an entire object, a material flow or the like, one would then have to record correspondingly detailed temperature courses at all relevant points, and then extract the relevant data through an individual analysis of each individual time course. the registration and the subsequent signal processing will thereby take so long that the pulse technique will also be too slow for real-time measurements; such utilization of the pulse technique has not been demonstrated either. The reasons for this lie in the fact that the pulse technique collects too much data, most of which is meaningless for the measurements to be carried out; this will be described in more detail during the presentation of the invention below.

The present invention is based on the fact that one

are usually not interested in dec the complete temperature course at each individual point in the object. Most often, it is sufficient to record any variations in temperature from point to point on the object's surface, in order to map and identify any differences in chemical and physical conditions in and below the surface. The invention is thus in its most general aspect, as stated in patent claim 1, a special utilization of the transient thermal waves in the object which makes it possible, continuously and in real time, exclusively to record the thermal radiation from a series of object points after a time delay corresponding thermal diffusion over a selectable length or depth d in the object. As can be seen from the patent claims, the invention includes both a method and an apparatus for giving the object a relative movement in relation to a source of continuous (and possibly constant) thermal pressure, where the relative speed between object and source, Vj, is greater than the effective thermal diffusion rate u( d) for the relevant distance in the object. This combination of continuous thermal excitation and physical movement will then be compared to a continuous series of independent, instantaneous thermal sources that move over the object. The thermal diffusion conditions for object points at a greater distance than d along the path of movement will be independent of each other because the source moves faster than the

transient thermal wavefront: After a time t s d2/ ir2K, the transient wavefront from each individual excitation point will have propagated a distance d in the object, at the same time that the excitation point relative to the source has moved a distance L <=> Vj' i > d. I this position then the object passes the field of view of a detector for thermal radiation, in such a way that also the relative speed v~ between the detector's field of view and the object is greater than u( d). The detector will then record the thermal radiation from each individual excited object point, independently of each other and delayed by a time t in relation to the original thermal excitation at the point. Any inhomogeneities in the object within a distance d from each individual excitation point will then, as explained above, result in corresponding variations in the delayed radiated thermal effect. In this way, it becomes possible to use the transient thermal wave for a selective probing of physical parameters in the object within distances d from each individual excitation point. The procedure then naturally also allows the registration and control of physical and chemical structures and patterns exposed to the object's surface, e.g. by spectrally selective thermal excitation; the delay time or length can then, if desired, be reduced to an absolute minimum to better separate the maximum temperature fluctuations in the surface from the thermal effects due to the transient thermal waves being able to propagate further out from the excitation points.

Below we will give a more detailed description of the invention

with reference to the drawings, where:

Figure 1 schematically shows a device for thermal pressure on the front

(upper side) of the object.

Figure 2 schematically shows the detection of thermal radiation from the front

(solid) or the back (dashed) of the object.

Figure 3 schematically shows examples of systematic and characteristic structures and patterns that can be incorporated into materials and objects.

Figure 4 schematically shows thermal pressure and detection at a distance

L apart.

Figure 5 schematically shows a spatial filter in connection with the detection system.

Figure 1 shows thermal pressure from a source 2 on an object 1, which moves with a speed Ui relative to the thermal pressure. The pressure is here idealized as point-shaped, while in practice it will have a certain extent a in the direction of movement. The thermal pressure can occur with a tow contact to a good thermally conductive body, possibly a "heat pipe", which is in: connection with a heated or cooled thermal reservoir, but it can also occur without mechanical contact using of cold or hot gas, electromagnetic radiation, electron or other particle radiation or also by acoustic waves. Gas and contact pressure will localize the initial heating or cooling to the object's surface. Electromagnetic radiation, on the other hand, will, as described above, need

into the object a typical distance a(X,)-1 equal to the inverse of the absorption coefficient for the radiation at the relevant wavelength; by choosing X, the thermal excitation can thus be adapted to some extent to the current situation, and especially in connection with the detection of substances exposed to the surface.

With electromagnetic radiation at two or more different wavelengths X arranged for pressure at a certain mutual distance across the direction of movement and with separate detectors for each wavelength, one also has the possibility of a more detailed and comprehensive spectral analysis of the object.

Particle beams similarly penetrate the object depending on the particles' energy and the object's material properties and composition of elements, and this can also be used to adjust the pressure in the volume of the object. Acoustic waves will penetrate and propagate in the object depending on its modulus of elasticity E, and will be able to selectively heat internal structures with E causing the waves to be absorbed. Together, this provides an arsenal of different thermal excitation modes, from which one will be able to choose a thermal pressure that is particularly suitable for each individual application of the invention.

It is also possible to combine two or more different thermal excitation modes to achieve special advantages. For example, can

the object is heated or cooled along the same path or along two parallel paths, with separate detection for each of these. The temperature fluctuations are then directed oppositely to each other, which gives complementary signals in the detection system. This can be used for e.g. to separate variations in emissivity from the thermally related signals.

Figure 2 shows detection of the resulting thermal radiation both from the same side (front) (a) as the thermal pressure as well as from the opposite side (back) (b). In a given measuring technical situation, you will usually only use one of the alternatives, but in many cases it will be relevant to have several detectors operating in parallel, e.g. in the form of an array. The field of view (5) in the detection system has a speed Vz relative to the object 1. The detection system consists of a device 3 which collects thermal radiation from the object, and is symbolized in the figure by a lens. Other relevant devices for collecting thermal radiation can be mirror arrangements, optical fibers, optical waveguides, etc. The device 3 collects the thermal radiation on a detector 4, which converts the radiation into electrical signals. Such detectors are often made of semiconductor materials and can be adapted to different spectral ranges for the thermal radiation, depending on the object's temperature, among other things , but there are also thermal detectors that have a flatter spectral characteristic; these are usually not as fast 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 special analysis in the unit 6 which contains an electronic signal processing system As mentioned later, the electrical bandwidth must ten l the units 4, 5 and 6 be such in relation to the speed V2 and to the size of the structures to be identified that equation (25) is satisfied; otherwise, there are no special requirements for these units. From the signal processing system, it will then be possible to retrieve information that is used to effect the management and control functions, etc. which the invention enables in each individual case, this is trivial and is not incorporated in the figure. In front of the device 3, in position (b) it is additionally shown how an optical filter 7 can be placed in relation to the detection system, both to further define the spectral range for thermal detection as well as to selectively discriminate against thermal and other electromagnetic radiation in other spectral ranges ( eg from the thermal excitation). As indicated in the figure, the vast majority of conditions are identical when detecting thermal radiation from the front or back. The choice of one or the other alternative depends significantly on the design of the object and on the purpose of the measurement, often both configurations will provide equivalent information. Detection from the back is relevant and particularly advantageous for thin objects and materials, where it may be to identify internal structures or possibly variations in thickness and other object parameters that are characteristic of the entire volume of the object; the same applies to objects and materials where the structures in question lie at a depth d which makes the diffusion-determined resolution of the front ( s d ) unreasonably poor. Examination of surface layers,

of objects where the structures in question are exposed to the surface and of internal structures in objects that are so thick that thermal diffusion to the backside takes an unreasonably long time and produces a corresponding reduction in resolution, on the other hand, detection from the front is best done.

Figure 3 gives examples of objects and materials that contain systematic and characteristic structures and patterns either in the surface (3a), inside the volume (3b) or as variations (e.g. in thickness, as shown) in parameters that affect the entire cross-section of the object (3c). Depending on the nature and character of the structures and patterns, you will then be able to

choose a suitable form of thermal pressure, as well as detection from the front or back, e.g. determined by the object's t} '..keise and other properties. In the case of a detection of the transient thermal processes delayed by a time x in relation to the thermal pressure, and where the time t is adapted to the extent d of the relevant patterns and structures according to equation (12), the device 3 in figure 2 will then give signals which contains information about these structures and patterns. With prior knowledge of possible patterns and structures in the object, this can thus be used to characterize each individual object in relation to a register of such structures and patterns, and thereby also to check that the object contains structures with a predetermined pattern.

The extent d to the structures and patterns in the object is in this context the central parameter, and which in particular determines the limits for the respective relative movement speeds of the object as given in claim 1. In a similar way, the invention can also be used to examine and characterize objects that has random patterns or structures on the surface, in the volume or throughout the cross-section. This can serve many purposes, e.g. to identify objects where the variation in such parameters lies outside certain limits. This can then also be used to establish feedback in industrial processes, so that these can be controlled in a way that ensures that the relevant parameters are kept within relevant limit values.

Figure 4 shows the device for thermal pressure together with the detection system mounted so that the distance between the pressure and the field of view in the detection system is given by the length L. If this length is kept fixed and constant, the object has the same relative speed V = Vi = V2 in relation to both the thermal the pressure and the detection system. There is then a fixed delay time 6t = L/v between thermal pressure and detection, and one will have the opportunity to specifically examine structures at a fixed distance d from the thermal pressure, where d satisfies (12) with t = 6t. The analyzes and presentations given above will then be applied; this will in many cases also be a practical implementation of the invention.

When checking material flows, etc. it is then natural to have stationary mounted devices for thermal pressure and detection, while the object is being moved. For imaging applications on objects (e.g. in a medical context) it may be more natural with a joint movement of impression and detection over a stationary object.

By varying v, you will then be able to sweep over a larger or smaller interval of distances d = n(K' 6t) v. This can also be achieved by letting L be variable, e.g. by that

where Li < LQ and F( ut) is a periodic function with amplitude equal to 1 and angular frequency oi. The time delay between thermal pressure and detection can then be varied between the limits 6tm^ n <=> ( LQ - Li)/ v and & tmax <=> ( L0 + Li)/ v; from (12) one will then find the corresponding range of diffusion-determined distances d that can be searched. However, the assumption is always that both Vi and V2 > u( d). This requires that the angular frequency w in the periodic function F( iiit ) is such that the velocity uLi associated with this motion satisfies

The conditions given in claim 1 will then apply during the entire cyclic variation of L. Such an embodiment may be of interest when one either does not know the relevant values of d, in cases where the distance d varies from object to object or between parts of the object, or where the object has several structures with different characteristic distances d. For reasons of electronic signal processing, in such contexts a report signal must be obtained from a unit 8, which controls the distance L, so that the ac signal processing system can correct for the different diffusion lengths, delay times and signal frequencies that possibly will perform. It would thus be most practical to vary the position of the thermal pressure, while the position of the field of view in the detection system is kept constant, so that the speed of the latter relative to the object is also constant; this will then make the frequency spectrum of the thermal signals easier to interpret.

A device for causing movement of the object 1 is on

Fig. 4 shown in the form of roller pairs 11 and 12 which, by their rotation, give the object the speed v.

Another embodiment will e.g. could consist of one thermal

pressure with two or more detectors one after the other at distances from the pressure corresponding to different diffusion times t. Such an arrangement will then make it possible to carry out detailed investigations of structures, e.g. at certain depths in the object.

Figure 5 shows shielding of the detector and detection system by means of a spatial filter 9. This can often only take the form of a screen, which prevents direct influence on the detector from the thermal pressure on the object. E.g. pressure by means of electromagnetic radiation could spread into the detection system, and since this radiation would be orders of magnitude stronger than the resulting thermal radiation, even a weak spread could disrupt and mask the actual signals. The same can happen with pressure, e.g.

with hot or cold gas, which could leak into the area between the object and the detector and affect the detection. Regardless of which type of thermal imprint is chosen, as a rule, one also always wants to shield the detector against recording the immediate maximum temperature fluctuations in the imprinted area on the object; the temperature fluctuation is usually only registered after a certain time delay. Especially in cases where this time delay is small, such spatial filtering is therefore important. Often, the spatial filter is made as a sleeve that completely encloses the units 3 and A, with only a small opening towards the object through which thermal radiation can escape.

It is emphasized that this invention differs both in principle and in practice from previously demonstrated stationary as well as pulse-based PTR techniques. The thermal excitation of the object is, in contrast to the known techniques, continuous and for most purposes constant in time. Secondly, however, it is important to note that the relative movements of the object serve at least six different purposes: I: Firstly, the relative movement between object and thermal source, with Vi > u( d), will transform the continuous (and possibly constant) the thermal imprint of a series of transient thermal excitations over the object, each with its own transient time evolution; in other words, the object movement makes it possible to exploit the advantages of the transient thermal waves without having to resort to pulse excitation.

II: Secondly, the object's movement relative to the source means that constantly new, transient thermal waves are initiated during the diffusion time t, while preceding transient waves are still en route; the larger Vi is in relation to u(d), the more such waves (i.e. measurement points) there is time for during time x, while the pulse techniques enable a maximum of one measurement per time x because the entire passage of time must be recorded; - this is where the invention's significant advantage lies in measurement speed compared to the pulse techniques, - more measurements are needed per unit of time, it is only necessary to increase Vi.

III: Thirdly, the movement of the thermally excited areas in the object a distance L = Vi-x (calculated in relation to the source) will selectively pick out for registration the thermal radiation at one and the same time of the transient temperature course from each individual excited object point ; in contrast to the pulse techniques, the object movement means that only one point on the temperature curve for each individual excitation point is recorded.

IV: For; fourthly, the relative movement between the object and the field of view in the detection system means that the thermal signals corresponding to the specific time t of the respective temperature courses, independently of each other (because V2 > u( d)), are converted by the detector into coherent, time-varying electrical signals, with an unambiguous correspondence between the electrical signal at a certain time and a specific excitation point on the object.

V: Fifthly, precisely because of the relative movement between the object and the field of view in the detection system, it becomes possible to adapt the relative speed v2 so that all the structural details to be examined produce signal variations that fall in frequency within the electrical bandwidth of the detection system ; these electrical signals can then be amplified and signal processed in the usual way.

VI: Sixth, the relative movement means that thermal impact and detection take place in physically separate places, which in a simple way separates the detection from the thermal impact, so that the impact takes place outside the field of view of the detection system. This means that, among other things, can use the same electromagnetic wavelengths for thermal pressure as for detection without crosstalk (mutual interference)..

The above description provides the necessary conditions for the invention to function. To simplify the presentation, it has been implicitly understood that the instantaneous thermal excitation only occurs in a thin surface layer of the object much thinner than the relevant distance d, and that the resulting thermal emission also comes from a correspondingly thin surface layer. In practice, this also corresponds to the most relevant materials and objects where the invention has advantages over existing methods. As the procedure is described, it will in principle also work for materials that are partially transparent at least in excitation and partly also in emission, because spectral or thermal inhomogeneities in the object will both give different starting conditions for the thermal waves and also affect the propagation of them'and the re-radiation from them differently. However, such cases can be more difficult to interpret, and it is therefore advantageous to choose spectral ranges for the detection, as indicated in (A), where the object material is as opaque as possible.

Depending on the design of the devices used to realize the invention, there are also certain prerequisites that must be satisfied:

1): Thermal waves from an excited point on the object propagate a distance d - t' u( d) along the surface (as well as in the volume) during the diffusion time x. The distance d therefore also determines the highest longitudinal resolution (ie in the object's direction of movement relative to the thermal pressure) that is possible to achieve with this method. If the thermal pressure has an extent a > d on the object, calculated along the direction of the relative motion, the relative velocity v must consequently be so great that a/ Vj < x, so that

The thermal pressure at each individual point on the object will then, as before, have a duration < x, which means that points at a distance > d can still be regarded as independent of each other for times < x in terms of diffusion.

If the detection system is to be able to realize this resolution in the longitudinal direction, the geometric extent of the field of view 0 on the object must, according to normal optical criteria, satisfy 3 S d. This assumes that the relative speed V o between the object and the field of view in the detection system satisfies so that areas with extent d of the object passes the field of vision at times < t. This is implicit in the invention according to section

IV above; the detection can then also be considered instantaneous in relation

to the thermal diffusion processes.

If 3 > d, the effective longitudinal resolution is given by the extent of the field of view 3 on the object. In order to achieve this resolution also from the thermal waves, it will then be sufficient to require

provided that 3 < a; for d < a < 3 only Vi in u($) is required. At the same time must be satisfied to provide the same longitudinal resolution for detection. The effective diffusion length in this case is equal to 3, the effective diffusion rate is and the associated diffusion time is r p = B<2>/it<2><. So far the longitudinal resolution. 2) A main point of the invention is to realize a high resolution transverse to the object's direction of movement. Assume that relevant structures with an extent d transverse to the direction of movement are to be identified with a resolution 6d « d. (For example, this may apply to the determination of thickness variations 2 Sd in the object or in a surface layer with an average thickness d). To propagate this extra distance, the thermal wavefront needs a time 6t which follows by differentiating (12), In order for structures at a mutually transverse distance 6d to be able to produce observable changes in the thermal course in the excited points on the object, it is consequently required that the object, in relation to the thermal pressure, moves at least a distance d (equal to the resolution element in the longitudinal direction) during the time 6t (while also (15) must apply), - this will make it marginally possible to detect differences in the thermal radiation at times t and t + St. From Vi'6t d then follows

An ideal optical detection of the thermal radiation has been assumed above, i.e. with 0 £ d. If 0 in d, it will be required that the thermal excitation runs through the distance 0 in a time < t.

From Vi'6t in 0 it then follows analogously above that

The longitudinal geometric resolution is then given at 0. This also requires that

The field of view with extent 0 will then be passed by every single point on the object during a time < x, and the detection can then still be regarded as instantaneous in relation to the thermal diffusion processes. Even with longitudinal resolution limited by the detection system's field of view 0 and not by the diffusion processes, it will e.g. it may still be possible to achieve the same transverse resolution, determined by the thermal waves, but then at higher relative velocities.

3): If the largest structures in the object to be characterized and/or checked have dimension D, these will pass the field of view of the detection system in a time D/v2. The inverse of this time corresponds to a frequency that must be greater than the lower limit frequency of the detection system, fmin, so that

Good detectors for heat radiation only exceptionally retain their detectivity for frequencies lower than 100 Hz. With e.g. D - 1 cm and fm^ n = 100 Hz, it will then follow from (24) that v2 > 100 cm/s in order for signals corresponding to 1 cm-sized structures to be registered by the electronic part of the detection system. Since typical thermal diffusion speeds over 1 cm distances vary from 10 cm/s for materials with low thermal diffusivity (plastic materials, paper, etc.) to 10 cm/s for good metallic heat conductors, the lower limit for the relative speed V2 between the object and the field of view in the detection system often be determined by (24), and with values that can far exceed the diffusion rates. Correspondingly, the highest frequencies in the signal will be given at V2/g, assuming a diffusion-limited longitudinal resolution g equal to the dimension of the smallest structure in the object to be identified. The relative speed between the object and the field of view in the detection system must

is therefore chosen so that all frequencies within the range

lies within the electrical bandwidth of the detection system.

Since the upper cut-off frequency of thermal radiation detectors is often of the order of MHz or more, (25) gives wide limits for the choice of V2.

It is emphasized that Uiogi>2 are different in principle and often also

in practice. Nor do they need to be directed along the same path relative to the object for the invention to work, as long as the conditions given above are satisfied. E.g. V\ and V2 can be directed along the same path on the object, while, for example, V2 is constant and V\ varies periodically around a mean value. The distance between thermal pressure and detection given by the diffusion time t will then also vary periodically,

so that one thereby essentially carries out a scan of the object over a range of diffusion lengths d corresponding to the range of different diffusion times t. This may be relevant in cases where the relevant distance d is not known or varies, possibly also where by running through a spectrum of diffusion lengths can probe a variety of structures each with its own characteristic length d.

A few examples will serve to make the invention concrete;

assume in the following that a detection system with lii Ulk ° 100 Hz is used. a) . Examination of internal structures in semiconductors can e.g. be applicable for depths of the order of 0.1 mm. Thermal diffusivity

is k = 0.3 cm<z>/s (as for Ge), which from (13) gives u = 3 m/s with d = 0.1 mm. With resolution 6c? = 10 mm then follows from (21) t>i S 15 m/s, which assumes thermal pressure of extent a < 0.5 mm (from (15)) and a well-focused detection system with 6 < 0.1 mm (from (22)). If the largest dimension to be registered is D = 1 mm, it follows from (24) V2 > 10 cm/s. Since this is a lower value for the relative velocity than that given by the diffusion velocity u, V2 will in this case be determined by the general claim for the invention reproduced in patent claim 1, V2 > u (= 3 m/s).

b). Protective coatings on surfaces are another important area of application for the invention. We assume that the coating is 50 ym

thick and has k = 10 cm<2>/s (glass etc.), is u = 20 cm/s. With thermal excitation of extent a = 0.5 mm, field of view 0 = 0.2 mm and requirement for resolution 6d = 5 ym when checking the thickness follows from (15) V\ > 2 m/s, while (22) gives V \ > 4 m/s; the latter value then becomes decisive for i>i. To investigate possible delamination over larger areas, it can

be applicable with D = 10 cm, which from (24) gives V2 2 10 m/s; correspondingly, (23) gives V2 in 0.8 m/s. In this case, the speed of the object relative to the detection system becomes the determining factor for choosing the relative movements.

c) . Measurement of thickness for e.g. aluminum products and identification of any defects in the material is another relevant issue

Example. The thickness can be d = 5 mm, thermal diffusivity is k = 1 cm<2>/s; from this follows u = 20 cm/s. With resolution 6c? = 0.1 mm then it follows from (21) that Vi in 5 m/s; here both a and B will do with ease

is done < d. If the largest structures sought are D = 5 cm, must

V2 2 5 m/s according to (24); in other words, the requirements for the two relative speeds will be, so to speak, identical.

d) . Paper and plastic materials are industrial products which

there is a need to control the thickness during production. Assume

that d = 0.1 mm and k — 10~<3>cm2/s, which gives u = 1 cm/s. With a = 1 mm,

3 = 0.5mm and 6c? = 5 ym follows from (15) V\ > 10 cm/s, while (22) gives Ul £ 50 cm/s. If defects, irregularities etc. of size D = 1 cm are to be registered, (24) gives v2 >A m/s; (23) gives for comparison V2 2 5 cm/s.

e). Chemical substances exposed to the object's surface can be identified using heating with electromagnetic radiation applied

the substances' characteristic absorption lines. Assume that the absorption coefficient is a(X) = 10<3> cm<-1>, which gives a penetration depth a(X)<-1> = 10~<3> cm. This then gives the thickness of the surface layer where most of the thermal pressure is deposited. With d 10 <3> cm and k = 10 2 cm<2>/s, u( d) = 1 m/s follows. If a 0.5 mm and B = 0.2 mm, the longitudinal resolution is given by 6 = 0.2 mm. If one wishes to control the object only to a depth equal to the absorption length a 1, it is then required that V\ i 50 m/s (from (15)) and V2 5 20 m/s (from (23)). In such cases, however, this would be unnecessarily high demands on the relative speeds. Since here one is unlikely to be interested in transverse resolution of the order of 10 <3> cm, one can content oneself with defining a diffusion velocity u(B) which corresponds to the resolution being given by the longitudinal resolution B. From (19) one then finds u(0) = 5 cm/s. (17) and (18) then respectively give Vi £ 12.5 cm/s and Vj £ 5 cm/s. The largest structure to be registered can be e.g. D = 2 mm (text, line drawings etc.), which gives V2 > 20 cm/s from (24).

Depending on the measuring technical situation in question, it will consequently be possible to make more accurate calculations of the limits for the relative velocities of the object in relation to the thermal pressure and in relation to the field of view in the detection system. The basic assumptions and limit values that define the procedure will nevertheless be given by patent claim 1, which must always be met.

The above examples only suggest a little of the scope for the applications of the invention: Inspection of semiconductor components and of surface coatings, thickness control and investigations of homogeneity in thin metal materials, foil materials, paper etc., reading of text, patterns, chromatograms etc., spectral thermography for medical analysis of skin and other organs, control of the composition of chemical products, etc., investigation and control of curing processes and other chemical processes and reactions, hot and glowing objects, etc., as well as liquid films. In all such cases the invention seems to offer

new measuring technical possibilities, as the procedure in its most general aspect is completely contact-free and does not disturb the object.

The technique can be done very quickly and thus adapted to industrial process and production requirements, the measurements are very sensitive, and technical equipment and components that are already well established are used.

The invention will also be able to be used to recognize characteristic structures and/or patterns found in materials and objects; these may occur naturally or they may be inserted on purpose. Such patterns and structures will then give characteristic signals when they pass the field of view of the detection system, and with the help of an electronic signal processing system which is specially designed to recognize precisely the signals in question, the relevant structures and patterns can then be identified among all possible others. This can e.g. is used to look for objects with special signatures that may be visually invisible, it can be used to characterize a group of objects according to certain criteria, etc. Another application lies in checking that the objects have a set of properties that guarantee that they are genuine, e.g. .ex. in connection with shares, other securities and the like.

For example, the object may contain characteristic structures of different materials, such as is embedded at a certain depth in the object (as in semiconductor materials). By using the procedure described above, it will then be possible to produce temperature variations on the surface that reproduce these internal patterns, and one can then check via the signal processing system that they have the correct shape. The same will apply to objects that contain systematic and characteristic mass changes that are not visible on the surface, e.g. holes, inclusions etc., systematic variations in thickness, characteristic patterns and structures of a chemical nature, e.g. dyes with distinct spectral absorption lines laid out as text, images etc. In all the cases mentioned here, the invention will be able to be used to recognize, control and/or characterize objects via the mentioned systematic and characteristic structures, by thermal pressure on one side of the object and detection of resulting thermal radiation from the same or opposite side of the object .

In conclusion, we would like to emphasize that the invention is based on exciting and utilizing transient thermal waves in a way that has not previously been demonstrated. The prerequisite for this is, as described in claim 1, that both the source of thermal pressure as well as the field of view in the detection system have relative velocities in relation to the object that are greater than the thermal diffusion velocity u(d), given by (13), corresponding to the time t where the thermal wave front at a distance d from the thermal excitation has reached approx. 1/3 of its maximum thermal fluctuation. Expressions (12) and (13) for the thermal time constant t and the thermal diffusion rate u( d) are therefore in and of themselves random; since thermal diffusion is a chaotic process, neither u(d) nor t can be given an exact definition. For many applications of the invention, it will thus be advantageous to define a higher value for u(d), based on a time when the temperature fluctuation has reached less than 1/3 of its maximum value, as this can contribute to increasing the thermal contrast between nearby structures.

In other contexts, it will be more natural to wait with thermal detection until the temperature fluctuation has reached close to 100% of its maximum, as this gives stronger thermal signals; the associated thermal diffusion speed will then be perceived as lower. The choice made in such respects essentially determines the definition of u(d), and has a direct influence on the limits for the relative movement speeds as given in claim 1. Claim 1 must therefore be understood in the light of what has been said here about the definition of u(d). The limit values for the relative velocities must therefore be understood as an approximation to the same degree that u(d) is only defined as an approximation, with room for practically determined adaptations in line with the above; in most cases there will only be small numerical corrections to the values for u(d) that can be calculated from (13). When constructing devices for realizing the invention, the above can be handled most simply by defining u(d) as in equation (13), and by allowing the time delay 6t between thermal application and detection to be larger or smaller than t in equation (12),

as described in claims 16 and 17. This will also take care of the case where the object is porous, which means that the thermal pressure will penetrate deeper than in a corresponding homogeneous material, at the same time that the thermal radiation will also be able to be written from deeper layers in the object than assumed in the above analysis; both of these effects contribute to increasing the apparent thermal diffusion rate.

Literature references.

(1) Nordal, P.-E. and Kanstad, S.O., Physiaa Scripta 20 659 (1979).

(2) Hendler, E., Crosbie, R. and Hardy, J.D., J. Appl. Physiol. \ 1_ 177 (1958).

(3) Rosencwaig, A. and Gersho, A., J. Appl. Phys. 4_7 64 (1976).

(4) Nordal, P.-E. and Kanstad, S.O., Appl. Phys. Easy. 38,486 (1981).

(5) Nordal, P.-E. and Kanstad, S.O., in Scanned Image Microscopy (E.Ash, Ed.), p. 341. Academic Press, London (1980).

(6) Busse, G., Infrared Physics 20 419 (1980).

(7) Carslaw, H.S. and Jaeger, J.C., Conduction of Heat in Solids,

Oxford University Press, Oxford (1959), p. 65.

(8) Parker, W.J, Jenkins, R.J., Butler, C.P. and Abbott, G.L.,

J. Appl. Phys. 32 1679 (1961).

(9) Deem, H.W. and Wood, W.D., Rev. scientific. Instrument. 33 1 107 (1962).

(10) Tam, A.C., Infrared Physics 25_ 305 (1985).

Claims (23)

1. Procedure for the characterization and/or control of substances, materials and objects by causing a relative movement between these (the object) and a device for external thermal pressure, that this pressure is continuous and preferably constant in time and is applied to the object as a continuous heating or cooling along a path defined by the object's relative movement, and that a relative movement is caused between the object and the field of view of a detection system designed to measure resulting thermal signals corresponding to changes in thermal radiation from the object, characterized in that the object's movement speed relative to the field of view in the detection system and relative to the external thermal pressure is greater, and preferably much greater, than the effective thermal diffusion speed in the object, and in that the detection system's electrical bandwidth includes the frequencies of the aforementioned thermal signals that appear when the object passes through the field of view of the detection system with said relative speed. 2. Method according to claim 1, characterized in that the effective thermal diffusion rate u( d) is determined by where k is the thermal diffusivity of the object material and d indicates the geometric dimension of the structures, lengths and/or thicknesses in the object to be characterized and/or controlled. 3. Method according to claim 1 or 2, characterized in that the electrical bandwidth of the detection system includes all the frequencies / that satisfy the relation where Vz is the object's speed of movement relative to the field of view in the detection system and where D and g respectively indicate the geometric dimension of the largest and smallest structure, length and/or thickness in the object to be characterized and/or controlled. A. Method according to one of claims 1-3, characterized in that the thermal pressure occurs by mechanical contact between the object and a device for thermal heating or cooling. 5. Method according to one of the claims 1 - 3, characterized in that the thermal pressure occurs by means of hot or cold gas. 6. Method according to one of the claims 1-3, characterized in that the thermal pressure occurs by means of a flame. 7. Method according to one of the claims 1-3, characterized in that the thermal pressure occurs by means of ultrasound adapted to be absorbed by components in the object. 8. Method according to one of the claims 1-3, characterized in that the thermal pressure occurs by means of an electron beam or other particle beam. 9. Method according to one of claims 1 - 3, characterized in that the thermal pressure occurs with electromagnetic radiation which is spectrally selected to be absorbed by components in the object. 10. Apparatus for carrying out the method according to one of claims 1-9, comprising a device for continuous thermal pressure on the object and a detection system arranged for measuring resulting thermal signals corresponding to changes in thermal radiation from the object, characterized by a device for causing relative movement between the object and the thermal pressure device, respectively the detection system, at speeds that are greater than, and preferably much greater than, the effective thermal diffusion speed in the object, and in that the detection system's electrical bandwidth includes the frequencies of the aforementioned thermal signals that appear when the object passes through the field of view of the detection system at said relative speed. 11. Apparatus according to claim 10, characterized in that the pressing device and the detection system have a mutual distance along the path of the said relative movement. 12. Apparatus according to claim 10 or 11, characterized in that the movement device is arranged to cause movement of the object and that the pressure device, respectively the detection system are essentially stationary. 13. Apparatus according to one of claims 10 - 12, characterized in that the movement device is designed to cause the same relative movement speed of the object in relation to the pressing device as well as to the detection system. 14. Apparatus according to one of claims 10 - 13, characterized in that the pressure device is arranged to place the thermal pressure on one side (the front) of the object and in that the detection system is arranged to measure the resulting thermal radiation from the same side of the object. 15. Apparatus according to one of claims 10 - 13, characterized in that the pressure device is arranged to place the thermal pressure on one side (the front) of the object and in that the detection system is arranged to measure the resulting thermal radiation from the other side (the back ) of the object. 16. Apparatus according to one of claims 10 - 15, characterized in that the distance L between the device for thermal pressure and the field of view in the detection system is fixed and given by where 6t > 0.5 d2/(ir2K) and V > u( d) is the relative speed of movement between the object and the device for thermal pressure or the field of view in the detection system. 17. Apparatus according to one of claims 10 - 15, characterized in that the distance L between the device for thermal pressure and the field of view in the detection system is variable according to the relation where Lo = V6t, with 6t > 0.5 d2/( vZK) and V > u( d), Li < L0 and F( uit ) is a periodic function of time t with amplitude equal to 1 and angular frequency u> that satisfies u < ( v - u( d))/ Li, where v is the average relative speed of movement between the object and the device for thermal pressure, respectively the field of view in the detection system. 18. Apparatus according to one of claims 10 - 17, for use with objects or products that are provided with systematic and characteristic structures and/or patterns of a physical and/or chemical nature, characterized in that it comprises an electrical signal processing system which is designed to recognize the corresponding systematic and characteristic thermal signals which appear when the aforementioned structures pass the field of view of the detection system. 19. Apparatus according to one of claims 10 - 18, characterized in that the detection system comprises a spectral filter for defining the spectral range for thermal detection and for discriminating against electromagnetic radiation applied to the object from the device for the thermal pressure. 20. Apparatus according to one of claims 10-19, characterized in that the detection system comprises at least two detectors placed at a distance from each other along the path for the said relative movement. 21. Apparatus according to one of claims 10 - 20, characterized in that the pressure device is arranged to emit separate electromagnetic rays of two or more different wavelengths and with placement on the object at a certain mutual distance across the direction of movement, and that the detection system includes correspondingly, separate detectors arranged for each pressure. 22. Apparatus according to one of claims 10 - 20, characterized in that the pressure device is designed to emit a first pressure and a second pressure independent of the first, parallel or coincident in path with this, that the detection system comprises a corresponding first, respectively second detector , and that one of the pressures takes the form of heating while the other takes the form of cooling. 23. Apparatus according to one of claims 10 - 22, characterized in that the movement device is designed to cause a periodically variable relative movement speed.