Abstract:
A device for producing and detecting induced heat radiation, in which movable mirror components are moved into and out of a crossed beam path by a stepping motor so as to be isogonal in the beam path. The stepping motor is controlled in such a manner that when the mirror components enter and leave the beam path, they leave at maximum speed. In another embodiment, a mirror component is pushed into and out of the beam path at a fixed angle of deviation.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an apparatus for producing and detecting induced heat radiation with a source of excitation radiation whose output radiation can strike an object, with the heat radiation induced by the source of excitation radiation being detectable by a detector, and with a beam deflector located in the beam path between the source of excitation radiation, the object, and the detector, with the output radiation and the heat radiation proceeding collinearly in a superimposed section between the object and the beam deflector, and with the beam deflector having at least one reflector element connected to a drive unit that limits the superimposed section with the drive unit. 
     Such a device is disclosed by DE-OS 23 00 436. In this device, there is a tilting mirror inclined with respect to a pulsed and focussed output beam of a laser as the source of excitation radiation, by which on the one hand the output beam can be directed to an object to be examined, and on the other hand, after tilting, infrared radiation collinear with the output beam emitted by the object from the irradiated area can be directed to a detector. 
     Tests can, in fact, be made with this device on the object with regard to heat dissipation as a reaction to the pulsed, focussed irradiation, but this method is unsuitable for serial tests in industrial quality control because of the necessity of maintaining the tilt angle precisely relative to the output beam, on the one hand, and on the other hand relative to the detector, and the resultant susceptibility to errors. Furthermore, additional devices have to be provided to keep the output beam away from the moving mirror when detecting the infrared radiation to prevent irradiation of the highly sensitive detector with stray scattered radiation. It is also a result of the tipping process that the end positions with the precise angles to be reached can be firmly assumed only after a relatively long period of time. 
     A device for separating and recombining optical radiation is disclosed by DE-PS 12 91 533, in which precisely reproducible dark pauses of equal length can be produced in two arms with a comparison substance and a test substance independently of the radiation path length, by means of rotating reflector disks with reflective, absorbing, and transparent sectors that have different sector angles. Certain tolerances are permissible here in the relative phase of the reflector disks and the congruence between the beam cross sections in the separating and combining regions, without giving rise to erroneous measurements. 
     A method and a device for controlling the production of welded seams are disclosed by GB 1 484 181, in which infrared radiation emitted from the two sides of the applied welded seam is directed in two focused beams to a detector by a sector wheel with reflective and perforated sectors. This is intended for reliable monitoring of the quality of the welded seam. 
     DE 40 15 893 A1 discloses a device for contactless and nondestructive examination of the internal and/or external structure of absorbing test specimens, in which induced temperature modulation can be produced locally with an intensity-modulated excitation beam. The back-emitted infrared radiation is separable from the excitation radiation by passing through a dichroic beam splitter that is also impacted by the excitation beam, and is diverted to a detector. It can in fact be determined with this device whether detected effects come from the interior or the surface of the material, but there is a high proportion of scattered light especially when evaluating close to the wavelength of the excitation radiation, which is detrimental to the signal/noise ratio and prevents measurements of weak signals. 
     DE 43 43 076 A1 discloses a device for the photothermal testing of a surface, in which excitation radiation falls on a surface to be tested through an opening in a focusing lens. The back-emitted heat radiation is fed to a detector through the focusing lens. Because of the separation of excitation radiation and heat radiation with no common optical elements, the individual components can be matched optimally to the individual wavelengths and in particular, back-emitted heat radiation can be evaluated even in the spectral region close to the excitation radiation without optical elements impacted by both radiations leading to superimposition of the measured signal and the excitation radiation, in which case either the excitation radiation would strike the detector with substantial intensity or the heat radiation would be prevented from reaching the detector. This device, however, has the drawback that the sizes of the optical elements have to be matched precisely to one another for a given beam geometry to produce minimal losses through the opening in the focusing lens. 
     The underlying purpose of this invention is to provide a device of the type mentioned initially that is stable in its optical precision in fast serial tests and is not susceptible to problems. 
     SUMMARY OF THE INVENTION 
     This problem is solved pursuant to the invention by providing that at least one reflecting element can be introduced completely into the beam path and removed completely from the beam path, and has a fixed angle of deflection with respect to the superimposed section. 
     Because of introduction at the proper angle and complete removal of the reflector from the beam path, on the one hand, a high rate is achieved during the alternation between impacting the object with output radiation and detection of the heat radiation, and on the other hand, high optical accuracy of collinearity is achieved between the output radiation of the excitation radiation source and the heat radiation, so that there is high spacing tolerance. Furthermore, because of the proper angular introduction into the beam path and removal of the reflector from the beam path, easily controlled deflection and absorption of the output beam can be accomplished. Low positioning precision is sufficient for the reflector with respect to the beam path during introduction and removal. 
     In a desirable embodiment, at least two reflecting elements, for example with circular segmental, circular, or oval shape, are attached to a rotating axle of a stepping motor and can be rotated into the beam path for the reflection position and out of the beam path for the transmission position with a fixed angle of deflection for the radiations. In another embodiment, one reflecting element can be shifted into the beam path and out of the beam path with a fixed angle of deflection. 
     It can be provided in both embodiments that either the output radiation or the heat radiation is reflected. It is especially desirable for the particular reflecting element to enter and to leave the beam path at high speed, so that any undesirable nonuniform irradiation of the object and only partial impacting of the detector with heat radiation, and any overlapping of the irradiation and emission phases, are minimized. This can be accomplished with technical ease both when rotating in and rotating out, or shifting in and shifting out, by providing that the reflecting elements are accelerated prior to ending the reflection when impacted with the radiation in question, so that the interruption of irradiation or pause in measurement is minimized before switching into the transmission position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other desirable refinements and advantages of the invention are found in the following description of examples of embodiment, with reference to the accompanying drawings wherein: 
     FIG. 1 is a schematic view of a device for producing and detecting induced heat radiation with two reflection elements that can be rotated into and out of the beam path, in which one reflecting element is impacted by output radiation from an excitation radiation source, to produce induced heat radiation; 
     FIG. 2 is a schematic view of the device pursuant to FIG. 1 with reflecting elements rotated out of the beam path to detect the induced heat radiation with a detector; 
     FIG. 3 is a schematic view of another embodiment of a device for producing and detecting induced heat radiation with a reflecting element that can be shifted into and out of the beam path, in a position in which it is impacted by output radiation from an excitation radiation source to produce the induced heat radiation; and 
     FIG. 4 is a schematic view of the device pursuant to FIG. 3 with the reflecting element shifted out of the beam path to detect the induced heat radiation with a detector. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows schematically the basic structure of an example of embodiment of a device for producing and detecting induced heat radiation pursuant to the invention. The device of FIG. 1 has a continuously radiating light source  1  as a broad-band excitation radiation source, for example a halogen lamp with a few tens of watts of power consumption, with which, after passing through beam-shaping optics and shielding diaphragms not shown in FIG. 1, a slightly divergent broad-band output beam  2  can be produced with a divergence angle of a few degrees. 
     In the reflection position shown in FIG. 1, the output beam  2  impacts the edge area  4  of a reflecting element  3  slanted from the output beam  2  in the form of a circular segment. The reflecting element  3  is part of a beam-diverting device  5  that has another circular segmental reflecting element  6  that is opposite the aforementioned reflecting element  3  and in the same reflecting plane. The reflecting elements  3 ,  6  are attached to a rotating axle  7  of a stepping motor  8 , shown partly covered in the illustration of FIG. 1, by which the reflecting elements  3 ,  6  can be rotated with identical fixed angles of deflection. Each of the reflecting elements  3 ,  6  has a segment angle  9  that is at most half of the full angle of 360 degrees divided by the number of reflecting elements  3 ,  6 . The positions of the reflecting elements  3 ,  6  are detectable by a fixed angular photodetector  11  placed on the outer edge  10  of a reflecting element  3 ,  6 , as a position detector. 
     After reflection on the reflecting element  3 , the output beam  2 , strikes an object  12 , on which the thickness of a coating applied to the surface, particularly a coating of paint, is to be determined, for example. 
     The device shown in FIG. 1 also has a detector  13  that is highly sensitive in the spectral region for heat radiation at room temperature, with which, as will be explained in detail later, the decay of the heat radiation induced by the output beam  2  can be detected through focusing optics  14 . The detector  13  has a spectral sensitivity characteristic that matches the spectrum of the induced heat radiation for maximum signal output in this example of embodiment at room temperature, or in modifications also at higher or lower characteristic temperature of the object  12 . Because of this, a relatively low-intensity light source  1  is sufficient to obtain satisfactory measurement signals. 
     There is a shielding or screening disk  16  that can be rotated by a shield disk drive  15 , just in front of the input window of the detector  13 . The shielding disk  16  has two opposite shielding segments  17 ,  18  that are opaque in the spectral region of the output beam  2 , and two transmission segments  19 ,  20  that are transparent at least for the heat radiation. In the position of the screening disk  16  shown in FIG. 1, the screening segment  18  is rotated in front of the input window of the detector  13  to protect the highly sensitive detector  13  against overloading or even destruction from stray radiation from the white light source  1  that may be very much more intense than the heat radiation to be detected. 
     There is also an electronic rotation-control unit  21  to which the stepping motor  8 , the angular photodetector  11 , and the screening disk drive  15  are connected. 
     FIG. 2 shows the device pursuant to FIG. 1 with the reflecting element  3 , which is rotated by control signals from the rotation-control unit  21  out of the beam path  23  formed by the output beam  2  and a thermal beam  22  produced during and after irradiation with the output beam  2  by thermooptical processes on the surface of the object  12 . After the reflecting element  3  is rotated out of the beam path  23  into the transmission position, the output beam  2  crosses the heat beam  22  at essentially a right angle in the arrangement according to FIGS. 1 and 2, and is absorbed, for example, in an absorber not shown in FIGS. 1 and 2. In the device shown in FIGS. 1 and 2, the output beam  2  and the heat beam  22  are collinear with one another between the reflection in the edge area  4  of the reflecting elements  3 ,  6  and the object, in a superimposed section  24 , which guarantees that measurement is largely independent of spacing. 
     In the transmission position of the reflecting elements  3 ,  6  shown in FIG. 2, the screening disk  16  is also in a position in which the transmission segment  20  is rotated by control signals from the rotation-control unit  21  to be in front of the input window of the detector  13  so that the heat beam  22  strikes the detector  13 . Besides broad-band energy utilization of the white light source  1 , the broad band also provides relatively high spectral independence from the color composition of the materials of the object  12  producing the heat radiation. 
     To obtain reliable measured data when determining the thickness of a coating applied to the surface of the object  12 , the stepping motor  8  is controlled by the rotation-control unit  21  by signals with increasing frequency, so that the reflecting element  3  has reached a maximum speed when rotating out of the beam path  23  along the largest possible angular acceleration distance between the edge area  4  located close to a leading edge  25  impacted by the output beam  2  and a trailing edge  26  of the reflecting element  3  opposite the leading edge  25 . The beam path  23  is traversed typically in a few milliseconds. This assures that there is only a very brief nonuniform illumination of the object  12  and only a very brief partial impacting of the detector  13  by the heat beam  22 , and only brief overlapping of irradiation and emission phases, with only correspondingly insignificant effect on the measured data when measuring the decay of intensity of the heat beam  22 , in particular also with short measurement times of a few milliseconds. 
     After the reflecting element  3  has rotated out of the beam path  23 , the stepping motor  8  is braked by control signals of decreasing frequency from the rotation-control unit  21 , and is rotated into a central position for unhindered passage of the output beam  2  and of the heat beam  22  between the reflecting elements  3 ,  6 . In a modification, the rotating axle  7  is slowly rotated further without hindrance to the passage of the heat beam  22  between the reflecting elements  3 ,  6  during the transmission phase. 
     To begin a new measurement, the stepping motor  8  is again accelerated, with a leading edge  27  of the other reflecting element  6  this time traversing the output beam  2  in the same direction of rotation at maximum speed. After braking the stepping motor  8  with continuous reflection of the output beam  2  at the reflecting element  6  before reaching a trailing edge  28  opposite the leading edge  27 , the reflecting element  6  is rotated by control signals from the rotation-control unit  21  in such a way that the output beam  2  impacts the reflecting element  6  corresponding to the arrangement shown in FIG. 1 in an edge area  4  adjacent to the leading edge  27 . 
     Just before the reflecting element  6  is rotated in, the screening disk  16  is rotated by control signals from the rotation-control unit  21  far enough for the shielding segment  17  to protectively cover the detector  13 . The angular photodetector  11  detects the passage of a leading edge  25 ,  27 , so that on the one hand the position of the rotating axle  7  of the stepping motor  8  can be detected, and on the other hand, the number of control signals from the rotation-control unit  21  can be corrected so that any rotational dislocations can be compensated for. 
     The design described above on the one hand provides spacing-independent irradiation of the object  12  by the rotating reflecting elements  3 ,  6  positioned in a plane, with fixed angle of deflection from the collinear orientation of the output beam  2  and the heat beam  22  in the superimposed section  24 , and on the other hand, it precludes mixing of beam fractions of the same frequency ranges of the output beam  2  with the heat beam  22  because of the timed beam separation, so that even heat radiation in a frequency range in which there is also intense emission from the broad-band light source, for example the white light source  1 , can be fed to the detector  13 . 
     FIG. 3 shows in perspective the basic structure of another example of embodiment of a device for producing and detecting induced heat radiation according to the invention. In the device according to FIG. 3, the parts of the device are assigned reference symbols to correspond to the device of FIGS. 1 and 2 and are not further explained in detail. The device according to FIG. 3 has a translation beam deflector  29  as the beam deflector, that has a translation reflector  31  driven by a translation motor  30 . The translation reflector  31  can be shifted on a running rail  32  at a fixed angle of deflection relative to the output beam  2 , between a reflection position and a transmission position, with the output beam  2  of the white light source  1  being reflected to the object  12  in the reflection position that is almost assumed in the position of the translation reflector  31  according to FIG.  3 . 
     The two positions of the translation reflector  31  are detectable by means of a reflection photodetector  33  and a transmission photodetector  34  as position detectors, with the reflection photodetector  33  and the transmission photodetector  34  being attached to the ends of the running rail  32 . In the position of the translation reflector  31  shown in FIG. 3, the end position for reflection is almost reached, inasmuch as a beam edge  35  of the transmission reflector  31  facing toward the output beam  2  has traversed the output beam  2 . In the reflection position of the translation reflector  31 , a signal emitted by the reflection photodetector  33  can be input to a translation control unit  36 , with which the shifting controlled by the translation control unit  36  can then be braked by the translation motor  30  connected to it, and comes to a stop with a reflection close to a reflection end  37  of the translation reflector  31  opposite the beam edge  35 . 
     In this reflection position, the detector  13  is protected against overload by the shielding disk  16  that can be rotated by control signals from the translation control unit  36 , with shielding segment  18  in front. 
     FIG. 4 shows the device according to FIG. 3 with the translation reflector  31  shifted into the transmission position after sufficient irradiation of the object  12  by the output beam  2 . A portion of the heat radiation then passes by the translation reflector  31  shifted out of the beam path  23 , and as a heat beam  22  through the focusing optics  14  and through the transmission segment  20  of the shielding disk  16  after it is rotated to the front, it reaches the detector  13  for measurement, whose output signal can be fed to the translation control unit  36  to evaluate the intensity of the heat beam  22  decaying with time. 
     The shifting of the translation reflector  31  by the translation control unit  36  in the example of embodiment shown in FIGS. 3 and 4 also occurs in such a way that the beam edge  35  cuts through the output beam  2  at a maximum speed, typically within a few milliseconds, so that in this case also the measurement errors resulting from nonuniform irradiation of the object  12  and only partial transmission of the heat beam  22 , and from overlapping of the irradiation and emission phases, are minimized. The collinear orientation of the output beam  2  and the heat beam  22  for the necessary spacing independence is likewise achieved by the shifting of the translation reflector  31  at a fixed angle of deflection. 
     In forms of embodiment modified from the examples of embodiment according to FIGS. 1,  2 ,  3 , and  4 , the positions of the white light source  1  and of the detector  13  preceded by focusing optics  14  and shielding disk  16  are interchanged, so that the output beam  2  strikes the object  12  in the transmission positions, and the heat beam  22  is reflected to the detector  13  in the reflection positions.