IL32419A - Infrared thermograph - Google Patents

Description

INFRARED THERMOGRAPH 1 This invention relates to an infrared thernr jjraph 2 for producing a thermal image of a field of view on a light- 3 sensitive surface, and more particularly to infrared ther¬ 4 mographs capable of producing a plurality of different visual displays of which one form is a color display. 6 The present invention relates to an infrared 7 camera called a thermograph of the type generally disclosed t 8 and described in U. S. Patent No. 2,895,049 entitled "Image 9 Transducer" by R. W. Astheimer et al. Such an infrared 10 thermograph is sensitive to infrared radiation and produces 11 a thermal image of a scanned field of view. In the afore12 said patent an infrared detector is scanned over a field of 13 view optically, and produces electrical signals in accord14 ance with infrared radiance of objects in the scanned field 5 of view of the thermograph. The signals from the infrared 6 detector are amplified, processed, and applied to a glow 7 modulator tube which is scanned over film synchronously with 8 the scanning of the field of view to provide a recorded 9 thermal image called a thermogram. The glow modulator tube 0 is thus intensity modulated in accordance with the signals 1 from the infrared detector to produce a black and white 2. picture in which the grayness of the picture is a prescribed 3 function of the infrared radiance of objects in the scanned 4 field of view of the thermograph. Although the aforesaid 5 system has proven quite satisfactory for a wide range of 6 applications in the medical and industrial areas, certain 7 problems exist with the use of the glow modulator tube and in destroying the tube. Though less apparent, but mor^1 important are the changes in spectral characteristics which result when the glow modulator tube is driven at varying levels. These changing spectral characteristics of the glow modulator tube make it difficult to provide a light sensitive surface or film which provides the proper response in accordance with the radiation level desired to be measured and recorded. Although the problem has been somewhat alleviated with the proper compromise selection of glow modulator tubes and film in the black and white, the problem takes on added stature when trying to produce colored thermal images of the field of view.

It is often advantageous in thermography to provide the thermal image of the field of view of the thermograph in a variety of patterns. For example, in a continu-ous spectrum or an analog presentation from black to white, it is sometimes difficult to distinguish areas on the thermograph which have nearly the same temperature. To provide such a showing in the system of the aforesaid patent, electronic means have been used in the form of a plurality of level setters so that distinct steps or levels of drive current are applied to the glow modulator tube which come out on the thermograph as plateaus having the same color. In a similar manner, electronic means have been provided to indicate isothermal lines by sending a spike of current when levels are shifted in the glow modulator tube drive current. These digital or step function thermograms, as well as the to provide a much simpler, more flexible means for deriving different forms of display.

Accordingly, it is an object of this invention to provide an Improved thermograph which offers a wide variety of visual displays and overcomes some of the disadvantages associated with prior thermographs.

Another object of this invention is to provide an improved thermograph which provides a multiplicity of readily changeable displays without the necessity of adding extensive and complex structure.

In carrying out this invention in one illustrative embodiment thereof, a constant brightness light source is scanned over a light sensitive surface in synchronism with the scanning of an infrared detector over a field of view of a thermograph. A filter means having a plurality of different filter components modulates the beam of the constant light source! before it strikes the light sensitive surface. In one form the filter means is driven in accordance with signals from the infrared detector, thereby producing a thermal Image of the field of view of the thermograph in a predetermined arrangement in accordance with the filter components of the filter means. In another form, the angular deflection on a small mirror of a galvanometer movement is controlled by the detector output to modulate the light source through the filter means . A large variety of filter patterns are disclosed, which include continuous or analog black and white as well as color, step function or digital patterns.

The invention, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which: Fig. 1 is a schematic diagram of a thermograph in accordance with this invention, Fig. 2 illustrates a variable:. density black and white filter means which may be used in the thermograph shown in Fig. 1, Fig. 3 illustrates a continuous spectrum color filter means which may be used in the thermograph shown in Fig. 1, Fig. 4 illustrates a digital or step function filter means either in black and white or color which may be used in the thermograph of Fig. 1, Fig. 5 illustrates a step function or digital type filter means including isotherms, and also illustrates the use of a hot background for the thermograph of Fig. 1, Fig. 6 illustrates a multiple choice type filter means and a drive mechanism for the filter means suitable for use in the thermograph of Fig. 1, Fig. 7 is a schematic diagram of another thermograph illustrating another embodiment of a light modulation system in accordance with this invention, and Fig. 8 is a schematic diagram of a side view of is provided having a plane scanning mirror 10 which i^ driven by a scan drive mechanism 13. The plane scanning mirror 10 provides an X-Y scan of the field of view of the thermograph, and radiation reflected therefrom is collected and focused by a Cassegrain optical system including a primary element 11 and a secondary element 12 on an infrared detector 14. Interposed in the path of the radiation collected and focused by the optical system on the detector 14 is a chopper 25 driven by a motor 16. The chopper 25 has alternating sections which are opaque and transparent to the radiation applied to the detector. Accordingly the detector 14 measures the difference in the radiation applied thereto from the field of view and the radiation from the opaque section of the chopper blade. Electrical signals generated by the infrared detector 14 In accordance with the differ- i ence in radiation received, are amplified and synchronously detected in processing circuitry 18. All the aforesaid structure is similar to that set forth in the aforesaid patent, as well as in commercial thermographs manufactured by Barnes Engineering Company, Stamford, Connecticut.

The present invention departs dramatically from the old system in the manner..and structure utilized in producing the thermogram or visual.presentation qf the. therma1 image of the field of view of the thermograph. In the present invention a bright light source 26 is energized from the power supply of the processing circuitry 18 to drive the light source 26 at a constant or steady brightness. Al this purpose. An aperture 28 is provided over the ligjet source 26 to provide a narrow beam of light from the source 26. The narrow beam of light from the source 26 is imaged through cpllimating lenses 23 on a light sensitive surface or film 24.by a reimaging mirror 22. The reimaging mirror 22 is conveniently attached to the scanning mirror 10. Accordingly as the plane scanning mirror 10 scans the field of view of the thermograph, the beam of light from the light source 26 is synchronously scanned across the film 24. In order to produce a thermal image of the field of view of the thermograph, the constant brightness light source 26 must have its beam of light modulated in accordance with the intensity of the radiation received on a point by point basis from its field of view. This is accomplished in the present invention using a filter means 30 which intercepts the light beam of the source 26 before it strikes the light sensitive surface of film 24. The filter means 30 is interposed in the system above the aperture 28 and is driven by a transducer 32 which in turn is driven by the electronic processing circuitry 18. The requirement of the transducer 32 is that it be able to provide small displacements of the filter means 30 in response to current from the processing circuitry 18 which, of course, is related to the intensity of radiation received by the detector 14 from the field of view of the thermograph. A commercially available integrated pen motor-amplifier taken from a standard paper recorder unit has been found suitable for this function. However, it will the filter means 30 includes a plurality of different filter components which are arranged in predetermined patterns as desired. By moving the filter means 30 by the transducer 32 across the aperture 28 in front of the light source 26, the beam of light is modulated by passing different amounts or different, colors of the spectrum in accordance with the plurality of filter components on the filter means 30. This in turn provides such a pattern on the film 24 to provide a thermal image of the field of view of the thermograph.

The beauty of the present system resides in the extreme flexibility of the system to provide a wide variety of visual presentations of the field of view of the thermograph in a very simple manner. Figs. 2-6 which show a variety of filter arrangements are merely illustrative of this point. In Fig. 2 the filter means 30 is comprised of a variable density filter which is continuous from white to black, which would provide the type of thermograms available from the conventional system. In the variable density filter different amounts of the beam of light from the source 26 would be applied to the film in accordance with the transparency of a given section of the filter means. Even this arrangement provides an advantage over the old system in being able to eliminate the intensity modulation of a glow tube by using a constant brightness source, which would provide a more uniform picture. Also appearing on the filter means of Fig. 2 is an opaque section 38. The opaque section 38 is utilized for blanking during retrace intervals 3.0 in front of the light source 26 during the retrace^ intervals, both horizontal and vertical. This is a very simple and convenient way to provide the blanking function which, in the aforesaid system, was .accomplished by reducing the drive current on the source, which suffers the disadvantage of putting a great strain on the source. Although blanking in the manner just described is preferred, as an alternative thereto an opaque vane driven by the scanning mechanism could be utilized if desired, to be actuated in covering the light source 26 on the retrace intervals.

Fig. 3 illustrates the use as filter means 30 of a continuous color spectrum from red to violet. With the use of this filter, conventional color film would be substituted for black and white film. In this case, thermograms are produced in color, in which the hot objects in the field of view appear red, and cold objects appear violet on the film.

The continuous spectrum, either black and white or color, provides an analog presentation of the thermal image of a field of view of the thermograph. Different types of visual presentation are simply provided in the present system. For example. Fig. 4 illustrates the step, or. digital type of presentation, in which the filter is made up of distinct bands or step-function filters. Fig. 4 illustrates ten such steps, which in black and white would be ten shades of gray from black to clear, and in color would represent ten different color filters. The thermo all areas on the thermogram of the same temperature puld be of the same color, making It easier to distinguish changes in temperature. It will be apparent that fewer steps may be used than are shown if only larger temperature changes are desired to be observed. It would also be apparent that, if desired, the temperature values could readily be reversed by merely reversing the order of the filters. Thus hot would become violet or black, while cold objects in the field of view would be depicted as red or white.

With respect to black and white presentations, greater temperature separations could be provided on the white end of the filter with smaller percentage changes on the black end to provide a sort of logarithmic filter. This would be of value because the eye is more susceptible to changes in black and less perceptive to changes in white. By making the steps greater on the white end, the thermograms become easier to read.

Fig. 5 illustrates a different form of the digital presentation illustrated in Fig. 4 in that the digital step filters 40 are separated by clear or transparent or opaque areas 42 between steps. The resulting thermogram, whether black and white or colored, and whether analog or digital, would contain the equivalent of isothermal white or black lines, depending on whether transparent or opaque areas 42 are used. Thus, areas of different temperature would be outlined by white or black lines, making it easy to differentiate areas of different temperatures. Also, once the so erms separate eac ter e ement, so t at any com nation or predetermined arrangement for black or white isotherms may be used. Fig. 5 also illustrates a clear or transparent section 44 next to the opaque blanking section 38. Again, either in the case of black and white or colored thermograms, the normally cold background associated with the subject being thermographed could be photographed as pure white by putting the clear filter 44 just below the "keep alive" or the coldest (violet or black) part of the filter 30. Such a simulated hot background is highly desirable in order that very cold objects such as hair, gangrenous toes, and cold extremities which are present in medical thermography could be silhouetted against a white background. In present thermograms, many of these extremities are cold and blend into the background such that they are hardly distinguishable in the thermogram. In providing the digital presentations, the one requirement that is necessary is that the filter component of the filter means be at least as wide as the light beam from the source 26 coming out of the aperture 28. A 0.02" aperture has been found to be satisfactory, but other sizes may be utilized. The positioning of the filter means 30 should be close to the aperture, since the light beam is diverging as it comes out of the aperture. However, this spacing has not been found to be critical, and it will depend on the aperture size and the optical design of the system.

The filter means 30 may be of any suitable form. The length as well as size will depend on the drive capability of the modulating transducer 32. One form of filter printing the variable density. or color spectrums directly on film and utilizing the exposed film as filter means. Such a filter has worked satisfactorily for scanning a line in 3/4 sec, which provides a 90- line picture in 1½ min. and a 180- line picture of the same field of view in approximatel 3 minutes Fig. 6 illustrates the further flexibility of the system by providing a multiple choice of filter patterns. The pen motor 32 drives an arm 36 which has a vane 34 mounted thereon, carrying a plurality of different filter patterns 50, By merely shifting vertically either the entire pen motor assembly or the filter patterns in the vane, a choice of data presentations is readily made available without the necessity of removing patterns from the vane and inserting new ones.

Another embodiment of the constant brightness source which is modulated to produce a thermal Image in a variety of displays is shown in Fig. 7. Although not re-stricted thereto, this embodiment is described with respect to' the type of thermograph shown and described in Italian patent No. 836,918, entitled "Termagrafo Infrarosso." Referring now to Fig. 7, an infrared thermograph is provided having an objective lens 51 which images a field of view after reflection from a plane scanning mirror 52 onto an infrared detector 60. The plane scanning mirror 52, which is driven by a scanning drive means 54, provides an scanning drive means 54 also operates a solenoid 56 a blackened vane 58 attached thereto. The solenoid 56 is energized after each horizontal line, which places the vane 58 in front of the detector 60 to provide a radiation reference for electronic clamping prior to each scan line.

Electrical signals are generated by the infrared detector 60 in accordance with the difference in radiation received from the field of view, and are amplified and synchronously detected in processing circuitr 62. All of the aforesaid thermograph structure is similar to that set forth in the aforesaid Astheimer application and which is manufactured by Barnes Engineering Company, Stamford, Connecticut.

The embodiment differs from prior thermographs in the manner and structure utilized in producing the thermogram or visual presentation of the thermal image of the field of view of the thermograph. In the present embodiment, a modulation system 64 (Figs. 7' and 8) is provided which includes a fixed intensity or constant brightness light source 66 which is focused by a condensing lens 68 by reflection from a mirror 72 onto a mirror 76. The mirror 76 is part of a galvanometer 74. The galvanometer, which is not shown in detail, is conventional and operates on the principle of a coil turning in a magnetic field. In such mechanisms the coil of wire is suspended between the poles of a permanent magnet by means of a torsion filament, and electrical connections to the coil are made through the filament suspension and a flexible spiral filament fastened restoring force and the turning movement due to the cuf?ent applied to the coil are equal, the galvanometer assumes a steady deflection. The mirror 76, which is illustrated, is one method of reading the deflection of the galvanometer. For illustrative purposes, a Minneapolis-Honeywell galvanometer M3300T has been found suitable for the present application, although others may be used. A lens 78 positioned in front of the galvanometer mirror 76 images the condensing lens 68 onto an aperture plate 80 by reflection from the galvanometer mirror 76. The light incident on the mirror 76 is collimated. A small aperture or hole 82 in the aperture plate 80 defines the recording scanning spot. All of the light admitted by the aperture 82 comes from a small area of the condensing lens 68 defined by the image of the aperture 82 formed on the condensing lens 68 by the galvanometer lens 78. Angular motion of the galvanometer 74 causes this point to scan a line across the condensing lens 68. A filter means 70 is placed at the condensing lens 68 or between its elements. Accordingly, the color or inten-sity of the light admitted by the aperture hole 82 will be Ί dependent upon the position of the galvanometer mirror 76. The light leaving the aperture 82 is reflected from a beam splitter 84 and applied to a concave mirror 85 which is mounted on the back of the scanning mirror 52. The concave mirror 85 images the aperture 82 on a light sensitive surface 86 such as film. The use of the beam splitter 84 locates the li^ht source centrally with respect to the as well as the light sensitive surface 86 are accomplished simultaneously, eliminating synchronization problems.

The type of visual presentation which may be presented depends on the type of filter means used as well as that of the light sensitive surface. For example, if the filter means is a color filter composed,, of: six to eight sections bracketing the visible spectral range from blue to red and the light sensitive surface 86 is sensitive to color, a color presentation may be provided . Such a filter produces a quantization of the record so that successive temperature intervals appear as different discrete color regions on the visual display. Color recordingrtaakes it easier to distinguish the differences in target* temperature since the ability of the -eye to discern differences in color is much greater than the ability to recognize difference in intensity or gray tones. However, gray tone representation can be provided by replacing the color filter- with a black and white gray scale transparency. Great flexibility is provided by the variety of filter means which,~may be utilized. Any of the filters shown and described with respect to Figs. 2-5 may be used in this embodiment,,. o provide a variety of displays. One of the advantages of. this embodiment resides in the fact that a stationary filter means is utilized. This eliminates weight and size problems accompanying a system such as shown in Fig. 1 which-uses a mov-able filter means. Another advantage of this- embodiment is the sensitivity of the galvanometer 34 which -requires very to detection of infrared radiation. The light modulation system of Fig. 7 may be utilized in the thermograph shown in Fig. 1.

Again summarizing the operation of the embodiment shown in Figs. 7 and 8, infrared detector 60 is scanned over a field of view of a thermograph by plane scanning mirror 52 which signals are applied to a galvanometer 74 of a modulation system 64. The deflection of the -galvanometer mirror 76 in accordance with the signals received from the infrared detector modulate a fixed intensity; light source 66 through a filter means 70 which, for exam le, -may be color filter means. This is optically accomplished by focusing the light source 66 by a condensing lens onto the galvanometer mirror 76 and utilizing a galvanometer lens 78 which images the condensing lens on an aperture. The angular motion of the galvanometer causes the aperture to describe a line across the condensing lens which contains the filter element and accordingly defines the color of the light passing through the filter means 70. -By^changing the filter means, a large variety of visual ..presentations are made available to the user.

An improved thermograph system is thus provided by replacing a modulated glow tube with a constant brightness source and modulating the constant brightness source with a plurality of filter means. The elimination of the glow tube provides a more consistent visual presentation whose results are not dependent on the vagaries of a vary provided without complexity. Color presentations, which were not available with the aforesaid system, are mad^ available The digitized or step-type presentations, which were available before only with elaborate electronic equipment, are now available merely by the selection of a filter.

Claims (9)

Claims

1. An infrared thermograph for producing visual displays in accordance with infrared radiation received from a field of view haying an infrared detector which is scanned over a field of view and develops signals in accordance with the infrared energy received therefrom, means for scanning a light source in synchronism with said detector across a light sensitive surface, said light source being modulated by said signals for producing a thermal image of said field of view, the improvement comprising providing a constant brightness light source, a filter means having a plurality of different filter components arranged in a predetermined manner, and means for modulating said light source with said filter means in accordance with signals received from said infrared detec-tor to produce a thermal image of the field of view of said thermograph.

2. An infrared thermograph in accordance with Claim 1 wherein said means for modulating said light source with said filter means comprises transducer means for moving said filter means in front of said light source in accordance with signals applied thereto from said infrared detector.

3. An infrared thermograph in accordance with Claim 1 wherein said means for modulating said light source second optical means for imaging said first optical means on said aperture,' means for driving said movable mirror in accordance with signals received from said infrared detector thereby modulating said light source, and means for imaging and scanning said aperture on said light sensitive surface in synchronism with the scanning of said detector over a field of view.

4. An infrared thermograph set forth in Claims 1, 2, or 3 wherein the plurality of filter components of said filter means comprise a continuous spectrum filter either black through white or red through violet.

5. An infrared thermograph set forth in Claims 1, 2, or 3 wherein said filter means includes an opaque section on one extremity thereof, said opaque section being positioned over said light source when said transducer is activated by said scanning means during retrace intervals of a scan cycle .

6. An infrared thermograph set forth in Claims 1, 2, or 3 wherein said filter means includes a transparent section positioned next to a filter component which represents the coldest color of the plurality of different filter components for providing a hot background on said light sensitive surface.

7. An infrared thermograph set forth in Claims

8. An infrared thermograph set forth in Claim 7 wherein at least two of said discrete bands of different color are separated by transparent sections .

9. An infrared thermograph set forth in Claim 7 wherein at least two of said discrete bands of different colo are separated by opaque sections.