US3778624A

US3778624A - Thermal imaging device

Info

Publication number: US3778624A
Application number: US00637036A
Authority: US
Inventors: N Jones; D Faulkner; R Johnston; M Meharry
Original assignee: Commonwealth of Australia
Current assignee: Commonwealth of Australia
Priority date: 1967-05-01
Filing date: 1967-05-01
Publication date: 1973-12-11
Anticipated expiration: 1990-12-11

Abstract

A thermal imaging device comprising collector means to focus infra-red rays, a fluorescent screen enclosed in a sealed evacuated capsule and including a phosphor on a membrane, said fluorescent screen being positioned at the focal plane of the collector means, an energizing source for the fluorescent screen and means for varying the fluorescence which results from the presence of an infra-red image on the screen.

Description

United States Patent [1 1 Jones et al.

THERMAL IMAGING DEVICE Inventors: Neal K. Jones, Brighton, South Australia; Douglas W. Faulkner, Salisbury Heights, South Australia; Robert T. Johnston, North Walkerville, South Australia; Murray R. Meharry, Elizabeth Park, South Australia, all of Australia Assignee: The Commonwealth of Australia,

Melbourne, Victoria, Australia Filed: May 1, 1967 Appl. No.1 637,036

US. Cl 250/333, 250/334, 250/365,

313/92 Int. Cl. 1101 39/00 Field of Search 250/833 IR, 71,

[ Dec. 11, 1973 [56] References Cited UNITED STATES PATENTS 2,482,815 9/1949 Urbach 250/833 lR 3,015,731 l/l962 Van Santen et al. 250/833 1R 3,365,576 1/1968 Teeg 250/833 [R 3,370,172 2/1968 Hora 250/833 1R Primary Examiner-Benjamin A. Borchelt Assistant Examiner-H. A. Birmiel Attorney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A thermal imaging device comprising collector means to focus infra-red rays, a fluorescent screen enclosed in a sealed evacuated capsule and including a phosphor on a membrane, said fluorescent screen being positioned at the focal plane of the collector means, an energizing source for the fluorescent screen and means for varying the fluorescence which results from the presence of an infra-red image on the screen.

30 Claims, 8 Drawing Figures THERMAL IMAGING DEVICE This invention relates to an improved thermal imaging device and in particular it relates to a device of the type which has as its object the collection of infra-red rays from a heat source near ambient temperature and the rendering visible of these rays to an operator.

The principle of utilising infra-red rays to produce an image is already known and such images are produced on a screen where they can be viewed directly, or by an optical magnification device, or alternatively they can be produced by a scanning device which permits sensitivity to be increased by electrical means.

Certain problems exist with this type of apparatus which it is the object of this invention to overcome, the chief problem being to obtain high resolution and sensitivity with rapidity of image production so that a picture displaying moving objects can be viewed by the operator with a minimum amount of relay and with sufficient definition and contrast.

It will be realised of course that temperature sensitive screens are still in early stages of development so far as resolution is concerned but apart from the screens themselves there are certain problems which require to be solved for practical purposes such as the reduction of the apparatus to a relatively small size for the particular purpose concerned, and a reduction of auxiliary gear which should be avoided where such a unit is suitable for field work.

The unit can actually be used for various purposes but one of the prime purposes is to provide a means for fire detection, or the detection of hot or warm spots, particularly where the source of the heat is obscured by smoke or haze or the like as is usually the case in fires.

According to an earlier patent application of ours a fire centre detector was provided which also used infrared rays from a heat source but in that case the mirror was so designed that it directed the incoming rays to a bolometer through a chopper and in that case a picture could only be obtained by traversing the unit and noting the fluctuations of the signal resulting therefrom, the object of the present invention of course being to provide a picture covering a required area all of which is visible at the same time to the viewer.

The objects of the invention are achieved by providing a unit comprising an optical collecting system for the infra-red rays and a temperature sensitive, light emitting screen in the path of such collected rays, means being provided to energise the screen and also to view the screen, to emit visible light such as a binocular viewer or scanning system using electrical means to amplify and display the picture signal.

A feature of the invention which, though not essential but is preferred, is a self-contained or capsulated detector unit or cell requiring no elaborate external apparatus and not requiring to be cooled with liquid nitrogen or like means to hold correct conditions within the unit.

Thus the invention in its basic form comprises collector means to focus infra-red rays, a fluorescent screen at the focal plane of the collector means, an energising source for the said fluorescent screen, and means to adjust the screen temperature to its optimum operating temperature, independently of ambient temperature, the screen comprising a support film with an absorber film thereon for infra-red rays, and on said absorber film phosphor particles in heat conductive contact with the absorber film, but with low thermal transfer between phosphor particles whereby the infra-red radiation produces a temperature pattern on the screen which in turn varies the fluorescence of the screen.

According to one form of the invention it comprises a housing which has towards its rear a primary aspheric mirror which is apertured at the centre to provide viewing through it by a binocular viewer or the like, which housing also carries a secondary aspheric mirror towards the front thereof and of a size sufficiently small to give the minimum obstruction of the field of the pri mary aspheric mirror, the two mirrors being set to direct the infra-red rays on to a detector disposed forwardly of the primary mirror, said detector containing a fluorescent screen which is capable of modification by an infra-red image and which is provided with energising means in the form of a lamp or electron beam to fluoresce the screen, and which may have, but does not necessarily have, a bias temperature control to maintain the temperature sensitive screen within it at the correct working conditions, the arrangement of the unit being such that infra-red rays striking the primary aspheric mirror are focused by the secondary mirror on to the axially disposed temperature sensitive screen through a suitable window which passes the infra-red rays to the screen, which screen is itself energised to fluorescence but is modulated by the incoming infrared light which in the case of phosphors reduces fluorescence and which modified image is visible to the viewer through the aperture in the primary mirror, thus enabling the viewer to see an actual image on the screen consistent with the heat values of the area covered by the collection mirrors.

As screens of this type present problems in preventing thermal spread with consequent loss of definition and to insure greatest possible sensitivity, a capsulated detector unit or cell is proposed which is completely sealed and evacuated and which need only have a fluorescing source applied to it apart from the infra-red image, thereby avoiding external vacuum applying means and consequent complication.

So that the invention can be fully understood an embodiment will now be described in more detail with reference to the accompanying drawings but it is to be clear that the invention need not necessarily be limited to this, the scope being defined in the claims forming part of this specification.

In the drawings:

FIG. 1 is a sectional perspective view of one form of the invention,

FIG. 2 is a schematic view of the invention,

FIG. 3 is a section of the capsule,

FIG. 4 shows a modified unit,

FIGS. 5, 6 and 7 are schematic views showing how the sensitive phosphor film can be formed, and

FIG. 8 shows phosphor brightness-temperature curves.

The outer housing 1 is of somewhat cylindrical form with a window 2 at the front through which the infrared rays can enter the housing, and a mirror 3 towards the rear of the housing which is the primary aspheric mirror collecting the infra-red rays and concentrating them on to a secondary aspheric mirror 4 near the front of the housing 1 which in turn directs the rays into the detector cell 5 which is disposed coaxially within the housing I and is supported by a mechanism projecting through an aperture 6 in the mirror 3 which allows axial movement of the supporting means 7 for the detector cell to take place under control of the operator, this movement being for the purpose of focusing the image on to the screen 8 in the detector cell 5.

Pockets 9 and 10 around the outside of the housing 1 carry the electronics 11 and batteries 12.

The detector cell 5 comprises a sealed housing 15 which is shaped to give a minimum interception of the infra-red rays from the mirror system and has at its forward end a lens 16 which may be made of potassium bromide or other material capable of efficiently transmitting infra-red radiation, and which together with the primary and secondary aspheric mirrors 3 and 4 constitutes an infra-red optical system with required minimum aberrations, field of view, and flat image plane, and through which the infra-red radiation enters the cell 5, and immediately behind this infra-red transmitting lens a temperature sensitive screen 17, this screen being arranged to glow when activated by an ultraviolet radiation source 18 such as a mercury vapour discharge lamp, but the glow being modified by the infra-red rays striking the screen 8.

The ultraviolet radiation source 18 is provided with a lens system 19 which directs the ultraviolet rays on to a multi-layer beam splitter 20 which is designed to reflect a very high percentage of the ultraviolet rays to the temperature sensitive screen 8 but allow good light transmission between the temperature sensitive screen 8 and the optical viewing system 21.

An ultraviolet filter 19a may be included.

The rear of the detector unit is sealed by a window 22 of fused silica or other material transparent to ultraviolet and visible radiation which preferably is arranged to be not quite normal to the axis of the unit so as to reduce reflection, and immediately behind this is the multi-layer beam splitter 20 which receives the rays from the ultraviolet radiation source 18 by means of which the phosphor screen 8 is energised, the multilayer beam splitter 20 of course being of such a nature that binocular or similar viewing mechanisms behind it allows the image screen to be viewed in an unobstructed manner.

Also within the capsule or cell so formed is a heater element 23 by means of which a thermal bias can be applied to the temperature sensitive screen 8, this preferably being in the nature of a grid which can be heated by passing an electric current through the grid so that the whole of the face of the temperature sensitive screen can be maintained at a required temperature for best results or for a required lower sensitivity.

In the modification shown in FIG. 4 the housing 30 has at its front a metal honey-comb 31 to form a protective window for the front of the unit but to allow infra-red rays to reach the mirror 32 at the rear of the housing, the detector cell 33 being disposed within the housing to receive infra-red rays through the potassium bromide lens 34, a corrector lens 35 being disposed between the honey-comb 31 and the mirror 32.

In this case the energising and viewing of the screen of the detector cell 33 is from the back of the unit and an ocular viewing arrangement is shown at 36 while the lamp supplying the ultraviolet light is disposed behind the mirror 32 but the rays are reflected by the multilayer beam splitter 38 through the lens system 39 to the detector cell 33, the visual rays from the phosphor screen of the detector cell 33 passing through the multi-layer beam splitter 38 and focusing in the ocular 36.

It will be realised that this is simply a rearrangement of the mechanism described with reference to the foregoing figures and it will also be realised that in both of these cases it is possible to modify the system for flying spot scanning or for television scanning where amplification of the signal is required.

In the case of a flying spot scanner the ultraviolet light source would be replaced by a flying spot scanner which would direct the electron beam onto the screen for scanning purposes.

In the case of the television scanner the eye-piece would be replaced by means of the television scanner so that an image of the screen would then be available for amplification by any required amount in the normal type of television system and also this would permit transmission of the picture so obtained by normal techniques.

To ensure that the temperature sensitive screen 8 will operate under best conditions the detector cell 15 is evacuated for the purpose of minimising heat transfer on the temperature sensitive screen itself and loss of heat from the screen to the surroundings, for it will be realised that, as the screen is actuated by thermal differentials, these thermal differentials must be maintained by the avoidance of heat transfer in the screen 8 itself and to any surroundings.

The actual construction of the screen can be varied but according to a convenient form it comprises a phosphor, which phosphor does not itself absorb the infrared radiation, used in conjunction with an absorber which rises in temperature under the action of infra-red radiation and raises the temperature of adjacent phosphor particles by thermal conduction, and in this regard it can be mentioned that the phosphor screen can be of the monolayer type or of a segmented type to avoid lateral conduction or thermal spread, the absorber itself being either coated on the phosphor particles or on to the very thin, low conductivity base on which the particles are carried, or the said very thin, low conductivity supporting film for the phosphor particles can itself embody the absorber, the above conditions requiring thin films such as single layer phosphor layers to preserve the correct temperature isolation but another system can be used in which groups of absorber and phosphor layers are provided in the nature of a grid giving the necessary thermal isolation to the sections of the grid without having to use the thin layer of phosphor.

The production of the temperature sensitive screen is critical but according to one form, as illustrated in FIG. 5, a collodion support film 35 which is about 750 Angstrom thick was prepared by allowing a drop or two of a 6 percent solution (weight per volume) of collodion in amyl acetate to spread over a water surface, and then lifting the hardened film on a ring. The phosphor particles 26, having a maximum effective diameter of about 2 microns, were then settled on to the film from a suspension in petroleum ether with lead oleate as a dispersing agent. The absorbing film 37 was lead oleate as a dispersing agent. The absorbing film 37 was then deposited by vacuum evaporation, the thickness being adjusted by monitoring the transmission at a selected wavelength.

In this way the phosphor particles adhere to the film by natural attraction, but sufficiently to give good adhesion unless physically brushed off. By depositing the phosphor particles from a liquid, particularly with a dispersing agent which provides a stable suspension of the particles in the liquid, the support deposition of the particles is such that the particles tend to remain separated and thermal transfer from particle to particle is minimised.

Another method of forming the support film, which in some cases may be an improvement on the first stated method in that it gives films having better thickness uniformity across the individual films and from batch to batch, consists in lowering a clean glass disc completely into. a 1.3 percent collodion solution and withdrawing at a constant slow rate. After being allowed to dry the film is removed from the glass plate by being placed at a slight angle from the horizontal in a dish which is slowly and steadily filled with water. The film rises to the surface of the water and is lifted off on a ring as before. Precoating of the plate with a release layer of alkali halide improves the stripping process.

In the case where the absorber such as gold black particles is to be embedded in the support film, see FIG. 6, the absorber particles 40 were first deposited on a glass disc and the second above process was then applied, the absorber particles being then entirely enclosed within the support film 41, thermal conduction between the absorber and the phosphor particles 42 being then assured by the collodion film, this leading to high sensitivity.

Using segmentedscreens, see FIG. 7, certain advantages result in that thermal transfer can only occur through the supporting membrane 45 at areas such as 46 where no phosphor particles 47 are deposited and where the absorber film 48 is absent, consequently high resolution is obtained with small segments. The phosphor particles do not have to be deposited as a monolayer with spacing between the individual particles to avoid thermal transfer, and thus there can be a denser layer. Provided all the phospor particles come to the temperature of the absorber there will be an increase in average screen luminance without loss of sensitivity.

, Also the greater number of particles in the picture element give greater uniformity over the area of the screen and from screen to screen. Deposition of the absorber film can be through a grid, and xerographic processes can then be used to deposit the phosphor electrostatically according to the grid pattern.

In FIG. 8 is shown a graph of phosphor brightness to temperature, from which it will be seen that by selecting the temperature of the phosphor as controlled by the heater 23, optimum results are obtainable when the correct temperature of the phosphor is maintained.

By using the detector unit 8 in the form of a completely sealed capsule or cell it will be obvious that the unit does not have to be supplied with vacuum lines to lower the pressure within same which is necessary to prevent thermal transfer within the capsule itself, and thus a simple and effective unit results which can be battery operated so far as heat or bias is concerned and also the mercury vapour discharge lamp which is the energising source for the phosphor can similarly be actuated from a battery supply, thus making the unit completely self-contained and allowing it to traverse an area where inspection is to be made such as for fire detection purposes or the like, the viewing screen according to this invention ensuring good resolution and a very effective type of control because of the variable heat or bias and also because of effective control of the amount of energisation of the screen from the fluorescent lamp. It will be appreciated however that if it was preferred not to use a sealed detector unit then a vacuum drawing nipple could be included, and the device would then have to be provided with vacuum drawing means to maintain optimum working condition of the unit. In the sealed unit the usual vacuum drawing con ditions may obtain using a getter if required so that a unit is assured of having no more complications'after manufacture than a thermionic valve and readily replaceable under field conditions.

What we claim is:

1. An improved thermal imaging device comprising a housing, an evacuated sealed capsule in said housing, said housing having a window for admission of infra-red radiation therein, collector means within said housing for focussing the admitted infra-red rays at a focal plane of the collector means, said capsule including a temperature sensitive screen therein disposed at said focal plane, said screen comprising a film with an absorber for infra-red rays and phosphor particles on said film in heat conductive contact with the absorber but with low thermal transfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means for detecting variation in fluorescence on said screen due to the presence thereat of an infra-red image, said collector means comprising at least one aspherical mirror arranged to direct infra-red radiation to said screen, an optical viewing system for said screen, and a beam splitter positioned to pass light to said optical viewer system, said energizing means comprising an ultraviolet ray generator directed towards the beam splitter to be focussed thereby onto said screen.

2. An improved thermal imaging device according to claim 1 comprising a thermal heating grid in said capsule adjacent said screen and means to energize the said grid.

3. An improved thermal imaging device according to claim 1 wherein the means for detecting variation in fluorescence on said screen comprises a television scanner focussed on the said screen.

4. An improved thermal imaging device according to claim 1 wherein said collector means comprises a correcting lens, said aspherical mirror being arranged behind said correcting lens to direct infra-red rays from the window to said capsule.

5. An improved thermal imaging device according to claim 1 wherein the phosphor particles on the screen are in the form of a mono-layer.

6. An improved thermal imaging device according to claim 1 comprising bias temperature control means for the said screen to control brightness of the screen.

7. An improved thermal imaging device according to claim 1, wherein said capsule includes a grid adapted to be heated to raise uniformly the temperature of the phosphor particles for brightness control.

8. An improvided thermal imaging device according to claim 1, wherein said screen is segmented both for the phosphor particles and the absorber film to prevent lateral spread of heat through the screen.

9. An improved thermal imaging device according to claim 1, wherein said absorber film is coated on one side of the said support film and the phosphor particles are on the other side of the support film.

10. An improved thermal imaging device according to claim 1, wherein the absorber film is coated onto the phosphor particles.

11. An improved thermal imaging device comprising a housing, an evacuated sealed capsule in said housing, said housing having a window for admission of infra-red radiation therein, collector means within said housing for focussing the admitted infra-red rays at a focal plane of the collector means, said capsule including a temperature sensitive screen therein disposed at said focal plane, said screen comprising a film with an absorber for infra-red rays and phosphor particles on said film in heat conductive contact with the absorber but with low thermal transfer between phosphor particles, energizing means in said housing for causing fluorescence of said phosphor particles and means for detecting variation in fluorescence on said screen due to the presence thereat of an infra-red image, the said absorber film being a particulate substance embedded in the said support film.

12. An improved thermal imaging device according to claim '11 wherein said collector means comprises at least one aspherical mirror arranged to direct infra-red rays to said capsule, said energizing means comprising ultraviolet ray producing means focussed onto the said phosphor screen.

13. An improved thermal imaging device comprising a housing, an evacuated sealed capsule in said housing, said housing having a window for admission of infra-red radiation therein, collector means within said housing for focussing the admitted infra-red rays at a focal plane of the collector means, said capsule including a temperature sensitive screen therein disposed at said focal plane, said screen comprising a film with an absorber for infra-red rays and phosphor particles on said film in heat conductive contact with the absorber but with low thermal transfer between phosphor particles, energizing means in said housing for causing fluorescence of said phosphor particles and means for detecting variation in fluorescence on said screen due to the presence thereat of an infra-red image, said collector means comprising at least one aspherical mirror arranged to direct infra-red radiation to said screen, an optical viewing system for said screen and a beam splitter positioned to pass light to said optical viewer system, said energizing means comprising a flying spot scanner directed towards the beam splitter to be focussed thereby onto said screen.

14. An improved thermal imaging device comprising a housing, an evacuated sealed capsule in said housing, said housing having a window for admission of infra-red radiation therein, collector means within said housing for focussing the admitted infra-red rays at a focal plane of the collector means, said capsule including a temperature sensitive screen therein disposed at said focal plane, said screen comprising a film with an absorber for infra-red rays and phosphor particles on said film in heat conductive contact with the absorber but with low thermal transfer between phosphor particles, energizing means in said housing for causing fluorescence of said phosphor particles and means for detecting variation in fluorescence on said screen due to the presence thereat of an infra-red image, said collector means comprising at least one aspherical mirror arranged to direct infra-red rays to said capsule, said energizing means comprising ultraviolet ray producing means focussed onto the said phosphor screen, and heater means within the capsule to raise uniformly the temperature of the phosphor particles.

15. An improved thermal imaging device comprising collector means to focus infra-red radiation, a fluorescent screen at the focal plane of the collector means, said screen comprising a support film with an absorber thereon for infra-red radiation, and phosphor particles on said film in heat conductive contact with the absorber on the film but with low thermal transfer between phosphor particles, an energizing source for said fluorescent screen, the infra-red radiation producing a temperature pattern on the screen which in turn varies the fluorescence of the screen, and means for detecting the variation of fluorescence due to the presence of the infra-red radiation on said screen.

16. An improved thermal imaging device according to claim 15 comprising a sealed evacuated capsule enclosing said fluorescent screen.

17. An improved thermal imaging device according to claim 16 wherein said capsule includes a grid adapted to be heated to raise uniformly the temperature of the phosphor particles for brightness control.

18. An improved thermal imaging device according to claim 16 wherein said collector means comprises at least one aspherical mirror arranged to direct infra-red rays to said capsule, said energizing means comprising ultraviolet ray producing means focussed onto the said phosphor screen.

19. An improved thermal imaging device according to claim 16 comprising heater means within the capsule to raise uniformly the temperature of the phosphor particles.

20. An improved thermal imaging device according to claim 16 comprising a thermal heating grid in said capsule adjacent said screen and means to energize the said grid.

21. An improved thermal imaging device according to claim 15 wherein said collector means comprises at least one aspherical mirror arranged to direct infra-red radiation to said screen, an optical viewing system for said screen, and a beam splitter positioned to pass light to said optical viewer system, said energizing means comprising an ultraviolet ray generator directed towards the beam splitter to be focussed thereby onto said screen.

22. An improved thermal imaging device according to claim 21 wherein said collector means comprises a correcting lens, said aspherical mirror being arranged behind said correcting lens to direct infra-red rays from the window to said capsule.

23. An improved thermal imaging device according to claim 15 wherein said collector means comprises at least one aspherical mirror arranged to direct infra-red radiation to said screen, an optical viewing system for said screen and a beam splitter positioned to pass light to said optical viewer system, said energizing means comprising a flying spot scanner directed towards the beam splitter to be focussed thereby onto said screen.

24. An improved thermal imaging device according to claim 15 wherein the means for detecting variation in fluorescence on said screen comprises a television scanner focussed on the said screen.

25. An improved thermal imaging device according to claim 15 wherein the phosphor particles on the screen are in the form of a mono-layer.

26. For an improved thermal imaging device, an evacuated sealed capsule containing a temperature sensitive screen, said capsule having a window to admit infra-red radiation, said screen comprising a support film with an absorber film thereon for infra-red radiation and phosphor particles on said support film in heat conductive contact with the absorber film but with low thermal transfer between phosphor particles, and a heater adjacent said support film for uniformly raising the temperature of the phosphor particles for optimum effect.

27. An improved thermal imaging device according to claim 26 wherein said screen is segmented both for the phosphor particles and the absorber film to prevent lateral spread of heat through the screen.

substance embedded in the said support film.

Claims

1. An improved thermal imaging device comprising a housing, an evacuated sealed capsule in said housing, said housing having a window for admission of infra-red radiation therein, collector means within said housing for focussing the admitted infra-red rays at a focal plane of the collector means, said capsule including a temperature sensitive screen therein disposed at said focal plane, said screen comprising a film with an absorber for infra-red rays and phosphor particles on said film in heat conductive contact with the absorber but with low thermal transfer between phosphor particles, energizing means in said housing for causing fluorescence of said phosphor particles and means for detecting variation in fluorescence on said screen due to the presence thereat of an infra-red image, said collector means comprising at least one aspherical mirror arranged to direct infra-reD radiation to said screen, an optical viewing system for said screen, and a beam splitter positioned to pass light to said optical viewer system, said energizing means comprising an ultraviolet ray generator directed towards the beam splitter to be focussed thereby onto said screen.

12. An improved thermal imaging device according to claim 11 wherein said collector means comprises at least one aspherical mirror arranged to direct infra-red rays to said capsule, said energizing means comprising ultraviolet ray producing means focussed onto the said phosphor screen.

28. An improved thermal imaging device according to claim 26 wherein said absorber film is coated on one side of the said support film and the phosphor particles are on the other side of the support film.

29. An improved thermal imaging device according to claim 26 wherein the absorber film is coated onto the phosphor particles.

30. An improved thermal imaging device according to claim 26 wherein the absorber film is a particulate substance embedded in the said support film.