BACKGROUND OF THE INVENTION Over the past several years, infrared radiation technology has evolved very quickly. One aspect of this evolution has been in the area of thermal imaging.

Thermal imaging systems generally include infrared radiation components, such as an infrared transmissive window through which infrared radiation enters the system, an infrared detector which detects the infrared radiation that passes through the window, a circuit that processes the detected infrared radiation information, and a monitor that displays infrared radiation information received from the circuit. These infrared radiation components cooperate to display a view of a scene based on the thermal energy emitted by objects in the scene. These thermal imaging systems have been generally adequate for their intended purposes, but have not been satisfactory in all respects.

A problem with existing thermal imaging systems is that the infrared transmissive windows are generally uniform in visual appearance, and are thus lacking in aesthetic appeal. This is particularly a problem when the thermal imaging system is included in a vehicle.

Thermal imaging systems that operate within a vehicle

generally include an infrared transmissive window in the grille of the vehicle. The window has a blank appearance which gives the impression that something is missing from the window or grille structure.

Another problem with existing thermal imaging systems is that some infrared transmissive windows reflect visible light in a manner which draws more attention to the window than desired. There is a need for a window which blends better with its surroundings.

SUMMARY OF THE INVENTION From the foregoing, it may be appreciated that a need has arisen for an apparatus and method that provide improved control over the manner in which an infrared transmissive window handles visible light. According to the present invention, an apparatus and method are provided to address this need, and involve: generating a visible image by causing visible radiation to cooperate with structure of an element in a manner so that the visible image is visible at a location spaced from the element; and passing infrared radiation through the element and the structure thereof without significant change in the infrared radiation.

BRIEF DESCRIPTION OF THE DRAWINGS A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: FIGURE 1 is a diagrammatic front view of a vehicle which includes an infrared imaging system that embodies the present invention;

FIGURE 2 is a diagrammatic perspective view of an infrared camera which is a component of the infrared imaging system of FIGURE 1; FIGURE 3 is a diagrammatic view of the infrared imaging system of FIGURE 1, showing details of the camera of FIGURE 2; FIGURE 4 is a diagrammatic sectional view taken along the line 4-4 in FIGURE 2; FIGURE 5 is a diagrammatic sectional view similar to FIGURE 4, but showing an alternative embodiment of the infrared camera; FIGURE 6 is a diagrammatic sectional view similar to FIGURE 4, but showing another alternative embodiment of the infrared camera; FIGURE 7 is a diagrammatic sectional view similar to FIGURE 4, but showing still another alternative embodiment of the infrared camera ; and FIGURE 8 is a diagrammatic perspective view of an alternative embodiment of an infrared transmissive window which is a component of the infrared camera of FIGURE 2.

DETAILED DESCRIPTION OF THE INVENTION FIGURE 1 is a diagrammatic front view of a vehicle 10 that includes an infrared imaging system 11. The system 11 includes an infrared camera 12 which is positioned in the center of the grille of the vehicle 10.

The infrared camera 12 includes an element which is a window 13, through which infrared radiation enters the infrared camera 12. The base material for the window 13 is a polymer material disclosed in U. S. Patent Number 5,629,074, the disclosure of which is hereby incorporated herein by reference. The material of the window 13 has

added to it a commercially available black dye. The dye absorbs some visible radiation, but also enhances the reflectivity and increases the index of refraction of the window 13 relative to visible radiation in general.

Alternatively, the window 13 could be made of some other suitable infrared transmissive material, such as silicon.

Infrared radiation having a wavelength in the range of 8- 12pm passes through the window 13 without significant change. The infrared radiation that passes through the window 13 is emitted by a scene which includes animate or inanimate objects that are within the field of view of the infrared camera 12.

The window 13 includes a structure shown diagrammatically in broken lines, which is a holographic fringe pattern 14, commonly referred to as a hologram.

The fringe pattern 14 is defined by undulations embossed in the center of the front surface of the window 13.

However, the fringe pattern 14 could be created in some other known manner, for example by using a laser to burn the pattern into the material of the window 13. The fringe pattern 14 generates a visible image based on light in the visible spectrum. Although the fringe pattern 14 of the disclosed embodiment happens to be in the center of the window 13, it could alternatively be off-center, or it could be made up of a plurality of separate images at different locations on the window 13.

Still another alternative is that the fringe pattern 14 could cover the entire window 13.

In FIGURE 1, the image produced by the fringe pattern is a symbol or trademark which identifies the manufacturer of the vehicle 10. However, the visible image provided by the fringe pattern could alternatively

be some other symbol, logo, text, pattern, picture, or image. If the camera 12 was being used in a security system to protect commercial premises, the image produced by the fringe pattern might be a random or other pattern that would allow the window to better blend in with its surroundings, thereby reducing the likelihood that an intruder would notice and disable the camera.

As shown in FIGURE 1, a conventional head up display (HUD) 19 generates a view 16 of a scene that correlates to infrared radiation emitted by objects within the field of view of the infrared camera 12. The view 16 is displayed in a known manner on a portion of an inner surface of the vehicle's windshield 17. The windshield 17 is a standard automotive glass windshield made of an infrared and visible light transmissive material.

Alternatively, the windshield 17 could be any transmissive material having a surface onto which an image could be projected by the (HUD) 19. The view 16 is monitored by a driver of the vehicle 10 while driving.

FIGURE 2 is a diagrammatic perspective view that shows only the infrared camera 12. The infrared camera 12 includes a body 22 and a frame 23. The frame 23 of the infrared camera 12 encircles and supports the window 13. The frame 23 is an annular plastic part that is coupled to the body 22 and supported by the grille of the vehicle 10. Alternatively, the frame 23 could be made of any suitable material that could be fabricated to extend around the window 13. The body 22 of the infrared camera 12 is an elongated metal piece having a cavity therein.

Alternatively, the body 22 could be made of any suitable material that can provide a strong degree of protection for components within the infrared camera 12.

A lens section 32 is included within the body 22 of the infrared camera 12, and is therefore shown diagrammatically in broken lines in FIGURE 2. The lens section 32 includes a silicon lens that refracts infrared radiation. Alternatively, the lens section 32 can be any suitable refractive material, capable of imaging infrared radiation. The lens section 32 images the infrared radiation 31 passing through the window 13 in a known manner onto a conventional infrared detector 33, which is also disposed within the body 22 and thus shown diagrammatically by a broken line. The infrared detector 33 is an uncooled detector having a two-dimensional array of detector elements that detect infrared radiation impinging thereon. The infrared detector 33 could alternatively be any other suitable infrared detection mechanism. The infrared detector 33 is at an end of the body 22 of the infrared camera 12, opposite the frame 23.

The infrared detector 33 receives the infrared radiation which has passed through the window 13 and the lens section 32. The lens section 32 and the infrared detector 33 will be discussed in more detail in association with FIGURE 3.

FIGURE 3 is a diagram showing in more detail the infrared imaging system 11, including the elements of the infrared camera 12, and how the infrared camera 12 is interfaced with a portion of the inner surface of the windshield 17 through the HUD 19. Light rays 27, in the visible spectrum, are reflected and/or scattered by the fringe pattern 14, so as to project at 28 in front of the vehicle 10 a visible image 29 which corresponds to the fringe pattern 14. Only a small portion of the visible light impinging on the window 13 is affected by the

pattern 14, but it is enough that the image 29 is distinctly visible to the naked eye. The dye in the window 13 helps to increase the degree of reflection and/or scattering of visible light. While the image 29 is being continuously generated with visible light, infrared radiation 31 continues passing through the window 13 and its fringe pattern 14 relatively unaffected. Although the image in this disclosed embodiment is visible at a location in front of the window 13, it will be recognized that, where appropriate for some other application, it could be formed behind the window or within the window.

The infrared radiation 31 passing through the window 13 moves to the lens section 32. The infrared detector 33 receives the imaged infrared radiation 31 from the lens section 32 and converts it into electrical information 36 that is transmitted to a circuit 37 of a known type. The circuit 37 generates electrical data 38 corresponding to the scene which is viewed by the infrared camera 12. The data 38 is transmitted to the HUD 41. The HUD 41 uses the data 38 to generate a visible image of the scene which is-projected as the view 16 onto a portion of the inner surface of the windshield 17. At night, the inner surface of the windshield will reflect enough light so that the view 16 will be clearly visible to the driver. The view 16 of the scene allows the driver of the vehicle 10 to monitor objects emitting thermal energy that are located in front of the vehicle 10. This view 16 of the scene allows the driver to monitor objects without detracting significantly from the view seen through the windshield 17 or the operation of the vehicle 10. At night, the driver may see in view 16

an animal or object which is beyond the range of the headlights, and thus not yet visible to the naked eye.

FIGURE 4 is a diagrammatic sectional view of the infrared camera 12, taken along the line 4-4 of FIGURE 2.

FIGURE 4 shows infrared radiation 31 passing through the window 13 of the infrared camera 12. The infrared radiation 31 moves to the lens section 32 where it is imaged onto the infrared detector 33. As discussed above, at least some visible ambient light rays 27 are reflected away the camera 12 by the fringe pattern 14.

The reflected light rays 28 form the image 29 (FIGURE 3) in front of the vehicle 10.

FIGURE 5 is a diagrammatic sectional view of an infrared camera 45 which is an alternative embodiment of the infrared camera 12 of FIGURE 4. FIGURE 5 shows artificial light sources 46 used to generate visible light that can produce the image 29. The light sources 46 are two units that are mounted on the inside of the body 22 of the infrared camera 45. The light sources 46 generate visible light rays 47 that propagate from within the body 22 of the infrared camera 45 toward the fringe pattern 14 of the window 13. As the visible light rays 47 pass through the window 13, they are affected by the fringe pattern 14 and thereafter carry the image 29 corresponding to the fringe pattern 14. The image 29 is thus projected in front of the vehicle 10. While this continuously occurs, the infrared radiation 31 continues to pass through the window 13 to the lens section 32 without being significantly affected. In this embodiment, the window 13 contains less dye than in the embodiment of FIGURES 1-4, so that in this embodiment more visible radiation will pass through the window 13

without being reflected back into the camera or absorbed within the window.

FIGURE 6 is also a diagrammatic sectional view of an infrared camera 49 which is another alternative embodiment of the infrared camera 12 of FIGURE 4. FIGURE 6 shows yet another pair of light sources 51 used to generate visible light that can produce the image 29.

The lights sources 51 are two units that are mounted in the front of the frame 53 of the infrared camera 49. The light sources 51 generate visible lights rays 52 that propagate from the front of the frame 53 to the fringe pattern 14 of the window 13. At least some of the visible light rays 52 are reflected and/or scattered away from the camera 49 by the fringe pattern 14 and generate the image 29 corresponding to the fringe pattern 14.

While this continuously occurs, the infrared radiation 31 continues to pass through the window 13 to the lens section 32 without being significantly affected.

FIGURE 7 is a diagrammatic sectional view of an infrared camera 55 which is yet another alternative embodiment of the infrared camera 12 of FIGURE 4. FIGURE 7 shows yet another pair of light sources 56 which are used to generate visible light that can produce the image 29. The light sources 56 are two units that are mounted in the frame 59 at the edges. of the window 13 of the infrared camera 55. The light sources 56 generate visible lights rays 57 that propagate from the edges of the window 13 toward the fringe pattern 14. At least some of the light rays 57 are reflected and/or scattered at the fringe pattern 14 and project the image 29 corresponding to the fringe pattern 14. While this continuously occurs, the infrared radiation 31 continues

to pass through the window 13 to the lens section 32 without being significantly affected. In this embodiment, the window 13 contains less dye than in the embodiments of FIGUREs 1-4 and 6, so that in this embodiment more visible radiation will pass through the window 13 without being reflected or absorbed.

The light sources 46,51, and 56, as shown in FIGURES 5-7, are all powered by the electrical system of the vehicle 10, including the battery and/or alternator.

Alternatively, these light sources 46,51, and 57 could be powered by any other suitable power source. In addition, FIGURES 5-7 illustrate the alternative light sources 46,51, and 56 as being two units in each embodiment, but it will be recognized that these sources 46,51, and 56 could alternatively be a single unit or multiple units either within or external to the infrared camera.

FIGURE 8 shows a diagrammatic perspective view of a window 60 which is an alternative embodiment of the window 13 in the infrared camera 12 of FIGURE 1. As illustrated in FIGURE 8, the window 60 is an element that includes a sheet 58 and a plate-like member 62 made of conventional infrared transmissive silicon. The sheet 58 is a thin piece of infrared transmissive material such as that disclosed in above-mentioned U. S. Patent No.

5,629,074. A structure that is a holographic fringe pattern 61 for a visible image is embossed into the sheet 58. The sheet 58 is then laminated in a known manner onto the member 62. If the materials of the member 62 and the sheet 58 have indices of refraction which are significantly different, these indices of refraction can be matched by providing between the member 62 and sheet

58 several layers or coatings of other materials with progressively increasing or decreasing indices of refraction. This matching of the indicies of refraction will avoid any significant reflection of infrared radiation at the interface between member 62 and sheet 58. Infrared radiation passes through the member 62 and its fringe pattern 61 substantially unaffected.

The visible image corresponding to the fringe pattern 61 is generated in the same basic fashion as described above in association with FIGURE 3.

A description will now be provided of the operation of the infrared camera 12 of FIGURES 1-4. As shown in FIGURES 1,2, and 3, infrared radiation passes through the window 13 without significant change. The infrared radiation that passes through the window 13 is emitted by a scene which includes animate or inanimate objects that are within the field of view of the infrared camera 12.

The infrared radiation passes through the window 13 to the lens section 32. The lens section 32 images the infrared radiation 31 passing through the window 13 in a known manner onto a conventional infrared detector 33 which is disposed within the body 22 shown diagrammatically by a broken line. The infrared detector 33 receives the infrared radiation which has passed through the window 13 and the lens section 32, and converts it into electrical information 36 that is transmitted to a circuit 37. The circuit 37 generates electrical data 38 corresponding to the scene viewed by the infrared camera 12. The data 38 is transmitted to the HUD 41. The HUD 41 uses the data 38 to generate a visible image of the scene which is projected as the view 16 onto a portion of the inner surface of the windshield

17. The view 16 of the scene allows the driver of the vehicle 10 to monitor objects emitting thermal energy that are in front of the vehicle 10, without detracting significantly from the view seen through the windshield 17 or the operation of the vehicle 10.

Light rays 27, in the visible spectrum, are reflected away from the camera 12 by the fringe pattern 14, so as to project at 28 in front of the vehicle 10 a visible image 29 which corresponds to the fringe pattern 14. While the image 29 is being continuously generated with visible light, infrared radiation 31 continues passing through the window 13 and its fringe pattern 14 relatively unaffected.

The present invention provides a number of technical advantages. One such technical advantage is the ability of the present invention to generate a visible image that enhances the aesthetic appeal of a window of a thermal imaging system. Particularly in the case of a vehicle, the present invention could cause the otherwise blank space of the infrared transmissive window of the thermal imaging system to appear to bear a trademark, logo, or other indicia representative of the manufacturer of the vehicle. This greatly enhances the commercial appeal of the vehicle.

A further advantage of the present invention is the ability to generate a visible image that can be used to make the infrared transmissive window less noticeable, so as to allow the infrared transmissive window to better blend in with its surroundings.

Although one embodiment has been illustrated and described in detail, it should be understood that various substitutions and alterations can be made therein without

departing from the scope of the present invention. For example, while the infrared imaging system has been described above with respect to a vehicle, it will be recognized that the invention could be used in virtually any infrared detecting system that has an infrared transmissive window. Also, although the present invention uses a holographic fringe pattern to generate a visible image, it would be possible to provide some other form of structure within the window, so long as it generated a visible image while passing infrared radiation substantially unchanged.

Other substitutions and alterations are also possible without departing from the spirit and scope of the present invention, as defined by the following claims.