WO2001081972A2

WO2001081972A2 - Method and apparatus for obtaining infrared images in a night vision system

Info

Publication number: WO2001081972A2
Application number: PCT/US2001/013268
Authority: WO
Inventors: Alexander L. Kormos
Original assignee: Raytheon Company
Priority date: 2000-04-25
Filing date: 2001-04-24
Publication date: 2001-11-01
Also published as: WO2001081972A3; AU2001257237A1

Abstract

A vehicle (10) includes a night vision system with an infrared camera unit (12) that provides information to a display unit (16), which in turn projects an image onto the vehicle windshield (17). The camera unit (12) includes a lens system (26) which images infrared radiation from a scene (21) onto a detector (31) that contains a two-dimensional array of detector elements. The lens system images onto each detector element incoming energy that corresponds to a portion of the scene having a size within the range of approximately 0.8 to 3.5 milliradians. The camera unit provides a horizontal field of view which is less than approximately 14°, while the lens system has an effective focal length which is less than approximately 2.25''. The night vision system is respectively capable and incapable of providing MRT recognition of a multi-bar target having a Nyquist spatial frequency that is respectively less than and greater than a selected frequency, the selected frequency being less than approximately 0.63.

Description

METHOD AND APPARATUS FOR OBTAINING INFRARED IMAGES IN A NIGHT VISION SYSTEM

TECHNICAL FIELD OF THE INVENTION

This invention relates in general to a night vision system and, more particularly, to a method and apparatus for obtaining infrared images in a night vision system.

BACKGROUND OF THE INVENTION

During daylight hours, the driver of a vehicle is able to readily detect and recognize objects which would be difficult or impossible to detect or recognize at night. For example, assume that a vehicle is driving along a road, and that a deer wanders into the road approximately 500 meters ahead of the vehicle. If this scenario occurs in the middle of a sunny day, the driver will not only be able to detect the fact that something is present ahead, but will readily recognize that it is a deer. On the other hand, if this same scenario occurs at night, particularly where there is little moonlight and the only artificial lighting is from the headlights of the vehicle, the deer will be beyond the range of the headlights. The driver will not be able to detect that anything is there, much less recognize that it is a deer. By the time the driver even realizes that something is in the road, and even before the driver can recognize what it is, the driver will be much closer to the deer than would be the case during daylight hours. Thus, the risk of a resulting accident is much higher at night than it would be during the day.

Consequently, in order to supplement the natural vision of a driver, and thus reduce the risk of accidents, night vision systems have been developed for vehicles, including automobiles sold in the consumer market. One existing night vision system has an infrared camera unit which is mounted in the center of the grill of the automobile, and has a head-up display which is mounted in the vehicle's dashboard, and which projects onto the inside surface of the windshield an image derived from information provided by the camera unit . The display provides a horizontal field of view of approximately 12° horizontal and 4° vertical, or in other words an aspect ratio of 3, with a 1:1 magnification factor. The display is a color display, and includes a backlit liquid crystal display

(LCD) , which is an array of 384 pixels horizontally by 220 pixels vertically, although the aspect ratio means that not all of the vertical pixels are used. As a result, the effective operational portion of the LCD display is 384 pixels horizontally by 107 pixels vertically. The image on the backlit LCD is reflected off an adjustable mirror and onto the inside surface of the windshield.

In this existing system, the camera unit has a lens system which takes incoming infrared radiation from the road ahead of the vehicle, and images it onto a detector. The detector is a focal plane or staring array detector which includes a two-dimensional matrix of pixel-like detector elements, where each detector element has a size of approximately 48.5 microns by 48.5 microns. In this existing system, the detector elements form an array of 320 detector elements horizontally by 240 detector elements vertically. Due to the effective aspect ratio of the head- up display, the portion of the detector actually used is 320 detector elements horizontally by 106 detector elements vertically. Given the size of the detector, and in order to image infrared radiation onto the entire width of the detector with the desired 12° horizontal field of view, the lens system includes an objective lens which has a diameter of 2.7", an effective focal length of 2.9", and an optical speed of f/1.2. While this existing system has been generally adequate for its intended purposes, it has not been satisfactory in all respects.

In particular, because of the need to mount the camera unit at the front of the vehicle, for example by integrating it into the design of the grill, it is desirable that the camera unit have a small size and a low weight. To the extent that the camera unit is integrated into the grill, it is exposed to hazards such as accidents and also small stones thrown by the wheels of trucks. Therefore, and because of the highly competitive nature of the commercial automotive market, it is desirable that the camera unit have a relatively low cost . With these considerations in mind, the camera unit in the existing system is generally regarded as having a size, weight and cost which are higher than would ideally be desirable. Nevertheless, despite prior efforts to find a way decrease the size, weight and/or cost of the camera unit, no satisfactory approach was found to decrease the size, weight and/or cost while maintaining the existing field of view and 1:1 magnification factor, and without sacrificing meaningful resolution. In this regard, it was generally thought that the detector in the camera unit needed a configuration of at least about 320 by 106 detector elements in order to avoid any loss of resolution in a 12° by 4° head-up display, which in turn dictated a minimum physical size for the detector under existing semiconductor fabrication technology. This in turn dictated the minimum size of the optics required to image a 12° horizontal field of view onto the detector width, and the size of the optics represented a significant contributing factor to the size, weight and cost of the camera unit . SUMMARY OF THE INVENTION

From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for providing night vision capability in a manner which provides a significant reduction in at least one of the size, weight and/or cost of a camera unit, while maintaining a desired magnification factor and field of view, and with no degradation in meaningful resolution.

According to a first form of the present invention, a method and apparatus are provided to address this need, and involve the operation of a night vision system which includes a detector having an array of detector elements, in particular by forming on the detector an image of information from a scene in a manner so that each of the detector elements receives incoming energy which corresponds to a portion of the scene having a size in the range of approximately 0.8 to 3.5 milliradians .

According to another form of the present invention, a method and apparatus involve operation of a night vision system containing an array of detector elements in a manner so that the system is capable of effecting MRT recognition of a multi -bar target having a Nyquist spatial frequency which is less than a selected Nyquist spatial frequency, and incapable of effecting MRT recognition of a multi-bar target having a Nyquist spatial frequency which is greater than the selected Nyquist spatial frequency, the selected Nyquist spatial frequency being selected to be less than approximately 0.63 cycles per milliradian.

According to yet another form of the present invention, a method and apparatus involve the provision of a night vision system having a detector which includes an array of detector elements, and an optical section which is operable to form on the detector an image of information from a scene, including selection for the optical section of a field of view less than approximately 14°, and an effective focal length less than approximately 2.25 inches.

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 a diagrammatic view of a vehicle which includes a night vision system that embodies the present invention;

FIGURE 2 is a diagrammatic view of the night vision system of FIGURE 1, showing in more detail the internal structure of a camera unit and a display unit which are components of the night vision system;

FIGURE 3 is a diagrammatic view of the camera unit of FIGURES 1 and 2, showing in more detail a lens system which is a component of the camera unit;

FIGURE 4 is a diagrammatic view of an infrared detector which is a component of the camera unit of FIGURES 1 and 2;

FIGURE 5 is a diagrammatic top view showing use of the camera unit of FIGURE 2 in association with standard detection and recognition techniques; FIGURE 6 is a diagrammatic view showing a portion of a scene which is imaged onto a single detector element of a detector disposed in the camera unit of FIGURE 2; and

FIGURE 7 is an enlarged view of a portion of FIGURE 6, diagrammatically showing additional detail within the camera unit . DETAILED DESCRIPTION OF THE INVENTION

FIGURE 1 is a diagrammatic view of a vehicle 10 which includes a night vision system that embodies the present invention. The night vision system includes a camera unit 12, which in the disclosed embodiment is mounted at the front of the vehicle, for example in the middle of the front grill. The camera unit 12 is electrically coupled at 14 to a display unit 16, which is also a part of the night vision system. The display unit 16 is mounted within an upwardly open recess provided in the top of the dashboard of the vehicle, and can project an image onto the inside surface of the windshield 17, as indicated diagrammatically by arrow 18. The display unit 16 is of a type which is commonly known as a head-up display (HUD) . The night vision system of FIGURE 1 is discussed in more detail later.

When a driver is operating a vehicle at night, the driver's ability to see the road ahead is substantially more limited than would be case for the same section of road during daylight hours. This is particularly true in a rural area, under conditions where there is little moonlight, no street lights, and no headlights of other vehicles. If an animal such as a deer happens to wander into the road at a location 500 meters ahead of the vehicle, the driver would readily notice the deer during daylight hours, whereas at night the deer may initially be beyond the effective reach of the illumination from the vehicle's headlights. Moreover, even when the headlights begin to illuminate the deer, the driver may not initially notice the deer, because the deer may be a brownish color which is difficult to distinguish from the surrounding darkness . Consequently, at the point in time when the driver first realizes that there is a deer in the road, the vehicle will be far closer to the deer in a nighttime situation than would be the case during daylight hours. There are other similar situations, for example where a pedestrian is walking along the road.

The purpose of the night vision system of FIGURE 1 is to provide the driver of the vehicle 10 with information above and beyond that which the driver can discern at night with the naked eye, based on an infrared (IR) image of the terrain ahead which is derived from the camera unit 12. In this regard, the camera unit 12 can detect infrared information at a distance well beyond the effective reach of the headlights of the vehicle. In the case of a life form such as an animal or a human, the heat signature of the life form, when presented in an infrared image, will usually have a significant contrast in comparison to the relatively hotter or cooler surrounding natural environment. As discussed above, this is not necessarily the case in a comparable night time image based on visible light.

Thus, in addition to the visible image which is directly observed by the driver through the windshield based on headlight illumination and any other available light, the night vision system provides a separate and auxiliary infrared image which is projected onto the inside of the windshield. This auxiliary image can provide a detectable representation of lifeforms or objects ahead that are not yet visible to a naked eye. Further, the auxiliary image can provide a much more striking contrast than a visible image between the lifeforms or objects and the surrounding scene. In an auxiliary vision system such as the night vision system of FIGURE 1, it is a generally accepted design principle that an object in the auxiliary image should appear to the driver to have substantially the same size that the corresponding real -life object appears to have when viewed by the driver through the windshield. Thus, for example, if an object along the roadway ahead appears to the driver to be approximately one inch tall when viewed through the windshield, the same object should appear to the driver to be one inch tall when viewed in the auxiliary image at that same point in time. This is known in the art as maintaining a 1:1 magnification factor for the auxiliary display. Of course, the magnification factor does not have to be precisely 1:1, and could for example be within 25% higher or lower than 1:1, but to facilitate an explanation of the present invention it will be assumed that the magnification factor is to be substantially 1:1.

The night vision system of FIGURE 1 involves competing design considerations with respect to the field of view (FOV) which it provides. It is, of course, desirable to minimize the size and weight of both the camera unit 12 and the display unit 16. However, if the effective FOV of the system is varied, the minimum sizes of the camera unit 12 and the display unit 16 tend to vary inversely. In particular, as the effective FOV is progressively increased, the size of the optics in the camera unit 12, and thus the size of the camera unit itself, can be progressively decreased, but the size of the display unit 16 must be progressively increased. Conversely, as the effective FOV is progressively decreased, the size of the display unit 16 can be decreased, but the size of the camera optics, and thus the size of the camera unit, need to be increased. Since the sizes of the camera unit 12 and display unit 16 thus vary inversely, a balance must be reached for purposes of any particular system.

As a practical matter, one side of this balance is determined by the display unit 16, due to yet another consideration. In particular, it is typically desirable to project onto the windshield a relatively large image. However, as a practical matter, the ability to achieve this is limited by the extent to which the dashboard area of an average automobile has space available to accommodate a display. In particular, if the display unit 16 is given the largest physical size which is reasonable for the dashboard area of an average automobile, and if 1:1 magnification is maintained, as discussed above, the display unit 16 will produce an image on the windshield which has an effective horizontal FOV of approximately 10° to 14°, and in the disclosed embodiment the FOV is approximately 12°. Of course, the invention is compatible with a FOV larger than 14° or smaller than 10°, for example a FOV of about 6° to 8°. However, the disclosed embodiment uses a FOV of about 12° because this value is typical of the displays presently used in existing night vision systems in the automotive industry.

Given this effective horizontal FOV of about 12° in the disclosed embodiment, as established by criteria relating to the display unit 16, the associated camera unit 12 must offer this needed horizontal FOV with a suitable degree of resolution. A feature of the present invention is a provision of an improved camera unit 12, which provides the needed FOV with a suitable level of resolution, but which has a substantially reduced size, weight and cost in comparison to preexisting camera units.

FIGURE 2 is a diagrammatic view of the night vision system of FIGURE 1, including a portion of the windshield

17 as well as a scene 21 observed by the camera unit 12, and showing in greater detail the internal structure of both the camera unit 12 and the display unit 16. More specifically, thermal radiation from the scene 21 enters the camera unit 12, and passes through a lens system 26 and a chopper 27 to a detector 31. The lens system 26 focuses the incoming radiation onto an image plane of the infrared detector 31. In the disclosed embodiment, the chopper 20 is a rotating disk of a known type, which has one or more circumferentially spaced openings. As the chopper is rotated, it periodically permits and prevents the travel of incoming infrared radiation to the detector 31.

In the disclosed embodiment, the detector 31 is a commercially available focal plane array or staring array detector, which has a two-dimensional matrix of detector elements, where each detector element produces a respective pixel of a resulting image. The circuitry 32 is provided to control the detector 31 and read out the images which it detects, and also to synchronize the chopper 27 to operation of the detector 31. Further, the circuitry 32 transmits the images obtained from detector 31 through the electrical coupling 14 to circuitry 41 within the display unit 16. The circuitry 41 controls a liquid crystal display (LCD) 42, which in the disclosed embodiment has a two-dimensional array of 384 by 220 pixel elements. The display unit 16 has a horizontal to vertical aspect ratio of 3 , as a result of which the portion of the LCD display 42 which is actually used is 384 by 107 pixel elements.

The circuitry 41 takes successive images obtained from the detector 31 through circuitry 32, and presents these on the LCD 42. In the disclosed embodiment, the LCD 42 includes backlighting that makes the image on LCD 42 visible at night. This visible image is reflected by a mirror 46, so as to be directed onto the inner surface of the windshield 17. Although the mirror 46 is shown diagrammatically in FIGURE 2 as a planar component, it actually has a relatively complex curvature which is known in the art, in order to compensate for factors such as the non-linear curvature of the windshield 17, parallax due to the inclination of the windshield 17, and so forth. The curvature also gives the mirror some optical power, so that it imparts a degree of magnification to the image that it projects onto the windshield 17. The mirror 46 is movably supported, and its position at any given time is determined by a drive mechanism 47. Using the drive mechanism 47, the driver may adjust the mirror 46 so that the image on the windshield is in a viewing position comfortable for that particular driver. This is analogous to the manner in which a driver may adjust a sideview mirror of a vehicle to a suitable and comfortable position. Once the driver has finished adjusting the mirror 46 to a suitable position, it remains in that position during normal operation of the night vision system.

FIGURE 3 is a diagrammatic view of the camera unit 12, and shows in more detail the optical structure within the lens system 26. More specifically, the lens system 26 includes an objective lens 56, which focuses incoming radiation in a manner forming an image at an image plane 57 which is adjacent the detector 31. The lens 56 has a diffractive surface pattern 59 on the side thereof nearest the detector 31. As discussed in more detail below, the disclosed embodiment uses only a subset of the pixel elements of the detector 31, and FIGURE 3 thus shows the radiation being imaged onto a portion of the detector 31, rather than the entire detector 31. In the disclosed embodiment, the lens 56 is made of

Chalcogenide glass of a type which is commonly known in the art as TI-1173, and which is available from Raytheon Company of Lexington, Massachusetts. Further, the objective lens 56 has a diameter of 1.5", an effective focal length (EFL) of 1.5" and an optical speed of f/l, where optical speed is a standardized measure of refractive strength per dimensional unit. Thus, the optical section 26 has a smaller aperture than preexisting systems. It is a feature of the night vision system according to the invention that the camera unit 12 can have an EFL less than about 2.25", and preferably less than about 1.9", while providing a horizontal FOV less than about 14° and preferably about 12°. A further feature is the use for lens 56 of an f/l optical speed, rather than the f/l.2 optical speed used in an existing system, because the f/l optical speed provides an increase of over 40% in the infrared energy imaged onto the detector 31, in comparison to use of an f/l.2 optical speed. When the lens 56 has an f/l optical speed, the resulting image information from the detector 31 has more contrast than it would if the lens 56 had an f/l.2 optical speed.

After passing through the lens 56, the converging radiation passes through an aperture stop 62, and then through a substantially flat diffractive field lens 64, which is made of a polymer material, and which has a diffractive surface pattern on one side. The diffractive surface patterns on the lenses 56 and 64 facilitate color correction. The lens system 26 is of a type disclosed in U.S. Patent No. 5,973,827. The specific prescription information for the lens system 26 of the disclosed embodiment is set forth in Table 1.

TABLE 1 - PRESCRIPTION FOR LENS SYSTEM 26

Parameter Lens 56 Lens 64

Radii : Rl 1.02740" Flat

R2 1.40648" Flat

Aspheric Coefficients: A4 Rl 0 0

A6 Rl 0 0

A8 Rl 0 0

A10 Rl 0 0

A4 R2 0.0342740 0

A6 R2 0.0110210 0

A8 R2 -0.0013182 0

A10 R2 0.0048045 0

Diffractive Coefficients: Cl Rl 0 0

C2 Rl 0 0

C3 Rl 0 0

Cl R2 0.012249 0.05157

C2 R2 0 -1.04960

C3 R2 0 -2.22110

Thickness : 0.320" 0.002"

Material : Chalcogenide Polymer Glass 1173

Refractive Index: 2.6 1.5

FIGURE 4 is a diagrammatic view of the detector 31. As discussed above, the detector 31 includes a two- dimensional array of detector elements which each correspond to a respective pixel of a detected image. In more detail, and as indicated in FIGURE 4, the detector 31 is an array of 320 detector elements by 240 detector elements. Each detector element has a size of approximately 48.5 microns by 48.5 microns. As discussed above, the disclosed embodiment effectively uses only a portion of the detector elements of the detector 31, as indicated diagrammatically by a broken line 71. The portion 71 of the detector 31 represents 25% of the detector elements, or in other word an array of 160 by 120 detector elements. The portion 71 shown in FIGURE 4 happens to be located in one corner of the detector 31, but it could be any portion of the detector 31. For example, it might be a portion of equal size which is located in the center of the detector 31. The camera unit 12 processes and outputs the information obtained from all of the 160 by 120 detector elements located in the portion 71 of the detector 31. However, in the disclosed embodiment, the particular display unit 16 discards some of this information. More specifically, and as discussed above, the display unit 16 has a horizontal to vertical aspect ratio of approximately 3. Consequently, while the display unit 16 uses a full 160 pixels of information from detector 31 in the horizontal direction, it uses only 53 pixels in the vertical direction, as indicated diagrammatically by dotted lines in FIGURE 4. A further consideration is that, since the circuitry 32 receives images with fewer pixels, the processing load is reduced, and thus less circuitry and/or slower circuitry can be used, which reduces the cost of the circuitry 32.

As evident from the foregoing discussion, the disclosed embodiment uses only 25% of the detector elements which are present in the detector 31. This is because the detector 31 is an existing part which is readily available. The present invention may alternatively be implemented with a similar but smaller detector, which has only 120 pixels by 160 pixels, and which corresponds functionally to the portion 71 of the detector 31. It will be recognized that, with respect to the manufacture of detectors, the number of detectors of size 71 which can be obtained from a given semiconductor wafer will be substantially greater than the number of detectors which could be obtained from the same wafer and have the size of the detector 31. The cost to process a given wafer will be approximately the same in either case, and thus a detector of the size 17 can be produced at a substantially lower cost than a detector having the size of detector 31. Consequently, above and beyond other cost reductions provided by the present invention, a further cost reduction will be realized through use of a detector having the size 71, rather than the larger size represented by detector 31.

As improvements are made in the technology for fabricating detectors, it will soon become possible to fabricate a detector in which the detector elements are smaller, for example on the order of 25 microns by 25 microns. In such a detector, a 120 by 160 array of detector elements would have a size which is only 25% of the size of an equivalent detector made using current fabrication technologies. With reference to FIGURE 3, this will allow the diameter of lens 56, the EFL of lens 56, the diameter of aperture stop 62 , and the diameter of lens 64 to each be reduced by a factor of approximately 2, which in three dimensions will reduce the overall volume of the lens system 26 by a factor of approximately 8. This represents a further reduction in size, weight and cost, which is within the scope of the present invention.

One feature of the present invention is the recognition that the size, weight and cost of a camera unit for a night vision system can be substantially reduced in comparison to preexisting camera units, without any change to the FOV, magnification, or effective level of resolution provided by the display unit 16. According to the invention, this can be achieved by using an array of detector elements in the camera unit which has a substantially smaller number of detector elements than has been thought necessary in preexisting camera units. In this regard, and assuming for purposes of discussion that the display unit 16 has an ideal ability to reproduce the resolution of an image received from the camera unit, there are limits on the ability of an infrared detection system and its operator to detect an object which is far away, or to recognize details of such an object. Further, the ability to detect an object and recognize detail may vary, depending on a variety of factors. As a result, there are industry standards which are used to measure the capability of an infrared detection system and its operator to detect an object at a distance, and the capability to recognize detail regarding the object.

One such industry standard involves the determination of a minimum resolvable temperature (MRT) and a minimum detectable temperature (MDT) , which each represent an average for several observers under a given set of ambient conditions. Since they include assessment by human observers of an image from an infrared detection system, when they are applied in the context of a night vision system, they take into account the limitations of the eye of a driver to resolve detail. These standards are discussed with reference to FIGURE 5, which is a diagrammatic top view of the camera unit 12, a standard target 86 used to measure recognition capability based on the MRT standard, and a standard target 87 used to measure detection capability based on the MDT standard. In practice, the targets 86 and 87 would actually each be oriented to extend perpendicular to the plane of FIGURE 5, so that each would face the camera unit 12. However, since FIGURE 5 is a diagrammatic view, they are oriented to be parallel to the plane of FIGURE 5 so they are clearly visible for purposes of the explanation which follows.

More specifically, the target 86 is square, and includes four black bars and three white bars of equal width which are parallel and interleaved. If the distance 91 to the target 86 is not too large, an operator of the system containing the camera unit 12 will be able to not only detect that the target 86 is present, but to readily distinguish the vertical bars from each other. If the target 86 is then moved progressively away from the viewpoint 89, so as to increase the distance 91, it will eventually reach a position where it is possible to detect the fact that the target 86 is present, but not to recognize details such as the presence of the black and white bars. This position represents the limit of recognition capability, as opposed to detection capability.

If the target is then moved progressively further away from the viewpoint 89, there will come a point at which the camera unit 12 can no longer detect that the target is even present. In order to measure this detection capability, as opposed to recognition capability, it is common in the industry to use a different type of target, such as that shown at 87. The target 87 has the same size as the target 86 and is also a square target, but has a square black outer border, and a square white center. As the target 87 is moved progressively further from the viewpoint 89, so as to increase the distance 92, there will come a point at which it is no longer possible for the operator to detect the presence of the target 87. According to the MDT industry standard, this is the limit of detection capability.

For purposes of the MRT and MDT standards, the position and size of a target relative to a viewpoint are not expressed in terms of the actual physical distance to the target, but in terms of the angle subtended by the target or a portion of it. For example, in FIGURE 5, the MDT target 87 subtends an angle 96. The target 86 is of equal size, but is closer to the viewpoint 89, and therefore subtends a larger angle 97. In the specific case of the MRT target 86, the position of the target 86 is normally expressed in terms of cycles per milliradian, where one cycle 98 is the combined width of one black bar and one white bar. The target 86 thus has a cumulative width of 3.5 cycles. If the angle 97 is X milliradians, then in FIGURE 5 the position of the target 86 would be expressed as 3.5/X cycles per milliradian.

Assume that the MRT target 86 in FIGURE 5 is at a position where, for purposes of the ambient conditions typical for night driving, it is at the maximum distance from the viewpoint 89 for which recognition is possible according to the MRT standard. In other words, if the target 86 was moved further away from the viewpoint 89, it would still be possible to detect that the target 86 is present, but it would not be possible to recognize details such as the presence of the black and white bars. Also assume that the camera unit 12 is at the viewpoint 89. In pre-existing night vision systems, it was assumed that each bar of the MRT target had to be imaged onto two or more pixels of the detector in the camera unit (in a direction along a line extending perpendicular to the bars of the target) , in order to avoid a loss of resolution. One feature of the present invention is the recognition that the pre-existing approach represents overdesign, because each bar of the target does not need to be imaged onto two or more pixels in order to have suitable resolution for purposes of achieving detection in a night vision system

More specifically, as a lower limit, it is sufficient if each of the black and white bars of the target are imaged directly onto one pixel element in a direction along a line extending perpendicular to the bars. This limit is referred to as the Nyquist spatial frequency for the camera unit 12. One feature of the present invention is that the camera unit 12 is designed so that the Nyquist spatial frequency has a relationship to the limits of MRT recognition which avoids use of the detector in a manner providing an excessive level of resolution that effectively increases the size, weight and cost of the camera unit. In this regard, the present invention provides a night vision system capable of providing MRT recognition of a multi-bar target having a Nyquist spatial frequency which is less than approximately 0.63 cycles per milliradian, and preferably less than 0.46 cycles per milliradian. In the disclosed embodiment, the Nyquist spatial frequency is approximately 0.38 cycles per milliradian.

FIGURES 6 and 7 show a different way of expressing the same basic relationship. FIGURE 6 is a diagrammatic view of the camera 12 at the viewpoint 89, while observing a remote scene 21. Reference numeral 102 designates a very small portion of the scene 102, which will be imaged onto a single detector element of the detector 31 within camera unit 12. This portion of the scene subtends an angle 106 with respect to the viewpoint 89. Stated differently, the angle 106, in milliradians, defines the portion of any scene which will be imaged onto a single detector element of the particular camera unit 12.

In this regard, FIGURE 7 is an enlarged view of the portion of FIGURE 6 which includes the camera unit 12. The chopper 27 and circuit 32 of the camera unit 12 have been omitted in FIGURE 7 for clarity. FIGURE 7 shows that the angle 106 represents the portion of any scene which, after optical processing by the lens system 26, is imaged onto a single respective detector element 111 of the detector 31. According to the invention, the portion of a scene imaged onto a single detector element has a size in the range of approximately 0.8 to 3.5 milliradians, and preferably in the range of 1.1 to 1.5 milliradians. In the disclosed embodiment, the camera unit 12 images approximately 1.3 milliradians of the scene onto each detector element 111.

The present invention provides a number of technical advantages. The physical volume of the lens system for the camera unit is reduced by a factor of approximately eight in comparison to preexisting systems, which results in a substantial reduction in the overall size and weight of the camera unit. A related advantage is that the size of the objective lens is substantially reduced. This reduces the amount of expensive material such as Chalcogenide glass which is required to fabricate the objective lens, and thus reduces the cost of the lens. Moreover, as the size of the lens is reduced, the effective yield from the lens manufacturing process is increased, which effects a further reduction in the cost of the lens. Still another reduction in size and cost is available from the fact that a smaller detector unit can be used, in particular a detector unit which has a substantially smaller number of detector elements than was used in preexisting night vision systems . This reduces the amount of costly infrared sensitive material required to make the detector unit, while increasing the number of detector units obtained from a given wafer for the approximately fixed cost of processing the wafer, and also increasing the effective yield of parts from each wafer. Moreover, since the size of the lens system is substantially reduced, it becomes more practical to use an improved optical speed for the lens, which in turn increases the performance of the system. Taken together, these advantages provide a substantially reduced cost, weight and size for the camera unit of a night vision system, with the option of improved performance, all of which are of critical importance in highly competitive commercial markets, such as the consumer automotive market. In addition, reducing the size and weight of the camera unit permits it to be more efficiently and less noticeably integrated into a front portion of a vehicle, such as the grill or a bumper. Although the one embodiment has been illustrated and described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the scope of the invention. For example, one possible optical configuration for the lens system has been illustrated and described in detail, but it will be recognized that there are variations and modification to this optical configuration which are within the scope of the present invention. In addition, the disclosed embodiment includes a chopper, but it will be recognized that there are variations of the present invention in which the chopper could be omitted.

Further, and as discussed above, the disclosed embodiment uses only a portion of the detector elements in the infrared detector, but the present invention encompasses use of a smaller detector having a number of detector elements which corresponds to the number of detector elements actually utilized by the disclosed embodiment. In addition, although the foregoing explanation of the disclosed embodiment sets forth the specific number of detector elements which are utilized, it will be recognized that the specific number of detector elements can be varied to some extent to accommodate various factors, including but not limited to a change in the effective field of view for the display unit. Also, although the disclosed embodiment uses a head-up display, it will be recognized that some other typ of display could be used, for example display that has a cathode ray tube (CRT) or LCD which is directly viewed by the driver. Other substitutions and alterations are also possible without departing from the spirit and scope of the present invention, as defined by the following claims.

Claims

WHAT IS CLAIMED IS:

1. An apparatus comprising a night vision system, including : a detector which includes an array of detector elements; and an optical section which is operable to form on said detector an image of information from a scene, said optical section imaging onto each said detector element incoming energy which corresponds to a portion of the scene having a size m the range of approximately 0.8 to 3.5 milliradians .

2. An apparatus according to Claim 1, wherein said incoming energy imaged by said optical section onto said each said detector element has a size m the range of approximately 1.1 to 1.5 milliradians.

3. An apparatus according to Claim 2, wherein said energy imaged by said optical section onto each said detector element has a size of approximately 1.3 milliradians .

4. An apparatus according to Claim 1, including a display section operable m response to information from said detector for displaying a visible image of the scene, said visible image having a horizontal field of view which is greater than approximately 6°.

5. An apparatus according to Claim 4, including a vehicle having said night vision system installed therein, and wherein said display section is a head-up display.

6. An apparatus comprising a night vision system, including : a detector which includes an array of detector elements ; and an optical section which is operable to form on said detector an image of information from a scene, which provides a field of view less than approximately 14°, and which has an effective focal length less than approximately 2.25 inches.

7. An apparatus according to Claim 6, wherein said effective focal length of said optical section is less than approximately 1.9".

8. An apparatus according Claim 7, wherein said effective focal length of said optical section is approximately 1.5".

9. An apparatus according to Claim 6, including a vehicle having said night vision system installed therein, and wherein said night vision system includes a display section operable in response to information from said detector for displaying a visible image of the scene, said display section being a head-up display.

10. An apparatus comprising a night vision system capable of providing MRT recognition of a multi -bar target having a Nyquist spatial frequency which is less than a selected Nyquist spatial frequency, and incapable of providing MRT recognition of a multi -bar target having a Nyquist spatial frequency which is greater than said selected Nyquist spatial frequency, said selected Nyquist spatial frequency being less than approximately 0.63 cycles per milliradian, and said night vision system including: a detector which includes an array of detector elemen s ; an optical section which is operable to form on said detector an image of information from a scene; and a display section operable in response to information from said detector for displaying a visible image of the scene .

11. An apparatus according to Claim 10, wherein said selected Nyquist spatial frequency is less than approximately 0.46 cycles per milliradian.

12. An apparatus according to Claim 11, wherein said selected Nyquist spatial frequency is approximately 0.38 cycles per milliradian.

13. An apparatus according to Claim 10, including a vehicle having said night vision system installed therein, wherein said display section is a head-up display and has a horizontal field of view which is less than approximately 14°.

14. A method of operating a night vision system which includes a detector having an array of detector elements, comprising the step of forming on the detector an image of information from a scene m a manner so that each of the detector elements receives incoming energy which corresponds to a portion of the scene having a size m the range of approximately 0.8 to 3.5 milliradians.

15. A method according to Claim 14, wherein said forming step includes the step of directing onto each of the detector elements incoming energy which corresponds to a portion of the scene having a size m the range of approximately 1.1 to 1.5 milliradians.

16. A method according to Claim 15, wherein said forming step includes the step of directing onto each of the detector elements incoming energy which corresponds to a portion of the scene having a size of approximately 1.3 milliradians .

17. A method of operating a night vision system which includes a detector having an array of detector elements, an optical section which forms on the detector an image of information from a scene, and a display responsive to information produced by the detector for displaying a visible image of the scene, said method comprising the steps of : operating said system in a manner so that said system is capable of effecting MRT recognition of a multi-bar target having a Nyquist spatial frequency which is less than a selected Nyquist spatial frequency, and incapable of effecting MRT recognition of a multi-bar target having a Nyquist spatial frequency which is greater than said selected Nyquist spatial frequency; and selecting as said selected Nyquist spatial frequency a Nyquist spatial frequency which is less than approximately 0.63 cycles per milliradian.

18. A method according to Claim 17, wherein said selecting step is carried out by selecting as said selected

Nyquist spatial frequency a Nyquist spatial frequency which is less than approximately 0.63 cycles per milliradian.

19. A method according to Claim 17, wherein said selecting step is carried out by selecting as said selected

Nyquist spatial frequency a Nyquist spatial frequency which is approximately 0.38 cycles per milliradian.