WO1995006388A1

WO1995006388A1 - Life extender and bright light protection for cctv camera system with image intensifier

Info

Publication number: WO1995006388A1
Application number: PCT/US1994/009190
Authority: WO
Inventors: Allan W. Scott; Norman H. Pond
Original assignee: Intevac, Inc.
Priority date: 1993-08-20
Filing date: 1994-08-12
Publication date: 1995-03-02
Also published as: AU7687994A

Abstract

A camera system for operating in nighttime lighting conditions without additional illumination includes an input lens (60) for receiving incoming light representative of an image, an image intensifier tube (61) for receiving the image from the input lens and producing an intensified image and a video camera (64) for receiving the intensified image. The life of the intensifier tube can be extended by automatically controlling its gain. The gain is sensed in an unused region of the intensifier tube, and the voltage applied to the intensifier tube is controlled to provide a desired gain. In addition, the intensifier tube can be gated on and off with a prescribed duty cycle to extend its life. A frame is stored when the intensifier tube is gated on, and the stored frame is repeated when the intensifier tube is gated off to provide a continuous series of images. The intensifier tube can be protected against bright light by turning it off for a prescribed time when the video signal exceeds a reference level that corresponds to a bright light.

Description

LIFE EXTENDER AND BRIGHT LIGHT PROTECTION FOR CCTV CAMERA SYSTEM WITH IMAGE INTENSIFIER

Field of the Invention

The present invention relates generally to surveillance systems, such as closed-circuit security or industrial inspection television systems and, more particularly, to techniques for extending the operating life of night vision camera systems which utilize an image intensifier tube.

Background of the Invention Image intensified television cameras suitable for use in closed circuit television (CCTV) systems are of two basic types. One involves the use of image orthicon television cameras utilizing lenses to focus the image onto the camera. To achieve better low-light sensitivity, such cameras are often supplemented by infrared illuminators. The second type consists of a lens to focus light onto the photocathode of a night-vision image intensifier tube, the image intensifier tube, and a direct coupled CCD device (charge coupled device) television camera or a transfer lens to focus the output of the intensified image onto the CCD input of the video camera. At the heart of such a CCTV system is the light amplifier image intensifier tube (usually referred to as a generation or Gen I, II or III type device).

Image intensifier tubes (also callec image enhancement tubes or simply image tubes) were first developed in the mid to late 1930's for military night vision applications. The early electro-optical low-light amplifiers were image converter infrared tubes, also known as Gen 0 and Gen I night amplifier tubes. These were used successfully for many years. A successor to these tubes was the microchannel intensifier. It was a great improvement in size, cost and performance. A microchannel intensifier tube basically consists of a photo-sensitive cathode, a microchannel plate (MCP), a phosphor output screen and means to create appropriate fields within the tube. The, photocathode converts incoming photons representing an image to a corresponding spatially positioned stream of electrons. The electrons are accelerated to an MCP which intensifies the flow of electrons. At the output of the MCP, the intensified electrons are accelerated again by another strong electric field to strike the luminescent phosphor screen on which an enhanced visible image is created. The MCP consists of a two-dimensional array of miniature microchannel multipliers. A description of microchannel image intensifiers and the fabrication of microchannel plates can be found in "The Microchannel Image Intensifier," The Scientific American. Vol. 245 (November 1981 ) pp. 46-55 by Michael Lampton.

Microchannel image intensifiers are frequently employed today in applications requiring high amplification of extremely low light levels. One obvious advantage of the current generation of microchannel image intensifiers is their light sensitivity which obviates the need for auxiliary irradiation either in the visible or near-infrared spectrum. They are particularly suited to nighttime surveillance in military or police applications, since they have high luminous gain, high image resolution and excellent light sensitivity. In addition, Gen III tubes are particularly sensitive in the near-infrared (NIR) spectrum, which makes them particularly useful in nighttime surveillance since night sky radiation is particularly high in the non-visible NIR region.

Image intensifier tubes and cameras have improved over the years, and today there are probably 50 different low light level camera systems made by a dozen or more manufacturers. Notwithstanding this, there are drawbacks that limit use of modern image intensifier tubes, even in the case of the latest generation units. Although relatively inexpensive when compared to earlier systems, there are many surveillance and security applications where the marketplace cannot accept the $9,000 to $20,000 price of the individual image intensifier

TV cameras. In addition to the initial acquisition cost, the annualized replacement costs of image intensifier tubes or SIT camera tubes (silicon intensifier target) are inordinately high due to their restricted shelf life and even shorter operating lifetime. The operating lifetime of these tubes is dramatically reduced by exposure to high light levels.

Also applications requiring continuous (24 hours/day) operation can shorten useful lifetime considerably.

Gating of the high-voltage power supply has been used to limit the on time of the intensifier tube in certain applications, but there are other tradeoffs with such modifications. In particular, fast gating has been used during a portion of each frame to control the light level and to augment the function of the auto iris lens used in such systems. Gating has also been used in strobe applications to "freeze" the movement of the subject being observed. However, these techniques require switching the intensifier power supply at speeds on the order of a microsecond and are expensive.

As indicated above, the operating life of an image intensifier tube may be reduced significantly by exposure to high light levels. Even at lower light levels, the gain of an image intensifier tube decreases gradually unϊil it reaches a level where the tube is no longer considered usable. In addition, a portion of an image intensifier tube may be damaged, or desensitized, when a bright light in an otherwise low light level scene is focused on that portion of the tube. It is desirable to provide night vision camera systems which have a long operating life, which have relatively stable gain and which are protected against bright lights.

Summary of the Invention

In accordance with one aspect of the present invention a camera system for operating in nighttime lighting conditions without additional illumination is provided. The camera system comprises an input lens for receiving incoming light representative of an image, an image intensifier tube for receiving the image from the input lens and producing an intensified image, a video camera for receiving the intensified image, a sensor for sensing the gain of the image intensifier tube, and a controller responsive to the sensor for controlling the gain of the image intensifier tube within a desired range.

In a preferred embodiment, the sensor comprises a light source for illuminating an unused region of the image intensifier tube and a light sensor for sensing an output of the image intensifier tube caused by the light source in the unused region. Preferably, the controller comprises means for controlling an operating voltage applied to the image intensifier tube. Typically, the voltage applied to the image intensifier tube is increased over time to maintain a substantially constant gain.

According to another aspect of the invention, a camera system for operation in nighttime lighting conditions without additional illumination comprises an input lens for receiving incoming light representative of an image, an image intensifier tube for receiving the image from the input lens and producing an intensified image, a video camera for receiving the intensified image, means for gating the image intensifier tube on and off so that the image intensifier tube is on less than full time during normal nighttime operation, and means for storing a frame representative of the intensified image when the image intensifier tube is gated on ar^J for repeating the stored frame when the image intensifier tube is gated off to provide a continuous sequence of frames.

Preferably, the image intensifier tube is gated on and off by the gating means with a prescribed duty cycle of 10% or less. In a preferred embodiment, the image intensifier tube has an on-time sufficient for obtaining one frame per second. Typically, the input lens comprises an auto iris lens, and the system preferably includes control means for controlling the auto iris lens to prevent further opening thereof when the image intensifier tube is gated off.

According to another feature of the invention, the camera system may include means for detecting motion in the image and means responsive to detection of motion in the image for controlling the gating means such that the duty cycle is increased to or near 100%. Thus, information is not lost when motion is detected.

According to a further feature of the invention, the camera system includes a light sensor for sensing an ambient light level, means responsive to the light sensor for increasing the duty cycle at low ambient light levels to provide multiple frames, and means for integrating the multiple frames to reduce noise. Under very low light level conditions, noise can be reduced by integrating multiple frames. Conversely, at high ambient light levels, the duty cycle can be decreased in order to protect the image intensifier tube and further extend its life. According to a further aspect of the invention, a camera sy _;m for operating in nighttime lighting conditions without additional illumination comprises an input lens for receiving incoming light representative of an image, an image intensifier tube for receiving the image from the input lens and producing an intensified image, a video camera for receiving the intensified image, means for comparing the amplitude of the video signal from the camera with a reference level that corresponds to a bright light, means for gating the image intensifier tube on for a prescribed time when the amplitude of the video signal exceeds the reference level to protect the image intensifier tube against bright light, and means for storing a frame representative of the intensified image when the image intensifier tube is gated on and for repeating the stored frame when the image intensifier tube is gated off to provide a continuous sequence of images. The above techniques can be applied to any night vision CCTV system which utilizes an image intensifier tube. Thus, the techniques can be applied to image intensified CCTV cameras and to camera systems which have daytime and nighttime operating modes, wherein the image intensifier tube is not utilized in the daytime mode. The techniques disclosed and claimed herein are utilized primarily during nighttime operation.

Brief Description of the Drawings

For a better understanding of the present invention, reference is made to the accompanying drawings, which are incorporated herein by reference and in which:

FIG. 1 is an enlarged cross-sectional representation of an image intensifier tube;

FIG. 2 is a block diagram representation of an intensified video camera system;

FIG. 3 is a cross-sectional view of a day/night camera system; FIG. 4 is a schematic block diagram of a system for controlling the gain of an image intensifier tube;

FIG. 5 is a block diagram of an image intensified camera system including a circuit for gating the image intensifier tube on and off; and FIG. 6 is a block diagram of the video processing circuit shown in FIG. 5.

Detailed Description A cross section of an image intensifier tube 10 is shown in FIG.

1. The intensifier tube 10 includes an input glass window 11 , a photocathode 12 bonded to the surface of the input window, a microchannel plate 13 spaced apart from the photocathode, and a phosphor screen 14 bonded to a glass output window 15 on the inner surface adjacent to the microchannel plate 13. Glass windows 11 and

15 also act as faceplates to the tubular housing 16 sealing the interior components 12, 13 and 14 in a vacuum. Housing 16 is preferably a solid ceramic body, although glass or other insulator materials could be used. Photocathode 12 may optionally be deposited onto the surface of input window 11. Greater detail regarding this intensifier tube may be found in a copending, commonly owned application in the names of Johnson, Scott and Bartz assigned Serial No. 08/063,234, filed May 18, 1993 and entitled "A Microchannel Image Intensifier Tube", which application is hereby incorporated by reference. All of the numerical dimensions and values that follow should be taken as nominal values rather than absolutes or as a limitation on the scope of the invention. These nominal values are examples only. Many variations in size, shape and types of materials may be used, as will readily be appreciated by one skilled in the art, as successfully as the values, dimensions and types of materials specifically set forth hereinafter. In this regard, where ranges are provided, these should only be understood as guides to the practice of this invention.

FIG. 2 shows a block diagram of a video camera system 50 suitable for use in a low cost CCTV system capable of operating under all environmental lighting conditions without additional external illumination. Operationally, light 59 representative of an image enters the camera system 50 through an automatic-iris camera objective lens 60. The auto iris lens 60 controls the amount of light allowed to pass to an image intensifier tube 61 , which may, for example, comprise the tube shown in FIG. 1. Power supply 62 is a high-voltage power supply for supplying the proper voltages to intensifier tube 61. In a preferred embodiment, the voltage requirements are for fixed voltages applied to various points in the tube as previously described. The output light is optically focused by a relay lens 63 on the input of a CCD camera 64. A feedback signal, proportional to the intensity of the light into camera

64, is applied through lead 65 to control the auto iris lens 60. A video output signal on lead 70 may be supplied to a CCTV monitor 72 for viewing of the received image.

A cross-sectional view of a day/night intensified camera system for operating in daytime and nighttime lighting conditions without additional illumination is shown in Fig. 3. An input auto iris lens 60 screwed on at input 58 and a CCD camera 64 screwed onto the threaded opening 74 are mounted on a housing 80 in optical alignment along an optical axis 57. Image intensifier tube 61 and an optical path length compensator (OPLC) 67 are mounted on a rotatable disk 101 within housing 80. Disk 101 is rotated such that the image intensifier tube 61 is positioned on optical axis 57 in the nighttime mode and such that OPLC 67 is positioned on optical axis 57 in the daytime mode. The OPLC 67 compensates for the optical gap left when the image intensifier tube 61 is rotated out of the optical axis 57. Relay lens 63 transfers the intensified image from the output screen of image intensifier tube 61 to the image sensor of CCD camera 64. A motor 68 provides the drive mechanism to move gear teeth 66, gear train assembly 69 and thus disk 101 about a rotatable shaft 99. A photosensor can be used to sense the ambient light level. When the sensed ambient light level indicates daytime operation, the OPLC 67 is rotated into the optical axis 57, and the image received by input auto iris lens 60 is transferred through OPLC 67 to camera 64. When the sensed ambient light level is too low for operation in the daytime mode, the image intensifier tube 61 is rotated into optical axis

57, and an intensified image is transferred to camera 64.

In a second embodiment, the OPLC 67 is replaced with a second relay lens. The relay lens associated with the image intensifier tube is mounted on the rotatable disk 101 and is rotated with the image intensifier tube.

In a third embodiment, a CCTV camera having an image sensor assembly that is separate from a camera electronics assembly is utilized. The image sensor assembly is connected to the electronics assembly by a ribbon cable and is easily movable. The image sensor assembly is positioned behind an image intensifier assembly in the nighttime mode and is moved along the optical axis directly behind the input lens in the daytime mode, thus eliminating the requirement for an OPLC.

In a fourth embodiment, a color CCTV camera is used in the daytime mode, and a black and white CCTV camera is used in the nighttime mode. Each camera has an image sensor assembly that is connected by a cable to its respective electronics assembly. The black and white image sensor assembly is located behind and in optical alignment with an image intensifier assembly, both of which are mounted on a rotatable disk. The color image sensor assembly is mounted on the rotatable disk to receive an image directly from the input lens in the daytime mode.

In a fifth embodiment, a single camera electronics assembly is used with two separate image sensor assemblies. The first image sensor assembly is mounted on a rotatable disk and is positioned directly behind the input lens in the daytime mode. The second image sensor assembly is located behind and in optical alignment with an image intensifier assembly, both of which are mounted on the rotatable disk. In the nighttime mode, the image intensifier assembly and the second image sensor assembly are rotated into the optical axis of the input lens. The first and second image sensor assemblies are switched to the electronics assembly in daytime and nighttime modes, respectively.

The operating life of a typical night vision image intensifier tube depends on the level of illumination on the intensifier tube faceplate. The relationship between faceplate illumination and life is given approximately by the relationship: illumination x life = one foot candle-hour For example, when the illumination on the intensifier faceplate is 10-4 foot candle, which is the illumination that the intensifier would receive on a moonlit night or when viewing a parking lot having artificial illumination, the intensifier life is approximately 10,000 hours. Since one year of operating life is approximately 8500 hours, the intensifier would last about one year when operated at 10-4 foot candle. During the operating life, the gain of the intensifier tube slowly decreases and by the end of the one year of operation, the gain will decrease to about 30% of the original gain. If the intensifier tube and CCTV camera are originally set up to provide a 100% video signal, then after one year of operation, the video signal will drop to about 30% of its original value. The resolution of the picture is degraded, the picture becomes noisier and the sensitivity of the intensified camera drops by an order of magnitude.

A similar decrease in gain will occur after one hour if the intensifier tube is operated at a one foot candle illumination level, which is equivalent to a bright light shining on the intensifier faceplate. In this case, after one hour, the gain will drop from its original value to a gain of 30% of the original value.

In the camera systems described above, the image intensifier tube is moved out of the optical path and is turned off during the daytime mode. Thus, the operating life is approximately doubled. In fact, the life may be increased by more than two times because of the gain recovery that occurs in intensifier tubes when they are turned off for a time period, such as 12 hours, before they are turned back on. Even taking this effect into account, the lifetime of an intensified camera, when it is operated only at night and is turned off during the day, is only about two years. During this two year time period, the performance continually deteriorates.

As the intensifier tube ages, as described above, the gain decreases. However, the gain can be restored by increasing the voltage across the microchannel plate (MCP). The gain of an image intensifier tube is typically adjusted to about 80,000, which is a measure of the output screen brightness compared to the input faceplate illumination. The amount of light from the output screen that reaches the CCD image sensor is controlled by the aperture of the relay lens and is typically reduced by a factor of about 160, so that the overall gain of the image intensifier tube and relay lens combination is about 500. When the intensifier tube is new, its gain can be increased by as much as four times this value by increasing the voltage across the MCP. Although this voltage change increases gain, it also adds to the noise of the tube, and a picture with scintillations is obtains . Therefore, the initial gain of the intensifier must be limited such that the scintillations are acceptable.

As the intensifier tube ages, the photo response of the cathode decreases, and consequently the gain of the intensifier tube decreases. The gain can be restored to its original value by increasing the voltage across the MCP. The increased gain with the low photo response of the aging cathode does not cause serious scintillation problems.

A circuit for automatically adjusting the gain of the intensifier tube, so that the gain is held substantially constant during the useful life of the tube and which extends the useful life by a factor by about 2 or 3, is shown in FIG. 4. The image from the intensifier tube 61 is focused onto a rectangular sensing element of the CCTV camera 64 (FIG. 3). Since the image intensifier tube screen is typically circular and the CCD sensor in the camera 64 is typically rectangular, there is a region at the top and the bottom of the intensifier output screen that is not imaged onto the CCD sensor. In this unused area, an optical gain measuring circuit is mounted. The gain measuring circuit does not interfere with the normal operation of the image intensifier tube. The optically measured gain is used to adjust the MCP voltage so that constant optical gain is obtained.

A reference signal is generated by a light-emitting diode (LED) 110. The light from the LED 110, which is at a relatively high intensity level, is attenuated by passing it through a neutral density filter 112 and then through an optical fiber 114 to the cathode faceplate of the intensifier tube 61. The neutral density filter 112 may, for example, have an attenuation of 10-4. The low light level optical signal, typically at 10-4 foot candle, is amplified by the intensifier tube 61 and is detected at the output faceplate by a photodetector 120. The output light can be transmitted to the photodetector 120 by an optical fiber 122. The photodetector 120 can, for example, be a photodiode.

The output signal from the photodiode is proportional to the gain of the intensifier tube 61 , since the intensifier tube is being illuminated in this area by the constant light level from the LED 110. The signal from photodetector 120 can be supplied to an operational amplifier 126 having adjustable gain. The output of the operational amplifier 126 is used to control the MCP voltage by controlling intensifier power supply 62. Typically, the voltage supplied by power supply 62 to intensifier tube 61 is increased as the intensifier tube ages, in order to automatically maintain a substantially constant gain. Typically, gain control within 10% is sufficient for most applications. The LED 110 and the photodetector 120 can be mounted directly to the intensifier tube input and output faceplates, respectively, or can be coupled through optical fibers 114 and 122 as shown in FIG. 4.

A great improvement in lifetime can be obtained by switching, or gating, the intensifier tube on for only a fraction of full time during the nighttime mode. For example, when the intensifier tube is gated on for only 1/10th of the time (a 10% duty cycle), the life of the intensifier tube is extended to over ten years. This technique can be used separately or in combination with controlling the gain of the intensifier tube with an automatic gain circuit and/or turning the intensifier tube off in the daytime mode.

The standard video signal for either color or black and white video provides 30 frames per second in the U.S.. This is the standard for commercial broadcast television. The human eye needs to see more than 40 pictures a second to avoid sensing individual pictures rather than continuous motion. The standard broadcast TV signal uses 60 half frames per second. Each half frame contains half of the 525 vertical lines, and the half frames are interlaced together to provide a full frame. In this way, the resolution that can be obtained using 525 lines is achieved, but the eye thinks it is seeing 60 pictures per second.

However, there are only 30 complete frames per second. Thus, 1/30th of a second is required for a complete video frame. In other countries, the standard video signal is 25 frames per second and 50 fields per second. Thirty frames a second provides much more information than is normally required for security surveillance. The scene that is being viewed in video surveillance typically changes very little in 1/30th of a second. Normally, the security system records the scene at a rate of one picture per second on a time lapse recorder to save recording space and to reduce reviewing time when the tape is viewed.

In view of the above, adequate information is provided for almost all security applications if the intensifier tube is turned on for only one frame (1/30th of a second) during each second of operation and then is gated off for the remaining 29 frames. This increases the life of the intensifier tube by up to 30 times, since the intensifier tube is on for only 1/30 of the time, thus providing a major increase in intensifier life.

If the image intensifier were simply gated on for one frame per second, the viewing monitor would see a flickering picture, since only one frame would appear at the monitor per second. Furthermore, the time lapse recorder could sample the video signal at a time when the intensifier tube was gated off and thus would not record the scene.

A block diagram of an intensified CCTV camera which gates the intensifier tube on and off and which overcomes the above drawbacks is shown in Fig. 5. Common elements in Figs. 2 and 5 have the same reference numerals. The video output of CCTV camera 64 is applied to a video processing circuit 150. The video processing circuit supplies a gating signal to intensifier power supply 62 for gating the power supply and the image intensifier tube 61 on and off with a prescribed duty cycle. In addition, the video processing circuit 150 includes a video frame memory to ensure that continuous video is supplied to the monitor 72 and to a time lapse recorder 152.

The duty cycle, for example, may be one frame per second, or about 3%, as described above. It will be understood that a gating duty cycle of one frame per second is given by way of example only and that other duty cycles can be used within the scope of the present invention. Lower duty cycles reduce the time that intensifier tube 61 is gated on and therefore extend the life of the tube. Usually, duty cycles of 10% or less are preferred. However, any duty cycle less than 100% is included within the scope of the invention.

A block diagram of the video processing circuit is shown in Fig. 6. A microprocessor 160 generates control signals that are synchronized to the frame generation of the CCTV camera 64, in response to a sync signal received from the camera 64. A video frame memory 162 digitizes and stores one frame of video information from the CCTV camera 64 when the intensifier power supply 62 is gated on. The video frame memory 162 also generates a standard analog video signal from the stored digital signal and repeats the analog version of the stored frame for the remainder of the one second interval while the image intensifier tube 61 is gated off. The stored frame video is supplied by the video frame memory 162 to the monitor 72 and to the time lapse recorder 152. The video frame memory can, for example, be a Minigrab video digital frame memory sold by Poynting Products, Inc. of Oak Park, Illinois. The monitor 72 and the time lapse recorder 152 receive the same picture for a full second. This causes no problem for the time lapse recorder, which normally records only one frame per second. This also poses no practical problem for the monitor. The picture is jerky, rather than having continuous movement, but no vital security surveillance information is lost. After one second, the video processing circuit 150 repeats the cycle, obtains a new frame of information and repeats the new frame for the next second.

The microprocessor 160 generates the gating signal for gating the intensifier power supply 62 on and off with a prescribed duty cycle. The gating signal is synchronized to the operation of the CCTV camera 64, so that the gating signal changes states between frames rather than in the middle of a frame. The required switching time for the gating signal is on the order of one millisecond. Furthermore, the microprocessor 160 supplies an override signal to the CCTV camera 64. The override signal prevents the CCTV camera 64 from automatically trying to increase the gain of the camera when the image intensifier tube 61 is gated off and prevents the auto iris lens 60 from being opened by the camera 64 in response to the reduced light level.

In most surveillance situations, the CCTV camera is looking at the same scene for long periods in which there is no change in the scene. When activity occurs that may be of interest from a security standpoint, each stored frame can be compared to the previous frame. Differences between frames indicate change or motion. Techniques for electronic motion detection are well known to those skilled in the art. In accordance with another feature of the invention, when motion is detected, a signal can be supplied to the video processing circuit 150 to return the camera system to its normal scanning rate of 30 frames a second, or to an intermediate duty cycle between 30 frames a second and one frame per second, so that no information is lost. Thus, the camera system operates at a low frame rate during normal periods of inactivity but increases the frame rate when motion or some other change in the scene is detected.

At low light levels, such as 10-5 foot candle, the intensifier tube will operate for 100,000 hours, and gating techniques to extend life are not required. A problem which occurs at these low light levels, which may occur on dark nights without moonlight or parking lot lights, is poor picture quality. The resolution is degraded and the signal-to-noise ratio decreases, so that scintillations occur in the picture. In accordance with a further feature of the invention, gating is stopped under these conditions and the full frame rate is used. However, instead of displaying each of the 30 frames per second, which would allow the scintillation noise to appear on the screen, the digitally stored frames are integrated to cancel noise. By storing only a few frames, the noise can be reduced by about 20 dB. When the noise cancellation feature is used, the camera system can adjust the gating rate as appropriate for the illumination situation. At light levels above 10-4 foot candle, the frame rate may be automatically adjusted to one frame per second. At higher light levels, approaching 10-3 foot candle, the gating rate may be reduced to less than one frame per second, such as a 1% duty cycle. At light levels below 10-4 foot candle, the duty cycle may be increased and may approach 100%. At light levels below 10-4 foot candle, the integrating feature is used to reduce noise. The light level can be sensed from the camera video itself using the same circuitry that controls the camera AGC and the auto iris lens. Alternatively, the light level can be determined from the intensifier tube brightness, either by monitoring the screen current, which is proportional to the output light level, or by monitoring the output light level with a photodiode, as described above in connection with automatic gain control. In this case, only an output light sensor is required.

When the intensified CCTV camera is viewing a nighttime scene, such as a parking lot, the light level is typically about 10-4 foot candle. This is the light level at which an intensifier tube typically works best and provides a clear, noise-free picture with good resolution. The camera 64 and the auto iris lens 60 maintain this ideal light level from this input light level upward. At lower light levels, the auto iris lens remains completely open, and the performance of the intensifier tube degrades as the light level decreases below 10-⁴ foot candle.

When the intensifier tube is looking at a parking lot, the scene from the parking lot is amplified by the intensifier tube and is received by the CCTV image sensor of the camera. The camera control adjusts the auto iris lens to the desired light level based on the average light of the scene. A special problem occurs when a bright light appears in the field of view. A typical example is an automobile headlight. If the automobile drives into the parking lot and parks with its lights on for a few minutes, a bright spot appears in the field of view. The bright spot may have an intensity of 50 foot candles. Since the bright spot appears only within a small portion of the total field of view and does not significantly increase the average light level in the scene, the auto iris lens does not sense this situation and remains open. However, the bright spot, when focused on the cathode of the intensifier tube, burns a spot in the cathode and desensitizes it in that area. If the bright spot is on for a short time, such as 15 minutes, the cathode may heal itself when the bright spot is removed. However, the cathode may take several hours to recover. The worst scenario occurs when the intensified CCTV camera is panned across the field of view, a common procedure with security cameras. Then, a streak is burned across the entire face of the intensifier tube cathode.

The digitizing and storing of the video frame by the frame memory 162 can be used to protect the intensified CCTV camera against bright spots in the field of view. Referring again to FIG. 6, the stored frame video from video frame memory 162 is applied to a comparator 170. The horizontal scan from the single stored frame is compared with a reference level which corresponds to a light of specified brightness in the field of view. When a bright light occurs in the field of view, a large value of the video signal occurs in at least a portion of the horizontal scan. When the video signal exceeds the reference level, the comparator supplies an output signal to microprocessor 160. When the bright light is detected for a specified number of frames, the microprocessor 160 can gate the intensifier power supply 62 off. During the time that the intensifier tube is gated off, the last digitized and stored frame is sent to the monitor 72 and the time lapse recorder 152. After a specified interval, such as one minute, the intensifier tube is turned on to determine if the bright light remains in the field of view. As long as the bright light remains in the field of view, the duty cycle can be maintained at a very low value, such as one frame per minute. For each new stored frame, the video is compared with the reference level and a decision is made as to whether to continue the reduced duty cycle (one frame per minute) or to revert to the normal duty cycle (one frame per second).

If the video level remains above the reference level for an extended time, such as several minutes, the camera system can be switched into the daytime mode so that an image is obtained but the intensifier tube is not used. Comparison of the video signal with the reference level continues, and when the bright light is no longer present, the system returns to the nighttime mode. This technique essentially eliminates high light level damage by keeping the image intensifier tube turned off during high light level conditions.

It will be understood that the techniques disclosed herein for automatic gain control, for gating the intensifier tube on and off and for protecting the intensifier tube against bright lights can be used separately or in combination. By combining these techniques, the life of the intensifier tube can be further extended.

While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

CLAIMS What is claimed is:

1 . A camera system for operation in nighttime lighting conditions without additional illumination, comprising: an input lens for receiving incoming light representative of an image; an image intensifier tube for receiving the image from said input lens and producing an intensified image; a video camera for receiving said intensified image; a sensor for sensing the gain of said image intensifier tube; and a controller responsive to said sensed gain for controlling the gain of said image intensifier tube within a desired range.

2. A camera system as defined in claim 1 wherein said sensor comprises a light source for illuminating an unused region of said image intensifier tube and a light sensor for sensing an output of said image intensifier tube caused by said light source in said unused region.

3. A camera system as defined in claim 1 wherein said controller comprises means for controlling an operating voltage applied to said image intensifier tube.

4. A camera system as defined in claim 1 wherein said image intensifier tube includes a microchannel plate and wherein said controller comprises means for controlling a voltage applied to said microchannel plate.

5. A camera system as defined in claim 1 further including: means for gating said image intensifier tube on and off with a prescribed duty cycle; and means for storing a frame representative of said intensified image when said image intensifier tube is gated on and for repeating said stored frame when said image intensifier tube is gated off to provide a continuous sequence of frames.

6. A camera system as defined in claim 1 further including: means for comparing the amplitude of a video signal from said camera with a reference level; means for gating said image intensifier tube off for a prescribed time when the amplitude of said video signal exceeds said reference level to protect said image intensifier tube against bright light; and means for storing a frame representative of said intensified image when said image intensifier tube is gated on and for repeating said stored frame when said image intensifier tube is gated off to provide a continuous sequence of images.

7. A camera system for operation in nighttime lighting conditions without additional illumination, comprising: an input lens for receiving incoming light representative of an image; an image intensifier tube for receiving the image from said input lens and producing an intensified image; a video camera for receiving said intensified image; means for gating said image intensifier tube on and off so that said image intensifier tube is on less than full time during normal nighttime operation; and means for storing a frame representative of said intensified image when said image intensifier tube is gated on and for repeating said stored frame when said image intensifier tube is gated off to provide a continuous sequence of frames.

8. A camera system as defined in claim 7 wherein said image intensifier tube is gated on and off by said gating means with a prescribed duty cycle.

9. A camera system as defined in claim 8 wherein said prescribed duty cycle is 10% or less.

10. A camera system as defined in claim 8 wherein said gating means causes said image intensifier tube to have an on time sufficient for obtaining one frame per second.

1 1 . A camera system as defined in claim 7 wherein said input lens comprises an auto iris lens and further including control means for controlling said auto iris lens to prevent further opening thereof when said image intensifier tube is gated off.

12. A camera system as defined in claim 8 further including means for detecting motion in said image and means responsive to detection of motion in said image for controlling said gating means such that said duty cycle is at or near 100%.

13. A camera system as defined in claim 8 further including a light sensor for sensing an ambient light level, means responsive to said light sensor for increasing said duty cycle at low ambient light levels to provide multiple frames and means for integrating said multiple frames to reduce noise.

14. A camera system as defined in claim 8 further including a light sensor for sensing an ambient light level and means responsive to said light sensor for decreasing said duty cycle at high ambient light levels.

15. A camera system as defined in claim 7 further including means for comparing the amplitude of a video signal from said camera with a reference level, said gating means including means for gating said image intensifier tube off for a prescribed time when the amplitude of said video signal exceeds said reference level to protect said image intensifier tube against bright light.

16. A camera system as defined in claim 15 further including means for switching to a daytime mode when said video signal exceeds said reference level for a prescribed time period, said camera receiving an image from said input lens without said image passing through said image intensifier tube in the daytime mode.

17. A camera system as defined in claim 7 further including: sensor means for sensing the gain of said image intensifier tube, and control means responsive to said sensor means for controlling the gain of said image intensifier tube at or above a desired level.

18. A camera system for operating in nighttime lighting conditions without additional illumination, comprising: an input lens for receiving incoming light representative of an image; an image intensifier tube for receiving the image from said input lens and producing an intensified image; a video camera for receiving said intensified image; means for comparing the amplitude of a video signal from said camera with a reference level; means for gating said image intensifier tube off for a prescribed time when the amplitude of said video signal exceeds said reference level, to protect said image intensifier tube against bright light; and means for storing a frame representative of said intensified image when said image intensifier tube is gated on and for repeating said stored frame when said image intensifier tube is gated off to provide a continuous sequence of images.

19. A method for operating a camera system in nighttime lighting conditions without additional illumination, comprising the steps of: providing an input lens for receiving incoming light representative of an image, an image intensifier tube for receiving the image from said input lens and producing an intensified image and a video camera for receiving said intensified image; sensing the gain of said image intensifier tube; and controlling the gain of said image intensifier tube within a desired range in response to the sensed gain.

20. A method as defined in claim 19 wherein the step of sensing the gain of the image intensifier tube includes illuminating an unused region of the image intensifier tube and sensing an output of the image intensifier tube caused by illuminating the unused region.

21 . A method as defined in claim 19 wherein the step of controlling the gain of the image intensifier tube includes controlling an operating voltage applied to said image intensifier tube.

22. A method for operating a camera system in nighttime lighting conditions without additional illumination, comprising the steps of: providing an input lens for receiving incoming light representative of an image, an image intensifier tube for receiving the image from the input lens and producing an intensified image and a video camera for receiving the intensified image; gating said image intensifier tube on and off so that said image intensifier tube is on less than full time during normal nighttime operation; and storing a frame representative of the intensified image when the image intensifier tube is gated on and repeating the stored frame when the image intensifier tube is gated off to provide a continuous sequence of frames.

23. A method as defined in claim 22 wherein the step of gating includes gating said image intensifier tube with a prescribed duty cycle.

24. A method as defined in claim 22 further including the step of comparing the amplitude of a video signal from the camera with a reference level and wherein the step of gating includes gating the image intensifier tube off for a prescribed time when the amplitude of said video signal exceeds said reference level to protect said image intensifier tube against bright light.