Abstract:
An infrared camera capable of automatically executing offset compensation so that fixed pattern noise is removed without the need of an operator comprises an offset compensation signal generation circuit and a shutter. The shutter is closed based on an offset compensation execution signal for carrying out offset compensation, the signal being automatically and periodically generated by the offset compensation signal generation circuit after execution of the first offset compensation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an infrared camera with offset compensation. 
     2. Description of the Related Art 
     FIG. 9 is a block diagram relating to a conventional infrared camera. The drawing shows an subject M, an infrared optical system  1 , an image pickup element formed on the imaging surface of the infrared optical system  1 , an element temperature monitor  3  thermally connected to the image pickup element  2 , a shutter  4  provided between the infrared optical system  1  and the image pickup element  2 , a bias power source  5  connected to the image pickup element  2 , a driver circuit  6  connected to the image pickup element  2 , pre-positioned amplifying circuit  7  connected to the image pickup element  2 , a display processing circuit  8  connected to the pre-positioned amplifying circuit  7 , an element temperature stabilizing means  9  thermally connected to the image pickup element  2 , a timing generation circuit  10  connected to the shutter  4 , the driver circuit  6 , and the display processing circuit  8 , a body  11 , an offset compensation execution switch  12  provided in the outside of the body  11  and connected to the timing generation circuit  10 . The display processing circuit  8  comprises an A/D converter  13 , an addition averaging circuit  14 , a frame memory  15 , a subtraction circuit  16 , and a D/A converter  17 . Also shown in FIG. 1 are a reference voltage power source  18 , a differential amplifying circuit  19  connected to the element temperature monitor  3  and the reference voltage power source  18 , a vacant element package  20  accommodating the image pickup element  2 , the element temperature monitor  3 , and the element temperature stabilizing means  9 , and an infrared window  21  typically made of germanium, leaving a vacant space enclosed by the vacant element package  20  and the infrared window  21 . 
     FIG. 10 shows an example structure of the image pickup element  2  which is, for the sake of brevity of explanation, comprised of 3×3 elements. The drawing shows infrared detectors  22  to  30 , transistors  31  to  48 , capacitors  49  to  51 , a vertical scanning circuit  52 , and a horizontal scanning circuit  53 . The infrared detectors  22  through  30  are microbolometers having a hollow structure, as described in Japanese Patent Laid-open No. Hei 7-509057, which are made of vanadium oxide or titanic oxide for reducing thermal conductance with respect to the surrounding so that heat quantity of absorbed infrared can be efficiently converted into an increase of temperature of the detector thereby achieving high sensitivity. 
     In operation, the reference voltage power source  18  outputs a reference voltage corresponding to an operation temperature of the image pickup element  2  to the differential amplifying circuit  19 . The differential amplifying circuit  19  compares the supplied output and an output from the element temperature monitor  3  to feed back a power corresponding to the difference between the outputs to the element temperature stabilizing means  9  for stabilizing the operation temperature of the image pickup element  2 . 
     Next, the bias power source  5  supplies bias voltage Vb and gate voltage Vg to the transistors  46  through  48  and the driver circuit  6  sends a driving clock to the vertical scanning circuit  52  according to a timing generated by the timing generation circuit  10  for selection of a row of infrared detectors. In response to the clock, the vertical scanning circuit  52  renders the transistors  31  through  33  conductive for a predetermined period, whereby a bias current defined according to gate voltage Vg is caused to flow into the infrared detectors  22  to  24 , so that the voltage corresponding to the respective resistance values will be caused at the infrared detectors  22  through  24 . 
     Subsequently, when a sample-hold clock is applied, the transistors  40  to  42  are made conductive so that the voltage according to the resistance values of the infrared detectors  22  to  24  is temporarily stored in the capacitors  49  to  51 . Then, after shutting off the transistors  40  to  42 , the horizontal scanning circuit  53  sequentially makes the transistors  43  through  45  conductive so as to output voltage according to the resistance values of the infrared detectors  22  through  24 . 
     Thereafter, the vertical scanning circuit  52  selects the row of infrared detectors  25  through  27  so that the voltage corresponding to the resistance values thereof will be output in the same procedure as that is applied to the infrared detectors  22  through  24 . 
     While repeating the above procedure, voltages corresponding to the resistance values of the infrared detectors  22  through  30  which constitute the image pickup element  2  are sequentially output and, after being amplified in the pre-positioned amplifying circuit  7 , are supplied to the A/D converter circuit  13 . Then, the timing generation circuit  10  sends a signal for closing the shutter so that the shutter is closed. 
     After the shutter was closed and consistent infrared were introduced into the infrared detectors  22  through  30 , the timing generation circuit  10  sends a signal to the display processing circuit  8 , for obtaining offset compensation data for the first time. Then, the A/D converter circuit  13  converts an output from the pre-positioned amplifying circuit  7  into a digital signal. Further, an addition average is obtained for every infrared detector in the addition averaging circuit  14  so that variation of the resistance values of the infrared detectors  22  through  30 , in other words, offset variation, is stored in the frame memory  15 . 
     Then, the shutter  4  is opened, and infrared radiation emitting from the direction of subject M is collected in the infrared optical system  1 . The converged infrared radiation then passes through the infrared window  21  to form an image on the infrared detectors  22  through  30 . This causes a slight increase of the temperatures of the infrared detectors  22  through  30  by an order of a few mK according to the strength of the collected infrared radiation. As a result, the respective resistance values of the infrared detectors are changed from those before the shutter  4  was opened. 
     Outputs from the infrared detectors  22  through  30  are then amplified in the pre-positioned amplifying circuit  7  and converted into digital signals in the A/D converter circuit  13 , similar to when offset compensation data is obtained. Then, the data stored in the frame memory  15  is subtracted from the digital signals for every pixel in the subtraction circuit  16  to remove fixed pattern noise due to offset variation of the infrared detectors, and the result is converted into an analogue video signal in the D/A converting circuit  17  before being output. 
     Here, a change in the inside temperature of the body  11  due to heat generation of an electric circuit or a change of ambient temperature may change an output voltage of the reference voltage power source  18 , characteristics of the element temperature stabilizing means  9 , the amount of heat discharged from the image pickup element  2 , or the amount of infrared radiation from the infrared optical system  1 , resulting in a slight change to an operation temperature of the image pickup element  2 . Accordingly, the resistant values of the infrared detectors  22  through  30  are changed for every pixel. Because the amount of change of the resistance value differs for every pixel, offset variation of an output is changed from that at the time when offset compensation data was first obtained, leaving outstanding fixed pattern noise in a video signal. In such a case, offset compensation data is obtained again by operating the offset compensation switch  12  to restore the image. 
     While the compensation operation described above as being applied to a non-cooling type of infrared camera whose image pickup element  2  has a two-dimensional array of microbolometers and operates as stabilized at a constant temperature around a room temperature, the operation may be similarly applied to a cooled infrared camera whose image pickup element  2  has a two-dimensional array of, for example, platinum and silicon Schottky barrier diodes, and operates as stabilized at a low temperature, including a typical value of around 77 K. 
     The structure of infrared cameras equipped as described above requires a manual switching operation for offset compensation. Therefore, an operator must stay near the camera, even for a long-time continuous use of the camera. 
     SUMMARY OF THE INVENTION 
     The present invention has been conceived to overcome the above problems and aims to provide an infrared camera which can automatically perform offset compensation to remove fixed pattern noise without requiring input or operation. 
     An infrared camera of the present invention comprises an offset compensation execution signal generation circuit for automatically generating an offset compensation execution signal, and a shutter arranged at a position covering the viewing field of the image pickup element. 
     With this arrangement, the shutter is closed according to an offset compensation execution signal automatically and periodically generated by the offset compensation signal generation circuit before carrying out offset compensation. This enables automatic offset operation without requiring an operator to manipulate the camera. 
     An infrared camera of the present invention may also comprise an offset compensation execution signal generation circuit for automatically generating an offset compensation execution signal, and a de-focus motor associated with an infrared optical system. 
     With this arrangement, the de-focus motor is operated based on an offset compensation execution signal which is automatically and periodically generated by the offset compensation signal generation circuit, to move the focusing plane of the infrared optical system for carrying out offset compensation. This enables automatic offset operation without requiring an operator. 
     An infrared camera of the present invention may further comprise an offset compensation execution signal generation circuit for outputting an offset compensation execution signal at a constant interval. 
     Further, an infrared camera of the present invention may comprise an offset compensation execution signal generation circuit for outputting an offset compensation execution signal in a shorter interval than the above mentioned constant interval during a predetermined period immediately after turning on the power. 
     With this arrangement, there can be provided an infrared camera which can produce a preferable image, even immediately after being powered up. 
     According to an infrared camera of the present invention, the offset compensation execution signal generation circuit may have a temperature sensor for measuring temperature around the structural elements of the infrared camera, and a temperature changing amount judging circuit connected to the temperature sensor. 
     With this arrangement, an offset compensation is applied when the temperature around the structural elements of the infrared camera has been changed by more than a predetermined value from the temperature at the time of previous execution of offset compensation. This enables production of an infrared camera which can produce a preferable image even during a period immediately after having turned on the power or when the temperature around the structural elements of the camera is changed while capturing an image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will become further apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a block diagram showing a structure of an infrared camera according to a first preferred embodiment of the present invention; 
     FIG. 2 is a diagram relative to an operation of the infrared camera according to the first preferred embodiment of the present invention; 
     FIG. 3 is a block diagram showing a structure of an infrared camera according to a second preferred embodiment of the present invention; 
     FIG. 4 is a diagram relative to an operation of the infrared camera according to the second preferred embodiment of the present invention; 
     FIG. 5 is a block diagram showing a structure of an infrared camera according to a third preferred embodiment of the present invention; 
     FIG. 6 is a block diagram showing a structure of a temperature changing amount judging circuit according to the third preferred embodiment of the present invention; 
     FIG. 7 is a diagram illustrating an operation of the infrared camera according to the third preferred embodiment of the present invention; 
     FIG. 8 is a block diagram showing a structure of an infrared camera according to a fourth present invention; 
     FIG. 9 is a block diagram showing a structure of an infrared camera according to the related art; and 
     FIG. 10 is a diagram showing a structure of an image pickup element of an infrared camera according to the related art. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 1 is a block diagram showing an infrared camera according to a first preferred embodiment of the present invention. The respective members  1  to  21  correspond to members described above for an infrared camera of the related art. The drawing additionally shows an offset compensation execution signal generation circuit  54  connected to the timing generation circuit  10 . 
     Operation of an infrared camera according to the present invention will next be described. Identical operations to those for an infrared camera of the related art will be applied from stabilization of the operation temperature of the image pickup element  2  at a desired temperature through operation of the element temperature stabilizing means  9 , through first-time automatic offset compensation, to output of a video signal from the D/A converter  17 . 
     FIG. 2 illustrates an operation of an infrared camera of the present embodiment. Specifically, during a predetermined period after execution of first offset compensation, the offset compensation execution signal generation circuit continues periodical output of an offset compensation execution signal to a timing generation circuit  10  in an interval To for performing automatic repetitive offset compensation. In addition to the automatic offset compensation, manual offset compensation may be applied if necessary by operating an offset compensation execution switch  12 . In this case, the next offset compensation may be applied after an interval To following the manual offset compensation, or following the last automatic offset compensation in ignorance of the manual offset compensation. 
     Embodiment 2 
     FIG. 3 is a block diagram showing an infrared camera according to a second preferred embodiment of the present invention. The respective members  1  to  21  correspond to those described above. The drawing additionally shows an offset compensation execution signal generation circuit  55  connected to the timing generation circuit  10 . 
     FIG. 4 shows operation of the infrared camera of this embodiment. Identical operations to those for an infrared camera of the related art will be applied from stabilization of the operation temperature of the image pickup element  2  at a desired temperature through operation of the element temperature stabilizing means  9 , through first-time automatic offset compensation, to output of a video signal from the D/A converter  17 . 
     The temperature of the inside of the body  11  generally increases rapidly one to two hours after power is turned on due to heat generation of an electronic circuit, and thereafter gradually attains balanced condition. Therefore, a great deal of fixed pattern noise will be caused in a unit immediately after power is supplied. To address this problem, an offset compensation execution signal as shown in FIG. 4 is applied from the offset compensation execution signal generation circuit to the timing generation circuit  10  so that offset compensation will be executed in a shorter interval after power is turned on in order to suppress fixed pattern noises and thereby produce a preferable image. Note that, in addition to the automatic offset compensation which is carried out in an interval set as described above, manual offset compensation may be applied by operating an offset compensation execution switch  12 . 
     Embodiment 3 
     FIG. 5 is a block diagram showing an infrared camera according to a third preferred embodiment of the present invention. The respective members  1  to  21  correspond to those members identified above using the same numerals. The drawing additionally shows a timing generation circuit  56 , and an offset compensation execution signal generation circuit  57  connected to the timing generation circuit  56 . The offset compensation execution signal generation circuit  57  comprises a temperature sensor  58  arranged in the inside of the body  11 , for measuring the temperature around the structural elements of the infrared camera, and a temperature changing amount judging circuit  59  connected to the temperature sensor  58 . 
     FIG. 6 is a block diagram showing a structure of a temperature changing amount judging circuit  59 , which comprises an amplifying circuit  60 , an A/D converter circuit  61 , a memory  62 , a D/A converter circuit  63 , a differential amplifying circuit  64  connected to the amplifying circuit  60  and the D/A converter circuit  63 , and a comparing circuit  66  connected to the differential amplifying circuit  64  and a reference voltage power source  65 . 
     Operation of an infrared camera according to the present invention will be described. Identical operations to those for an infrared camera of the related art will be applied from stabilization of the operation temperature of the image pickup element  2  at a desired temperature through operation of the element temperature stabilizing means  9 , through first-time automatic offset compensation, to output of a video signal from the D/A converter  17 . 
     In an infrared camera according to this embodiment, an output of the temperature sensor  58  at the time of performing offset compensation is amplified by the amplifying circuit  60 , and then converted into a digital signal in the A/D converting circuit  61  to be stored in the memory  62 . The digital signal is then converted into an analogue signal in the D/A converting circuit  63 , and supplied to the differential amplifying circuit  64 . Meanwhile, an output from the temperature sensor  58  while taking an image after offset compensation, is also supplied to the differential amplifying circuit  64  so that a difference between this output and an output from the D/A converting circuit  63 , i.e., a change of an output from the temperature sensor  58  from that at the time of performing last offset compensation, is supplied to the comparing circuit  66 . When the amount of change exceeds a value set according to the reference voltage power source  65 , an offset compensation execution signal is output to the timing generation circuit  56  for execution of another offset compensation. 
     Subsequently, after completion of the offset compensation, the timing generation circuit  56  sends a signal for newly obtaining an output from the temperature sensor  58  to the temperature changing amount judging circuit  59  for updating outputs of the memory  62  and the D/A converting circuit  63 . 
     FIG. 7 is a diagram showing example operation of an infrared camera according to the present embodiment, in which offset compensation is automatically applied immediately after turning on the power or at the time when the temperature around the body  11  is changed, for suppressing generation of fixed pattern noises. Note that, in addition to the automatic offset compensation, manual offset compensation may be applied upon necessity by operating the offset compensation execution switch  12 . 
     Embodiment 4 
     FIG. 8 is a block diagram showing an infrared camera according to a fourth preferred embodiment of the present invention. The respective members  1  to  3  and  5  to  21  correspond to those described above, and the member  54  is identical to that employed in the first preferred embodiment of the present invention. The drawing additionally shows a de-focus motor  67  associated with the infrared optical system  1 . 
     Operation of an infrared camera according to the present embodiment will be described. Identical operations to those for an infrared camera of the related art will be applied from stabilization of the operation temperature of the image pickup element  2  at a desired temperature through operation of the element temperature stabilizing means  9 , through first-time automatic offset compensation, to output of a video signal from the D/A converter  17 . 
     After first offset compensation, the offset compensation execution signal generation circuit  54  periodically outputs, an offset compensation execution signal, similar to the first embodiment, to the timing generation circuit  10 . The timing generation circuit  10  then sends an operating signal to the de-focus motor  67  for activation, instead of closing the shutter  4 , to move the focusing plane for attaining an out-of-focus state so that infrared can be constantly introduced into the infrared detectors  22  to  30 . 
     In such a situation, the timing generation circuit  10  sends a signal for obtaining offset compensation data to the display processing circuit  8  so that an output voltage of the infrared detectors  22  to  30  is stored in the frame memory  15  as offset compensation data, similar to a conventional infrared camera. The de-focus motor  67  is then activated again so that the image of the subject M is formed on the infrared detectors  22  to  30 . Subsequently, offset variation with the infrared detectors  22  to  30  is removed by the subtraction circuit  16 , and the signal is converted into an analog video signal by the D/A converter circuit  17  before being output.