FIELD OF THE INVENTION 
       [0001]    The present invention relates to detecting changes in a vacuum state. More specifically, the present invention relates to detecting a change in a vacuum state within a sealed thermal detector package which is a part of a thermal camera. 
       BACKGROUND OF THE INVENTION 
       [0002]    An uncooled infrared thermal camera creates an image that represents the distribution of radiation that originates from a scene. The detector of an uncooled infrared thermal camera is enclosed in a vacuum package that is evacuated and sealed during manufacture of an uncooled detector. 
         [0003]    Sometimes, after the manufacture process, the gas pressure inside the vacuum package increases and the vacuum degrades, Degradation of the vacuum in the vacuum package could lead to degradation of the accuracy of the image created by the camera. If the vacuum loss is detected when the loss of vacuum is still small, the loss of vacuum may be correctable by a simple corrective action, possibly on site. Such corrective maintenance could include relatively simple actions such as flashing a getter. However, a small loss of vacuum that is correctable by simple means would likely not have a sufficiently noticeable effect on image quality to be detected. 
         [0004]    A more serious loss of vacuum inside the vacuum package may require repairs involving more complex, time-consuming, and expensive procedures. 
         [0005]    On the other hand, flashing a getter as a preventative measure, without any indication of loss of vacuum, is also not desirable. Flashing a getter more often than required could lead to deterioration of the detector. 
         [0006]    Therefore, there is a need for timely detection of loss of vacuum in the vacuum package surrounding the detector of an uncooled infrared camera. 
         [0007]    It is an object of the present invention to provide for timely detection of loss of vacuum in the vacuum package surrounding the detector of an uncooled infrared camera during the course of routine use of the camera, and to inform the camera operator of such loss. 
       SUMMARY OF THE INVENTION 
       [0008]    There is thus provided, according to embodiments of the present invention, a method for detecting a change in a vacuum state within a sealed thermal detector package which is a part of a thermal camera, the package housing a thermal detector array and at least one temperature sensor, the method comprising: 
         [0009]    measuring an initial signal from said thermal detector array; 
         [0010]    concurrently measuring an initial signal from said at least one temperature sensor; 
         [0011]    measuring a later signal from said thermal detector array; 
         [0012]    concurrently measuring a later signal from said at least one temperature sensor; 
         [0013]    performing a first calculation of a ratio of the difference between the later and initial signals from said thermal detector array to the difference between the later and initial signals from said at least one temperature sensor; and 
         [0014]    periodically measuring the initial and later signals from said thermal detector array and from said at least one temperature sensor and calculating the ratio to determine changes in the ratio indicative of changes in the vacuum state within the package. 
         [0015]    Furthermore, according to embodiments of the present invention, the first calculation is performed during a manufacturing process of the thermal camera. 
         [0016]    Furthermore, according to embodiments of the present invention, the thermal camera is provided with a shutter the method further comprising using the shutter to block thermal radiation from entering into the thermal detector package during the periodical measurements. 
         [0017]    Furthermore, according to embodiments of the present invention, the thermal camera is directed at a scene characterized by homogeneous thermal radiation. 
         [0018]    Furthermore, according to embodiments of the present invention, the signals from said thermal detector array are converted to grey-scale values. 
         [0019]    Furthermore, according to embodiments of the present invention, the sealed thermal detector package comprises at least one temperature stabilizer in thermal contact with the thermal detector array, and the method further comprises operating the temperature-stabilizing element so as to produce the difference between the later and initial signals from said thermal detector array. 
         [0020]    Furthermore, according to embodiments of the present invention, the thermal stabilizer comprises a thermo-electric element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    In order to better understand the present invention, and appreciate its practical applications, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals. 
           [0022]      FIG. 1  is a block diagram of an uncooled infrared camera in accordance with embodiments of the present invention. 
           [0023]      FIG. 2  is a block diagram of control of an uncooled infrared camera in accordance with embodiments of the present invention. 
           [0024]      FIG. 3A  is a flow chart of acquisition of a reference value in accordance with embodiments of the present invention. 
           [0025]      FIG. 3B  is a variation of the flow chart of  FIG. 3A . 
           [0026]      FIG. 4A  is a flow chart of checking for possible loss of vacuum in accordance with embodiments of the present invention. 
           [0027]      FIG. 4B  is a variation of the flow chart of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0028]    Embodiments of the present invention provide for checking for changes in the level of vacuum in a vacuum package that surrounds the detector of an uncooled thermal infrared camera. 
         [0029]    The principles and operation of checking for changes in the vacuum level in a vacuum package surrounding the detector array of an uncooled infrared camera, according to embodiments of the present invention, may be better understood with reference to the drawings and the accompanying description. 
         [0030]      FIG. 1  is a block diagram of an uncooled thermal infrared camera in accordance with embodiments of the present invention. The main function of camera  10  is to create an image on display device  38  that represents radiation that originates from scene  12 . Shutter  18  may be opened or closed by a suitable mechanism (not shown). When shutter  18  is open, scene  12  may irradiate upon the surface of detector array  22  through optics  16 . Detector array  22  comprises an array of individual detector elements. When shutter  18  is closed, direct irradiation from scene  12  upon detector array  22  is blocked. 
         [0031]    Detector array  22  is located within vacuum package  20 . A temperature stabilizing element, for example thermoelectric element  28 , is in thermal contact with detector array  22 . Thermoelectric element  28  generates or absorbs heat in accordance with a voltage that is applied to it. Temperature sensor  30  is affixed to vacuum package  20 . 
         [0032]    Readout circuits  24  are associated with detector array  22 . Each detector element of detector array  22  is associated with one of readout circuits  24 . Each readout circuit  24  creates an analog electrical signal. 
         [0033]    Detector array  22  is encapsulated in a vacuum package  20 . During manufacture of uncooled detector  20 , a vacuum pump pumps gas out of vacuum package  20  via nozzle  14 . Once a vacuum is formed inside vacuum package  20 , nozzle  14  is sealed. 
         [0034]    In order to assist in maintaining a vacuum inside vacuum package  20 , a getter  26  is provided inside vacuum package  20 . When getter  26  is heated or flashed, material from getter  26  is deposited in a layer  27  on an inner wall of vacuum package  20 . The material in layer  27  traps gas that is present in vacuum package  20 , thus maintaining the level of the vacuum inside vacuum package  20 . 
         [0035]    Analog-to-digital converter  28  converts analog electrical signals to digital signals. Analog signals include the signals that are created by readout circuits  24 , and the output of temperature sensor  30 . 
         [0036]    Image processing module  34  processes the digital output of analog-to-digital converter  32 . Image processing module  34  includes processing circuitry and programmed instructions. Image processing module  34  communicates with data-storage device  35 . During image creation, image processing module  34  calculates pixel gray-level values based on the digital output of analog-to-digital converter  32 . Pixel gray-level values may be displayed graphically on display device  32 . Pixel gray-level values may also be stored on data-storage device  35 . 
         [0037]      FIG. 2  is a block diagram of control of an uncooled infrared camera in accordance with embodiments of the present invention. Serial port  36  communicates with external devices. External devices may include operator controls, or external displays, data storage devices, or processors. Instructions may be entered via serial port  36 . Serial port  36  communicates with controller  40 . Controller  40  may control components of camera  10 . For example, controller  40  may cause shutter  18  to open or close, may apply a voltage to thermoelectric element  28  causing it to generate or absorb heat, and may cause the flashing of getter  26 . Controller  40  communicates with image processing module  34 . 
         [0038]    Image processing module  34  may receive digital input from detector array  22  via the readout circuits  24  and analog-to-digital converter  32 . Image processing module  34  may convert digital input from detector array  22  to gray level values. Image processing module  34  may also receive digital input from temperature sensor  30  via analog-to-digital converter  32 . Image processing module  34  may convert digital input from temperature sensor  30  to temperature data. Image processing module  34  may display image and text data on display device  38 . Image processing module  34  may save data on data-storage device  35 , or retrieve data from data-storage device  35 . Image processing module  34  may communicate with controller  40  to send and receive data via serial port  36 . 
         [0039]    Referring to  FIG. 1 , and in accordance with the method of embodiments of the present invention, changes in the outputs of both detector array  22  and temperature sensor  30  under the influence of thermoelectric element  28  are measured when the vacuum level in vacuum package  20  is assumed to be at the desired level. Should a similar measurement made at a later date indicate different changes in output, this would imply a change in the vacuum level. 
         [0040]    In embodiments of the present invention, the average output of detector elements of detector array  22  is expressed by the average of the gray-level values that correspond to those detector elements. Average gray-level values are calculated by image-processing module  34  on the basis of digitized data from readout circuits  24 . 
         [0041]    When shutter  18  is open, exchange of radiation between scene  12  and detector array  22  may affect the output of detector array  22 . The content of scene  12  would be likely to vary from output measurement to output measurement. The measured change in output of detector array  22  could then be influenced by the changes in the content of scene  12 . Measured changes in output of detector array  22  then would not reliably correlate with the level of vacuum. Therefore, when measuring the output of detector array  22 , shutter  18  is closed to prevent the direct exchange of radiation between scene  12  and detector array  22 . When shutter  18  is closed, shutter  18  presents detector array  22  with a source of radiation that, in general, is more uniform and reproducible than scene  12 . Alternatively, camera optics  16  could be aimed at a non-reflecting surface that emits radiation uniformly and homogenously. For example, camera optics  16  could be aimed at a black body surface or cavity, where the black body is kept at a uniform temperature and fills the field of view of camera  10 . 
         [0042]    In addition, in accordance with embodiments of the present invention, changes in the digitized output signal of temperature sensor  30  are measured. 
         [0043]    The outputs of detector array  22  and temperature sensor  30  are measured concurrently, and at least twice during determination of the output changes. In between measurements, thermoelectric element  28  is operated. Operation of thermoelectric element  28  may cause the temperatures of detector array  22  and temperature sensor  30  to change, each at its own rate. The ratio of the change in the output of detector array  22  to the change in the output of sensor  30  may be calculated. This output-change ratio, in essence, expresses the rate of the change in the output of detector array  22  as a multiple or fraction of the rate of the change in the output of temperature sensor  30 . The value of the output-change ratio correlates the state of the vacuum inside vacuum package  20 . 
         [0044]    In accordance with embodiments of the present invention, the output-change ratio is first measured during the process of manufacturing an uncooled infrared camera. Gas is evacuated from vacuum package  20  to a desired level during the manufacturing process of the detector. It may be assumed that the gas pressure in vacuum package  20  shortly after evacuation is at a desired level. The value of the output-change ratio that is measured during the manufacturing process of the infrared camera can be recorded as a reference value. Measurement of the output-change ratio at a later date may be expected to correlate with the state of the vacuum inside vacuum package  20 . A significant difference between the output-change ratio measured at a later date and the recorded reference value would imply a change in gas pressure, i.e. a change in the level of vacuum, inside vacuum package  20 . 
         [0045]      FIG. 3A  is a flow chart of acquisition of a reference value in accordance with embodiments of the present invention.  FIG. 3B  is a variation of the flow chart of  FIG. 3A . In the description of the acquisition of a reference value, reference is made to steps of the flow charts in  FIG. 3A  and  FIG. 3B , and to control components in  FIG. 2 . 
         [0046]    During the manufacture process of an uncooled infrared camera  10 , the vacuum inside vacuum package  20  may be assumed to be at an acceptable level. Power to the camera is turned on (step  42 ). Controller  40  causes shutter  18  to close (step  44 ). Image processing module  34  acquires output data from detector array  22  and readout circuits  24  via analog-to-digital converter  32 , and processes the data to yield initial gray-level values for detector elements of detector array  22  (step  46 ). Concurrently, image processing module  34  acquires an initial output value from temperature sensor  30  via analog-to-digital converter  32 . Controller  40  causes shutter  18  to open (step  48 ). At this point, the camera is allowed to operate for an interval of time, during which the temperature of components in vacuum package  20  may change (step  49  of  FIG. 3A ). Alternatively, controller  40  operates thermoelectric element  28  to generate or absorb heat (step  50  of  FIG. 3B ) for an interval of time. The length of the interval of step  49  or step  50  may be determined by a timer circuit incorporated into, or associated with, image processing module  34 , or may be determined by sampling output of temperature sensor  30  until a predetermined output value, or change in output value, is attained. At the end of the interval, controller  40  causes shutter  18  to close (step  52 ). Image processing module  34  collects output data from detector array  22  and processes the data to yield final gray-level values for detector elements. Concurrently, image processing module  34  acquires a final value from temperature sensor  30  (step  54 ). Controller  40  causes shutter  18  to open to enable normal camera operation (step  55 ). For each detector element, the initial gray-level value is subtracted from the corresponding final gray-level value. This difference result is referred to in step  56  as ΔGray_level. Also, the initial temperature sensor output value is subtracted from the final temperature sensor output value to yield ΔTemperature. The average value of ΔGray_level is calculated. The values of ΔGray_level may be averaged for all detector elements, or for a subset of the detector elements. The average value of ΔGray_level is divided by ΔTemperature (step  56 ). Image processing module  34  permanently stores this quotient, the initial output-change ratio, as a reference value in data storage device  35  (step  58 ). The stored reference value may be compared at a later date with a value of the output-change ratio calculated on that later date. 
         [0047]      FIG. 4A  is a flow chart of checking for possible loss of vacuum in accordance with embodiments of the present invention.  FIG. 4B  is a variation of the flow chart of  FIG. 4A . In the description of checking for possible loss of vacuum, reference is made to steps of the flow charts in  FIG. 4A  and  FIG. 4B , and to control components in  FIG. 2 . 
         [0048]    In embodiments of the present invention, acquisition and calculation of a value for comparison with a stored reference value occurs whenever electric power supply  39  is connected to controller  40  of uncooled infrared camera  10  is turned on (step  60 ). Controller  40  causes shutter  18  to close (step  62 ). Image processing module  34  acquires output data from detector array  22  and readout circuits  24  via analog-to-digital converter  32 , and processes the data to yield initial gray-level values for detector elements of detector array  22 . Concurrently, image processing module  34  acquires an initial output value from temperature sensor  30  via analog-to-digital converter  32  (step  64 ). Controller  40  causes shutter  18  to open (step  66 ). At this point, the camera is allowed to operate for an interval of time, during which the temperature of components in vacuum package  20  may change (step  67  of  FIG. 4A ). Alternatively, controller  40  causes thermoelectric element  28  to generate or absorb heat (step  68  of  FIG. 4B ) for an interval of time. At the end of the interval, controller  40  causes shutter  18  to close (step  70 ). Image processing module  34  collects output data from detector array  22  and processes the data to yield final gray-level values for each detector element. Concurrently, image processing module  34  acquires a final output value from temperature sensor  30  (step  72 ). Controller  40  causes shutter  18  to open (step  74 ). For each detector element, the initial gray-level value is subtracted from the corresponding final gray-level value. This difference result is referred to in step  76  as ΔGray_level. Also, the initial temperature sensor output value is subtracted from the final temperature sensor output value to yield ΔTemperature. The average value of ΔGray_level is calculated and divided by ΔTemperature (step  76 ). Image processing module  34  temporarily stores this quotient, the output-change ratio, as a comparison result (step  58 ). The comparison result is stored until power to camera  10  is shut off. 
         [0049]    Once the comparison result is calculated and temporarily stored, the comparison result is compared with the permanently stored reference value (step  82 ). This comparison may be made immediately after storing the comparison result, as part of a built-in test procedure that is performed upon camera startup. Alternatively, the comparison may be initiated by a command received via serial port  36 . Alternatively, the comparison may be initiated by a component of the camera, for example image processing module  34 , when predetermined conditions are met. Comparison of the comparison result with the reference value entails checking whether the value of the current comparison result is within a predefined tolerance range of the reference value. Such a tolerance range may be defined, for example, in terms of a fraction or percentage of the reference value. In this case, the comparison result would first be subtracted from the reference value. The absolute value of the difference would then be divided by the reference value. If the value of the resulting quotient is found to be below a defined tolerance value, the comparison result is considered to fall within the tolerance range of the reference value. 
         [0050]    If the comparison result falls within a predefined tolerance range of the permanently stored reference value, the comparison is taken to indicate that the vacuum in vacuum package  20  is intact. Operation of the camera then continues. Image processing module  34  may then display text or symbols on display device  38  indicating that the vacuum is intact, or may send such an indication to an external device via serial port  36 . 
         [0051]    If the comparison result does not fall within the predefined tolerance range of the permanently stored reference value, the comparison is taken to indicate that gas is present within vacuum package  20 . Image processing module  34  may then display text or symbols on display device  38  indicating the loss of vacuum, or may send such an indication to an external device via serial port  36 . 
         [0052]    When loss of vacuum is indicated, one or more courses of action may be taken. Getter  26  may be flashed to remove trace gasses from vacuum package  20 . Flashing of getter  26  may be caused by controller  40  in response to instructions received via serial port  36  from an external device. Alternatively, getter  26  may be flashed by means of a device that is connected directly to leads that are connected to getter  26 . If a vacuum check performed after flashing getter  26  continues to indicate loss of vacuum, other courses of action may be taken. Vacuum may be reestablished in vacuum package  20 , for example, by opening nozzle  14  of vacuum package  20 , using a vacuum pump to remove gas from inside vacuum package  20 , and resealing nozzle  14 . If vacuum cannot be reestablished in vacuum package  20 , the detector can be declared as a damaged. 
         [0053]    Alternatively, the comparison of the comparison result for the output-change ratio with the reference result may be calibrated to yield an indication of the extent of vacuum loss. An indication of the extent of vacuum loss may then immediately indicate a recommended course of remedial action. 
         [0054]    As described above, embodiments of the present invention provide for checking the status of the vacuum in a package surrounding the detector array of an uncooled infrared camera. Checking the vacuum may be performed routinely within the camera during camera startup, without the need for external equipment. 
         [0055]    It should be clear that the description of the embodiments and attached Figures set forth in this specification serves only for a better understanding of the invention, without limiting its scope. 
         [0056]    It should also be clear that a person skilled in the art, after reading the present specification could make adjustments or amendments to the attached Figures and above described embodiments that would still be covered by the present invention.
A method for detecting a change in a vacuum state within a sealed thermal detector package which is a part of a thermal camera, the package housing a thermal detector array and at least one temperature sensor. The method comprises measuring an initial signal from said thermal detector array; concurrently measuring an initial signal from said at least one temperature sensor; measuring a later signal from said thermal detector array; concurrently measuring a later signal from said at least one temperature sensor; performing a first calculation of a ratio of the difference between the later and initial signals from said thermal detector array to the difference between the later and initial signals from said at least one temperature sensor; and periodically measuring the initial and later signals from said thermal detector array and from said at least one temperature sensor and calculating the ratio to determine changes in the ratio indicative of changes in the vacuum state within the package.