Thermal imaging calibration systems and methods

A calibration assembly employs a thermoelectric cooler to provide a calibration temperature for one calibration flag and a different calibration temperature for a second calibration flag. Either calibration flag is immediately available for insertion into the optical path when calibration of the thermal imaging device is required. Consequently, the time required to calibrate a thermal imaging device within a thermal imaging system is greatly reduced.

BACKGROUND

1. Field of the Invention

The present invention relates generally to thermal imaging systems and, more particularly, to systems and methods for calibrating thermal imaging devices, such as focal plane arrays.

2. Related Art

A focal plane array (FPA), which detects infrared radiation, is well known in the art. An FPA, for example, may be formed from an array of microbolometers, with each microbolometer functioning as a pixel to produce a two-dimensional image. The change in resistance of each microbolometer due to incident infrared radiation is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC). The combination of the ROIC and the microbolometer array is commonly known as a microbolometer FPA or microbolometer infrared FPA. Microbolometers and FPAs are described in further detail in U.S. Pat. Nos. 5,756,999 and 6,028,309, which are herein incorporated by reference in their entirety.

Thermal imaging device performance, such as with a FPA for example, is typically degraded due to non-uniform responses among the individual microbolometer detectors to uniform incident infrared radiation. Factors contributing to the performance degradation include variations in the infrared radiation absorption coefficient, resistance, temperature coefficient of resistance (TCR), heat capacity, and thermal conductivity of the individual detectors. Because the magnitude of the non-uniformity can be substantially larger than the magnitude of the actual response due to the incident infrared radiation, various techniques are typically required to compensate for the non-uniformity and acquire the portion of the signal representing the incident infrared radiation.

FIG. 1is a graph of pixel output (e.g., output voltage) for two microbolometers within an FPA as a function of photon flux (i.e., received incident infrared radiation). As lines102and104illustrate for the two corresponding microbolometers, the pixel outputs for a certain initial level of photon flux (e.g., at point106) differs by a certain amount of pixel output offset. Furthermore, the amount of gain exhibited by the two microbolometers varies over a range of photon flux (e.g., between points106and108), as indicated by the difference in the slope of lines102and104.

Typically, the offset and gain of each infrared detector in the FPA is calibrated so that a more uniform response is obtained from the microbolometer FPA over the desired range of photon flux. For example, as shown inFIG. 2, the initial offsets for the two microbolometers are calibrated at point106to remove the non-uniform offset. The gain is then normalized, as shown inFIG. 3, for the two microbolometers over the photon flux range defined by points106and108to produce a more uniform response. Further details of calibration procedures may be found, for example, in U.S. Pat. Nos. 5,756,999 and 6,028,309.

Thermal imaging devices are typically periodically calibrated, such as upon power-up or at certain intervals during use, to minimize the non-uniform response from the FPA. For example, the FPA of the thermal imaging device may be calibrated over two or more levels of photon flux by inserting into the optical path a calibration flag (i.e., an optical obscuration). The temperature of the calibration flag is raised or lowered to provide the desired level of photon flux for the FPA. When the calibration flag reaches the required temperature and is inserted into the optical path of the thermal imaging device, the FPA takes one or more data frames or snapshots of the calibration flag to calibrate its response at that temperature. The temperature of the calibration flag can then be changed and the calibration process repeated at the new temperature. The data collected at the calibration points can then be used to calibrate the FPA to provide a more uniform response, as discussed above.

The calibration flag is typically coupled to a thermoelectric cooler (TEC), which is a small heat pump that heats or cools the calibration flag to the desired temperature (i.e., desired level of photon flux). The TEC is coupled to a heat sink, which is used to help maintain the desired temperature and prevent temperature elevation drift or overheating of the TEC or calibration flag. In some implementations, a fan may be used to further aid in maintaining the desired temperature range. A small motor or servo is typically used to translate or rotate the calibration flag into the optical path when calibration is desired.

One drawback of this technique is the significant time delay between calibration points due to the time required by the TEC to heat or cool the calibration flag from one temperature calibration point to the next. For example, one or more minutes may be required by the TEC to transition or slew the calibration flag between calibration temperature points. Another drawback is that the fan, servo, and heat sink detrimentally add to the size, weight, cost, and complexity of the thermal imaging device. Furthermore, the fan and servo are an additional power draw and may contribute undesired electromagnetic interference. As a result, there is a need for improved techniques for providing calibration for thermal imaging devices.

BRIEF SUMMARY

Thermal imaging calibration systems and methods are disclosed herein. In accordance with an embodiment of the present invention, a thermal imaging sensor calibration assembly is disclosed that employs a TEC to provide simultaneously two calibration temperatures for two corresponding calibration flags. Either calibration flag is immediately available for insertion into the optical path when calibration of the thermal imaging device is required. Consequently, the time required for calibration is significantly reduced (e.g., from minutes to seconds) because the time delay for slewing the temperature of a single calibration flag between calibration points has been eliminated. Furthermore, as described in greater detail herein, because the fan may be eliminated in some embodiments, the size, weight, cost, and complexity of the thermal imaging device is reduced along with its power demands.

More specifically, in accordance with one embodiment of the present invention, a calibration flag assembly includes a main body, a first calibration flag coupled to the main body, a first thermoelectric cooler coupled to the main body, and a second calibration flag coupled to the first thermoelectric cooler.

In accordance with another embodiment of the present invention, a method of calibrating a thermal imaging device includes operating a first thermoelectric cooler to adjust a temperature of a first calibration flag and a second calibration flag to a first calibration temperature and a second calibration temperature, respectively; inserting into an optical path of the thermal imaging device the first calibration flag; and inserting into the optical path the second calibration flag.

In accordance with another embodiment of the present invention, a thermal imaging system includes a thermal imaging sensor adapted to receive thermal radiation through an optical path and a calibration flag assembly that provides thermal calibration images, when inserted into the optical path, to assist in performing non-uniformity calibration. The calibration flag assembly includes a main body; a first calibration flag coupled to the main body; a first thermoelectric cooler coupled to the main body; and a second calibration flag coupled to the first thermoelectric cooler.

DETAILED DESCRIPTION

FIGS. 4 and 5show top and bottom perspectives, respectively, of a calibration flag assembly400in accordance with an embodiment of the present invention. Calibration flag assembly400includes a calibration flag402, mounting holes404,406, a thermoelectric cooler (TEC) mounting area408, and a main body410. As explained further herein, calibration flag402, which is coupled to main body410, provides a surface having a controllable temperature that can be used by a thermal imaging device to calibrate its sensor (e.g., calibration procedures for an FPA such as disclosed in U.S. patent application Ser. No. 10/085,226 entitled “Microbolometer Focal Plane Array Methods and Circuitry” filed Feb. 27, 2002, which is herein incorporated by reference in its entirety, or in U.S. Pat. Nos. 5,756,999 and 6,028,309 referenced earlier).

Mounting holes404and406are used to attach calibration flag assembly400to a thermal imaging system (e.g., in conjunction with screws, rivets, etc.). For example, calibration flag assembly400may be coupled to a thermal imaging sensor calibration assembly (TISCA), as discussed herein, which is part of a thermal imaging system. TEC mounting area408allows for the attachment of a TEC and an additional calibration flag, as shown in the following figures.

FIGS. 6 through 9show various perspectives of a calibration flag assembly600, which incorporates calibration flag assembly400ofFIGS. 4 and 5with a thermoelectric cooler (TEC)412and a calibration flag414, in accordance with an embodiment of the present invention. Calibration flag assembly600is shown with mounting screws416,418to secure calibration flag assembly600within the thermal imaging system, such as shown in the exemplary embodiment of FIG.16. For example, mounting holes404,406may be threaded to assist in securing calibration flag assembly600.

TEC412may be secured to calibration flag414and to calibration flag assembly400by various methods, such as adhesive bonding, compression using thermal grease, or solder. TEC412is a small heat pump that operates on direct current and may be used for heating or cooling depending upon the direction of current flow, which moves heat from one side of TEC412to the other by the use of current flow and the laws of thermodynamics. The direction of current flow through TEC412is controlled by the application of a direct current source to a negative terminal420and a positive terminal422of TEC412.

TEC412may be any type of heating/cooling device. For example, Marlow Industries, Inc.™ of Dallas, Tex. produces a number of different types of TECs suitable for one or more embodiments of the present invention. In general, TEC412has a cold side, which calibration flag414is coupled to, and a hot side, which calibration flag402is coupled to through main body410of calibration flag assembly400. Thus, calibration flag assembly400serves the dual purpose of a heat sink for TEC412and as a thermal channel between calibration flag402and TEC412.

In operation, by the application of an appropriate direct current source to terminals420,422(i.e., a positive voltage applied to positive terminal422and a less positive voltage applied to negative terminal420), TEC412will cool calibration flag414and heat calibration flag402simultaneously. Consequently, calibration flags402,414may each reach a desired calibration temperature at approximately the same time, with either available for insertion into the optical path to calibrate the thermal imaging device.

The time required for calibration flags402,414to reach the desired temperature could be estimated by the following equation:
t=[(ρ)(V)(Cp)(T1−T2)]/Q
where t is the time (seconds), ρ is the density (g/cm3), V is the volume (cm3), Cp is the specific heat (J/g ° C.), T1−T2is the temperature change (° C.), and Q represents (Qto+Qtt)/2 (J/s with 1 J/s=1 W). Qtois the initial heat pumping capacity when the temperature difference across the cooler is zero. Qttis the heat-pumping capacity when the desired temperature difference is reached and heat-pumping capacity is decreased. Qtoand Qttare used to obtain an average value.

Calibration flags402and414can be designed to reach their respective calibration temperatures by the appropriate selection of a TEC and the size and type of materials used for calibration flag assembly600(including calibration flags402,414). For example,FIGS. 10 through 14show exemplary design specifications for calibration flag assembly400ofFIGS. 4 and 5andFIG. 15shows exemplary design specifications for calibration flag414in accordance with an embodiment of the present invention.FIGS. 11 and 13are bottom and top views, respectively,FIG. 10is a side view ofFIG. 11,FIG. 12is a cross-sectional side view opposite that of FIG.10and taken along line A—A, andFIG. 14is a cross-sectional front view taken along line B—B of FIG.11.

Calibration flag assembly400and calibration flag414, for the exemplary design specifications of correspondingFIGS. 10 through 15, are made of 6061-T6 aluminum. TEC412for the exemplary design specifications ofFIGS. 10 through 15is Marlow Ind. Inc.™ TEC model no. MI2012T, which is adhesive bonded to calibration flag assembly400. However, it should be understood that this exemplary embodiment is not limiting and, for example, the type of TEC or the size, shape, and materials for calibration flag assembly600will vary depending upon the design parameters required and the calibration temperatures desired. For example, calibration flag assembly400and calibration flag414may be made of any type of thermally conductive material, such as for example a metal or metal alloy (e.g., copper or aluminum) or ceramic.

One or more temperature-measuring devices may be coupled to calibration flag assembly600to allow for the monitoring of the temperature of calibration flag assembly600, including calibration flags402,414. For example, as can be seen inFIG. 7, a thermistor424is coupled to calibration flag assembly600to allow for temperature monitoring. Thermistor424can be used, as an example, to monitor the overall temperature of calibration flag assembly600to ensure that temperature elevation drift does not occur. This may occur when successive calibrations occur without allowing sufficient time for the calibration flag assembly600to return to its normal non-operating temperature range. A fan or other cooling device may optionally be provided, if rapid successive calibrations are expected to occur, to maintain calibration flag assembly600within a desired operating range.

FIG. 16shows an exemplary TISCA1600, which incorporates calibration flag assembly600ofFIGS. 6 through 9, in accordance with an embodiment of the present invention. TISCA1600includes a motor1602having a drive gear1604that meshes with teeth1606of an assembly1612to rotate assembly1612into the optical path (indicated by the optical path arrow in FIG.16). For example, attached to assembly1612is calibration flag assembly600and optical filters1608and1610.

Although motor1602is shown, any type of prime mover, which includes not only a motor but also for example a servo or a solenoid, may be employed to place calibration flag assembly600into the optical path. By controlling motor1602, optical filters1608,1610or calibration flags402,414can be placed directly into the optical path. If optical filter1608or optical filter1610is placed into the optical path, the incident infrared radiation is appropriately filtered prior to reaching an infrared sensor1614of the thermal imaging system. If calibration flag402or calibration flag414is placed into the optical path, the incident infrared radiation traveling along the optical path is blocked and infrared sensor1614can calibrate using the known thermal radiation emitted from the calibration flag (i.e., either calibration flag402or calibration flag414) inserted into the optical path.

More than one calibration flag assembly600can be employed within a thermal imaging system to provide any desired number of calibration points for the thermal imaging system's sensor. For example, assembly1612may be modified so that two calibration flag assemblies600could be attached to provide four calibration flags (i.e., four calibration points) for the thermal imaging system's sensor. Alternatively, one calibration flag assembly600could be operated to provide two calibration points and then, with TEC412continuing to operate, provide two more calibration points within the desired operating range of the sensor as calibration flag412continues to decrease in temperature as calibration flag402continues to increase in temperature. This technique could be extended to provide numerous calibration points within the sensor's operating range.

FIG. 17shows a side view of a calibration flag assembly1700in accordance with an embodiment of the present invention. Calibration flag assembly1700includes TECs1702,1704and calibration flags1706,1708. By employing two TECs within calibration flag assembly1700, a greater degree of temperature control can be maintained for calibration flags1706,1708and a faster ramp-up to the desired temperature for calibration flag1706is possible. For example, TEC1704could be operated solely to cool calibration flag1708and heat calibration flag1706. Alternatively, TEC1704could be operated in conjunction with TEC1702to heat calibration flag1706at a faster rate. By operating TECs1702and1704simultaneously, the amount of time to reach the desired difference in temperature between calibration flags1706and1708is reduced.

Furthermore, TEC1702could function to regulate or maintain the overall temperature of calibration flag assembly1700. For example, TEC1704would be operated to cool calibration flag1708and heat calibration flag1706. However, when the overall temperature of calibration flag assembly1700drifts below the desired operating temperature (e.g., after rapid successive calibrations), TEC1702could be operated to add heat to calibration assembly1700.

It should be understood by the description herein that embodiments of the present invention provide numerous advantages over conventional techniques. For example, some embodiments provide a two-point calibration (i.e., two calibration flags at different temperatures) for a sensor within one temperature slewing cycle rather than two slewing cycles as with conventional techniques, which greatly reduces the time required to calibrate a thermal imaging sensor. Also some embodiments do not require active cooling, such as a heat sink or a fan, which reduces the associated size, cost, complexity, and power requirements of the calibration process.

Consequently, one or more embodiments described herein can be incorporated into a filter wheel (e.g., assembly1612of FIG.16), which allows the elimination of a separate prime mover (e.g., motor) to rotate the calibration flags into the optical path. Alternatively, calibration assembly600may be moved into and out of the optical path independent of the optical filters by having its own prime mover (e.g., a motor, a servo, or a solenoid), but the time required for calibration is still greatly reduced.