Abstract:
A blackbody radiation source includes an insulated enclosure having a viewing aperture defining a line of sight through the wall of the enclosure. A heat sink, preferably a pool of liquid nitrogen, is located within the insulated enclosure. There is a viewing surface in thermal contact with the heat sink but having an unobstructed view through the viewing aperture. The viewing surface is inclined to the line of sight through the viewing aperture. A sensor is calibrated by placing the sensor in facing relation to the aperture and measuring an output black body signal of the sensor. The viewing surface may be radiatively heated concurrently with the measuring to produce a higher equivalent radiometric temperature.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a blackbody radiation source operable at low temperatures. 
     Sensors such as imaging focal plane array sensors are calibrated prior to, and sometimes during, service. In one calibration procedure, the sensor is placed into a facing relation with a calibration source which produces a standard condition. The standard condition typically includes a simulated background field comparable with the background field expected in service for the wavelength range of the sensor but without any target present. The sensor is operated while facing the calibration source. From the data collected in the calibration procedure, the operability of, zero values for, and/or scaling factors for individual pixel sensor elements of the array are determined and stored for later use. 
     An important class of sensors includes infrared sensors for use in space or in viewing space from earth. The relevant background for the calibration of such sensors is a blackbody radiation source having a low temperature of 200®K. or less. One blackbody radiation source previously used for this calibration procedure has been a bath of liquid nitrogen or a flat metal container whose exterior is painted black and which is filled with liquid nitrogen. This type of source yields somewhat unpredictable equivalent radiometric temperatures. Reflections from the ambient environment may interfere with the measurement of the source. Additionally, this source is limited to a single source temperature and lacks the flexibility required for many applications. 
     There is a need for an improved low-temperature blackbody radiation source for use in calibrating sensors and in other applications. The present invention fulfills this need, and further provides related advantages. 
     SUMMARY OF THE INVENTION 
     The present invention provides a low-temperature blackbody radiation source which is operable for the calibration of sensors and other applications. The blackbody radiation source is operable at temperatures below 200®K. It is effective in reducing ambient radiometric reflections that otherwise interfere with the calibration performed using the blackbody radiation source. The aperture of the source may be made quite large, as may be required for large sensors. The equivalent radiometric temperature of the blackbody radiation source, in terms of spectrally integrated radiance, may be varied over a range of temperatures. 
     In accordance with the invention, a blackbody radiation source comprises an insulated enclosure having a viewing aperture defining a line of sight therethrough, and a heat sink within the insulated enclosure. The insulated enclosure preferably has a bottom, a top, and a lateral side, and the viewing aperture is through the lateral side of the insulated enclosure. The heat sink is preferably a pool of a liquefied gas lying around or below the level of the viewing aperture. The heat sink preferably has a temperature of no greater than the boiling point of nitrogen, and most preferably is a pool of liquid nitrogen. The blackbody radiation source further includes a viewing surface in thermal contact with the heat sink but having an unobstructed view through the viewing aperture, wherein the viewing surface is inclined to the line of sight through the viewing aperture. The viewing surface has a high-emissivity, diffuse surface over the radiation wavelength range of interest, with an emissivity that is preferably greater than about 0.97. 
     The viewing surface is desirably constructed as at least a portion of one side of the inner surface of a closed box having an outer surface in contact with the heat sink. The closed box is positioned inside the insulated enclosure, and the heat sink such as the pool of liquefied gas lies between an inner surface of the insulated enclosure and the outer surface of the closed box. The viewing surface is preferably inclined such that the viewing aperture lies within an acute angle formed between the viewing surface and a horizontal plane. 
     By making the substrate of the viewing surface of a relatively thin piece of a high thermal conductivity material such as aluminum, liquid oxygen from the atmosphere condenses on and flows over the viewing surface. The movement of the liquid oxygen prevents the buildup of condensed water on the viewing surface 
     In one embodiment, the blackbody radiation source includes a heater whose radiant output is directed toward the viewing surface but which itself is not within the line of sight and therefore is not itself directly measured by the sensor being calibrated. The viewing surface is cooled from its back side by conduction from the heat sink, and is radiatively heated on its front side by the heater. Although the temperature of the viewing surface stays approximately constant at the temperature of the heat sink, radiant energy from the heater is reflected back from the viewing surface, through the aperture, and to the sensor being calibrated. By adjusting the power level of the heater, the amount of reflected energy reaching the sensor may be controllably varied, producing a condition wherein the equivalent radiometric temperature of the viewing surface source may be controllably varied. The ability to achieve a range of radiometric temperatures near that of the heat sink is an important advantage of the invention. 
     In operation, a sensor to be calibrated is placed into a facing relation with the aperture of the insulated enclosure and thence into a facing (but inclined) relation with the inclined viewing surface inside the enclosure. The blackbody radiation source is brought to the desired temperature state. The sensor is operated, and calibration is performed against the blackbody field. 
     The present invention thus provides an important advance in the field of blackbody radiation sources for use in the calibration of sensors and other applications. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the apparatus arrangement in the calibration of a focal plane array sensor; 
     FIG. 2 is a sectional view of a first embodiment of the blackbody radiation source used in the calibration process; 
     FIG. 3 is a sectional view of a second embodiment of the blackbody radiation source used in the calibration process; and 
     FIG. 4 is a sectional view of a third embodiment of the blackbody radiation source used in the calibration process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an arrangement whereby a sensor  20  is calibrated using a blackbody radiation source  22 . The sensor  20  is placed in a facing relationship to a viewing aperture  24  of the blackbody radiation source  22  along a line of sight  26 . The sensor  20  views a viewing surface  28  within the blackbody radiation source  22 , through the viewing aperture  24 . The viewing aperture  24  subtends an arc α relative to the intersection of the line of sight  26  with the viewing surface  28 . The angle α is preferably less than 1 radian, and most preferably less than 0.1 radian. The sensor  20  is typically located in a housing  32  and views the viewing surface  28  through appropriate optics, here indicated schematically by a single lens  34 . The sensor  20  converts the incident radiation, in the wavelength band of the operation of the sensor  20 , to an electrical signal which is monitored and analyzed by sensor electronics  30 . Sensors  20  and sensor electronics  30  are well known in the art. The sensor  20  typically operates in the infrared band of from about 3 to about 12 micrometers wavelength, or in the subbands of from about 3 to about 5 micrometers wavelength or from about 7 to about 12 micrometers wavelength. 
     FIG. 2 depicts one preferred embodiment of the blackbody radiation source  22  of the invention. An insulated enclosure  40  having the viewing aperture  24  therethrough is provided. The insulated enclosure  40  has a bottom  42 , a top  44 , and a lateral side  46 . The viewing aperture  24  is preferably in the lateral side  46  at an intermediate location between the bottom  42  and the top  44 . The insulated enclosure  40  is made of an insulating material such as Styrofoam™ foam material of thickness about 2-4 inches that insulates an interior  48  maintained at a temperature of below about 200®K. 
     A heat sink  50  is maintained within the insulated enclosure  40 . The temperature of the heat sink is no greater than the boiling point of nitrogen, about 77®K. The heat sink is preferably a pool of a liquefied gas such as liquid nitrogen (LN 2 ) or liquid helium (LHe). Liquid nitrogen is preferred for the majority of the applications where equivalent radiometric temperatures above about 100 K are required. 
     In the embodiment of FIG. 2, the heat sink  50  is a pool of liquid nitrogen in a tray  52  that rests against the bottom  42  of the insulated enclosure  40 . The level of the pool is below the aperture  24 . 
     The viewing surface  28  is in thermal contact with the heat sink  50 . There is an unobstructed view of the viewing surface  28  through the viewing aperture  24 . The viewing aperture has a width W A  and is spaced a distance L A  from the viewing surface  28 . The viewing surface  28  is inclined at an angle β to a horizontal plane. In this case, the line of sight  26  lies in the horizontal plane. In the embodiment of FIG. 2, the angle β is an acute angle of greater than 0 degrees but less than 90 degrees, and the viewing aperture  24  lies within that acute angle. In another embodiment that will be discussed in relation to FIG. 4, the angle β is obtuse, and the viewing aperture  24  lies within the obtuse angle. Most preferably, the angle β is from more than about 40 degrees to less than 90 degrees, or from more than 90 degrees to less than about 160 degrees. That is, the viewing surface  28  is inclined to the line of sight  26  and is not perpendicular to the line of sight  26 . The inclining of the viewing surface  28  downwardly in FIG. 2 reflects specular thermal energy incident upon the viewing surface  28  through the viewing aperture  24  downwardly into the pool of liquid nitrogen. The thermal energy therefore cannot specularly reflect back through the viewing aperture  24  into the sensor  20  to alter the apparent radiometric temperature of the viewing surface  28 . Stated another way, any energy directed to the viewing aperture  24  and thence to the sensor  20  comes from diffuse scattering from viewing surface  28  from input radiation from the viewing aperture  24 , or from radiative emission from the viewing surface  28 . 
     In the illustrated case, the viewing surface  28  comprises at least a portion of an inner surface  54  of a box  56  having an outer surface  58  in contact with the heat sink  50 . The box  56  is closed on all sides except for an opening  60  corresponding to the viewing aperture  24 . In the embodiment of FIG. 2, the side of the box  56  adjacent to the viewing aperture  24  need not be sealed to the wall of the insulated enclosure  40 , because the liquid level of the liquid nitrogen is below the level of the viewing aperture  24 . 
     The box  56  is preferably made of aluminum (including pure aluminum and aluminum alloys) having a thickness of more than about 0.1 inch. Other high thermal conductivity materials such as copper, silver, and beryllium (including their alloys) may also be used The inner surface  54  of the box  56 , at least in the region of the viewing surface  28 , has a high emissivity in reflection in the infrared ranges of interest. The high emissivity surface is typically achieved by anodizing the aluminum to a black color. 
     The embodiment of FIG. 3 is like that of FIG. 2 in major respects, and the above description and reference numerals are incorporated herein. The embodiment of FIG. 3 adds at least one heater  70  that is out of the line of sight  26  from the sensor  20  through the viewing aperture  24 . The heater  70  may be placed inside the box  56  and just above the opening  60 . Most preferably, the heater  70  includes an electrical resistance heating element  72  and a reflector  74  that directs the radiant energy of the heating element  72  toward the viewing surface  28  and shields the line of sight  26  from the viewing aperture  24 . When an electrical current is passed through the heating element  72 , heat is generated and directed toward the viewing surface  28 . The magnitude of the electrical current determines the amount of heat generated and consequently the radiant energy directed toward the viewing surface  28 . Some of the photons of energy, typically about 1 to 4 percent, are reflected from the viewing surface  28  back through the aperture  24  and to the sensor  20  being calibrated. The viewing surface  28  remains at approximately the temperature of the heat sink, but its apparent radiometric temperature increases as more photons are produced by the heating element  72  and reflected back through the aperture  24  to the sensor  20 . Accordingly, the apparent radiometric temperature of the viewing surface  28 , in terms of photons per second per square centimeter per steradian integrated over the sensor response range, may be increased from that of the heat sink to greater apparent temperatures over a controllable range by increasing the current to the heating element  72 . This ability to controllably vary the apparent radiometric temperature of a calibratior source operating at cryogenic temperature is valuable in calibrating sensors for use in space applications or which otherwise view space. 
     FIG. 4 illustrates another embodiment of the blackbody radiation source  22 . Many of the elements are the same as in FIG. 2, and the above description and reference numerals are incorporated herein to the extent of the similarity. The embodiment of FIG. 4 differs in that the box  56  is sealed to the insulated enclosure at the viewing aperture  24 . The liquid level of the liquid nitrogen may therefore extend above the viewing aperture  24  and above the box  56 , achieving more efficient cooling of the box  56 . A fill tube  80  and a gas vent  82  are provided through the top  44  of the insulated enclosure  40  in this embodiment. This embodiment may also be provided with a heater  70 , as discussed in relation to FIG.  3 . 
     The embodiment of FIG. 2 was built with a two-inch aperture  24 , and the embodiment of FIG. 4 was built with a one-inch aperture  24 . These aperture sizes may be readily scaled upwardly or downwardly. As a practical matter, the embodiment of FIG. 2 is more preferred for applications where the source and aperture must be large, as it requires less coolant than the embodiment of FIG.  4 . Both embodiments achieved radiometric temperatures of less than 160®K. 
     Although a particular embodiment of the invention has been described in detail for purposes of illustration, various modifications and enhancements may be made without departing from the spirit and scope of the invention. Accordingly, the invention is not to be limited except as by the appended claims.