Abstract:
A thermal test target with a uniform surface temperature which can be used to characterize and measure thermal image degradation due to atmospheric propagation of the image radiation field. This thermal test target board produces very uniform spatial frequency patterns with near perfect transitions between hot and cold portions which do not change during the diurnal cycle and which are not impacted by environmental changes.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured, used and licensed by or for the United States Government for Governmental purposes without payment to us of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is in the field of thermal (infrared) target simulators used in testing imaging systems. 
     2. Description of the Prior Art 
     In order to test thermal target viewers, imagers, and the like, simulators must be used. In addition, a target which can be used to characterize the thermal image degradation due to atmospheric propagation of the image radiation field from close to the target to the thermal image at the imager acquisition range is required. In the area of target acquisition the visible is the most convenient because the human eye has adapted to distinguishing objects by the dominant source of reflected spectral radiation, the sun a 6000 degree K blackbody radiator whose radiation peak is between 0.4 and 0.7 microns. The sun is not always present to illuminate the target and obscurants can prevent the observer from receiving the reflected solar radiation; hence, another spectral region (the 8 to 14 micron, far-infrared, or thermal) is often used to detect targets because ambient temperature radiation from the targets themselves are peaked in the 8 to 14 micron spectral region. In order to assess how well a thermal imaging system will perform for target detection the mechanisms which produce changes in the received thermal image must be quantitatively measured. These mechanisms which produce contrast include: solar radiation which is often variable for partly cloudy conditions, windspeed which cools surfaces convectively or through an evaporative process, precipitation which can rapidly cool warm objects, and a variety of atmospheric obscurants and turbulence which produce attenuation and distortion of the received scene radiation distribution. It is here that the present invention is utilized. The near field/far-field image comparison technique of the Target Contrast Characterizer (described in The Proceedings of the SPIE International Symposium on Optical, Infrared, and Millimeter Wave Propagation Engineering, VOL. 926. Orlando, Fla. [1988]) can be used to separate target contrast change components of interest or closeup target contrast changes from the propagation degradation of the inherent contrast to the distant observation location. To properly quantify the propagation degradation a stable thermal spatial target is needed because the inherent signature of a target against a background can change rapidly and even go through periods of no contrast during the thermal reversals of the diurnal cycle. The target board which is the subject of the invention presented herein can be used to produce very uniform spatial frequency patterns with near perfect transistions between hot and cold portions which do not change during the diurnal cycle and which are not impacted by environmental changes. To be useful such a target board must be large (on the order of two meters by two meters), yet lightweight for ease in transporting over rough terrain to typical target locations, and the spatial patterns must be changeable to meet specific target spatial feature characterization requirements. No prior art thermal test target with a uniform surface temperature is known which can be used to characterize and measure thermal image degradation due to atmospheric propagation of the image radiation field. Nothing currently exists which meets these surface temperature uniformity standards requirements. Those target boards which use other methods such as temperature controlled cooling liquids to create uniform surface temperatures weigh several times more than the thermal target test board, which is the subject of this invention. 
     SUMMARY 
     This invention is a thermal target test board (TTTB) with a uniform surface temperature which can be used with the Target Contrast Characterizer mentioned above or other thermal imaging systems to characterize the thermal image degradation due to atmospheric propagation of the image radiation field from close to the target to the thermal image representing the target at the imager acquisition range. 
     The thermal target test board (TTTB) has several features which are essential for its utility. The heated surface must be large on the order of two (2) meters on a side to be of any use at typical one (1) to two (2) kilometer ranges of interest. The heated surface must have a uniform plus or minus one (1) degree centigrade temperature variation over the surface for a 10 to 20 degree centigrade elevated temperature over background temperature. The target board must have ambient temperature bar patterns with variable spatial frequencies which can be used with the heated surface to form near perfect transition between hot and cold bars. The entire thermal target board must be shielded from solar loading and winds to maintain uniformity under changing environmental conditions. Also, the thermal target board must be lightweight for ease in positioning in rough terrain where it is to be used. When used in conjunction with the Target Contrast Characterizer (TCC), the thermal target board (TTTB) was used to measure optical turbulence distortion in thermal imagery which severely impacts the use of aided target recognition systems. 
     The TTTB was also used to measure contrast transmission with the TCC. Prior to this, attempts to extract contrast transmission values from imagery in the 8 to 12 um region over a 1.5 kilometer path were only within a factor of two in magnitude of predictions. The TTTB can also be used to directly measure the optical transfer function of the imagers in the field environment. The TTTB can be used alone to quantify prevailing atmospheric degradation for a wide range of imager testing and use. The bar pattern is produced by flat white and black panels which can be illuminated by artificial visible light sources in its solar loading shield and provide a day/night visible calibration target pattern in addition to the infrared spectral region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     A better understanding of the invention will be obtained when the following detailed description of the invention is considered in connection with the accompanying drawing(s) in which: 
     FIG. 1 shows the thermal target test board design. 
     FIG. 2 shows the heater arrangement in the thermal oven. 
     FIG. 3 shows the construction of the thermal oven box. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 and FIG. 3, the Thermal Target Test Board (TTTB) 30, thermal oven 40, consists of a oven front surface board 31 made of aluminum sheet with a flat black front surface measuring six feet wide and seven feet high. Said front surface board 31 rests on phenolic insulators 32 supported on side aluminum channels 33. The side aluminum channels 33 are used on all four sides of the thermal oven 40 to totally enclose the heaters 34 within the thermal oven 40. FIG. 3 is a cutaway drawing and does not show the bottom and top aluminum side channels 33, which are constructed exactly the same way as the side channels 33 shown. Twenty Chromalox (Trademark) Strip Heaters 34, Model No. SE2450W (240 volt, 250 watt) are supported on four heater support aluminum channels 36, which are welded to side channels 33 at the top and bottom of the thermal oven 40, and placed as shown in FIG. 2, spaced from and beneath the oven front surface board 31. The strip heaters 34 are arranged beneath the oven front surface board 31 in the positions shown in FIG. 2. The positions are numbered individually from one to twenty four, and no heaters are placed in positions 5, 7, 9, and 11 to help create a uniform oven front surface board 31 temperature. Although a variety of heater 34 arrangements are possible, the arrangement shown in FIG. 2 is a preferred arrangement, and when this preferred arrangement is used in conjunction with the tilting of the thermal oven 40, a uniform oven front surface temperature is achieved. In the preferred arrangement, the heaters 34 are arranged in a pattern three columns wide and eight rows in height. The empty spaces at positions 5, 7, 9, and 11 help to eliminate a hot spot on the oven front surface board 31. The row spacing in this arrangement of FIG. 2 increases two inches for every row starting at row four which is spaced six inches above row three, and row eight is spaced sixteen inches above row seven. Row one is located at the bottom of the oven front surface board 31. A panel of foam insulation 35 is placed beneath the heaters 34 and around the inside of the aluminum side channels 33, said foam insulation panel 35 is supported by the side channels 33. A metal oven bottom cover sheet 37 seals the bottom of the thermal oven 40. The thermal oven 40 is tilted at an angle to the vertical, about 30 degrees from vertical, as shown in FIG. 1, to enable the oven front surface board 31 temperature to be uniform. 
     Referring to FIG. 2, it is seen that a flat white surfaced aperture panel 41 is placed in a vertical position with respect to the flat black toven front surface board 31. When the thermal oven 40 is placed at an angle behind the aperture panel 41, it appears as a square when it is viewed fron in front of the aperture panel 41. The front surface of the aperture panel 41 is flat white and the rear surface of aperture panel 41 is silver colored and faces the flat black oven front surface board 31. A weather shield 51 made of metal and canvas and of a tent like nature covers the aperture panel 41 and the thermal oven 40 to prevent solar loading of the thermal oven 40 and panel 41. The weather shield 51 also protects the thermal oven 40 and aperture panel from wind, precipitation, and other obscurants. The weather shield 51 contains a flap 91 which together with the shape of the roof of the weather shield 51 allows the heat buildup due to the thermal oven 40 to flow out through the front top of said weather shield 51. FIG. 1 shows a blown up view of the Thermal Target Test Board 30. In actual use, the artificial lighting 71 is enclosed within the weather shield 51 which also encloses the aperture panel 41, and the thermal oven 40. In a typical setup a power supply 61 supplies power to the strip heaters 34 mounted in the thermal oven 40. A source of artificial light 71 is utilized within the weather shield 51 to evenly illuminate the aperture panel 41 and the thermal oven 40 to provide day and night visible target calibration. The power supply 61 also supplies power to the source of artificial light 71. Power conversion devices such as flourescent ballasts 81 can be used, as shown in FIG. 1. A trailer 63 is used as a vehicle to mount the Thermal Target Test Board 30. The trailer 63 aids in transporting said Thermal Target Test Board 30 over rough terrain. 
     Having described this invention, it should be apparent to one skilled in the art that the particular elements of this invention may be changed, without departing from its inventive concept. This invention should not be restricted to its disclosed embodiment but rather should be viewed by the intent and scope of the following claim.