Abstract:
A system is disclosed which utilizes the substantially steady-state temperature of a coated object, in conjunction with an optical detection system, to selectively view defects and features of the object below the coating without the necessity of transient heating or IR illumination and reflectance imaging. The optical detector, such as an IR camera, may be tailored for the wavelengths at which the coating material is substantially transparent, thereby maximizing the viewing clarity of the defects and features under the coating, and distinguishing them from any spurious features on the top surface of the coating. The present system enables the inspection of small or large areas in real time, without requiring complex image acquisition, storage and image processing equipment and software.

Description:
GOVERNMENT CONTRACT  
       [0001]     The United States Government has certain rights to this invention pursuant to Contract No. DACA 72-99-C-01 awarded by SERDP. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to detection of structural features, and more particularly relates to a system which utilizes blackbody self-illumination to observe defects and other structural features of coated objects such as aircraft components.  
       BACKGROUND INFORMATION  
       [0003]     Aircraft components are subject to constant degradation such as corrosion and cracking caused by environmental and operational conditions. Although the application of coatings, such as paints, reduces corrosion problems substantially, they typically cannot eliminate them entirely. Furthermore, stress experienced during flight can result in damage which a coating of paint cannot mitigate, such as stress defects and cracking. In order to ensure that aircraft are ready for flight, periodic inspections are necessary.  
         [0004]     Inspection of aircraft components traditionally includes visual inspection. When visually inspecting aircraft components, the coating used to protect the components becomes an obstacle because it may hide structural defects or features beneath the coating. It is therefore necessary to strip the component assembly or aircraft in question of its paint before a proper visual inspection can be performed. Afterward, a new coating of paint must be applied. This process results in substantial expense in the form of labor and materials, raises environmental concerns, and requires a great amount of time.  
         [0005]     Apart from the inefficiency of visual inspection methods, another problem is that visual inspection is not always effective. While a skillful eye may pick up most human-visible defects with a satisfactory degree of consistency, some defects may be very small or lie under the surface of the component. In many cases these defects will go unnoticed by visual inspection regardless of the skill and experience of the observer.  
         [0006]     In addition to visual inspection, active thermography techniques have been proposed for inspection of various components. One such technique utilizes a transient heat source to heat the component, followed by detection of a transient heat signature on the surface of the component to determine the presence of anomalies or defects. However, such techniques require specialized equipment and controls to generate the necessary transient heating, and are inefficient because detection of the transient thermal signature can require a significant amount of time.  
         [0007]     U.S. Published Patent Application No. US 2004/0026622 A1 discloses a system for imaging coated substrates which utilizes an infrared (IR) light source. The IR light shines on the object and is reflected to a focal plane array. While such a system may be useful for some applications, an IR light source is required and the incident IR radiation must make two passes through the coating. Furthermore, a portion of the incident radiation may reflect off the surface of the coating, thereby obscuring the image of the underlying substrate.  
         [0008]     The present invention has been developed in view of the foregoing.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention utilizes the substantially steady-state temperature of a coated object, in conjunction with an optical detection system, to selectively view defects and features of the object below the coating without the necessity of transient heating or IR illumination and reflectance imaging. The optical detector, such as an IR camera, may be tailored for the wavelengths at which the coating material is substantially transparent, thereby maximizing the viewing clarity of the defects and features under the coating, and distinguishing them from any spurious features on the top surface of the coating. The present system enables the inspection of small or large areas in real time, without requiring complex image acquisition, storage and image processing equipment and software.  
         [0010]     An aspect of the present invention is to provide a method of inspecting a coated object. The method includes maintaining substantially steady state blackbody radiation from the object, and detecting structural features of the object under the coating based on the blackbody radiation.  
         [0011]     Another aspect of the present invention is to provide a system for inspecting a coated object. The system comprises means for maintaining substantially steady state blackbody radiation from the object, and means for detecting structural features of the object under the coating based on the blackbody radiation.  
         [0012]     A further aspect of the present invention is to provide a system for inspecting a coated object comprising a camera structured and arranged to detect structural features of the object under the coating based on substantially steady state blackbody radiation generated from the object.  
         [0013]     These and other aspects of the present invention will be more apparent from the following description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a schematic illustration of a system for detecting structural features of a coated object utilizing blackbody self-illumination of the object.  
         [0015]      FIG. 2  is a schematic flow diagram illustrating the detection of blackbody radiation from an object to be inspected in accordance with an embodiment of the present invention.  
         [0016]      FIGS. 3-26  are blackbody infrared radiation images of coated graphite panels and coated aluminum panels in accordance with various embodiments of the present invention.  
         [0017]      FIGS. 27   a - d  are visible, IR reflectance, and IR blackbody radiation images of a coated aircraft panel with rivets, showing features of the rivets underneath the coating in accordance with a blackbody self-illumination embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  schematically illustrates a detection system in accordance with an embodiment of the present invention. A coated object  10 , such as an aircraft component, composite panel, painted panel, ship hull, ground vehicle, aircraft assembly, aircraft landing gear, metallic substrate, honeycomb bonded assembly or the like, includes a substrate or object  12  at least partially covered with a coating  14  such as paint, composite matrix material or the like. Examples of some specific coatings include coatings manufactured to the following specifications: BMS 10-72; BMS 10-11; BMS-10-79, BMS 10-60; MiL-PRF-23377; MiL-PRF-85582; MiL-PRF-85285 and TT-P-2760. In accordance with the present invention, the object  12  emits blackbody radiation B toward a detector  16  such as an infrared (IR) camera, IR detector or the like.  
         [0019]     In accordance with the present invention, the blackbody radiation B from the object  12  is generated in a substantially steady state. As used herein, the term “substantially steady state blackbody radiation” means the radiation naturally generated from the object to be inspected due to its maintenance at a temperature above zero degrees Kelvin, typically at room temperature or a slightly elevated temperature. Steady state blackbody radiation results from maintaining the object or a portion thereof at a substantially uniform temperature, i.e., in the absence of significant thermal gradients throughout the object or portion thereof being inspected.  
         [0020]     Since the object  12  is at or near room temperature, it emits a significant amount of substantially steady state infrared (IR) blackbody thermal radiation B. In contrast, the coating  14  may be substantially transparent at some of the wavelengths at which the underlying object emits the blackbody radiation B. Many organic polymers that may be used in the coating  14  are significantly IR-transmissive in certain spectral bands. The blackbody radiation B of the object can penetrate the organic coating  14  covering the object  12  and reveal the surface condition of the object  12  under the coating  14 . The radiation B transmitted through the coating  14  is thus used to provide images from the self-illuminated object  12  that reveal any defects such as corrosion, cracks and pits, as well as other structural features under the coating  14 . The object  12  to be inspected becomes observable by its own IR radiation B, which is a function of the temperature of the object  12 .  
         [0021]     As shown in  FIG. 1 , the object  12  to be inspected may include various types of structural features. The structural features may be located on the surface of the object  12  under the coating  14 , or may be located below the surface of the object  12 . For example, surface features  22  may be provided on the surface of the object  12  below the coating  14 . Examples of surface features  22  include indicia such as alphanumeric symbols, marks, codes, part numbers, bar codes and the like. The object  12  may also include surface defects such as corrosion  24 , pits  26 , cracks  28 , gouges, and other structural defects. As shown in  FIG. 1 , the object  12  may also include structural features below the surface of the object  12 , such as corrosion  32 , cracks  34 , composite reinforcements  36  and pits  26 .  
         [0022]      FIG. 2  schematically illustrates a blackbody radiation detection process in accordance with an embodiment of the present invention. Blackbody radiation from an object such as the coated object  10  shown in  FIG. 1  is transmitted to a detector such as an IR camera. After detection, an image of the coated object  12 , including structural features of the object  10  under the coating  14  may be displayed and/or stored. In addition, the image may be transmitted by any suitable means such as the Internet, wireless, cable or satellite for display and/or storage at any desired location.  
         [0023]     In accordance with an embodiment of the present invention, the steady state blackbody radiation B from the object to be inspected may be generated by holding the object at room temperature. The entire object may be maintained at a substantially uniform temperature at or near room temperature. As used herein, the term “room temperature” means the surrounding ambient temperature found in an area such as a testing laboratory, production facility, warehouse, hanger, airstrip, aircraft cabin or ambient exterior temperature. Room temperatures are typically within a range of from about 60 to about 80° F. However, temperatures above or below such a range may exist. For example, in cold environments such as unheated hangers or warehouses in cold regions, the room temperature may be 32° F. or lower. In warm environments such as non-air-conditioned hangers and warehouses in desert or tropical regions, the “room temperature” may be well above 80° F., e.g., up to 100 or 110° F., or even higher.  
         [0024]     In accordance with another embodiment of the present invention, the object to be inspected is held at an elevated temperature, e.g., above room temperature, to maintain the substantially steady state blackbody radiation. Such an elevated temperature may be up to about 120° F. or higher, typically in a range of from 80 to about 110° F. The elevated temperature may be maintained by any suitable means, such as exposure to sunlight, heat gun, heat lamp, thermal blanket, hot packs, human contact and the like.  
         [0025]     The detector  16  may selectively detect radiation at certain wavelengths at which the coating  14  is substantially transparent. In this manner, the coating  14  does not substantially interfere with the image from the object  12 . The detector  16  may include any suitable device such as an IR camera, IR detector, IR focal plane or the like. For example, the camera may be an analog or digital camera, and may record still or video images. Infrared cameras may be used, for example, cameras which detect mid-infrared radiation, e.g., having wavelengths between about 3 and about 5 microns. Such mid-IR wavelengths have been found to produce relatively sharp images with minimal interference from several types of coatings. Other infrared cameras include near-infrared cameras which detect wavelengths between about 0.7 and about 3 microns, and far-infrared cameras which detect wavelengths between about 3 and about 12 microns.  
         [0026]     In addition to the camera  16 , standard filters and/or polarizers (not shown) may be positioned in the optical path of the blackbody radiation B between the object  12  and the detector  16 . Such filters and/or polarizers may remove a portion of the blackbody radiation B having wavelengths at which the coating  14  is non-transparent.  
         [0027]     The detector  16  may include a portable or movable camera such as a hand-held camera or a camera that may be mounted on a tripod or the like that can be moved by means of a pan feature and/or a tilt feature.  
         [0028]     In accordance with an embodiment of the present invention, the detected image of the object  12 , including the detected structural features, may be compared with a reference image. For example, a reference image may be generated from another object similar to the coated object that is known to be substantially free of defects. By comparing a substantially defect-free reference object to the coated object being inspected, manual or automated evaluations may be performed. The reference image used as the standard could be preprogrammed into a database and a comparison made between the reference image and the image created from paint under test. Acceptability criteria could be preprogrammed as well, unacceptable areas could be highlighted in red and acceptable areas in green. Other colors could be selected, as well, such as gray for an area requiring more evaluation.  
         [0029]     The following examples are intended to illustrate the various aspects of the present invention and are not intended to limit the scope of the invention.  
       EXAMPLE 1  
       [0030]     As shown in  FIG. 3 , a painted graphite panel comprising epoxy graphite with an epoxy primer and urethane top coat paint was imaged with a mid-IR camera at the wavelength of 3 to 5 microns. During the imaging process, the panel was held at 89° F. The panel was subjected to a room temperature calibration which involved adjusting pixel intensity to make focal plane uniform and linear within selected room temperature (RT) calibration.  
       EXAMPLE 2  
       [0031]     As shown in  FIG. 4 , a painted graphite panel comprising epoxy graphite and epoxy primer and urethane top coat paint was imaged with a mid-IR camera at a wavelength of 3 to 5 microns with the panel held at a temperature of 90° F. The panel was subjected to hot calibration at a temperature of 84° F. The hot calibration process involved adjusting pixel intensity to make focal plane uniform and linear within selected 84° F. calibration.  
       EXAMPLE 3  
       [0032]     As shown in  FIG. 5 , a composite panel comprising epoxy graphite and laminated copper fiber painted with epoxy primer and urethane top coat was imaged with a mid-IR camera at a wavelength of 3 to 5 microns, with the panel maintained at a temperature of 90° F. The panel was subjected to hot calibration at 84° F., as described above.  
       EXAMPLE 4  
       [0033]     As shown in  FIG. 6 , a painted graphite and copper fiber panel similar to the panel of Example 3 was imaged at a temperature of 74° F. The panel was subjected to hot calibration at 84° F.  
       EXAMPLE 5  
       [0034]     As shown in  FIG. 7 , a panel comprising epoxy graphite with a laminated copper weave painted with epoxy primer and urethane top coat was imaged at a temperature of 91° F. The panel was subjected to room temperature calibration.  
       EXAMPLE 6  
       [0035]     As shown in  FIG. 8 , a painted graphite and copper weave panel similar to that of Example 5 was imaged at 87° F. after room temperature calibration.  
       EXAMPLE 7  
       [0036]     As shown in  FIG. 9 , a painted graphite and copper weave panel similar to that of Examples 5 and 6 was imaged at 82° F. after room temperature calibration.  
       EXAMPLE 8  
       [0037]     As shown in  FIG. 10 , an epoxy graphite panel painted with epoxy primer and urethane top coat was imaged at 90° F. after room temperature calibration.  
       EXAMPLE 9  
       [0038]     As shown in  FIG. 11 , a painted graphite panel similar to that of Example 8 was imaged at 86° F. after room temperature calibration.  
       EXAMPLE 10  
       [0039]     As shown in  FIG. 12 , a painted graphite panel similar to that of Examples 8 and 9 was imaged at 82° F. after room temperature calibration.  
       EXAMPLE 11  
       [0040]     As shown in  FIG. 13 , a painted graphite panel similar to that of Examples 8-10 was imaged at 78° F. after room temperature calibration.  
       EXAMPLE 12  
       [0041]     As shown in  FIG. 14 , a panel comprising epoxy graphite was primed with epoxy primer and painted with epoxy primer and urethane top coat on the right side of the panel.  FIG. 14  is a visible image of the painted and primed panel.  
       EXAMPLE 13  
       [0042]     As shown in  FIG. 15 , a panel comprising graphite with copper weave was primed with epoxy primer in lower right hand side and painted with urethane top coat in upper right of panel. In  FIG. 15 , the left side of the panel is unprimed and unpainted, while the right side is primed and painted.  FIG. 15  is a visual image of the panel.  
       EXAMPLE 14  
       [0043]      FIG. 16  is a visible image of an aluminum panel comprising a corroded aluminum substrate coated with epoxy primer and urethane top coat.  
       EXAMPLE 15  
       [0044]      FIG. 17  is an IR reflectance image of the panel of Example 14 at 77° F. The IR reflectanct image was generated by reflecting IR radiation off the aluminum substrate detecting the reflected energy in an IR camera or detector. The corrosion is indicated in dark areas.  
       EXAMPLE 16  
       [0045]      FIG. 18  is an IR reflectance image of the panel of Example 14 taken 75° F. The corrosion is indicated in dark areas.  
       EXAMPLE 17  
       [0046]      FIG. 19  is a blackbody radiation image made in accordance with the present invention of the coated aluminum panel of Example 14. The panel was maintained at a temperature of 84° F. with a room temperature calibration. The corrosion is indicated in light areas.  
       EXAMPLE 18  
       [0047]      FIG. 20  is a blackbody radiation image made in accordance with the present invention of the coated aluminum panel of Example 14, at 78° F. with a room temperature calibration. The corrosion is indicated in light areas.  
       EXAMPLE 19  
       [0048]      FIG. 21  is a blackbody radiation image made in accordance with the present invention of the coated aluminum panel of Example 14, at 72° F. with a room temperature calibration. The corrosion is indicated in light areas.  
       EXAMPLE 20  
       [0049]      FIG. 22  is an IR reflectance image of a corroded aluminum panel coated with an epoxy low IR primer and urethane top coat. The IR reflectance image was made by reflecting IR radiation off the coated aluminum substrate and detecting the reflected energy in an IR camera. The corrosion is indicated in dark areas.  
       EXAMPLE 21  
       [0050]      FIG. 23  is a blackbody radiation image produced in accordance with the present invention taken from the same primed and top coated aluminum panel described in Example 20. The blackbody radiation procedure was performed at 96° F. with a 78° F. hot calibration. The corrosion is indicated in light areas.  
       EXAMPLE 22  
       [0051]      FIG. 24  is a blackbody radiation image produced in accordance with the present invention taken from the same primed and top coated aluminum panel described in Example 20. The blackbody radiation procedure was performed at 86° F. with a 78° F. hot calibration. The corrosion is indicated in light areas.  
       EXAMPLE 23  
       [0052]      FIG. 25  is a blackbody radiation image produced in accordance with the present invention taken from the same primed and top coated aluminum panel described in Example 20. The blackbody radiation procedure was performed at 79° F. with a 78° F. hot calibration. The corrosion is indicated in light areas.  
       EXAMPLE 24  
       [0053]      FIG. 26  is a visible image of the primed and top coated aluminum panel of Examples 20 to 23.  
         [0054]     The foregoing examples demonstrate that blackbody type IR radiation is capable of passing through coatings and producing an image. External illumination is not required, i.e., the parts are self-illuminating.  
       EXAMPLE 25  
       [0055]     A bolted aluminum aircraft panel was coated with Epoxy primer MIL-PRF-23377TYI and Urethane MIL-PRF-85285TYI paint, as shown in  FIG. 27   a . It was inspected using visible imaging ( FIG. 27   b ), IR reflectance imaging ( FIG. 27   c ), and IR blackbody imaging ( FIG. 27   d ). The blackbody self-illumination image was made with a mid-IR camera at a wavelength of 3 to 5 microns. During the black body imaging process, the painted aluminum panel was held at a temperature of 85 to 95° F. As shown in  FIG. 27   d , details of the bolt heads, including alphanumeric symbols, can be seen in the IR blackbody image which are not detectable from the visible image of  FIG. 27   b.    
         [0056]     An advantage of the present blackbody self illumination system is that an independent IR illumination source is not needed. In some cases, an object&#39;s IR radiation at ambient temperature may be sufficient to allow imaging of the object through the coating, while in other situations moderate heating of the object to a slightly elevated temperature may be desirable. Such heating can be achieved naturally, e.g., by sunlight, or by a heat gun, thermal blankets, an IR heat lamp, or by other means that produce a substantially steady-state temperature of the object.  
         [0057]     Another advantage of the present blackbody system is that the IR radiation only has to make one pass through the coating. This is more efficient compared to IR reflectance techniques, in which IR radiation from an external illuminator must first penetrate the coating, reflect off the substrate or object and pass through the coating again. An additional advantage of the present blackbody method is the reduction or elimination of the coating surface reflection. In the reflectance method, IR energy is reflected off the coating surface partially obscuring the image from the substrate underneath.  
         [0058]     Whereas particular embodiments of this invention have been described above for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details of the present invention may be made without departing from the invention as defined in the appended claims.