Patent Publication Number: US-2012025079-A1

Title: Infrared led source for thermal imaging

Description:
BACKGROUND 
     The present disclosure is related to thermal imaging, and particularly to an LED array for a flash thermography based thermal imaging device. 
     When inspecting parts for quality control, one aspect reviewed for quality is the thickness of the part. Various techniques are used in the art to determine thickness, and thereby determine if the work piece meets quality control standards. One technique used is thermal imaging via flash thermography. Flash thermography based thermal imagers operate by subjecting the work piece to a short flash of light (a “pulse”) from a flash lamp such as an xenon strobe. A sensor in the thermal imaging device detects Infra-Red (IR) radiation from the heat being emitted from the part, and determines the magnitude of the sensed IR radiation. A controller then creates a thermal image of the work piece based on the magnitude of the sensed IR radiation and the time of maximum occurrence. 
     SUMMARY 
     Disclosed is a thermal imaging device which has an LED flash array, a sensor capable of detecting IR radiation, and a controller. The controller is coupled to the LED flash array and the sensor. The controller is capable of causing the LED flash array to emit a pulse of radiation. 
     Also disclosed is an LED flash array. The LED flash array has a substantially cylindrical component with a first opening, a substantially cup shaped component with a second opening, and a passageway through the substantially cup shaped component and the substantially cylindrical component that joins the two components, such that electromagnetic radiation may pass through the LED flash array. Arranged about the cup shaped component is a plurality of LED sockets. 
     Also disclosed is a method for creating a thermal image which has the steps of generating an IR pulse using an array of LEDs which are controlled by a controller, sensing emitted IR radiation using a sensor, and determining the thickness of an object based on the magnitude of the emitted IR radiation. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic example of a thermal imager. 
         FIG. 1A  illustrates a schematic example of a controller for the thermal imager of  FIG. 1 . 
         FIG. 2  illustrates another schematic example of a thermal imager. 
         FIG. 3A  illustrates a side view of an example IR LED housing for a thermal imager. 
         FIG. 3B  illustrates a top view of an example IR LED housing for a thermal imager. 
         FIG. 3C  illustrates a bottom view of an example IR LED housing for a thermal imager. 
         FIG. 4  illustrates a flow chart of a method by which a thermal imager captures a thickness of a part. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Schematically illustrated in  FIG. 1  is an example thermal imaging device  10 . The thermal imaging device  10  includes a controller  20 , a flash source  30  and an IR sensor  40 . In front of the thermal imaging device  10  is a work piece  50 . The work piece  50  can be of any shape or material. When the thermal imager  10  is activated, the flash source  30  emits a pulse of IR radiation. The pulse is a short duration pulse. By way of example, the pulse could be under 3 ms. The electromagnetic emissions from the pulse impact the work piece  50 . The electromagnetic emissions are absorbed and reflected by the work piece  50  and a portion of the thermal energy radiated by the work piece  50  and strike the sensor  40 . The reflected radiation should be minimized. 
     The sensor  40  detects the magnitude of the IR portion of the emitted electromagnetic emissions, and communicates the magnitude to a controller  20  such as a computer, a microprocessor, or programmable logic controller. The controller  20  also determines the magnitude of the emitted IR radiation from the pulse based on the duration of the pulse and the magnitude of power provided to the flash source  30  during that duration. The controller  20  determines the thickness of the work piece  50  by calculating the Temperature-time curve of the sample surface temperature decay history. The controller  20  can then generate a thermal image of the work piece  50  using known techniques. 
     IR radiation covers wavelengths of approximately 770 nm to 1 mm, whereas the spectrum emitted by xenon strobe light, as is used in conventional systems, covers the full visible spectrum and ultraviolet spectrum as well as the IR portion of the electromagnetic spectrum. Therefore, when using a xenon strobe to create the IR pulse, a full spectrum burst containing high levels of visible light and UV light along with the IR radiation is created. Such a burst requires a large expenditure of energy creating the pulse in the portions of the electromagnetic spectrum aside from the IR portion. Contrary to the burst created by a xenon strobe, a flash source  30  which is constructed out of an array of IR LEDs will emit minimal radiation outside of the IR portion of the electromagnetic spectrum, thereby reducing energy spent generating electromagnetic radiation outside of the IR portion of the spectrum. IR LEDs are LEDs which emit no visible light and UV light when an electric charge is applied, yet still emit a high magnitude of IR radiation. 
       FIG. 2  illustrates another schematic diagram of an example thermal imager  100  using an IR LED array  130  as the flash source. As in the example of  FIG. 1 , the thermal imager  100  has a sensor  140 , a controller  120  and an IR LED array  130  in place of the flash source  30 . The IR LED array  130  has a housing with a cup shaped component  132  and a cylindrical component  134 . The cup shaped component  134  has multiple LED sockets with installed IR LED&#39;s  138  arranged about its surface. The cylindrical component  134  is hollow, thereby allowing IR radiation  142  to pass through the LED array housing and strike the sensor  140 . An example physical arrangement of the IR LED array housing is illustrated in  FIGS. 3A ,  3 B, and  3 C, and a more detailed description of the example arrangement is provided below. 
     The sensor  140  is aligned with the cylindrical component  134  such that IR radiation  142  from a pulse that is reflected off the work piece  150 , will pass through the cylindrical component  134  and strike the sensor  140 . Multiple IR LEDs are arranged on the inside of the cup shaped component  132 , and are angled such that emitted IR radiation  136  strikes the work piece  150  at a single focal point  280  which is shared by all the IR LEDs. When the IR radiation  136  strikes the work piece  150 , a large portion of the IR radiation is absorbed by the work piece  150 , and a small portion reflects off the work piece  150 . The sensor  140  senses the magnitude of the emitted IR radiation  142 . 
     The sensor  140  is connected to the controller  120 . The controller  120  receives a signal from the sensor  140  when the emitted IR radiation  142  strikes it and determines the magnitude of the emitted IR radiation  142 . The controller  120  may also include a simple electrical control circuit for providing and limiting power to the IR LED array  130 . The control circuit can be constructed according to any number of known principles using standard components. The controller  120  can additionally determine the magnitude of the IR radiation  136  which was output from the IR LED array  130  based on the duration of the pulse as well as the magnitude of electrical power which was transmitted through the control circuit during the pulse. 
       FIGS. 3A ,  3 B, and  3 C illustrate an example LED array housing, such as could be used in the example thermal imagers of  FIGS. 1 and 2 .  FIG. 3A  illustrates a side view,  FIG. 3B  illustrates a top view, and  FIG. 3C  illustrates a bottom view of the IR LED array housing. The IR LED array housing has a cylindrical component  134  and a cup shaped component  132 . The cylindrical component  134  has an opening  260  on a first end, and is connected to the cup shaped component  132  at a second end. The cup shaped component  132  has an opening  262  on a first end opposite the cylindrical component  134 . The cup shaped component  132  and the cylindrical component  134  are joined at respective second ends. The cylindrical component  134  opening  260  and the cup shaped component  132  opening are connected via a passageway, thereby forming a substantially tube shaped IR LED array housing through which electromagnetic radiation can pass. 
     Spaced around the cup shaped component  134  are multiple LED sockets  272 . In the illustrated example of  FIGS. 3A ,  3 B, and  3 C the sockets  272  are arranged in two circular patterns approximately concentric to a cross-section of the first and second openings  260 ,  262  of the cup shaped component  132  and the cylindrical component  134 . The sockets  272  are further arranged such that when LEDs are installed in the sockets  272  and illuminated, each of the LEDs will share at least a single focal point  280  away from the cup shaped component  132 . This arrangement is illustrated in the Figures using axis lines  282  which define a vertical axis of each socket opening and a corresponding axis line  282  of each installed LED. 
     While the LED sockets  272  are arranged such that they share a focal point  280  while illuminated, it is understood that an alternate arrangement could be used which causes a broader beam without a single focal point, thereby encompassing the entire part, and generating an image based on the IR reflection from the broader beam. While the cup shaped component  132  and the cylindrical component  134  are illustrated having concentric circular cross sections, other shaped cross sections could be used, as well as non-concentric cross sections. 
     Referring again to  FIG. 1  a controller  20  executes a thermal imaging process. The functions of the process are disclosed in terms of a functional block diagram (see  FIG. 4 ) and may be executed in either dedicated hardware circuitry, or in a programmed software routine capable of executing in a microprocessor based environment. The controller  20 , typically includes a processor  20 A, a memory  20 B, and an interface  20 C. The memory  20 B may be any known memory type which is capable of storing instructions for performing the thermal imaging process. 
     Illustrated in  FIG. 4  is a method for generating a thermal image of a work piece using a thermal imager. Initially the thermal imager generates a pulse of IR radiation using an array of IR LEDs which are controlled by a controller in the Generate IR LED Pulse step  310 . In order to emit the pulse, the controller allows a short electrical current to pass from a voltage source to the IR LED array, thereby causing the IR LED array to begin emitting radiation. The magnitude of electrical current required to emit the necessary amounts of IR radiation is sufficiently small due to the nature of IR LEDs, and therefore the controller can use standard power circuitry to control the current. The controller allows the IR LEDs to emit radiation for a predetermined duration, typically less than 3 ms, and then prevents electrical power from reaching the IR LED array, thereby stopping the pulse. 
     During the IR radiation pulse, the IR radiation impacts a work piece which is in front of the thermal imager and a portion of the thermal radiation by the work piece  50  is radiated back toward the thermal imager. The emitted portion of the IR radiation is sensed using a sensor within the thermal imager in the Sense Emitted IR Radiation step  320 . The controller is communicatively coupled to the sensor and receives the sensed data. Based on the sensed data, the duration of the pulse, and the magnitude of the electrical power provided during the duration the thermal imager can determine the thickness of the work piece according to known thermal imaging techniques in the Determine Thickness Based on emitted IR Radiation step  330 . 
     While an example method has been illustrated above, it is understood that minor variations to the apparatus or method fall within this disclosure. Such variations include varying the duration of the pulse time, and varying the portion of the electromagnetic spectrum, which is used in the imaging process. 
     Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this disclosure.