Abstract:
A compact infrared (IR) scene generator capable of generating multiple-color mid-IR scenes through the use of readily available commercial near-IR lasers and a fluorescent conversion material (FCM). Such a scene generator would be useful to test IR imaging sensors in a controlled laboratory environment. In operation, each laser emits energy at an initial wavelength outside the operating band of an IR imaging sensor. This energy of a first set of wavelengths is written onto the FCM in patterns, which collectively form an IR scene. The FCM absorbs the energy and radiates it at wavelengths longer than the initial wavelengths, i.e., a second set of wavelengths. As these longer wavelengths are within the operating waveband of the IR imaging sensor, the patterns written onto the FCM are detectable by it.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to the testing of sensor systems and, in particular, to a system of generating an infrared scene using a fluorescent conversion material (FCM). 
     2. Background Information 
     The use of infrared (IR) imaging sensors, both mid-wave infrared (MWIR) operating approximately in the 3-5 μm waveband and long-wave infrared (LWIR) operating approximately in the 7-12 μm waveband, have become common in both the commercial and military applications. In military and security systems, for example, IR imaging sensors have often been used to monitor and visually describe spatial areas to determine the presence or movement of objects within the area. 
     A sensor system accomplishes this by detecting the various temperatures in an area and then by generating an image showing the distribution of temperatures. Such an image or “scene” is used to determine, among other information, the shapes of objects in the area and their proximity to the sensor system. An example of a sensor system used for scene generation is described in U.S. Pat. No. 5,710,431, the disclosure of which is hereby incorporated by reference in its entirety. In a military setting, for example, objects detected may be enemy troops or vehicles; therefore, it is essential that the IR imaging sensors be precisely calibrated, sighted, and in otherwise perfect working order. 
     To accurately and completely characterize the performance of a system utilizing such sensors, it is advantageous to be able to generate synthetic infrared scenes in a controlled laboratory environment. The most widely used IR scene generator produces images with an array of resistive heaters, also known as a microbolometer. However, this type of device has numerous shortcomings. 
     First, a microbolometer takes time to both heat up and cool off, approximately 15 milliseconds for each transition. Second, with resistive heaters, long periods of time spent simulating a bright source will cause adjacent areas of the array to heat up, causing the scene in these areas to be wiped out. And third, the microbolometer must have a very large number of small resistors to achieve high resolution. Any failure of one of these heating elements will reduce the quality and capability of the scene generator. 
     What is needed is a system for testing IR imaging sensors by generating laboratory-controlled IR scenes in a way that is fast, accurate, and relatively inexpensive to manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system using a fluorescent conversion material for generating detailed, dynamic IR scenes that can be used in testing IR imaging sensors. 
     According to a first embodiment of the present invention, an infrared scene generator is provided, comprising a plurality of sources emitting energy at a first set of wavelengths, a fluorescent conversion material, a plurality of beam steering optics directing the emitted energy onto the fluorescent conversion material, wherein the first set of wavelengths are absorbed and radiated by the fluorescent conversion material as a second set of wavelengths, and an infrared imaging sensor detecting energy of the second set of wavelengths. 
     According to a second embodiment of the present invention, a system is provided for generating an infrared scene, comprising first emitting means for emitting energy of a first wavelength, converting means for converting the energy of a first wavelength to energy of a second wavelength using fluorescent conversion, and sensing means for detecting energy of the second wavelength. 
     According to a third embodiment of the present invention, a method is provided for generating an infrared scene, comprising the steps of emitting energy of a first wavelength, converting the energy of a first wavelength into energy of a second wavelength using a fluorescent conversion material, and detecting energy of the second wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments, when read in conjunction with the accompanying drawings wherein like elements have been represented by like reference numerals and wherein: 
     FIG. 1 illustrates a monochromatic scene generator in accordance with one embodiment of the present invention; 
     FIG. 2 illustrates a polychromatic scene generator in accordance with another embodiment of the present invention; 
     FIG. 3 is an exemplary diagram illustrating possible radiative transitions resulting from the use of Praseodymium doped into LaCl3 as a fluorescent conversion material; and 
     FIGS. 4 a  and  4   b  illustrate the process of fluorescence. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Exemplary embodiments of the present invention use a class of materials referred to as fluorescent conversion materials (FCM&#39;s). An example of a system using a fluorescent conversion material is described in U.S. Pat. No. 4,302,678, the disclosure of which is hereby incorporated by reference. FCM&#39;s possess the ability to absorb laser radiation of one wavelength and then, through a process of non-radiative and radiative transitions, emit one or more photons at a longer wavelength. This method offers at least three advantages over the microbolometer approach. 
     First, a system using fluorescent conversion allows much faster changes to be made to an IR image and produces radiation almost immediately, without the need to heat-up or cool-down waiting periods associated with a microbolometer. In addition, when a stimulating energy (i.e., radiation) directed onto a fluorescent material is stopped, the material will continue to fluoresce (i.e., radiate the absorbed energy at a second wavelength) for anywhere from 100 microseconds to 20 milliseconds depending on the specific material properties selected. This faster response time will permit the displaying of more dynamic scenes and the testing of higher frame rate sensors. Second, the simulated scene will not bloom during the generation of bright objects. In other words, unlike with a microbolometer, the areas where a stimulating radiation is incident on a fluorescent material will not increase in size and “blur out” adjacent areas when the material radiates. Third, the monolithic nature of the fluorescent conversion material will not result in single point failures, unlike a microbolometer, which depends on each and every resistor to achieve high resolutions. Because of this, the use of a fluorescent conversion material will lead to higher yields and require lower fabrication costs. 
     The quantum or photonic process known as fluorescent conversion is shown in FIG. 4 a , whereby at time t 1  an ion or a molecule in the ground state S 0  absorbs an incident photon that falls within the characteristic absorption bands of that particular ion or molecule, and is elevated to an excited state S 1 . At time t 2 , the excited ion/molecule relaxes, through vibrational or rotational interactions to a lower (less energy) excited state S 2  and then finally relaxes back to the ground (unexcited) state at t 3  through the emission of a photon of longer wavelength λ 2  than the wavelength of the exciting photon λ 1 . In this way, a material that possesses the ability to perform fluorescent conversion can, for example, be optically “pumped” in near-IR region (˜0.7 to 3.0 μm) and then fluoresce in mid-IR region (˜3.0 to 5.0 μm). 
     FIG. 4 b  illustrates the relationship between energy intensity and wavelength. Energy (i.e., a photon) of a shorter wavelength, as seen in FIG. 4 b , is absorbed by an ion or a molecule but may not be “seen” by a passive sensor of a particular operating band (shown by the dotted line). Once the photon is emitted (as at t 3  in FIG. 4 a ), the wavelength is increased and the energy may be detected by the sensor. 
     Two exemplary embodiments of an IR scene generator according to the present invention are shown in FIGS. 1 and 2. Both the monochromatic scene generator  100  of FIG.  1  and the polychromatic scene generator  200  of FIG. 2 comprise four primary modules. 
     The first module comprises one or more first emitting means for emitting energy at either a first wavelength or a first set of wavelengths. These energy input means or sources are represented as a single laser  101  in FIG.  1  and as a plurality of lasers  201   a -N in FIG.  2 . If the IR scene to be generated can be monochromatic (only one IR color) then only one input laser  101  would be required. If a polychromatic (multicolor) IR image is desired, then multiple input lasers  201   a -N can be used to address multiple absorption/emission bands in the fluorescent conversion material  203 . In the case of FIG. 1, laser  101  provides an input energy with an wavelength λ 1  that the FCM  103  would then convert up to a energy with a longer wavelength λ 2  that is within the sensitive waveband of the IR imaging sensor  105 . 
     FIG. 2 illustrates a polychromatic scene generator  200  generating an N color IR image and using N input lasers  201   a -N. N can be any arbitrary number greater than 1, but in exemplary embodiments can be practically limited by the number of emission bands present in the FCM  203 . In addition, the intensity of the generated scene features can be modulated by modulating the intensity of the input laser  101  or lasers  201   a -N. These energy sources do not have to be lasers, but the use of commercial diode lasers or commercial solid state lasers (such as Nd:YAG, operating in the visible- to near-IR wavelength regions, 0.7 to 3.0 μm) would be an easy and effective solution. 
     The second module is the beam steering optics, represented as single beam steering optic  102  in FIG. 1 and a plurality of beam steering optics  202   a -N in FIG.  2 . The beam steering optic  102  or beam steering optics  202   a -N act to direct emitted energy onto a fluorescent conversion material (e.g., FCM  103  or  203 ) and can be any type of 2-axis computer controlled beam steering devices, such as fast steering or scanning mirrors, prisms, Acousto-Optic modulators, or Electro-Optic modulators. The use of these devices for “writing” patterns with lasers is very well developed, being used extensively in both commercial entertainment (laser light shows) and the semiconductor industry (writing complex patterns for lithographic exposures). For monochromatic scene generators, only one beam steering optic  102  is required, and no beam combining optics (elements  208   a -N in FIG. 2) are needed. For a polychromatic scene generator, a separate beam steering optic ( 202   a, b, c , or N) may be required for each input laser ( 201   a, b, c , or N, respectively), so that each laser energy can be “written” onto the FCM in a different pattern. The beam combining optics  208   a -N may be dichroic beam combiners that reflect the wavelength being inserted into the common path  206  and transmit all of the other wavelengths in the path  206 . For example, beam combiner  208 N reflects the wavelength of laser  201 N and transmits the wavelengths of lasers  201   a-c.    
     The third module is a converting means using fluorescent conversion, represented by FCM  103  and  203  in FIGS. 1 and 2, respectively. The FCM  103  or  203  can be a plate of arbitrary cross sectional shape that allows the imaging sensor  105  or  205  to “see” the incident laser energy by converting the laser wavelength(s) to a wavelength(s) that falls within the sensitivity waveband of the imaging sensor  105  or  205 . During the generation of a polychromatic IR image, for example, energy from lasers  201   a -N are focused onto the FCM  203  and scanned (or “written”) onto the surface in the pattern desired, absorbed by the FCM  203 , and then at the areas where the laser energys are incident, the FCM  203  will radiate them at wavelengths that are detectable by the IR imaging sensor  205 . 
     The FCM  103  or  203  can be fabricated as a thin flat plate with two major surfaces. A first surface of the plate, which is nearest to laser  101  or lasers  201   a -N, can be anti-reflection (AR) coated for the laser wavelength(s) and rejection coated for the waveband of the passive imaging sensor  105  or  205 . The back surface of the plate, nearest to the IR imaging sensor  105  or  205 , can be AR coated for the waveband of the passive imaging sensor  105  or  205 , but can be coated with a blocking filter for the laser wavelength(s) to provide protection to the IR detector array (comprised in IR imaging sensor  105  of  205 ) against direct illumination by the laser  101  or lasers  201   a -N. 
     One candidate for use as the FCM  103  or  203  in a mid-wave IR (MWIR) scene generator is the trivalent rare earth ion Praseodymium (Pr 3+ ) 1  doped into various hosts. FIG. 3 shows the radiative transitions that can occur between the lower level manifolds of Pr 3+  when doped into LaCl3 as the host material. The numbers in each downward pointed arrow represent the nominal wavelength (in micrometers) of the energy emitted when the material relaxes from a higher level to a lower level as indicated by the ends of the arrow. For instance, exciting the ion from the  3 H 4  (ground) level to the  3 H 6  level and then having it relax back down to the  3 H 5  level would produce fluorescence at around 4.7 μm. Further radiative transitions from the  3 H 5  back to the  3 H 4  levels would again emit at around 4.8 μm. 
     Table I gives a summary of how this material can be used to generate a polychromatic MWIR scene generator. By pumping the FCM  203  with multiple lasers  201   a -N, different spectral components in the image can be generated at specific locations, and the relative intensities of the spectral components can be adjusted by varying the intensity of the input laser. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Potential Input and Output Wavelength from Pr 3+ :LaCl 3   
               
             
          
           
               
                 Pump (absorption) 
                 Pump wavelength (μm)/ 
                 Fluorescent wavelength 
               
               
                 transition 
                 potential input laser type 
                 (μm) 
               
               
                   
               
               
                   3 H 4 → 1 G 4   
                 1.01-1.06/Nd: YAG, Nd: 
                 4.7-4.8 
               
               
                   
                 Glass 
               
               
                   3 H 4 → 3 F 2   
                 2.0-2.1/Tm: YAG 
                 3.7, 3.7-4.8 
               
               
                   3 H 4 → 3 F 3   
                 1.55-1.58/Nd: YAG 
                 5.2 
               
               
                   
                 OPO, diodes 
               
               
                   
               
             
          
         
       
     
     The fourth module is a sensing means for detecting energy of a second wavelength or of a set of second wavelengths, that is, energy radiated by FCM  103  or  203 . The sensing means is represented by IR imaging sensor  105  in FIG.  1  and IR imaging sensor  205  in FIG.  2 . 
     In this way, the present invention provides a simple and effect method of generating a multiple-color IR scene, such as would be seen in an operational setting where multiple objects of different temperatures are in a single image, or when an object with a temperature distribution across its projected surface is present in the scene. 
     It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced within.