Abstract:
A thermal imager including a cell containing a fluid whose refractive index varies with temperature and, optical components for focusing the infrared image of a scene to be viewed onto the cell to produce local temperature differences across the fluid. The local temperature differences give rise to local refractive index differences. The thermal imager also includes a Schlieren optical system for reading out the local refractive index differences to reproduce the scene.

Description:
GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to thermal imagers, and more particularly, to hyper-spectral imagers. 
     BACKGROUND OF THE INVENTION 
     Staring array imagers use a solid semiconductor focal plane surface to generate a television-type thermal image display. The focal plane surfaces are extremely thin and may be subject to permanent thermal shock damage from excessive thermal energy. Since these focal planes are relatively expensive to fabricate, a new type of self-healing focal plane array lower in cost is desirable. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a more damage-resistant staring thermal imager than is available in the current art. 
     This and other objects of the invention are achieved in one aspect by an improved thermal imager. The thermal imager includes a cell containing a fluid whose refractive index varies with temperature, means for focusing the infrared image of a scene to be viewed onto the cell to produce local temperature differences across the fluid which give rise to local refractive index differences, and means for reading out the local refractive index differences to reproduce the scene. 
     Another aspect of the invention involves an improved method of generating thermal images comprising the steps of providing a cell containing a fluid whose refractive index varies with temperature, focusing the infrared image of a scene to be viewed onto the cell to produce local temperature differences across the fluid, the local temperature differences giving rise to local refractive index differences, and reading out the local refractive index differences to reproduce the scene. 
     Additional advantages and features will become apparent as the subject invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a first embodiment of a thermal imager embodying the invention. 
         FIG. 2  is a schematic illustration of a second embodiment of a thermal imager embodying the invention. 
         FIG. 3  is a schematic illustration of a third embodiment of a thermal imager embodying the invention. 
         FIG. 4  is a schematic illustration of a fourth embodiment of a thermal imager embodying the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing, wherein like reference numerals designate like or corresponding parts,  FIG. 1  shows a first embodiment of the thermal imager. The optical components of the thermal imager are enclosed in an airtight container  11  with an IR-transparent window  13 . The container  11  is either evacuated or filled with a dry non-IR-absorbing gas such as Nitrogen. IR light  15  from a scene to be viewed enters the window  13  and is focused by means for focusing, e.g., an LWIR/MWIR objective lens  17  onto a thin optical cell  19 , after being reflected off a dichloric (visible/NIR transmitting, SWIR/LWIR reflecting) beam splitter  21 . The optical cell  19  has entrance and exit windows that are LWIR/MWIR as well as visible/NIR transparent. The optical cell  19  contains an IR-absorbing, but optically-transparent fluid, such as the gas Sulfur Hexafluoride, with IR absorption characteristics consistent with the desired IR image wavelength pass-band (and optical/NIR transparency characteristics consistent with visible/NIR optical readout). The IR energy from the scene to be viewed that is focused on the fluid in the optical cell  19  gives rise to local temperature differences across the optical cell corresponding to the focused IR light image of the scene to be viewed. These local temperature differences give rise in turn to local optical refractive index differences. The resulting optical refractive index map in the cell is then read out by means of a Schlieren optical system wherein a parallel beam of visible/NIR light  23  is passed through the optical cell  19  from a point source illuminator  25  and a collimating mirror  27 , and then focused with a secondary objective mirror  29 , after traversing the beam splitter  21 , onto a visible/NIR, CCD or CMOS camera focal plane array  31  which transforms the image of the scene to be viewed into a suitable electronic format for computer or TV presentation. The use of Schlieren optical systems to optically read out a high-resolution thermal refractive index map is well known in the art and is disclosed in U.S. Pat. No. 6,181,416 to Falk, and in the article R. Aaron Falk, “Backside Thermal Mapping Using Active Laser Probe,” Electronic Device Failure Analysis News, May 2000, the disclosures of which are hereby incorporated by reference. 
       FIG. 2  shows a second embodiment of the thermal imager which differs from the first embodiment by the substitution of a single spherical or parabolic mirror  33  for the collimating mirror  27 , the secondary objective mirror  29 , and the beam splitter  21 , in order to greatly simplify the optical system. As with the first embodiment, the total optical system can be enclosed in a gas-tight container  11 , but with a larger IR-transparent window  13 . The container  11  is either evacuated or filled with a dry, SWIR-LWIR-transparent gas such as Nitrogen. A visible or NIR emitting point source  25  is positioned slightly off axis of the mirror  33  at or near its radius (twice the focal length). A visible/NIR focal plane imager  35  is placed slightly off axis (and at a distance greater that the mirror radius) such that the reflected point source light will pass through an optical cell  19  and also such that the image blur circle of the point source  25  is sufficient to fill the entire focal plane imager  35 . The optical cell  19  is also placed slightly off the mirror axis such that the reflected point source light will pass through the optical cell, but not the direct light traveling to the mirror  33 . The design of this optical system is therefore a compromise between off-axis image quality at the mirror focal plane, the desired diameter of the optical cell and the size of the focal plane array, the object being to avoid mechanical interference and light obstruction between the point source  25  and the visible/NIR focal plane imager  35 . 
     In operation, IR light  37  from the scene to be viewed passes through the IR transparent window  13  and is focused by means for focusing, e.g., the mirror  33  on the optical cell  19 . The resulting optical refractive index map is read out by means of light  39  emitted by the visible or NIR-emitting point source  25  which is reflected off the mirror  33 , passes through the optical cell  19 , and is intercepted by the visible/NIR focal plane imager  35 . The focal plane imager  35  may optionally have a pass-band filter  41  placed just in front of it, as shown. The purpose of this filter is to pass only the near IR or visible narrowband wavelength emitted by the point source  25 , and to block any other wavelengths, thereby reducing the cost, band-pass, and material requirements of the larger entrance window  13 . 
       FIG. 3  shows a third embodiment of the thermal imager which differs from the first embodiment by the substitution of a Bragg cell  43  for the optical cell  19 . That is, optical components of the thermal imager can be enclosed in an airtight container  11  with an IR-transparent window  13 . The container  11  can be either evacuated or filled with a dry non-IR-absorbing gas such as Nitrogen. IR light  15  from a scene to be viewed enters the window  13  and is focused by means for focusing, e.g., an LWIR/MWIR objective lens  17  onto a Bragg cell  43 , after being reflected off a dichloric (visible/NIR transmitting, SWIR/LWIR reflecting) beam splitter  21 . Further, a parallel beam of visible/NIR light  23  can be passed through the Bragg cell  43  from a point source illuminator  25  and a collimating mirror  27 , and then focused with a secondary objective mirror  29 , after traversing the beam splitter  21 , onto a visible/NIR, CCD or CMOS camera focal plane array  31  which transforms the image of the scene to be viewed into a suitable electronic format for computer or TV presentation. 
       FIG. 4  shows a fourth embodiment of the thermal imager which differs from the second embodiment by the substitution of a Bragg cell  43  for the optical cell  19 . That is, the total optical system can be enclosed in a gas-tight container  11 , but with a larger IR-transparent window  13 . The container  11  can be either evacuated or filled with a dry, SWIR-LWIR-transparent gas such as Nitrogen. A visible or NIR emitting point source  25  is positioned slightly off axis of means for focusing, e.g., the mirror  33 , at or near its radius (twice the focal length). A visible/NIR focal plane imager  35  is placed slightly off axis (and at a distance greater that the mirror radius) such that the reflected point source light will pass through a Bragg cell  43  and also such that the image blur circle of the point source  25  is sufficient to fill the entire focal plane imager  35 . The Bragg cell  43  can also be placed slightly off the mirror axis such that the reflected point source light will pass through the optical cell, but not the direct light traveling to the mirror  33 . The resulting optical refractive index map can be read out by means of light  39  emitted by the visible or NIR-emitting point source  25  which is reflected off the mirror  33 , passes through the Bragg cell  43 , and is intercepted by the visible/NIR focal plane imager  35 . The focal plane imager  35  may optionally have a pass-band filter  41  placed just in front of it, as shown. 
     In both the third and the fourth embodiments, the IR energy from the scene to be viewed that is focused on the Bragg cell gives rise to local temperature differences across the cell corresponding to the focused IR light image of the scene to be viewed. These local temperature differences give rise in turn to local acoustic refractive index differences. The resulting acoustic refractive index map in the cell is then read out by means of the Schlieren optical system. The temperature-induced variation of the acoustic refractive index of a fluid such as air is well known to be roughly a million times greater than the corresponding optical refractive index variation, mainly due to the difference between the speeds of light and sound. Therefore, the third and fourth embodiments produce a greatly enhanced refractive index map to be read out by the Schlieren optical system. 
     It is obvious that many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as described.