Abstract:
In an uncooled camera having a focal plane array and an optical lens for focusing radiation from infrared radiation sources onto the focal plane array, the improvement comprising providing a thermochromic optical switch for blocking radiation from an extremely hot infrared radiation source from reaching the focal plane array.

Description:
GOVERNMENT INTEREST 
       [0001]    The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to an optical switch window for an uncooled Focal Plane Array (FPA) package and in particular to a thermochromic optical switch window coated with crystalline vanadium oxide (VO 2 ). 
       BACKGROUND OF THE INVENTION 
       [0003]    Some micro-bolometer-based uncooled infrared focal plane arrays can be temporarily or permanently damaged if exposed to infrared radiation coming from extremely hot or bright sources. 
         [0004]    What is needed is a low cost solution to minimize the degradation or damage to uncooled focal plane arrays from infrared radiation. Previously, it was known in U.S. Pat. No. 7,259,925 to Byong H. Aim of the Army Night Vision Laboratory that a layer of VO 2  would generally protect infrared sensors in a Forward Looking Infrared imager (FLIR) against radiation from high power lasers. But, this patent does not disclose how the VO 2  layer should be incorporated in an uncooled sensor, or how it can be designed to block radiation that is less intense than lasers, such as sunlight as a special case. 
       SUMMARY OF THE INVENTION 
       [0005]    The sensor system has a housing which contains optical elements disposed within the housing and at a first end of the housing to converge light rays entering the housing onto a focal plane. A focal plane array is disposed within an FPA package inside the sensor housing substantially coincident with the image plane. A means for protecting the sensor system from intense infrared radiation is disposed upon the window of the FPA package or between the window and the FPA. It is therefore an object of this invention to protect a focal plane array from extremely hot or bright infrared radiation sources. 
         [0006]    This and other objects of the invention are achieved in one aspect by an uncooled camera having a focal plane array package with an optical lens for focusing infrared radiation from infrared radiation sources onto the FPA. The improvement comprises providing in addition a means for blocking radiation from an extremely hot infrared radiation source from reaching the FPA. The blocking means may be located on the window of the FPA package or between the window and the FPA. 
         [0007]    Another embodiment of the invention is where the blocking means is a layer of VO 2 . 
         [0008]    A further embodiment of the invention involves a method for protecting an uncooled focal plane array by blocking radiation from an extremely hot infrared radiation source from reaching the focal plane array. 
         [0009]    Additional advantages and features will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a conventional uncooled camera with a Focal Plane Array. 
           [0011]      FIG. 2  shows the rear area of the camera of  FIG. 1  with an optical switch located on a window of Focal Plane Array. 
           [0012]      FIG. 3  shows an FPA package with an optical switch located between the window and the Focal Plane Array. 
           [0013]      FIG. 4  shows  FIG. 3  with an exploded view of the optical switch located near the pixels of the FPA. 
           [0014]      FIG. 5  shows the rear area of the camera of  FIG. 1  in the presence of an extremely hot radiation source. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Referring to the drawing,  FIG. 1  shows a conventional uncooled camera with a lens having optical elements  1 ,  2  and Focal Plane Array (FPA) package  3 . Infrared radiation from the scene travels through the lens and is focused into the FPA package. The figure shows ray traces at three specific field angles  4 ,  5 ,  6 , but those familiar with the art will recognize that in fact the entire FPA is illuminated with varying degrees of infrared radiation due to the infrared scene. 
         [0016]      FIG. 2  focuses on the rear area from  FIG. 1  and shows that the FPA package  3  is further composed of the FPA  7  and the FPA package window  8 . In accordance with the invention, a means is provided for blocking radiation from an extremely hot infrared radiation source from reaching the focal plane array. The blocking means is optical switch  9  which is shown located on the rear of the FPA window, where the optical gain is the highest. The optical switch is a thin layer of crystalline VO 2 , a material which undergoes a reversible semiconductor-to-metal phase transition upon heating at approximately 67 degrees Celsius. Upon switching, high quality VO 2  films (previously transparent to long wavelength infrared) become opaque, both reflecting and absorbing the incident radiation. For normal scene intensities, the infrared radiation from all fields is transmitted through the window and the VO 2  switch. 
         [0017]      FIG. 3  discloses an FPA package  3  with window  8 , FPA  7  and optical switch  9  located on thermal insulation  10  within the package between the window and the FPA. 
         [0018]      FIG. 4  is similar to the FPA package  3  of  FIG. 3  in that the VO 2  switch is between the window and the FPA. However, this is an alternate configuration, seen best in the exploded view which shows individual optical switches  9  on thermal insulation  10  in front of each individual detector pixel  11 . Each optical switch is designed to have a thermal time constant in the range of milliseconds to promote heating of the optical switch and therefore the VO 2  layer to switch from a normally transparent state to an opaque state when heated by infrared radiation from an extremely hot infrared radiation source and returning to the normally transparent state when the extremely hot infrared radiation source is removed. 
         [0019]    As shown in  FIG. 5 , if a region of the scene becomes extremely bright in the infrared, the optical switch heats up in the corresponding region of the window, and switches locally  12 , blocking the intense infrared radiation from reaching the FPA. 
         [0020]    A thin film VO 2  layer can be incorporated into the anti-reflection coating on the window of the uncooled FPA package. The window may be made out of germanium, ZnSe or other material transparent to far-infrared and amenable to the deposition of VO 2 . It is near the image plane and is subject to approximately the same scene irradiance as the FPA for extended sources. The VO 2  layer is designed to absorb approximately 10% of the in-band infrared radiation. Exposure to an extremely hot source causes the film to heat beyond the transition temperature, thus switching to the reflective state. In the reflective state, the irradiance on the FPA is limited to levels below those which cause adverse effects. In the reflective state, the layer then cools until it becomes transmissive again. If the hot source of radiation is still present, the cycle repeats and thus, the device is self-regulating. 
         [0021]    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 present invention may be practiced otherwise than as described. 
         [0022]    Although the invention has been shown and described with respect to certain preferred embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular with regard to the various functions performed by the above described components (assemblies, devices, sensors, circuits, etc), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. For example, the VO 2  optical switch can also protect the FPA from CW and pulsed inband lasers. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments, such feature may be combined with one or more other features of the other embodiments as may be desired and advantageous for any given or particular application.