Abstract:
An apparatus and method for evaluating window transmission loss comprising taking a plurality of photographs through a filter of a window to be evaluated, determining a percentage of the window shown in each photograph that is undamaged, and computing an estimate of transmission loss for the window from the percentages determined.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     COPYRIGHTED MATERIAL 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention (Technical Field) 
     The present invention relates to devices and methods for evaluating damage to windows, including aircraft-mounted infrared windows with surface damage due to high speed impacts with sand and dust particles. 
     2. Description of Related Art 
     Infrared windows must periodically be evaluated for transmission loss resulting from surface damage due to high speed impacts with sand and dust particles in flight. Currently, these windows have to be removed for evaluation back at the window supplier. Typically this requires removal of the window from the aircraft in which the window is mounted, sending it back to a supplier, and measuring it with an infrared instrument costing on the order of $250,000. Thus, a need exists for making such evaluations in the field and/or in situ with inexpensive equipment. 
     It has been suggested that a portable scatter meter would make adequate measurement, but such an instrument was tried on the same samples used in testing of the present invention and the correlation was poor. Furthermore, this purportedly alternative device contains a laser, complicating its use in the field. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is of an apparatus and method for evaluating window transmission loss, comprising: taking a plurality of photographs through a filter of a window to be evaluated; determining a percentage of the window shown in each photograph that is undamaged; and computing an estimate of transmission loss for the window from the percentages determined. In the preferred embodiment, taking comprises taking the plurality of photographs through a filter and a prism, most preferably a Wollistan prism. The window is illuminated, preferably using a light passed through a polarizing filter. Computing comprises averaging the determined percentages, preferably also adding a transmission loss of an original sample window to a slope multiplied by the computed average. The apparatus is preferably portable. The photographs are preferably magnified, most preferably at least about 100×. 
     Further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one or more embodiments of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating one or more preferred embodiments of the invention and are not to be construed as limiting the invention. In the drawings: 
         FIG. 1  is a schematic diagram of a device according to the invention; and 
         FIG. 2  is a graph illustrating the basis for the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is of a device and method allowing surface damage area to be determined by a series of photos of the surface of a window (e.g., a silicon window), preferably taken at a magnification (most preferably 100×) using a filter, thereby allowing calculation of transmission loss. The invention uses a camera, filter, and an algorithm to determine the extent of area damaged. This damaged area prevents transmission and correlates very well with transmission loss. Analyses of test samples from sand and dust testing have shown that results from the invention correlate well with actual transmission loss in the 3 to 5 mid-wavelength range. With 13 sample reference points, measurement of transmission loss via the invention would take approximately 13 minutes or less. 
       FIG. 1  is a schematic diagram of the device  10  according to the invention for analyzing window under test  32 , the device comprising camera  12 , analyzer filter  14 , processing unit  16 , power supply  18 , light source  20 , polarizing filter  22 , reflector  24 , prism  26 , stand-off  28 , and interchangeable nosepiece  30 . Camera images are passed through the analyzing filter and the results are analyzed by processing unit  16 . Camera  12  is preferably a digital camera (e.g., a camera chip with lenses) providing for magnification, preferably of at least 100×. Filters  14  and  22  are preferably rotatable. Processing unit  16  can be any appropriate computing hardware, software, firmware, or a combination thereof, including central processing units, field-programmable gate arrays, and like units. 
     The light source is preferably a lamp (e.g., a halogen lamp), light-emitting diode (LED), or plurality of LEDs. The prism is preferably a Wollistan prism. The interchangeable nosepiece provides for varying amounts of magnification. Preferably, the overall illumination technique used is known as differential interference contrast. 
     One or more (preferably a plurality) micro photographs are taken of a window to be evaluated (e.g., 13 micro photographs) which are compared to an original sample. Each micro photograph is evaluated to determine percentage of undamaged area of the window (based upon intensity levels of the matching individual pixels within the corresponding digital images). An average over all the micro photographs is determined. This percentage is compared to that of the original sample (usually zero). The transmission loss is calculated by adding the transmission loss of the original sample to a slope multiplied by the average percentage loss calculated for the window. 
       FIG. 2  shows test results and a concomitant calculation method for a test sample. With R 2  of 93%, the maximum transmission loss is 0.0019x+0.0044, where x is the percentage change in the undamaged area of sample as against the original specimen. 
     A hand-held version of the present invention preferably uses a camera chip and lenses to provide images which are evaluated by an on board processor. The resulting device is quite similar to a camera phone, and may even be included as part of a camera phone. 
     Note that in the specification and claims, “about” or “approximately” means within twenty percent (20%) of the numerical amount cited. 
     Although the invention has been described in detail with particular reference to these preferred embodiments, other embodiments can achieve the same results. Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above are hereby incorporated by reference.