You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
FIELD OF THE DISCLOSURE  
       [0001]    This disclosure relates generally to a fabric power door and more specifically to a flexible window for a door. 
       BACKGROUND  
       [0002]    Basically, there are two types of fabric power doors, those that have a curtain that moves generally vertically and those that move generally sideways. For vertically operating doors, a horizontal drum or roller above the doorway typically takes up or pays out a flexible curtain to respectively open or close the door. An example of such a vertically operating rollup door is shown in U.S. Pat. No. 7,151,450. Other vertically acting fabric power doors may have a fabric panel that does not roll up on a drum. For example, the fabric panel may take a spiral configuration in the stored open position, by being guided by spiral guideways at its edges, as it moves from the closed to the open position. Alternatively, the door may have a fabric panel that does not roll up or spiral at all, but rather maintains a generally planar configuration above the doorway when in the open position. Although stored this way, such doors still typically undergo some significant bending, such as a 45 or 90 degree turn around a roller as it moves between the open and closed positions. 
         [0003]    Horizontally acting fabric power doors, such as the one disclosed in U.S. Pat. No. 4,096,902; typically include at least one flexible curtain wrapped around at least one vertical roller at a lateral edge of the doorway. A generally vertical leading edge of the curtain translates across the doorway to open or close the door. As with vertically acting doors, a rolled configuration of the stored panel is one among many possible. 
         [0004]    To provide visibility through the curtain when the door is closed, often the curtain will include at least one window or “vision” panel. The window might be comprised of a transparent sheet of material that overlaps a window opening in the curtain. VELCRO or double-sided flexible tape between the overlapping sections of the sheet and the curtain can be used to hold the sheet in place. 
         [0005]    The differing material properties of the curtain material, and the material forming the window can lead to undesirable degradation of the window over time. The curtain is typically formed of a fabric base with a flexible coating, while the window is usually a unitary material, such as urethane, vinyl, or a polycarbonate (such as LEXAN). In general, the heavy curtain material is stiffer and more resistant to bending than the material forming the window. Moreover, the different constituents of the curtain (base fabric, coating, etc.) may allow it to better handle and distribute the stresses induced thereon by door operation, such as, rolling or spiraling into a stored configuration, or bending around a roller as it moves between open and closed positions, as compared to the ability of the unitary material of the window to handle such stresses. As a result of these differing properties, it has been seen that the resulting stresses placed on the window can degrade the material, particularly in the area of the connection between the window and the curtain. Even the conventional presence of a flexible or resilient connection between the overlapping edges of the window opening in the curtain and the window material itself (the aforementioned tape or VELCRO connection) is not adequate to relieve the stress and tension on the window caused by the flexing (or the reduced amount of flexing) of the adjacent curtain material, leading to the window&#39;s undesirable degradation. 
         [0006]    At the same time, the existing window materials have the desirable properties of providing a material both that provides adequate transparency, but that can handle being rolled or spiraled repeatedly. Some of the window materials, such as the LEXAN windows, are also more resistant to scratching (which can be desirable in a roll-up configuration where the window panels my rub together as they are wound onto the drum) and have better UV stability. The more flexible window materials (such as vinyl or urethane) also have desirable properties while maintaining adequate transparency. So, while the materials for fabric power door windows are acceptable for their purpose, their degradation owing to their differing material properties as compared to the remainder of the curtain limits their ability to provide a long-lasting clear window for the door. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0007]      FIG. 1  is a front view of a fabric power door in the form of a rollup door shown in a closed position. 
           [0008]      FIG. 2  is a front view of the door of  FIG. 1  but showing the door partially open. 
           [0009]      FIG. 3  is a front view of the door of  FIG. 1  but showing the door fully open. 
           [0010]      FIG. 4  is a closer front view of the window installed on the door of  FIG. 1 . 
           [0011]      FIG. 5  is a front view similar to  FIG. 4  but showing some parts partially pulled away from the completed assembly. 
           [0012]      FIG. 6  is a cross-sectional view taken along line  6 - 6  of  FIG. 1 . 
           [0013]      FIG. 7  is a cross-sectional view similar to  FIG. 6  with the components in an alternate arrangement. 
       
    
    
     DETAILED DESCRIPTION  
       [0014]    Certain examples are shown in the above-identified figures and described in detail below. In describing these examples, like or identical reference numbers are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic for clarity and/or conciseness. Additionally, several examples have been described throughout this specification. Any features from any example may be included with, a replacement for, or otherwise combined with other features from other examples. 
         [0015]    The illustrated examples of  FIGS. 1-3  show a fabric power door in the form of a rollup door  10  with a curtain  12  having a leading edge  14  that translates at a doorway  16  to open and close the door  10 . Other forms of vertically acting fabric power doors exist (the above-referenced spiral-storing doors, and those that store in a generally planar configuration) and would also benefit from the window construction to be described herein. Moreover, although leading edge  14  is horizontal and moves vertically, this disclosure further applies to horizontally acting fabric power doors where the leading edge is vertical and translates horizontally across the doorway. In any of these cases, the curtain  12  undergoes some bending stress, such as, the curtain  12  wrapping around a roller  18  that selectively takes up or pays out the curtain  12  to respectively open or close the door  10 , or the curtain  12  coiling up into a stored configuration, or turning 45 or 90 degrees around the roller  18  as it is driven at its edges between open and closed positions. For the purposes of this description, such bending of the curtain  12  as it travels will be referred to as the curtain assuming “at least a partially curved configuration as it moves between open and closed positions.” Door  10 , for example, includes the roller  18  that is driven by a motor  20 . Depending on the motor&#39;s  20  direction of rotation, curtain  12  moves to either open or close the door  10 . While the window construction  22  described herein could apply to any of this variety of fabric power doors, the roll-up style has been depicted as it may represent the most extreme case of the attendant stresses on the window, and its interface with the curtain  12  and its different material properties. 
         [0016]    To provide visibility through the door when it is closed, door  10  includes the window construction  22  for use with a fabric power door. In the window construction  22 , as shown in  FIGS. 4-7 , a transparent sheet  24  is installed within a window opening  26  in curtain  12 . The term, “transparent” means that one can see through the material. Transparent does not necessarily mean perfectly clear, and the material could be tinted. The sheet  24  has some or all of the desirable properties for a window in a power door application, such as its transparency, combined with flexibility for rolling up, spiraling or bending; scratch resistance; UV stability, etc. In some cases, sheet  24  is made of polycarbonate, while curtain  12  is comprised of a nylon or polyester base material with a polypropylene coating. Materials other than polycarbonate and polypropylene coated base fabric are certainly possible, but the two materials will typically have different properties, such as resistance to bending (the curtain  12  is typically stiffer and more resistant to bending), and the ability to absorb and resist applied stresses due to door operation—with the curtain  12  material typically having better stress-resistance owing to the curtain  12  being made up of differing components as compared to the unitary material of the sheet  24 . 
         [0017]    To minimize the strain on sheet  24  caused by the differing flexibility and stress-resistance of curtain  12 , sheet  24  is appreciably smaller than window opening  26  to create a strain-relieving gap  28  between the outer perimeter of sheet  24  and the edges of opening  26 . To hold sheet  24  in place, a flexible frame  30  bridges gap  28  to couple sheet  24  to curtain  12  without overlap between the two, wherein frame  30  is preferably more flexible than at least curtain  12 . In this example, frame  30  is made of a polyester base material coated with polyurethane with a material thickness that is less than that of at least curtain  12 . This allows frame  30  to be less stiff and more flexible than the curtain material. At the same time, it also has the desirable stress-distribution properties of a fabric like the curtain, made up of a coated base material. Frame  30 , thus, has the ability to handle and/or absorb the stresses that would otherwise be placed on the sheet  24  were it directly attached around its edges to the curtain  12  (e.g., stresses that are introduced to the curtain  12  during, for example, opening and/or closing). Frame  30 , thus, serves some or all of several functions: bridging the physical gap between the sheet  24  and the larger window opening  26  in the curtain  12 ; creating a lateral perimeter connection between the sheet  24  and the curtain  12 ; and better absorbing stresses and forces that would otherwise have to be borne by the sheet  24  were it itself directly connected to the curtain  12 , e.g., the frame  30  is a buffer or stress-absorbing structure between the window sheet  24  and the curtain  12 . Frame  30  may be made of any other suitable material such as, for example, other flexible or stretchy materials (e.g., a resiliently stretchable knit fabric). 
         [0018]    The actual assembly and construction of window  22  may vary. An outer perimeter  32  of frame  30 , for instance, can be joined to curtain  12  in various ways including, but not limited to, adhesive bonding, thermal bonding, sonic welding, snaps, zipper, touch-and-hold fastener (e.g., VELCRO) and sewing. Likewise, an inner perimeter  34  of frame  30  can be joined to an outer edge  36  of sheet  24  in various ways including, but not limited to, adhesive bonding, thermal bonding, sonic welding, snaps, tape, zipper, touch-and-hold fastener (e.g., VELCRO) and sewing. In some cases, frame  30  is comprised of flexible adhesive tape.  FIGS. 5 and 6  show a touch-and-hold fastener  38  that removably holds sheet  24  to frame  30 , and a layer of adhesive  40  holds frame  30  to curtain  12 .  FIG. 7  shows an alternate assembly arrangement of sheet  24 , frame  30  and curtain  12 . As can be seen from the  FIG. 7 , the arrangement of  FIG. 7  allows the window  24  to be substantially co-planar with the remainder of the curtain. Such an arrangement was not possible in previous configurations where the edge of the window overlapped with the curtain material surrounding the opening. This co-planar arrangement of window  22  and curtain  12  may also help reduce the stresses on the window that would otherwise be present if it were directly connected to the curtain  12 . 
         [0019]    At least some of the aforementioned examples include one or more features and/or benefits including, but not limited to, the following: 
         [0020]    In some examples, a flexible frame couples a flexible transparent sheet to a door curtain having a window opening that is appreciably larger than the sheet. 
         [0021]    In some examples, the frame is more flexible than the curtain. 
         [0022]    In some examples, the frame is thinner than the curtain. 
         [0023]    In some examples, the frame overlaps the transparent sheet and the curtain, but the transparent sheet does not overlap the curtain. 
         [0024]    In some examples, the frame absorbs and/or reacts stresses that would otherwise be exerted on the sheet if it were directly attached to the curtain. 
         [0025]    In some examples, the stress-resistance of the frame is provided by its being comprised of a coated base fabric. 
         [0026]    In some examples, the sheet and surrounding curtain are coplanar, with the frame providing a perimeter lateral connection between the two. 
         [0027]    Furthermore, although certain example methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Summary:
A fabric power door with a vertical or side-winding curtain includes window comprised of a transparent sheet that is attached to the curtain by way of a flexible frame. The transparent sheet is appreciably smaller than the window opening in the curtain to create a strain-relieving gap between the sheet and the curtain. The flexible frame extends across the gap and overlaps both the curtain and the transparent sheet. As the door opens and closes, the flexible frame protects the transparent sheet from stresses from the flexing movement of the curtain.