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You are an expert at summarizing long articles. Proceed to summarize the following text: 
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwan Patent Application No. 104112986, filed on Apr. 23, 2015 the contents of which are incorporated by reference herein. 
       FIELD 
       [0002]    The subject matter herein generally relates to building construction. 
       BACKGROUND 
       [0003]    Instructions of emergency exit are installed near exits of buildings. The instruction of emergency exit includes a light source, a shell, an acrylic plate, and an opaque green tape. The structure of the exit signal is complex. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Implementations of the present technology will now be described, by way of example only, with reference to the attached figures. 
           [0005]      FIG. 1  is an isometric view of a door with an exit signal in an open state. 
           [0006]      FIG. 2  is an isometric view of the door of  FIG. 1  with exit signal in a closed state. 
           [0007]      FIG. 3  is an enlarged view of a predetermined pattern of the door of  FIG. 1   
           [0008]      FIG. 4  is an enlarged view of a top surface of a first plate of the door of  FIG. 1 . 
           [0009]      FIG. 5  is an isometric view of a door with an exit signal in an open state, according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. The drawings are not necessarily to scale and the proportions of certain parts may be exaggerated to better illustrate details and features. The description is not to be considered as limiting the scope of the embodiments described herein. 
         [0011]    The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series, and the like. 
         [0012]      FIGS. 1 and 2  illustrate a door  10  includes a door frame  11 , a door panel  12 , and a light source  30 . The door frame  11  includes a top rim  111 , a bottom rim  112 , a first lateral rim  113 , and a second lateral rim  114 . The top rim  111  is opposite to the bottom rim  112 . The first lateral rim  113  and the second lateral rim  114  are connected between the top rim  111  and the bottom rim  112 . The top rim  111  and the bottom rim  112  are configured to be fixed within walls. The bottom rim  112  is configured to be fixed to a floor. A first guiding groove  116  is defined at a side of the bottom rim  112  opposite to the top rim  111 . A second guiding groove (not shown) is defined at a side of the top rim  111  opposite to the bottom rim  112 . 
         [0013]    The door panel  12  includes a first plate  121  and a second plate  122 . The first plate  121  is parallel to the second plate  122 . The first plate  121  and the second plate  122  are positioned between the top rim  111  and the bottom rim  112 . Two opposite ends of the first plate  121  are slidably received in the first guiding groove  116  and the second guiding groove. The second plate is fixed to the top rim  111 , the bottom rim  112 , and the first lateral rim  113 . The first plate  121  is slidable between the first lateral rim  113  and the second lateral rim  114 . The first plate  121  can overlap the second plate  122 , the door  10  then being in an opened state. When the first plate  121  slides to touch the second lateral rim  114 , the first plate  121  spreads with the second plate, and the door  10  is then in a closed state. The first plate  121  and the second plate  122  are made of translucent glass. 
         [0014]    The first plate  121  includes a top surface  1211 , a bottom surface  1212 , a front surface  1213  and a rear surface  1214 . The top surface  1211  is opposite to the bottom surface  1212 . The top surface  1211  and the bottom surface  1212  are connected between the top surface  1211  and the bottom surface  1212 . The top surface  1211  is opposite to the top rim  111 . The  FIG. 3  illustrates the top surface  1211  including an optical structure  1217 . The optical structure  1217  includes a number of depressions  1218 . The depressions  1218  are connected. The front surface  1213  defines a predetermined pattern  1216 . As  FIG. 4  illustrates, the predetermined pattern  1216  is formed by a number of pits  1219 . In the embodiment, the pits  1219  are arranged to display the word “EXIT.” 
         [0015]    The light source  30  is fixed to the top rim  111  and opposite to the bottom rim  112 . The light source  30  is light emitting diode. When the door  10  is in the closed state, the light source  30  is positioned above the first plate  121 , and light emitted from the light source  20  passes through the top surface  1211  to enter the interior of the first plate  212 . Then the light enters the front surface  1213 . When the light passes through the front surface  1213 , the light is scattered at the pits  1219 . Thus, the predetermined pattern  1216  is illuminated. When the door  10  is opened, the first plate  121  moves away from the light source  20 , and the light is withdrawn from the interior of the first plate  121 . Thus, the determined pattern  1216  is not illuminated. 
         [0016]      FIG. 5  illustrates a door  20  in another embodiment, similar to the door  10  in  FIGS. 1-4 . The door frame  21  does not define the first guiding groove  115  and the second guiding groove  116 . The door panel  22  only includes the first plate  221 . The first plate  221  is rotationally connected to the first lateral rim  213  through a rotation shaft (not shown). The light source  23  is also fixed to the top rim  211  and opposite to the bottom rim  212 . When the first plate  221  rotates to touch the second lateral rim  214 , the door  20  is in a closed state, and the determined pattern  2216  is illuminated. When the first plate  221  rotates away from the second lateral rim  214 , the door  20  is in an opened state, the determined pattern  2216  is not illuminated. 
         [0017]    The embodiments shown and described above are only examples. Even though numerous characteristics and advantages of the present technology have been set forth in the foregoing description, together with details of the structure and function of the present disclosure, the disclosure is illustrative only, and changes may be made in the details, including in matters of shape, size, and arrangement of the parts within the principles of the present disclosure, up to and including the full extent established by the broad general meaning of the terms used in the claims.

Summary:
A door with an “EXIT” signal which operates automatically includes a door panel and a light source. The door panel includes a first plate. The first plate includes a front surface. The front surface defines a plurality of pits. The pits form a predetermined pattern, in the shape of a word. The light source is fixed above the first plate. The light source emits light to toward the pits to illuminate the predetermined pattern in the shape of a word when the door is in a closed state.