Abstract:
A backlight assembly that allows a more efficient light usage and thinner device size is presented. The backlight assembly includes a first light guide and a second light guide that each has a light incident face, a light exiting face, and a light interfacing face. The light interfacing face of the first light guide has a first interfacing portion and a second interfacing portion while the light interfacing face of the second light guide has a third interfacing portion and a fourth interfacing portion. A light reflective member has a first reflection face arranged at the second light reflection face portion and a second reflection face arranged at the fourth light reflective face portion, and the light source supplies light to the first and second light incident faces. The light incident faces are on the same side of the light guiding members.

Description:
RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 10/452,319 filed on May 30, 2003 now U.S. Pat. No. 7,161,644, which claims priority under 35 USC §119 to Korean Patent Application Serial No. 2002-58319 filed on Sep. 26, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosure relates to a backlight assembly and a liquid crystal display device using the same. More particularly, the disclosure relates to a backlight assembly and liquid crystal display device using the same in which light generated from a single light source is respectively supplied to two light guiding plates to display two different images, to minimize light loss and to decrease the thickness of elements of the backlight assembly while minimizing the light loss. 
     2. Description of the Related Art 
     Generally, liquid crystal is a material in an intermediate state between solid and liquid. As for its electrical property, liquid crystal may change its aligned angle according to an applied electric field. As for its optical property, liquid crystal may change transmissivity thereof according to its aligned angle. The liquid crystal display (LCD) device displays images using these properties of liquid crystal. 
     A generalized liquid crystal display device has an LCD panel assembly for controlling the liquid crystal and a backlight assembly for supplying light to the LCD panel assembly. 
     The LCD panel assembly includes an LCD panel and a driving module. The LCD panel precisely controls the aligned angle of the liquid crystal in an area ranging from a few μm 2  to a few hundred μm 2 , and the driving module generates driving signals for driving the LCD panel. 
     The backlight assembly includes a light source, a light guiding plate, and optical sheets. 
     As the light source, either cold cathode fluorescent lamp (CCFL) or light emitting diode (LED) is frequently used because of its long life span, easy manufacturability, and low heat generation. The CCFL is mainly used in large-screen LCD devices, whereas LED is mainly used in small-screen LCD devices. 
     The light guiding plate changes the optical distribution of the light generated from the light source and supplies the redistributed light to the LCD panel. Specifically, the light guiding plate changes a line source of light or a point source of light into a surface source of light. 
     The optical sheets are installed between the LCD panel assembly and the light guiding plate. The optical sheets make uniform the brightness of the light emitted from the light guiding plate and also control the viewing angle. 
     In the conventional LCD device having the above construction, images are displayed while light passes through the light guiding plate, optical sheets and the LCD panel assembly sequentially. As a consequence, image can be displayed only in one direction. 
     Recently, an improved LCD device called “dual liquid crystal display device” is disclosed, and the dual liquid crystal display device can overcome the one directional displaying limitation of the conventional LCD device and can display information in two different directions. 
     In a conventional dual LCD device, two LCD panels, each performing image display independently, are arranged in an overlapping configuration to display images in two different directions. Specifically, the conventional dual LCD device includes a first LCD device and a second LCD device. The first LCD device includes a first LCD panel, a first light source, a first light guiding plate, first optical sheets, a first light reflective plate and a first receiving container. The second LCD device includes a second LCD panel, a second light source, a second light guiding plate, second optical sheets, a second light reflective plate and a second receiving container. 
     The first LCD device displays images in a first direction, and the second LCD device display images in a second direction. 
     The conventional dual LCD device enables displaying images in two different directions, but does so at the expense of other desirable qualities. For example, the extra parts necessary to achieve this dual-display quality increase volume and thickness of the LCD device, making the LCD device bulkier and less desirable. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is provided to substantially obviate one or more problems due to limitations and disadvantages of the related art. 
     In one aspect, the present invention is a backlight assembly that includes a first light guiding member, a second light guiding member that overlaps the first light guiding member, a light reflective member, and a light source. The first light guiding member includes i) a first light incident face, ii) a first light exiting face, and iii) a first interfacing a face that has a first interfacing portion and a second interfacing portion connected to the first interfacing portion. The second light guiding member includes i) a second light incident face, ii) a second light exiting face, and iii) a second interfacing face that has a third interfacing portion and a fourth interfacing portion connected with the third interfacing portion. The first light guiding member and the second light guiding member each has a stepped portion. The light reflective member has a first reflection face arranged at the second interfacing portion and a second reflection face arranged at the fourth interfacing portion. The light source supplies light to the first and second light incident faces, which are disposed on the same side of the first and second light guiding members, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and other advantages of the present invention will become more apparent by describing in detail the preferred embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a backlight assembly according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a sectional view taken along the line II-II of  FIG. 1 ; 
         FIG. 3  is a sectional view taken along the line III-III of  FIG. 1 ; 
         FIG. 4  is a sectional view showing a coupling structure of a first light guiding plate and a second light guiding plate according to a first exemplary embodiment of the present invention; 
         FIG. 5  is a sectional view showing a first light guiding plate, a second light guiding plate and a light reflective plate according to a first exemplary embodiment of the present invention; 
         FIG. 6  is an enlarged view of the portion “A” in  FIG. 5  according to a first exemplary embodiment of the present invention; 
         FIG. 7  is an enlarged view of the portion “A” in  FIG. 5  and shows a modification in the attachment of the first light guiding plate and the second light guiding plate according to a first exemplary embodiment of the present invention; 
         FIG. 8  is an enlarged view of the portion “A” in  FIG. 5  and shows another modification in the attachment of the first light guiding plate and the second light guiding plate according to a first exemplary embodiment of the present invention; 
         FIG. 9  is a sectional view showing yet another modification in the attachment of the first light guiding plate and the second light guiding plate according to a first exemplary embodiment of the present invention; 
         FIG. 10  is a sectional view showing a backlight assembly according to a second exemplary embodiment of the present invention; 
         FIG. 11  is an exploded perspective view of a liquid crystal display device according to an exemplary embodiment of the present invention; and 
         FIG. 12  is a sectional view taken along the line B-B of  FIG. 11  after the liquid crystal display device is assembled. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a perspective view of a backlight assembly according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , a backlight assembly  100  includes a light source  110 , a first light guiding plate  120 , a second light guiding plate  130  and a light reflective plate  140 . 
     Alternatively, the backlight assembly  100  can further include a receiving container  150  in which the first light guiding plate  120 , the second light guiding plate  130 , the light reflective plate  140  and the light source are received. 
     The light source  110  generates a first light radially. In  FIG. 1 , the light source  110  may be a point light source such as a light emitting diode. Alternatively, the light source  110  may be a line light source such as a cold cathode fluorescent lamp (CCFL). 
       FIG. 2  is a sectional view taken along the line II-II of  FIG. 1 . 
     Referring to  FIG. 2 , the first light guiding plate  120  has a three-dimensional shape such as a rectangular parallelepiped plate. Specifically, the first light guiding plate  120  has at least three side faces, a top face and a bottom face, wherein the top and the bottom faces are connected with the side faces. For example, the first light guiding plate  120  has four side faces. 
     Here, the light source  110  supplies a first light  160  into at least one of the four side faces. Hereinafter, the side face onto which the first light  160  from the light source  110  is supplied is referred to as a first light incident face  121 . 
     The first light  160  generated from the light source  110  is refracted to change the advancing direction thereof while the first light  160  passes through the first light incident face  121 . This is because the first light guiding plate  120  has a different refractive index than air. Hereinafter, the light that has passed through the first light incident face  121  is referred to as a second light  162 . 
     A first interfacing face  123  reflects the second light  162  toward a first light exiting face to be described later. 
     The first interfacing face  123  is comprised of a first interfacing portion  123   a  and a second interfacing portion  123   b . The first interfacing face  123  has a first width (W 1 ) and a first length (L 1 ). 
     The first interfacing portion  123   a  has the first width (W 1 ) and a second length (L 2 ) that is less than the first length (L 1 ) when measured from the first light incident face  121  toward an opposite direction of the first light incident face  121 . 
     The second interfacing portion  123   b  is connected with the first interfacing portion  123   a , and has a third length (L 3 ) that is somewhat shorter than a length remaining after the second length (L 2 ) is subtracted from the first length (L 1 ). The second interfacing portion  123   b  lies in a plane that is different from and substantially parallel to the plane of the first interfacing portion  123   a.    
     The first interfacing portion  123   a  is connected with the second interfacing portion  123   b  by an inclined face  123   c . The inclined face  123   c , i.e., the connecting portion (or stepped portion) has a length that corresponds to a value remaining after the second length (L 2 ) and the third length (L 3 ) are subtracted from the first length (L 1 ). 
     The portion of the inclined face  123   c  that meets the first interfacing portion  123   a  and the portion of the inclined face  123   c  that meets the second interfacing portion  123   b  may have smooth curve shapes. This is because the inclined face  123   c  causes occurrence of discontinuous bright line in the first light guiding plate  120 . 
     The first light exiting face  125  allows the second light  162  reflected by the first interfacing portion  123   a  and the second interfacing portion  123   b  of the first interfacing face  123  to be output therethrough. The light output from the first light exiting face  125  proceeds with a different angle from the proceeding direction of the second light  162 , and accordingly it is referred to as a third light  164 . 
     The first light exiting face  125  is in parallel with the first interfacing portion  123   a  and the second interfacing portion  123   b . The portion between the first light exiting face  125  and the first interfacing portion  123   a  has a first thickness (T 1 ), and the portion between the first light exiting face  125  and the second interfacing portion  123   b  has a second thickness (T 2 ) that is thinner than the first thickness (T 1 ). 
       FIG. 3  is a sectional view taken along the line III-III of  FIG. 1 . 
     Referring to  FIG. 3  and  FIG. 1 , the second light guiding plate  130  overlies the first light guiding plate  120 . The second light guiding plate  130  is similar in shape to the first light guiding plate  120 , and has a second width (W 2 ) and a fourth length (L 4 ). The second width (W 2 ) is wider than the first width (W 1 ), and the fourth length (L 4 ) is longer than the first length (L 1 ). 
     The second light guiding plate  130  is comprised of a second light incident face  131 , a second interfacing face  135  and a second light exiting face  133 . 
     The second light incident face  131  is arranged such that it does not disaccord with the first light incident face  121  of the first light guiding plate  120 . In other words, the second light incident face  131  is aligned with the first light incident face  121  of the first light guiding plate  120  to receive the first light  160 . The first light  160  generated from the light source  110  is partially incident onto the second light incident face  131 . Hereinafter, the first light  160  that is incident onto the second light incident face  131  is referred to as a fourth light  166 . 
     The second interfacing face  135  connected to the second light incident face  131  faces the first interfacing portion  123   a  and the second interfacing portion  123   b  of the first light guiding plate  120 . The second interfacing face  135  reflects the fourth light  166  to direct the reflected fourth light  166  toward the second light exiting face  133 . 
       FIG. 4  is a sectional view showing a coupling structure of a first light guiding plate and a second light guiding plate according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , the second interfacing face  135  is directly in contact with the first interfacing portion  123   a . Between the second interfacing  135  and the second interfacing portion  123   b , there is a gap (G). In other words, a part of the second interfacing  135  is spaced apart from the second interfacing portion  123   b  by the gap (G). 
     Referring again to  FIG. 3 , the fourth light  166  reflected by the second interfacing  135  is directed toward the second light exiting face  133 . Hereinafter, the light output from the second light exiting face  133  is referred to as a fifth light  168 . The second light exiting face  133  faces the second interfacing  135 . The second light exiting face  133  and the second interfacing  135  have a uniform third thickness of T 3  throughout an entire area of the second light guiding plate  130 . The third thickness (T 3 ) of the second light guiding plate  130  is thinner than the first thickness (T 1 ) of the first light guiding plate  120 . 
       FIG. 5  is a sectional view showing a first light guiding plate, a second light guiding plate and a light reflective plate according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 1  and  FIG. 5 , the light reflective plate  140  is interposed between the first light guiding plate  120  and the second light guiding plate  130 . The light reflective plate  140  has a thickness that is equal to the gap between the second interfacing portion  123   b  and the second interfacing face  135 . 
     Also, among the edges of the light reflective plate  140 , the edge facing the inclined face  123   c  of the first light guiding plate  120  has an inclined face  141  that is designed to fit with the inclined face  123   c  so as to be closely in contact with the inclined face  123   c.    
     The first light guiding plate  120  and the second light guiding plate  130  are attached to each other by means of an attaching member, which is to prevent the first light guiding plate  120  from getting displaced with respect to the second light guiding plate  130 . 
       FIG. 6  is an enlarged view of the portion “A” in  FIG. 5  according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , a double-layered adhesive tape  160  is interposed between the first interfacing portion  123   a  of the first light guiding plate  120  and the second interfacing face  135  of the second light guiding plate  130  facing the first interfacing portion  123   a . The double-layered adhesive tape  160  includes a transparent base film  161 , and adhesive layers  162  and  163  coated on both surfaces of the base film  161 . 
       FIG. 7  is an enlarged view of the portion “A” in  FIG. 5  and shows a modification in the attachment of the first light guiding plate and the second light guiding plate according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , a thin adhesive layer  170  is interposed between the first interfacing portion  123   a  of the first light guiding plate  120  and the second interfacing face  135  of the second light guiding plate  130  facing the first interfacing portion  123   a . The thin adhesive layer  170  allows the first light guiding plate  120  and the second light guiding plate  130  to adhere to each other. 
       FIG. 8  is an enlarged view of the portion “A” in  FIG. 5  and shows another modification in the attachment of the first light guiding plate and the second light guiding plate according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , at least one adhering protrusion  135   a  is formed on the second interfacing face  135  of the second light guiding plate  120 . In a state that the first interfacing portion  123   a  of the first light guiding plate  120  is in contact with the adhering protrusion  135   a  formed on the second interfacing face  135  of the second light guiding plate  130 , ultrasonic waves or heat is applied to the adhering protrusion  135   a . As a result, the adhering protrusion  135   a  is melted and the melted adhering protrusion  135   a  allows the first interfacing portion  123   a  and the second interfacing face  135  to adhere to each other. The adhering protrusion  135   a  is preferably processed to form a minimum gap between the first interfacing portion  123   a  and the second interfacing face  135 . 
       FIG. 9  is a sectional view showing yet another modification in the attachment of the first light guiding plate and the second light guiding plate according to a first exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , in a state that the first light guiding plate  120  and the second light guiding plate  130  overlap with each other, some portions of the first light guiding plate  120  may be fixed to the second light guiding plate  130  by means of a transparent clip  180  or the like. Specifically, a portion of the clip  180  presses the first light exiting face  125  of the first light guiding plate  120  against the second light guiding plate  130  and the remainder of the clip  180  presses the second light exiting face  133  of the second light guiding plate  130  against the first light guiding plate  120 . 
     The first light guiding plate  120 , the light reflective plate  140 , the second light guiding plate  130  and the light source  110  having the aforementioned coupling structure are received in the receiving container  150  shown in  FIG. 1  during the assemblage of the backlight assembly  100 . 
     The receiving container  150  encloses the sidewalls of the first light guiding plate  120  and the sidewalls of the second light guiding plate  130 , and has an opening or an opened bottom face such that the first light exiting face  125  of the first light guiding plate  120  and the second light exiting face  133  of the second light guiding plate  130  are exposed. 
     Also, the light source  110  is installed within the receiving container  150  such that the light source  110  faces the first light incident face  121  and the second light incident face  131 . 
     Embodiment 2 
       FIG. 10  is a sectional view showing a backlight assembly according to a second exemplary embodiment of the present invention. 
     Referring to  FIG. 10 , a backlight assembly  100  includes a light reflective plate  140 , a first light guiding plate  120 , a second light guiding plate  130 , a light source  110  and a receiving container  150 . 
     The light reflective plate  140  has two interfacing faces  143  and  144 . One of the two interfacing faces is the first interfacing face  143  and the other is the second interfacing face  144 . 
     The first light guiding plate  120  is arranged to face the first interfacing face  143 , and the second light guiding plate  130  is arranged to face the second interfacing face  144 . 
     The first light guiding plate  120  is comprised of a first light incident face  121 , a first interfacing face  123  having a first interfacing portion  123   a  and a second interfacing portion  123   b , and a first light exiting face  125  facing the first interfacing face  123 . Reference numeral  123   c  represents a transitional region between the first interfacing portion  123   a  and the second interfacing portion  123   b . The transitional region  123   c  may have a smooth curve structure or an inclined surface structure. 
     A first thickness of T 1  between the first interfacing portion  123   a  and the first light exiting face  125  is different from a second thickness of T 2  between the second interfacing portion  123   b  and the first light exiting face  125 . Preferably, the first thickness of T 1  is greater than the second thickness of T 2 . 
     The first interfacing face  143  of the light reflective plate  140  is laid over the first light guiding plate  120  and aligned with the second interfacing portion  123   b  of the first light guiding plate  120 , so that the overall thickness of the backlight assembly can be decreased. 
     The second light guiding plate  130  is comprised of a second light incident face  131 , a second interfacing face  135  having a third interfacing portion  135   a  and a fourth interfacing portion  135   b , and a second light exiting face  133  facing the second interfacing face  135 . Reference numeral  135   c  represents a transitional region between the third interfacing portion  135   a  and the fourth interfacing portion  135   b . The transitional region  135   c  may have a smooth curve structure or an inclined surface structure. 
     A third thickness of T 3  between the third interfacing portion  135   a  and the second light exiting face  133  is different from a fourth thickness of T 4  between the fourth interfacing portion  135   b  and the second light exiting face  133 . Preferably, the third thickness of T 3  is thicker than the fourth thickness of T 4 . 
     The second interfacing face  144  of the light reflective plate  140  is positioned to lie over the second light guiding plate  130  and align with the fourth interfacing portion  135   b , so that an overall thickness of the backlight assembly can be decreased. 
     By the above arrangement, the light reflective plate  130  is interposed between the first light guiding plate  120  and the second light guiding plate  130 . 
     The first light guiding plate  120 , the light reflective plate  140 , the second light guiding plate  130  and the light source  110  having the aforementioned coupling structure are installed in the receiving container  150  shown in  FIG. 10  during the assemblage of the backlight assembly  100 . 
     The receiving container  150  encloses the sidewalls of the first light guiding plate  120  and the sidewalls of the second light guiding plate  130 , and has an opening or an opened bottom face such that the first light exiting face  125  of the first light guiding plate  120  and the second light exiting face  133  of the second light guiding plate  130  are exposed. 
     Also, the light source  110  is installed within the receiving container  150  such that the light source  110  faces the first light incident face  121  and the second light incident face  131 . 
     Embodiment 3 
       FIG. 11  is an exploded perspective view of a liquid crystal display device according to an exemplary embodiment of the present invention and  FIG. 12  is a sectional view taken along the line B-B of  FIG. 11  after the liquid crystal display device is assembled. 
     Referring to  FIG. 11  or  FIG. 12 , a liquid crystal display device  900  includes a light source  800 , a backlight assembly  100 , optical sheets ( 200 ,  300 ), a first display panel  400 , a second display panel  500  and receiving containers ( 600 ,  700 ). 
     The light source  800  generates light radially. The light source  800  may be a line light source such as a cold cathode fluorescent lamp (CCFL) or a point light source such as a light emitting diode. 
     The backlight assembly  100  includes a first light guiding plate  120 , a second light guiding plate  130 , and a light reflective plate  140 . 
     Since the backlight assembly  100  has the same construction as those of the first and second exemplary embodiment, repeated description thereof will be omitted. Hereinafter, like reference numerals identify identical elements of the backlight assembly  100  throughout the drawings. 
     The first display module  400  is installed to face a first light exiting face  125  of the first light guiding plate  120 . The first display module  400  has a plain surface area that is substantially equal to the surface area of the first light exiting face  125 . 
     Any construction for the first display module  400  may be allowed as long as the first display module  400  can display images by using the liquid crystal and the light passing through liquid crystal molecules. 
     Preferably, the first display module  400  includes a first color filter substrate  410 , a first TFT substrate  430 , and a first liquid crystal  420  interposed between the first color filter substrate  410  and the first TFT substrate  430 . 
     The second display module  500  includes a second color filter substrate  520 , a second TFT substrate  510 , and a second liquid crystal  530  interposed between the second color filter substrate  520  and the second TFT substrate  510 . The second display module  500  is installed to face a second light exiting face  133  of the second light guiding plate  130 . The second display module  500  has a surface area that is substantially equal to the area of the second light exiting face  133 . Since the area of the second light exiting face  133  is larger than that of the first light exiting face  125 , the area of the second display module  500  is larger than that of the first display module  400 . 
     The optical sheets are comprised of the first optical sheets  200  and the second optical sheets  300 . 
     The first optical sheets  200  are installed between the first display module  400  and the first light guiding plate  120 , and the second optical sheets  300  are installed between the second display module  500  and the second light guiding plate  130 . 
     In order to enhance light efficiency in the LCD device, each of the first and second optical sheets ( 200 ,  300 ) includes at least one sheet. Preferably, each of the first and second optical sheets ( 200 ,  300 ) includes a diffusion sheet and a prism sheet. A brightness enhanced film may be further added to the diffusion sheet and the prism sheet. 
     The first display module  400 , the optical sheets ( 200 ,  300 ), the backlight assembly  100 , and the second display module  500  are received in the receiving containers ( 600 ,  700 ). 
     The receiving containers ( 600 ,  700 ) are comprised of a top chassis  600  and a bottom mold frame  700 . “Top” and “bottom,” as used herein, refers to the top and the bottom in reference to the orientation of the figure in  FIG. 11 . 
     The bottom mold frame  700  has a square frame structure that is sized to receive four sidewalls of the second light guiding plate  130 . 
     The bottom mold frame  700  is comprised of sidewalls only, and allows the second display module  500 , the optical sheets ( 200 ,  300 ), the backlight assembly  100  and the first display module  400  to be received therein. The bottom mold frame  700  has a fixing rib  710  which the second display module  500  is hung on and fixed to. Also, in the bottom mold frame  700 , the sidewall adjacent to the first light incident face  121  of the first light guiding plate  120  and the second light incident face  131  of the second light guiding plate  130  has a light source receiving groove  720  into which the light source  800  is placed. 
     The top chassis  600  is coupled with the bottom mold frame  700  to keep the first display module  400  inside the bottom mold frame  700 . 
     The top chassis  600  has an opening  610  such that users can view images displayed through the first display module  400 . 
     Hereinafter, an assembling method of the LCD device is described. 
     First, the first display module  500  for displaying images is placed in the bottom mold frame  700  such that a first face of the first display module  500  faces the inner bottom face of the bottom mold frame  700 , and the first face of the first display module  500  is fixed to the bottom mold frame  700  by using the fixing rib  710 . Then, the second optical sheets  300  are mounted on a second face of the first display module which is the face opposite the first face. 
     After the second optical sheets  300  are placed in the bottom mold frame  700 , an assembly including the first light guiding plate  120 , the light reflective plate  140  and the second light guiding plate  130  coupled with each other is mounted on the upper face of the second optical sheets  300 . The second light exiting face  133  of the second light guiding plate  130  is coupled to the bottom mold frame  700  such that the second light exiting face  133  faces the second optical sheets  300 . 
     The first optical sheets  200  are closely mounted on the first light guiding plate  120 , and then the first display module  400  is mounted on the first optical sheets  200 . 
     After the second display module  500  and the first display module  400  are sequentially placed in the bottom mold frame  700 , the top chassis  600  is coupled with the first display module  400 . The top chassis  600  is coupled with the bottom mold frame  700  while the top chassis  600  presses edges of the first display module  400 . 
     While the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by appended claims.