Patent Publication Number: US-7905647-B2

Title: Prism sheet, back light unit using the same and liquid crystal display device having the back light unit

Description:
This application claims the benefit of the Korean Patent Application No. P2005-57012, filed on Jun. 29, 2005, which is hereby incorporated by reference as if fully set forth herein. 
     TECHNICAL FIELD 
     The present application relates to a back light unit, and more particularly, to a prism sheet, a back light unit using the same and liquid crystal display device having the back light unit, which is suitable for improvement of light concentration, frontal luminance and viewing angle. 
     BACKGROUND 
     Various flat panel displays that can reduce weight and volume with respect to that of a cathode ray tube display have been developed. Examples of flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and a light emitting display (LED). 
     The LCD displays desired images by controlling light beams emitted from a back light unit using an LCD panel. The LCD panel includes a plurality of liquid crystal cells and a plurality of control switches for switching video signals to be supplied to the respective liquid crystal cells. 
     As shown in  FIG. 1 , the related art back light unit includes a lamp  10  generating light, a light guide plate  20  emitting a light source through inner dispersion of the light generated from the lamp  10  and entering the light guide plate  20  through an incident surface  22 , a lamp housing  12  arranged to surround the lamp  10  and disposed opposite the incident surface  22  of the light guide plate  20 , a reflecting plate  30  arranged below the light guide plate  20 , a diffusion sheet  40  arranged above the light guide plate  20  to diffuse the light passing through the light guide plate  20 , and a prism sheet  50  controlling a direction of the light passing through the diffusion sheet  40 . 
     Generally, the lamp  10  is a cold cathode fluorescent lamp. The lamp  10  is lit by a lamp driving voltage from an inverter (not shown). 
     The lamp housing  12  has a reflecting surface therein to reflect the light from the lamp  10  toward the incident surface  22  of the light guide plate  20 . 
     The light guide plate  20  allows the incident light from the lamp  10  to reach a portion away from the lamp  10 , and guides the incident light to the diffusion sheet  40 . 
     A reflective pattern is formed at a lower side of the light guide plate  20  to reflect the light from the incident surface  22  at a tilted rear surface of the light guide plate  20  and direct the reflected light to the diffusion sheet  40 . 
     The reflecting plate  30  is arranged below the light guide plate  20  so as to reflect a light emerging through the rear surface of the light guide plate  20  back to the light guide plate  20  so as to reduce light loss. The diffusion sheet  40  diffuses the light passing through the light guide plate  20  and emits the light towards the prism sheet  50 . The prism sheet  50  serves to condense the light passing through the diffusion sheet  40 . 
     As shown in  FIG. 2 , the prism sheet  50  includes a condensing film  52  of polyester (PET) and a plurality of prism peaks  54  formed on the condensing film  52  in a stripe shape. 
     The prism peaks  54  have first and second tilt surfaces tilted at a predetermined angle. Each of the first and second tilt surfaces is tilted with respect a top surface of the condensing film  52  at an angle of 45°. 
     The incident light with a predetermined angle θ 1  to the prism sheet  50  having a refractive index n 1  is refracted by the prism sheet  50  at a predetermined angle θ 2  in accordance with the Snell&#39;s law of refraction expressed in the following equation 1 the region outside of the prism sheet  50  has a refractive index n 2 . 
     
       
         
           
             
               
                 
                   
                     
                       n 
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                       n 
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                   = 
                   
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
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                       sin 
                       ⁢ 
                       
                           
                       
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                       θ 
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                   equation 
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     The light emitted from the lamp  10  proceeds to the diffusion sheet  40  arranged above the light guide plate  20 . The diffused light is condensed through the prism sheet  50  and condensed light is finally emitted to the outside. 
     However, the incident light to the prism sheet  50  can split into three regions, as shown in  FIG. 3 : viz., a total reflection region, a condensing region, and a side lobe region. 
     Light ray A of the total reflection region enters the condensing film  52  vertically and is totally internally reflected by the first and second tilt surfaces of the prism peaks  54 . The totally reflected light proceeds again to the light guide plate  20 . In this way, the light A is condensed by recycling by the reflecting sheet  30 . 
     Light ray B of the condensing region enters the condensing film  52  at an angle and is condensed in such a manner that it is refracted by the lower surface of the prism sheet  50  and one of the first and second tilt surfaces of the prism peaks  54 . 
     Light ray C of the side lobe region enters the condensing film  52  at an angle and is totally internally reflected by one of the first and second tilted surfaces of the prism peaks  54 . In this case, light efficiency and viewing angle characteristics are deteriorated. 
     As shown in  FIG. 4  and  FIG. 5 , a bright region exists at both sides of each of the prism sheet  50  around a symmetrical point due to the side lobe. 
     As a result, the related art back light unit has some problems due to the structure of the prism sheet  50 . That is, viewing angle characteristics are deteriorated by luminance asymmetry in the vertical and horizontal axes (Y and X axes), and condensing efficiency is deteriorated by the side lobe. 
     SUMMARY 
     A prism sheet includes a light-condensing film, and a plurality of prism peaks, each prism peak having a triangular cross section surface which is inclined from a top surface of the light-condensing film at an angle of about 20° to about 40°. 
     In another aspect, a back light unit includes a lamp for generating light; a light guide plate having an incident surface at a side thereof, which directs the light towards a prism sheet, provided on an upper surface the light guide plate, and the prism sheet condenses the light with a plurality of prism peaks, each prism peak having an inclined angle of about 20° to about 40°; and, a reflecting type polarizer, provided on the prism sheet, for increasing any one of S-polarization and P-polarization in the light condensed by the prism sheet. 
     In another aspect, a back light unit includes a plurality of lamps generating light; a bottom cover supporting the plurality of lamps; a diffusion plate disposed above the lamps; a prism sheet, provided disposed above the diffusion sheet, condensing the light with a plurality of prism peaks, each prism peak having an inclined angle of about 20° to about 40° with respect to the prism sheet; and a reflecting type polarizer, disposed above the prism sheet, for increasing one of a S-polarization or a P-polarization in the light condensed by the prism sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic view of a back light unit according to the related art; 
         FIG. 2  illustrates a perspective view of a prism sheet shown in  FIG. 1 ; 
         FIG. 3  illustrates the light characteristics of a prism peak shown in  FIG. 2 ; 
         FIG. 4  illustrates the light characteristics and angle distribution of a prism sheet shown in  FIG. 2 ; 
         FIG. 5  illustrates the luminance angle distribution of light passing through a prism sheet shown in  FIG. 2 ; 
         FIG. 6  illustrates a back light unit according to a first example; 
         FIG. 7  illustrates a cross sectional view of a prism sheet shown in  FIG. 6 ; 
         FIG. 8A  illustrates the luminance angle distribution of light passing through a prism sheet shown in  FIG. 6 ; 
         FIG. 8B  illustrates the luminance angle distribution of light passing through a reflecting type polarizer shown in  FIG. 6 ; 
         FIG. 9  illustrates a cross sectional view of a back light unit according to a second example. 
         FIG. 10  illustrates a cross sectional view of a back light unit according to a third example. 
         FIG. 11  illustrates a cross sectional view of a liquid crystal display device having the back light unit according to the first example. 
         FIG. 12  illustrates a cross sectional view of a liquid crystal display device having the back light unit according to the second example. 
         FIG. 13  illustrates a cross sectional view of a liquid crystal display device having the back light unit according to the third example. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to examples of embodiments which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The back light unit according to the first example, as shown in  FIG. 6 , includes a lamp  110 , a light guide plate  120 , a lamp housing  112 , a reflecting plate  130 , a diffusion sheet  140 , a prism sheet  150 , and a reflecting type polarizer  160 . 
     The lamp  110  emits the light, and the light guide plate  120  guides the light which is emitted from the lamp  110  and then is incident upon an incident surface  122 . The lamp housing  112  surrounds lamp  110  and is disposed opposing the incident surface  122  of the light guide plate  120 . The reflecting plate  130  is provided below the light guide plate  120 . The diffusion sheet  140 , provided on the light guide plate  120 , diffuses the light passing through the light guide plate  120 . The prism sheet  150  condenses the light passing through the diffusion sheet  140 . The reflecting type polarizer  160  is provided on the prism sheet  150 , wherein the reflecting type polarizer  160  increases one of a S-polarization or a P-polarization in the light condensed by the prism sheet  150 . 
     The lamp  110  may be a cold cathode fluorescent lamp. Although not shown, an inverter applies a driving voltage to the lamp  110 . The light emitted from the lamp  110  is incident upon the incident surface  122  provided at the side of the light guide plate  120 . 
     The lamp housing  112  is provided at the side of the light guide plate  120 , wherein the lamp housing  112  surrounds the lamp  110 , and opposes the incident surface  122  of the light guide plate  120 . The lamp housing  112  has an inner reflecting surface, whereby the light emitted from the lamp  110  is reflected on the reflecting surface of the lamp housing  112  toward the incident surface  122  of the light guide plate  120 . 
     The light guide plate  120 , guides the light to an area apart from the lamp  110 , and the incident light is guided toward the diffusion sheet  140 . Since the rear surface of the light guide plate  120  is formed of a reflecting pattern, the incident light is reflected on the inclined rear surface of the light guide plate and the light is guided to the diffusion sheet  140 . 
     A reflecting plate  130  is provided below the light guide plate  120 . When the light reaches the reflecting plate  130  through the rear surface of the light guide plate  120 , the reflecting plate  130  reflects the light back toward the light guide plate  120 , thereby decreasing the loss of light. 
     The diffusion sheet  140  diffuses the light passing through the light guide plate  120 , such that the diffused light reaches the prism sheet  150 . Then, the prism sheet  150  condenses the light. The prism sheet  150  includes a light-condensing film  152  and a plurality of prism peaks  154 . The light-condensing film  152  is formed of polyester PET. The prism peaks  154 , each having a triangular cross section of an inclined surface at an angle of about 20° to about 40° with respect to a surface of the prism sheet  150 , are formed in a stripe or pyramid shape. 
     A pyramid shape (not shown) is a configuration which may have a second pair of opposing inclined surfaces, the second pair of surfaces rotated by 90° with respect to the first set of surfaces about an axis perpendicular to the surface of the prism sheet  150 . Multiple prisms are disposed along a direction of the stripe. 
     Each of the prism peaks  154  has an apex angle of about 100° to about 140° and first and second inclined surfaces at an angle of about 20° to about 40° with respect to the surface of the prism sheet  150 . Each angle of the first and second inclined surfaces is in an optimized range for removing light rays which would be emitted in a side lobe area. 
     The incident light ray A which is perpendicular to the diffusion sheet  140  is totally reflected on the first and second inclined surfaces of the prism peaks  154  toward the reflecting plate  130 , and the light is recycled. Also, the incident light B and C is refracted on the first and second inclined surfaces of the prism peaks  154 , whereby the light is either condensed or totally reflected. Thus, it is possible to substantially eliminate the side lobe. 
     The reflecting type polarizer  160  increases one of a S-polarization or a P-polarization in the light condensed by the prism sheet  150 , thereby improving the frontal luminance and light efficiency. 
     The reflecting type polarizer  160  transmits the S-polarization of the light passing through the prism sheet  150 , and reflects the P-polarization toward the reflecting plate  130 . Alternatively, the reflecting type polarizer  160  may transmit the P-polarization, and may reflect the S-polarization to the reflecting plate  130 . The luminance angle distribution P(θ) of the reflecting type polarizer  160  may be expressed as equation 2. 
     
       
         
           
             
               
                 
                   
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     In formula 2, ‘σ’ is a full-width half-maximum angle of the luminance distribution which is related with the diffusion intensity of light by a Gaussian characteristic, and ‘θ’ is an incident angle of the light coming through the prism sheet  150 . 
     The full-width half-maximum angle σ of the reflecting type polarizer  160  has a value of about 0° to about 10°. Accordingly, the value of the luminance angle distribution is optimized in the range between about 0° to about 10°. Thus, it is possible to improve the light-condensing efficiency of the light passing through the prism sheet  150 , and to improve the frontal luminance by decreasing or substantially eliminating the light emitted in the side lobe area. 
       FIG. 8A  illustrates the luminance angle distribution of light passing through the prism sheet shown in  FIG. 6 .  FIG. 8B  illustrates the luminance angle distribution of light passing through the reflecting type polarizer shown in  FIG. 6 . 
     As shown in  FIG. 8A , if the inclined angle of the prism peak  154  is optimized to about 37°, it is possible to minimize the side lobe area in the luminance angle distribution of the light passing through the prism sheet  150 . 
     Also, as shown in  FIG. 8B , if the inclined angle of the prism peak  154  is optimized to about 37°, and the full width half maximum angle σ is optimized to about 1°, it is possible to substantially eliminate the side lobe area from the luminance angle distribution of the light passing through the reflecting type polarizer  160 . 
     In the back light unit of the first example, the inclined angle of the prism peak  154  is formed in the optimized range between about 20° to about 40°, and the full width half maximum angle σ of the reflecting type polarizer  160  is formed in the optimized range between about 0° to about 10°. As a result, it is possible to substantially eliminate the side lobe area and to improve the light-condensing efficiency and the frontal luminance. 
       FIG. 9  illustrates a cross sectional view of a back light unit according to the second example. Referring to  FIG. 9 , the back light unit according to the second embodiment of the present invention includes a planar-type light guide plate  220 , at least one lamp  110 , two lamp housings  112 , a reflecting plate  130 , a diffusion sheet  140 , a prism sheet  150 , and a reflecting type polarizer  160 . 
     Incident surfaces  222  are provided at both sides of the planar type light guide plate  220 . At least one lamp  110  is provided in correspondence with each of the incident surfaces  222 . Each of the two lamp housings  112  surrounds the incident surface  222  of the light guide plate  220  and the lamp  110 . Also, the reflecting plate  130  is provided below the light guide plate  220 . The diffusion sheet  140 , provided on the light guide plate  220 , diffuses the light passing through the light guide plate  220 . The prism sheet  150  condenses the light passing through the diffusion sheet  140 . The reflecting type polarizer  160  is provided on the prism sheet  150 , wherein the reflecting type polarizer  160  increases any one of S-polarization and P-polarization in the light condensed by the prism sheet  150 . 
     At least one lamp  110  is provided in correspondence with each of the incident surfaces  222  formed at both sides of the light guide plate  220 , the back light unit according to the second example is similar in structure as to back light unit according to the first example. 
     The light emitted from the lamp  110  is incident through the incident surfaces  222  formed at both sides of the light guide plate  220 , so it is possible to improve the luminance of light. 
     An inclined angle of a prism peak  154  formed in the prism sheet  150  is determined in the range between about 20° to about 40°, and a full-width half-maximum angle σ of the reflecting type polarizer  160  is in an optimized range between about 0° to about 10°. As a result, it is possible to substantially eliminate a side lobe area and to improve light-condensing efficiency and frontal luminance. 
       FIG. 10  illustrates a cross sectional view of a back light unit according to the third example. The back light unit includes a plurality of lamps  310 , a bottom cover  312 , a diffusion plate  320 , a prism sheet  150 , and a reflecting type polarizer  160 . 
     The plurality of lamps  310  mounted on the back cover  312  emit light. The diffusion plate  320  is disposed above the lamps  310  such that the diffusion plate  320  diffuses the light emitted from the plurality of lamps  310 . The prism sheet  150 , provided over the diffusion plate  320 , condenses the light diffused by the diffusion plate  320 . The reflecting type polarizer  160  is provided on the prism sheet  150 , whereby the reflecting type polarizer  160  increases one of a S-polarization or a P-polarization in the light condensed by the prism sheet  150 . 
     Each of the lamps  310  may be a cold cathode fluorescent lamp. Although not shown, an inverter applies a driving voltage to the lamp  310 . The light emitted from the lamps  310  is incident upon a rear surface of the diffusion plate  320 . 
     The bottom cover  312  supports the plurality of lamps  310 . In addition, a reflecting sheet (not shown) may be adhered to the inner surface of the bottom cover  312 , such that the reflecting sheet reflects the light emitted from the lamps  310  to the diffusion plate  320 . 
     The diffusion plate  320  covers the top of the bottom cover  312  above the lamps  310 . Thus, the light emitted from the lamps  310  and reflected on the reflecting sheet of the bottom cover  312  is diffused by the diffusion sheet  320 , and then the diffused light is incident upon the prism sheet  150 . 
     The prism sheet  150  of the back light unit of third example is similar in structure to a prism sheet of the back light unit of the first example shown in  FIG. 7 . Accordingly, a detailed explanation for the prism sheet  150  of the back light unit of the third example. 
     The reflecting type polarizer  160  of the back light unit of the third example is similar in structure to the reflecting type polarizer of the back light unit of the first example shown in  FIG. 6 . Accordingly, a detailed explanation of the reflecting type polarizer  160  will be omitted. 
     The light emitted from the lamps  310  is may be directly incident on the rear surface of the diffusion sheet  140 , thereby improving the luminance of light. 
     An inclined angle of a prism peak  154  formed in the prism sheet  150  is in an optimized range between about 20° to about 40°, and a full-width half-maximum angle σ of the reflecting type polarizer  160  is formed in a range between about 0° to about 10°. As a result, it is possible to substantially eliminate a side lobe area and to improve light-condensing efficiency and frontal luminance. 
     Each of the back light units of the first to third examples may be used as a light source for emitting light to a liquid crystal display panel  400  shown in  FIG. 11  to  FIG. 13 . The liquid crystal display panel  400  may be driven in any of an IPS (In Plane Switching) mode, a VA (Vertically Aligned) mode, a TN (Twisted Nematic) mode, or an MD (Multi-Domain) mode. 
     The liquid crystal display panel  400  includes a thin film transistor substrate, a color filter substrate, and a liquid crystal layer. The thin film transistor substrate includes a plurality of liquid crystal cells arranged in regions defined by a plurality of gate and data lines, and a plurality of thin film transistors serving as switching elements formed in the respective liquid crystal cells. The liquid crystal layer is formed between thin film transistor substrate and the color filter substrate, wherein the thin film transistor substrate is disposed at a separation interval from the color filter substrate. The liquid crystal display panel  400  displays desired images by forming an electric field in the liquid crystal layer depending on data signals to control light transmission. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the following claims and their equivalents.