201104150 六、發明說明： 【發明所屬之技術領域】 本發明S關於-種背光模組，特別是關於一種設計有 散熱結構之背光模組。 【先前技術】 近年來，由於液晶顯示器的製造成本顯著地降低，因 • 此液晶顯示器已逐漸成為家庭電視及電腦用螢幕的主流。 -般來說’液晶顯示器包括背光模組與液晶面板兩大部 二，而貪光模組的主要功用在於提供—背光源供液晶顯示 器使用。請參照圖1A，圖1A所繪示為習知背光模組之電 路板俯視圖。該電路板〇〇包括有一晶片〇11)、一電 源（116 )、多個LED燈（112)及多條導線（115)。其中， 該晶片（111)、該電源（116)及多個LED燈（112)/、乃是 透過該些細長的導線（115)而串連形成一封閉迴路。接$ 來請參照圖1B，圖1B所繪示為習知背光模組之側視剖面 • 圖。該背光模組（1)在該電路板（11)的上方更設置有一 擴散板（12)及一光學膜（13)。該電路板（u)係由一基 材（113)組成’且該晶片（111)與多個led燈（112)"· 乃是設置於該基材（113)的上方。該LED燈（112)係藉 由6亥導線（115 )與相鄰的晶片（111 )或另一相鄰的[ED 燈（112)串連相接。 然而，如圖1A與圖1B所示’上述之背光模組（1)， 因為s亥些LED燈（112 )所產生的高熱僅透過細長的導 (115)來將熱量帶離開該LED燈（112)，造成其散熱:欠 201104150 率差。還有，因為該晶片（111)被該擴散板（12)與該電 路板（11)包覆在該背光模組（1)内部，故無法將該晶片 (111)產生的熱量散逸出去。因此，改善該背光模組（1) 的整體散熱能力是本領域具有通常知識者努力的目標。 【發明内容】 本發明主要目的在於利用金屬的散熱面積來加強熱傳 導效應所增加的散熱效率。 本發明另一目的在於利用空氣對流效應來加強晶片散 熱的效率。 為達上述目的，本發明提供一種背光模組，該背光模 組包括一電路板及一背板。該電路板包括有一基材、一晶 片及複數個LED (Light Emitting Diode，發光二極體）燈， 該些LED燈設置於該基材的正面，每一 LED燈係藉由一 第一金屬箔片與相鄰的LED燈串連相接，每一 LED燈與 第一金屬箔片間隔有一阻焊漆且透過一接腳相連接。該背 板設置於該電路板的背面之外。藉此，該背光模組可藉由 熱傳導效應，將該些LED燈產生的高熱透過該些接腳傳導 至該些第一金屬箔片，加強散熱功效。 如上所述之背光模組，其中，該基材更包括有多個垂 直貫通該基材的穿透孔，且在該基材的背面更設置有多個 第二金屬箔片，該第一金屬箔片與該第二金屬箔片相對 應。在每一穿透孔的孔壁上鍍有一金屬膜，藉以連接該第 一金屬箔片與該第二金屬箔片。 201104150 如上所述之背光模組，其中，該基材更包括有多個垂 直貫通該基材的穿透孔，且在該基材的背面更設置有多個 第二金屬箔片，該第一金屬箔片與該第二金屬箔片相對 應。在每一穿透孔内設置有一金屬材質的柱狀體，該柱狀 體的兩端分別與該第一金屬箔片、該第二金屬箔片相接。 如上所述之背光模組，其中，該些第二金屬箔片外覆 有一層阻焊漆。 如上所述之背光模組，其中，該晶片設置於該基材的 背面，並透過至少一導線與該基材正面的第一金屬箔片相 連接。5玄導線垂直貫通該基材。該背板上更設置有一通風 孔，該通風孔與該晶片相對應，藉以加強對流效應。 藉此’本發明之背光模組利用金屬快速導熱的特性而 將LED燈上的熱量迅速傳導至第一金屬箔片及第二金屬箔 片。還有’將控制用的晶片設置於電路板的最外侧，配合 背板上的通風孔之設計，便可利用熱對流效應來對晶片散 熱。藉此’可大幅改善背光模組的整體散熱能力。 為使熟悉該項技藝人士瞭解本發明之目的、特徵及功 效’茲藉由下述具體實施例，並配合所附之圖式，對本發 明詳加說明如後。 【實施方式】 請參閱圖2A，圖2A所繪示為本發明之實施例的電路 板俯視圖。該電路板（21)包括有一晶片（211)、一電源 (216)、多個LED燈（212)、多條導線（215)及多個第 一金屬箔片（217)。其中，該電源（216)係用以提供該電 5 201104150 路板（21)上的多個LED燈（212)所需之電力，使該些 LED燈（212 )產生光亮。該晶片（21丨）係用以控制該些 LED燈（212)的開關及亮度。該晶片（211 )、電源（216)、 導線（215)、多個第一金屬箔片（217)及多個LED燈（212) 串連而形成一封閉迴路。在兩個串連而相鄰的LED燈（212 ) 之間，以一第一金屬箔片（217)相連接。接下來請參閱圖 2B，圖2B所繪示為本發明之實施例的電路板側視剖面圖。 如圖2B所示，請同時參閱圖2A，該電路板（21)係由一 基材（213)組成，該些LED燈（212)設置於該基材（213) 的上面，母一 LED燈（212)係藉由一第一金屬箔片（217) 與相鄰的LED燈（212)串連相接。每一 LED燈（212) 與第一金屬箔片（217)間隔有一阻焊漆（214)而透過一 接腳（2121)相連接。該阻焊漆（214)將該些第一金屬箔 片（217)包覆，用以絕緣並保護該些第一金屬箔片（217)。 還有，該基材（213)下方亦包覆一層阻焊漆（214)，用以 保濩並隔絕該基材（213)。藉此，本發明可藉由熱傳導效 應，將該些LED燈（212)產生的高熱，透過該接腳（2121 ) 傳導至該些第一金屬箔片（217)。因為該些第一金屬箔片 (217)為一大面積金屬平面的結構，利用金屬快速導埶的 特性而將熱量迅速地傳導至第一金屬箔片（217)的整個平 面，藉以加強散熱功效。 如上所述的電路板為單層板結構，但，本發 用於雙層板結構。請參閱圖3,目3料示為本發明另二 實施例的電路板侧視剖面圖。如圖3所示，該電路板 包括有-基材（313)、多個LED燈（312)、多個接聊⑶、 多個第-金屬箔片（317)，以及包覆在該些第一金屬箔片 201104150 (317)之上的阻焊漆（314)。如圖3所示，該基材（313) 更包括有多個垂直貫通該基材（313)的穿透孔（318)，且 在該基材（313)的下面更設置有多個第二金屬箔片（319)。 在每一穿透孔（318)的孔壁上鍍有一金屬膜（未繪示）， 藉以連接該第一金屬箔片（317)與該第二金屬箔片（319)。 該第一金屬箔片（317)與該第二金屬箔片（319)面積相 同且位置上下互相對應，且在該些第二金屬箔片（319)外 覆有一層阻焊漆（314)。如此，本實施例之電路板（31) ❿可藉由金屬良好的熱傳導能力而將該些第一金屬箔片 (317)上的熱量，迅速地透過該穿透孔（318)内的金屬 膜而傳導至該些第二金屬箔片（319);因為本實施例用以 散熱的金屬平面結構之面積較前述實施例增加一倍，故其 散熱能力可再進一步地增加。另外，本領域具有通常知識 者也可以在每一穿透孔（318)内設置一金屬材質的柱狀體 (未繪示），用以取代前述鍍在孔壁上的金屬膜，使該柱狀 體的兩端分別與該第一金屬箔片（317)、該第二金屬箔片 (319)相接，藉以傳導熱量；該柱狀體可為任意細長形的 • 金屬導線。 以上實施例乃是利用熱傳導來增加背光模組的散熱能 力，本發明還可以藉由熱對流來加強其散熱效果。請參閱 圖4圖4所繪示為本發明之實施例的背光模組側視剖面 圖。如圖4所示，該背光模組（4)包括有一電路板（41)、 一擴散板（42)、一光學膜（43)及一背板（44)，該電路 板（41)包括有一基材（413)、一晶片（411)、多個led 燈（412)、兩條導線（415)、多個第一金屬箔片（4i7)及 兩層阻焊漆⑷4)。其中，該晶片（4⑴係設置於該基材 201104150 (413)的下面，且該晶片（411)透過一垂直貫通該基材 (413)的導線（415)來與上方的第一金屬箔片（417)相 連接，進而與該些LED燈（412)相串連。該背板（44) 设置於該電路板（41)的下面。為了將該晶片（41◦上的 熱量散逸出去，在該背板（44)上設置有一通風孔（441)， 使該通風孔（441)與該晶片（411)相對應；如此，藉由 熱對流效應而將該晶片（4 Π )週圍的熱量帶走。 請參閱® 5 ’ ® 5所繪示為本發明另一實施例的背光 模組側視剖面圖。如圖5所示，該背光模組（5)包括有一 電路板（51)、一擴散板（52)、一光學膜（53)及一背板 (54)，該電路板（51)包括有一基材（513)、一晶片（511)、 多個LEDS (512)、祕導線（515)、多個第—金屬绪片 (517)及兩層阻焊漆（514)，該背板（54)包括有”通風 孔（541)。為了增加熱傳導散熱的能力，本實施例採雙層 板設計，在該電路板（51)上更設置有多個穿透孔（518) 及多個第二金射I片（519)，藉以進-步地增加熱傳導散 熱的面積。 前述為使用擴散板的直下式之背光模組，另外，本發 明之背光模組還可以使用導光板，使該LED燈從側邊入 光。請參閱圖6, ® 6所繪示為本發明又—實施例的背光 模組側視剖面圖。如圖6所示，該背光模組（6)包括有一 電路板（61)、一光學膜（63)、一導光板（65)及一背板 (64)〇該電路板（61)包括有一基材（613)、一 LED燈 (612)、一接腳（6121)、兩層阻焊漆（614)、多個穿透孔 (618)、一第一金屬箔片（617)及一第二金屬箔片（619)。 如上所述’背光模組（6)可藉由金屬快速導熱的特性，將 201104150 δ玄些LED燈（612)產生的高熱，透過該接腳（6121) 導至該第一金屬箔片（617)及該第二金屬箔片（619)，藉 以加強散熱功效。在本實施例中，將該電路板（61 )及該 LED燈（612)叹置在該導光板（65)的側邊，使光線由該 導光板（65)的側邊進入該導光板（65) μ。藉此，本實 施例之背光模組（6)可配合導光板（65)的使用，以側邊 入光的方式來達到液晶顯示器（未繪示）薄型化的要求。 綜上所述，本發明之背光模組利用金屬快速導熱的特 φ 性而將LED燈上的熱量迅速傳導至第一金屬箔片及第二金 屬箔片。還有，將控制用的晶片設置於電路板的最外側， 配合背板上的通風孔之設計，便可利用熱對流效應來對晶 片散熱。藉此’可大幅改善背光模組的整體散熱能力。 本發明以實施例說明如上’然其並非用以限定本發明 所主張之專利權利範圍。其專利保護範圍當視後附之申請 專利範圍及其等同領域而定。凡本領域具有通常知識者， 在不脫離本專利精神或範圍内，所作之更動或潤飾’岣屬 g 於本發明所揭示精神下所完成之等效改變或設計，且應包 含在下述之申請專利範圍内。 【圖式簡單說明】 圖1A所繪示為習知背光模組之電路板俯視圖。 圖1B所繪示為習知背光模組之側視剖面圖。 圖2A所繪示為本發明之實施例的電路板俯視圖。 圖2B所繪示為本發明之實施例的電路板側視剖面圖。 201104150 圖3所繪示為本發明另一實施例的電路板側視剖面 圖。 圖4所繪示為本發明之實施例的背光模組側視剖面 圖。 圖5所繪示為本發明另一實施例的背光模組側視剖面 圖。 圖6所繪示為本發明又一實施例的背光模組側視剖面 圖。 【主要元件符號說明】 <習知> 1 :背光模組 115 :導線 11 :電路板 116 :電源 111 :晶片 12 .擴散板 112 : LED 燈 13 :光學膜 113 :基材 <本發明〉 4、5、6 :背光模組 212、312、412、512、612 : 21 、 31 、 41 、 51 、 61 :電 LED燈 路板 2121、3121、6121 :接腳 211、411、511 :晶片 213、313、413、513、613 : 201104150 基材 214'314'414'514'614： 阻焊漆 215、415、515 :導線 216 :電源 217、317、417、517、617: 第一金屬箔片 318、518、618 :穿透孔 319、519、619 :第二金屬 箔片 42、 52 :擴散板 43、 53、63 :光學膜 44、 54、64 :背板 441、541 :通風孔 65 :導光板201104150 VI. Description of the Invention: [Technical Field] The present invention relates to a backlight module, and more particularly to a backlight module designed with a heat dissipation structure. [Prior Art] In recent years, since the manufacturing cost of a liquid crystal display has been remarkably lowered, the liquid crystal display has gradually become the mainstream of home televisions and computer screens. Generally speaking, the liquid crystal display includes two parts of the backlight module and the liquid crystal panel. The main function of the greedy module is to provide a backlight for the liquid crystal display. Referring to FIG. 1A, FIG. 1A is a top view of a circuit board of a conventional backlight module. The circuit board includes a chip cassette 11), a power source (116), a plurality of LED lamps (112), and a plurality of wires (115). The wafer (111), the power source (116) and the plurality of LED lamps (112)/ are connected in series to form a closed loop through the elongated wires (115). Referring to FIG. 1B, FIG. 1B is a side cross-sectional view of a conventional backlight module. The backlight module (1) further includes a diffusion plate (12) and an optical film (13) above the circuit board (11). The circuit board (u) is composed of a substrate (113) and the wafer (111) and the plurality of led lamps (112) are disposed above the substrate (113). The LED lamp (112) is connected in series with an adjacent wafer (111) or another adjacent [ED lamp (112) by a 6-wire wire (115). However, as shown in FIG. 1A and FIG. 1B, the backlight module (1) described above, because the high heat generated by the LED lamps (112), only passes the elongated guides (115) to carry heat away from the LED lamps ( 112), causing its heat dissipation: owing 201104150 rate difference. Further, since the wafer (111) is covered inside the backlight module (1) by the diffusion plate (12) and the circuit board (11), the heat generated by the wafer (111) cannot be dissipated. Therefore, improving the overall heat dissipation capability of the backlight module (1) is an objective of those skilled in the art. SUMMARY OF THE INVENTION The main object of the present invention is to utilize the heat dissipation area of metal to enhance the heat dissipation efficiency increased by the heat conduction effect. Another object of the present invention is to utilize the air convection effect to enhance the efficiency of wafer heat dissipation. To achieve the above objective, the present invention provides a backlight module including a circuit board and a back board. The circuit board includes a substrate, a wafer, and a plurality of LED (Light Emitting Diode) lamps. The LED lamps are disposed on a front surface of the substrate, and each LED lamp is coated by a first metal foil. The chip is connected in series with adjacent LED lights, and each LED lamp is separated from the first metal foil by a solder resist and connected through a pin. The backing plate is disposed outside the back of the circuit board. Thereby, the backlight module can transmit the high heat generated by the LED lamps to the first metal foils through the heat conduction effect to enhance the heat dissipation effect. The backlight module as described above, wherein the substrate further comprises a plurality of through holes vertically penetrating the substrate, and a plurality of second metal foils are further disposed on the back surface of the substrate, the first metal The foil corresponds to the second metal foil. A metal film is plated on the wall of each of the through holes to connect the first metal foil and the second metal foil. 201104150 The backlight module as described above, wherein the substrate further comprises a plurality of through holes vertically penetrating the substrate, and a plurality of second metal foils are further disposed on the back surface of the substrate, the first The metal foil corresponds to the second metal foil. A columnar body made of a metal material is disposed in each of the penetrating holes, and the two ends of the columnar body are respectively in contact with the first metal foil piece and the second metal foil piece. The backlight module as described above, wherein the second metal foils are covered with a layer of solder resist paint. The backlight module as described above, wherein the wafer is disposed on the back surface of the substrate and is connected to the first metal foil on the front surface of the substrate through at least one wire. 5 mysterious wires run through the substrate vertically. The back plate is further provided with a ventilation hole corresponding to the wafer to enhance the convection effect. Thus, the backlight module of the present invention utilizes the rapid thermal conductivity of the metal to rapidly conduct heat from the LED lamp to the first metal foil and the second metal foil. Also, the control wafer is placed on the outermost side of the board, and the design of the vent holes on the back board allows the heat convection effect to be used to dissipate the wafer. This can greatly improve the overall heat dissipation capability of the backlight module. The present invention has been described in detail with reference to the accompanying drawings and drawings [Embodiment] Please refer to FIG. 2A, which illustrates a top view of a circuit board according to an embodiment of the present invention. The circuit board (21) includes a wafer (211), a power source (216), a plurality of LED lamps (212), a plurality of wires (215), and a plurality of first metal foils (217). The power source (216) is used to supply the power required by the plurality of LED lamps (212) on the circuit board (21), so that the LED lamps (212) are illuminated. The chip (21丨) is used to control the switching and brightness of the LED lamps (212). The wafer (211), the power source (216), the wires (215), the plurality of first metal foils (217) and the plurality of LED lamps (212) are connected in series to form a closed loop. Between two serially adjacent LED lamps (212), a first metal foil (217) is connected. Referring next to Figure 2B, Figure 2B is a side cross-sectional view of the circuit board in accordance with an embodiment of the present invention. As shown in FIG. 2B, please refer to FIG. 2A at the same time, the circuit board (21) is composed of a substrate (213), and the LED lamps (212) are disposed on the substrate (213), and the mother LED lamp (212) is connected in series with an adjacent LED lamp (212) by a first metal foil (217). Each of the LED lamps (212) is separated from the first metal foil (217) by a solder resist (214) and connected through a pin (2121). The solder resist (214) coats the first metal foil sheets (217) for insulating and protecting the first metal foil sheets (217). Further, a solder resist (214) is also coated under the substrate (213) to protect and insulate the substrate (213). Thereby, the present invention can conduct the high heat generated by the LED lamps (212) through the pins (2121) to the first metal foils (217) by the heat conduction effect. Because the first metal foil (217) is a large-area metal plane structure, the heat is quickly transmitted to the entire plane of the first metal foil (217) by utilizing the characteristics of the metal quick guiding, thereby enhancing the heat dissipation effect. . The circuit board as described above is a single-layer board structure, but the present invention is applied to a two-layer board structure. Referring to Fig. 3, a third side view of a circuit board according to another embodiment of the present invention is shown. As shown in FIG. 3, the circuit board includes a substrate (313), a plurality of LED lamps (312), a plurality of contacts (3), a plurality of metal foils (317), and cladding layers. A solder resist (314) over a metal foil 201104150 (317). As shown in FIG. 3, the substrate (313) further includes a plurality of through holes (318) vertically penetrating the substrate (313), and a plurality of second portions are disposed under the substrate (313). Metal foil (319). A metal film (not shown) is plated on the wall of each of the penetration holes (318) to connect the first metal foil (317) and the second metal foil (319). The first metal foil (317) and the second metal foil (319) have the same area and corresponding positions above and below, and the second metal foil (319) is covered with a solder resist (314). Thus, the circuit board (31) of the embodiment can rapidly transfer the heat on the first metal foil (317) through the metal film in the penetration hole (318) by the good thermal conductivity of the metal. The heat transfer capability can be further increased because the area of the metal planar structure for dissipating heat in the embodiment is doubled compared with the previous embodiment. In addition, a person having ordinary knowledge in the art may also provide a metal columnar body (not shown) in each of the penetration holes (318) to replace the metal film plated on the hole wall, so that the column The two ends of the body are respectively connected to the first metal foil piece (317) and the second metal foil piece (319) to conduct heat; the columnar body may be any elongated metal wire. The above embodiment utilizes heat conduction to increase the heat dissipation capability of the backlight module, and the present invention can also enhance the heat dissipation effect by thermal convection. Referring to FIG. 4 and FIG. 4, a side cross-sectional view of a backlight module according to an embodiment of the present invention is shown. As shown in FIG. 4, the backlight module (4) includes a circuit board (41), a diffusion plate (42), an optical film (43) and a back plate (44). The circuit board (41) includes a circuit board (41). A substrate (413), a wafer (411), a plurality of led lamps (412), two wires (415), a plurality of first metal foils (4i7), and two layers of solder resist (4) 4). The wafer (4(1) is disposed under the substrate 201104150 (413), and the wafer (411) passes through a wire (415) perpendicularly penetrating the substrate (413) to the upper first metal foil ( 417) connected in series, and further connected in series with the LED lamps (412). The backplane (44) is disposed under the circuit board (41). In order to dissipate heat from the wafer (41◦, The back plate (44) is provided with a venting hole (441) corresponding to the radiant hole (441); thus, the heat around the wafer (4 Π) is taken away by the heat convection effect. Please refer to ® 5 ' ® 5 for a side view of a backlight module according to another embodiment of the present invention. As shown in FIG. 5 , the backlight module ( 5 ) includes a circuit board ( 51 ) and a diffusion. a plate (52), an optical film (53) and a back plate (54), the circuit board (51) comprising a substrate (513), a wafer (511), a plurality of LEDS (512), and a secret wire (515) ), a plurality of first metal sheets (517) and two layers of solder resist paint (514), the back sheet (54) includes a "venting hole (541). In order to increase the heat conduction heat dissipation capability, The embodiment adopts a double-layer board design, and a plurality of penetration holes (518) and a plurality of second gold-emitting I sheets (519) are further disposed on the circuit board (51), thereby further increasing the area of heat conduction and heat dissipation. The foregoing is a direct-type backlight module using a diffusion plate. In addition, the backlight module of the present invention can also use a light guide plate to allow the LED lamp to enter light from the side. Referring to FIG. 6, ® 6 is shown as In another embodiment, the backlight module (6) includes a circuit board (61), an optical film (63), a light guide plate (65), and a backlight module. The backplane (64) 〇 the circuit board (61) includes a substrate (613), an LED lamp (612), a pin (6121), two layers of solder resist (614), and a plurality of penetration holes (618). a first metal foil (617) and a second metal foil (619). As described above, the backlight module (6) can be used for the rapid thermal conductivity of the metal, and the 201104150 δ Xuan LED lamp (612) The generated high heat is guided to the first metal foil (617) and the second metal foil (619) through the pin (6121) to enhance the heat dissipation effect. In this embodiment The circuit board (61) and the LED lamp (612) are slanted on the side of the light guide plate (65), so that light enters the light guide plate (65) μ from the side of the light guide plate (65). The backlight module (6) of the present embodiment can be used with the light guide plate (65) to achieve the requirement of thinning the liquid crystal display (not shown) by the side light entering. In summary, the present invention The backlight module rapidly transfers heat on the LED lamp to the first metal foil and the second metal foil by utilizing the special properties of the rapid thermal conduction of the metal. Further, the wafer for control is placed on the outermost side of the circuit board, and the venting hole on the back plate is used to heat the wafer by the heat convection effect. This can greatly improve the overall heat dissipation capability of the backlight module. The present invention is described by way of example only, and is not intended to limit the scope of the claims. The scope of patent protection is subject to the scope of the patent application and its equivalent fields. Appropriate changes or designs made by those skilled in the art, without departing from the spirit or scope of the present invention, may be included in the following application. Within the scope of the patent. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a top plan view of a circuit board of a conventional backlight module. FIG. 1B is a side cross-sectional view of a conventional backlight module. 2A is a top plan view of a circuit board according to an embodiment of the present invention. 2B is a side cross-sectional view of a circuit board according to an embodiment of the present invention. 201104150 FIG. 3 is a side cross-sectional view of a circuit board according to another embodiment of the present invention. 4 is a side cross-sectional view of a backlight module in accordance with an embodiment of the present invention. FIG. 5 is a side cross-sectional view showing a backlight module according to another embodiment of the present invention. FIG. 6 is a side cross-sectional view showing a backlight module according to still another embodiment of the present invention. [Explanation of main component symbols] <Primary> 1 : Backlight module 115: Conductor 11: Circuit board 116: Power supply 111: Wafer 12. Diffusion plate 112: LED lamp 13: Optical film 113: Substrate <The present invention 〉 4, 5, 6: backlight modules 212, 312, 412, 512, 612: 21, 31, 41, 51, 61: electric LED lamp board 2121, 3121, 6121: pins 211, 411, 511: wafer 213 , 313, 413, 513, 613 : 201104150 substrate 214 '314 '414 '514 '614: solder resist 215, 415, 515: wire 216: power supply 217, 317, 417, 517, 617: first metal foil 318, 518, 618: penetration holes 319, 519, 619: second metal foil 42, 52: diffusion plates 43, 53, 63: optical films 44, 54, 64: back plates 441, 541: ventilation holes 65: Light guide
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