Backlight of a liquid crystal display device

[0002] This application claims the benefit of Korean Patent Application No. 1999-8111, filed on Mar. 11, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.

[0003] 1. Field of the Invention

[0004] The present invention relates to a liquid crystal display device (LCD), and more particularly to a backlight of the liquid crystal display device.

[0005] 2. Discussion of the Related Art

[0006] A liquid crystal display device is widely used for a note book computer or a portable TV and generally comprises a liquid crystal panel and a backlight to provide a uniform collimated light to the liquid crystal panel. The liquid crystal panel basically comprises an upper substrate, a lower substrate, and a liquid crystal layer between the two substrates.

[0007] As shown in FIG. 1, the backlight generally comprises a light source 1; a light source housing 3 both for protecting the light source 1 and for concentrating light from the light source 1; a light pipe or light guide 4; a reflecting plate 6 on a lower surface of the light pipe 4; a light dispersing film 7 over an upper surface of the light pipe 4; one or two lens films 8 over the light dispersing film 7; and a protecting film 9 over the lens films 8. The backlight provides a uniform collimated light to the liquid crystal panel.

[0008] Scattering patterns are formed on a predetermined region of the light pipe 4 to scatter light which is trapped within the light pipe 4 by total internal reflection. The light pipe 4 is divided into a non-display region A and a display region B.

[0009] Light emitted from the light source 1 is incident on a light entering surface 2 of the light pipe 4, and being totally reflected, the incident light travels within the light pipe 4 by the non-display and display regions A and B in the named order. The light trapped within the light pipe 4 is scattered by the scattering patterns, destroying the total internal reflection condition, and the light partially comes out of the light pipe 4 through the upper surface so that a uniform collimated light is provided to the liquid crystal panel. The light reflecting plate 6 under the light pipe 4 reflects light coming out of the light pipe 4 through its lower surface, improving the light utilization ratio. The scattering film 7 imparts an optimum uniformity and directionality to the exit light. The lens film 8 concentrates the light into a predetermined direction, preventing a bad image. The protecting film 9 protects the lens film 8, increases the light uniformity, and enlarges the viewing angle range by properly dispersing the light.

[0010] As mentioned above, after entering the light pipe 4 through the light entering surface 2, the light exits from the light pipe 4 when scattered by the scattering patterns. The density of the scattering patterns is determined so that the display region B is provided with a uniform collimated light.

[0011] However, as shown in FIGS. 2aor 2b, light can enter the upper and lower surfaces of the light pipe 4 or the rounded surfaces 2aand 2bthrough a gap between the light source housing 3 and the light pipe 4. Most of the light which enters the light pipe 4 through the upper surface of the light pipe 4 is directed to the display region B without total internal reflection and light dispersion, resulting in a bright line. Referring to light paths shown in the Figures, it can be understood that the position of the bright line on the upper surface of the light pipe 4 is changed according to its incident angle, and is changed according to the exit angle.

[0012] FIG. 2cis a graph showing the exit position (Y, mm) with respect to the exit angle (X, degree) of the bright line mentioned above. where the tested value T is compared with the computed value C. As shown in FIG. 2d, the exit position Y is defined as a horizontal distance between the incident surface 2 and the exit position, and the exit angle X is defined as an angle between the normal direction and the exit direction S. This test and computation are performed with a light pipe having a thickness of 2.6 mm and without any optical film such as a dispersing film, and a lens film.

[0013] Two causes of the bright line can be seen by FIG. 2aand FIG. 2b, respectively. The relationship shown in FIG. 2C applies to both cases. When there is a large gap between the light source housing 3 and the light pipe 4, the bright line of FIG. 2aincreases, and when there are large rounded surfaces 2aand 2bon the light pipe 4, the bright line of FIG. 2bincreases. The rounded surfaces 2aand 2bare results from an inaccurate molding process. It is more difficult to prevent the bright line caused by the rounded surfaces in comparison, with the bright line of the gap between the light source housing 3 and the light pipe 4. Thus, the light pipe 4 is typically manufactured to make the edges without the rounded surfaces. Accordingly, the following explanations are mainly given for the bright line caused by the gap between the light pipe and the light source housing.

[0014] As shown in FIG. 3, printed regions 11, 12, and 13 are conventionally formed, respectively, on the light pipe 4, reflecting plate 6, and the dispersing plate 7 in the light path of the bright line in order to prevent the bright line. In this figure, the same elements as in FIG. 1 are indicated by same numerals without further explanation of the structure. Light incident on the upper surface of the light pipe 4 is weakened by absorption and scattering by the black printed region 11 on the upper surface of the light pipe 4, and the weakened light is further weakened by absorption, reflection and scattering by the black printed region 12 on the reflecting plate 6. Finally, the weakened light is scattered and removed by the white printed region 13 on the dispersing film 7.

[0015] Light entering a gap between the lower surface of the light pipe 4 and the reflecting plate 6 also is weakened by absorption and scattering by the black printed region 12, and then scattered and removed by the white printed region 13 on the dispersing film 7.

[0016] The structure described above to prevent the bright line has the following problems.

[0017] First, the manufacturing process is complicated to form the printed regions on the light pipe 4, the reflecting plate 6, and the dispersing film 7, and print degradation is easily generated, increasing the manufacturing cost.

[0018] Second, the printing regions are liable to weaken light which does not cause the bright line, decreasing the brightness of the collimated light provided to the liquid crystal panel. When using all of the printing regions on the light pipe 4, reflecting plate 6, and the dispersing film 7 to entirely remove the bright line, the light brightness is lowered by 3%-6%.

[0019] Third, a black stripe image comes in sight when using a deep black printing region, namely, it is difficult to determine a proper depth of black.

[0020] Fourth, there is a possible error of 0.5 mm in the vertical positions of the reflecting plate 6 and the dispersion film 7. Thus, the reflecting plate 6 may enter the light source housing 3, so that the black printing region 12 absorbs light incident on the light entering surface 2, decreasing the light brightness of the backlight. Further, the reflecting plate 6 may be so far removed from the light source housing 3 as to fail to remove the bright line by the black printing region 12. Further, if the dispersion film 7 is not correctly located, white and black stripes can be seen vaguely. Actually, the vague white and black strips are inevitable unless the reflecting plate 6 and the dispersing film 7 are fixed.

[0021] Accordingly, the present invention is directed to a backlight of a liquid crystal display device that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.

[0022] An object of the present invention is to provide a backlight of a liquid crystal display device which can provide a bright and uniform collimated light without generating a bright line.

[0023] Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0024] To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a backlight of a liquid crystal display device which comprises:

[0025] a light pipe having a first surface as a top surface, a second surface as a bottom surface, and an end surface as a light entering surface, wherein the first surface is connected to the end surface through a first boundary and the second surface is connected to the end surface through a second boundary;

[0026] a light source facing the light entering surface; and

[0027] a light source housing including a light concentrating portion and a fixing portion; wherein the light concentrating portion has an inner space where the light source is located and an opening facing the end surface; wherein the opening is defined by first and second sides and at least one of the two sides is located vertically between the first boundary and the second boundary; and wherein the fixing portion is connected to the light concentrating portion through the opening and has an end located on at least one of the first and second surfaces.

[0028] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

[0029] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain principles of the invention.

[0030] In the drawings:

[0031] FIG. 1 is a sectional view showing a conventional backlight;

[0032] FIGS. 2aand 2bare views for explaining how a bright line is generated in the related art;

[0033] FIG. 2cis a graph showing the exit position Y with respect to the exit angle X in the related art;

[0034] FIG. 2dis a view for explaining the exit angle X and the exit position Y;

[0035] FIG. 3 is a sectional view showing a backlight according to a related art technique for preventing the bright line;

[0036] FIG. 4ais a backlight of a first embodiment according to the present invention;

[0037] FIGS. 4band 4care views showing modifications of the first embodiment;

[0038] FIGS. 5aand 5bare views showing the bright line causing light of the prior art;

[0039] FIGS. 5cand 5dare views for explaining the bright line preventing effect of the first embodiment;

[0040] FIG. 5eis a view showing a luminance L of the bright line causing light with respect to a distance F;

[0041] FIG. 6 is a view showing light paths of the prior art and the first embodiment;

[0042] FIG. 7ais a sectional view showing a second embodiment of the present invention;

[0043] FIGS. 7b, 7cand 7dare views showing modifications of the second embodiment;

[0044] FIG. 8ais a sectional view showing a third embodiment of the present invention;

[0045] FIGS. 8bto 8fare views showing various modifications of the third embodiment;

[0046] FIG. 9ais a view showing a path of the bright line causing light in the light pipe of the related art;

[0047] FIGS. 9band 9care views for explaining the bright line preventing effect of the third embodiment; and

[0048] FIG. 10 is a sectional view showing a fourth embodiment.

[0049] Reference will now be made in detail to the preferred embodiments, examples of which are illustrated in the accompanying drawings.

[0050] FIG. 4ashows a first embodiment. As shown in this figure, a backlight of this embodiment comprises a light pipe 24 having a light entering surface 22; a reflecting plate 26 facing a lower surface of the light pipe 24; a dispersing film 27, lens film 28 and protecting film 29 over an upper surface of the light pipe 24; a light source 21 facing the light entering surface 22; a light source housing 33 which protects the light source 21 and comprises a light concentrating portion 31, and upper and lower fixing portions 32aand 32b. The dotted lines indicate the boundaries between the light concentrating portion 31 and the fixing portions 32 and 32b, respectively.

[0051] The present invention is characterized in that the light concentrating portion 31 has an opening defined by its upper and lower sides, and at least one of the two sides of the opening is located vertically between upper and lower sides of the light entering surface 22.

[0052] The fixing portions 32aand 32bare formed to be integrated with the light concentrating portion 31, and have upper and lower ends which define an opening of the fixing portions 32aand 32band encompass the light entering surface 22. Accordingly, the upper and lower ends of the fixing portion 32aand 32bface the upper and lower surfaces of the light pipe 24, respectively. Although the figure shows that the fixing portions 32aand 32bare bent at a right angle, they may be bent at any angle, or bent with a rounded curve to have a continuous surface.

[0053] As shown in FIG. 3, there is a gap between the light source housing 3 and the upper and lower surfaces of the light pipe 4, and a bright line causing light enters the gap. Whereas the prior art prevents the bright line by scattering and absorption, the present invention has a bright line preventing structure of the light source housing 33 which blocks the bright line causing light from entering the gap. Light beams shown in FIG. 4awould have become a bright line in the prior art. However, because the upper and lower sides of the light concentrating portion 31 opening are located vertically between upper and lower sides of the light entering surface 22, the light beams are prevented from entering the gap, and reflected on inner surfaces of the light concentrating portion 31 to enter the light pipe 24 through the light entering surface 22. The light beam contributes to improve the brightness of the collimated light. Also when one of the upper and lower sides of the opening is located vertically between the upper and lower sides of the light entering surface, the bright line can be prevented. Accordingly, in the backlight, only an upper part of the light source housing 33amay have a bright line preventing structure as shown in FIG. 4b, and conversely only a lower part of the light source housing 33bmay have a bright line preventing structure as shown in FIG. 4c. In FIGS. 4band 4c, the same elements as in FIG. 4aare indicated by same numerals without explanation of the structure.

[0054] Hereinafter, the bright line preventing principles are explained in detail referring to FIGS. 5ato 5d. In these Figures, the light source housing and the light pipe are represented only by their outlines, and the same elements as in FIG. 4aare indicated by same numerals without further explanation of the structure.

[0055] In the prior art, as shown in FIG. 5a, lights which enter the gap at any incident angle cause a bright line. In the prior art as shown in FIG. 5b, lights incident on the rounded surface 2aproduce a bright line if an angle Q is less than sin⁻¹(1/1.49) according to Snell's law.

[0056] On the other hand, in the embodiment as shown in FIG. 5C, light cannot enter the gap if an inner angle between its traveling direction and the horizontal direction of the light pipe 4 is less than an angle C₁. Further, as shown in FIG. 5d, when a bent angle G is not 90 degrees, light cannot enter the gap if the inner angle is less than an angle C₂. Accordingly, the amount of the bright line causing lights is reduced, and the bright line can be removed using only the dispersion film and the scattering patterns of the light pipe 24, which are essential elements of a backlight, without the printing regions for removing the bright line.

[0057] In FIGS. 5cand 5d, D₁and D₂indicate vertical distances between the upper side of the opening of the light concentration portion 31 and the upper side of the light entering surface. F₁and F₂indicate horizontal distances between the upper side of the opening of the light concentration portion 31 and the light entering surface 22. E₁and E₂indicate a minimum refracted angle of the bright line causing light with respect to the upper surface of the light pipe 24. In FIG. 5a, E₃indicates a minimum refracted angle of the bright line causing light with respect to the upper surface of the light pipe 4 in the prior art.

[0058] Equations for the above-mentioned parameters are as follows.

[0059] C₁=tan⁻¹(D₁/F₁), C₂=tan⁻¹(D₂/F₂),

[0060] E₁=90°−sin⁻¹(sin(90°−C₁)/refractive index of the light pipe)),

[0061] E₂=90°−sin⁻¹(sin(90°−C₂)/the refractive index), and

[0062] E₃=90°−sin⁻¹(1/the refractive index).

[0063] In FIG. 6, explanations are made for each path of the bright line causing light incident at a maximum incident angle onto the upper surface of the light pipe as shown in FIGS. 5a, 5c, and 5d.

[0064] Because the angle E₃of FIG. 5amust be less than angles E₁and E₂of FIGS. 5cand 5d, the corresponding bright line causing light H₃of the related art, as shown in FIG. 6, reaches farther than the bright line causing lights H₁and H₂according to the first embodiment. Accordingly, the bright line causing light H₃has more possibility to be seen in the display region B than the bright line causing lights H₁and H₂. Further, gaps F₁and F₂denoted in FIG. 5 can be controlled to be sufficiently small in order to completely remove the bright line or limit most of the bright line causing light to the non-display region A. Even though the gaps F₁and F₂are not zero, most of the bright line causing lights which would become bright lines in the related art enters the light entering surface and does not produce a bright line, even without the use of the printing regions for removing the bright line.

[0065] The following equations are relations of the horizontal distances h₁, h₂, and h₃between the light entering surface and the exit position of the bright ling causing light on the upper surface of the light pipe 24; a vertical width I of the light entering surface; and refractive angles E₁, E₂, and E₃denoted in FIGS. 5a, 5c, and 5d.

[0066] h₁=2Itan(90−E₁), h₂=2Itan(90−E₂),

[0067] h₃=2Itan(90−E₃)

[0068] When the bright line causing light encounters first the upper surface in the non-display region A, the exit light H₁and H₂may be seen in the display region only after being subjected to a lot of absorption and scattering in the backlight. Therefore, the lights H₁and H₂do not cause bright lines.

[0069] When the bright line causing lights G₁and G₂exit through positions g₁and g₂after internal reflection, the exit lights G₁and G₂do not cause bright lines because they are twice scattered by the scattering patterns in the light pipe 24 during internal reflection.

[0070] Thus, the amount of the bright line causing light is reduced, and also the bright line causing light exits toward the display region B only after undergoing lots of the scattering.

[0071] It is understood that the present invention has an increased effect of preventing bright lines more than the related art also when the gaps F₁and F₂are not zero. As mentioned above, the bright line preventing effect occurs because the upper and lower sides of the opening of the light concentration portion 31 are located vertically between the upper and lower sides of the light entering surface 22. Accordingly, the bent angle G denoted in FIG. 5dmay have an angle of 0 to 180 degrees, and the fixing portion 32ahas a rounded bend.

[0072] Also, it is reasonable that the bright line preventing effect is expected in a backlight whose light pipe has a rounded surface 2aas shown in FIG. 5b, if only employing the light source housing 33 of the present invention.

[0073] Many of bright lines are caused in the backlight whose light pipe has the rounded surface 2aas shown in FIG. 5b, and also when the light pipe is tightly held by the light source housing. Therefore, the light pipe is generally manufactured so as not to have the rounded surface 2a. Although the rounded surface 2amay be formed by an inaccurate molding process, in most cases the rounded surface 2ahas a size less than 0.05 mm. Therefore, a larger amount of bright lines are caused by the case of FIG. 5athan the case of FIG. 5b.

[0074] Accordingly, the relationship between the gap F (F₁or F₂) illustrated in FIGS. 5cand 5dand the bright line quantity ratio L with respect to the related art device are calculated by simulation only for the case where there is no rounded surfaces. FIG. 5eis a graph showing the relationship. For the calculation, the gap P between the upper surface of the light pipe 24 and the end of the upper fixing portion 32awas assigned a worst value of 0.1 mm, considering the tolerance. The vertical distance between the opening's upper side of the light concentrating portion 31 and the end of the upper fixing portion 32awas assigned a value of 0.3 mm, and the vertical distance D (D₁or D₂) between the upper edge of the light entering surface 22 and the opening's upper side of the concentrating portion 31 was assigned a value of 0.2 mm. As shown in FIG. 5e, more than 80% of the bright line causing light of the related art device enters the light pipe with the gap F larger than 0.3 mm. That is, the bright line preventing effect of the first embodiment is reduced for a large value of the gap F. Accordingly, it is preferable that the fixing portions 32aand 32bare bent so as to closely encompass the upper and lower side of the light entering surface 22, respectively.

[0075] The selection of a preferable value for D/F depends on what kinds of dispersion film, lens film, and the light pipe are used. Considering the typical position and luminance of the bright line, it is preferable to select D/F to be greater than one, where C₂is less than 45 degrees. In this case, the bright line preventing effect of the first embodiment is remarkable compared to the related art device. Of course, even when D/F is less than one, the first embodiment has a high bright line preventing effect in comparison with the related art device.

[0076] When D has a large value, F is permitted to have a large value as well. Because the vertical width of the light entering surface is 2 to 3 mm, D has an upper limit. Because vertical position error or width error of the light entering surface is about 0.1 mm, D cannot be less than 0.1 mm. However D is permitted to have a lower limit of 0.05 mm, provided that with the reflecting plate 26 being outside of the light source housing 33, the light pipe 24 is tightly held by the light source housing 33 so as to have minimum position or width error. Accordingly, D is preferably in the range of 0.05 mm to 0.4 mm. It is preferable to select a value of between 0.05 mm and 0.5 mm for the vertical distance between the upper side of the opening of the light concentrating portion 31 and a portion of the inner surface of the upper fixing portion 32alying in an imaginary plane extending from the light entering surface 22. From the explanation above, it can be understood that it is preferable to select F to be less than 0.5 mm to effectively prevent the bright line.

[0077] There are various methods to limit F to be less than 0.5 mm: (1) fixing the light source housing to the light pipe with a screw or a pin therethrough; (2) fitting a protrusion of the light pipe into a hole of the light source housing; (3) protrusions or holes on end surfaces, except the light entering surface, are coupled to holes or protrusions of a main support as a base frame of the backlight; and (4) attaching the inner surfaces of the fixing portion of the light source housing to the light pipe with an adhesive.

[0078] A second embodiment is described as follows referring to FIGS. 7aand 7b.

[0079] This embodiment is different from the first embodiment in that lower fixing portions 42band 42dare in contact with the lower surface of the light pipe 24. In this embodiment, the lower fixing portion 42bmay cover a reflecting plate 46 as shown in FIG. 7a, or the lower fixing portion 42dmay not cover the reflecting plate 46 as shown in FIG. 7b.

[0080] As shown in FIGS. 1 and 4a, the illuminance of the bright line is proportional to the quantity of the light entering the gap between the light source housings 3 and 33 and the upper and lower surfaces of the light pipes 4 and 24. Therefore, the light pipes 4 and 24 should be tightly held by the light source housings 3 and 33 in order to reduce the gap. However, the tight holding causes furrows in the reflecting plates 6 and 26 when thermally expanded. For preventing the furrows, the lower fixing portion of the light source housing 41 is once more bent so as to hold only the light pipe 24 without the reflecting plate 46 being held as shown in FIG. 7a. Further, the reflecting plate 46 is outside of the light source housing 43aand the lower fixing portion 42dof the light source housing 41 holds only the light pipe 24. Therefore, the reflecting plate is not furrowed also when the light pipe 24 is tightly gripped by the light source housing made of a hard material in order to decrease the gap between the light source housing and the upper and lower surfaces of the light pipe 24. Thus, there are many freedoms for selecting the material for the light source housing 43a, and the bright line can be much more prevented by decreasing the gap.

[0081] In this embodiment, only the lower part of the light source housing 43bmay have the bright line preventing structure as shown in FIG. 7c, or only upper part of the light source housing 43cmay have the bright line preventing structure as shown in FIG. 7d. Further, the light source housing 43amay not cover the reflecting plate 46.

[0082] FIG. 8ais a view showing a third embodiment. As shown in this Figure, in this embodiment, the light pipe 54 has upper and lower transparent protrusions 54aand 54b, respectively, on the upper and lower surfaces in a region adjacent to its light entering surface 57. Each of the transparent protrusions 54aand 54bhas such an inclined surface 55 such that the light pipe 54 has a larger thickness, the closer to the light entering surface 57. Namely, an inner angle between the inclined surface 55 and the light entering surface 57 is set to be less than 90 degrees.

[0083] An upper fixing portion 52aof the light source housing 53ahas an inner surface encompassing the upper transparent protrusion 54a. Therefore, the upper transparent protrusion 54ais trapped in the upper fixing portion 52aso that the light pipe 54 is fixed to the light source housing 53a. In this case, it is possible that each of the upper and lower fixing portions 52aand 52bhas a hole through which each of the upper and lower protrusions 54aand 54bare inserted to fix the light pipe 54.

[0084] This embodiment has a bright line preventing structure as in the first embodiment where the opening's upper and lower sides of the concentrating portion 51 are located vertically between the upper and lower sides of the light entering surface 57. Due to the transparent protrusions 54aand 54b, the light entering surface 57 a larger area than that of the first embodiment, and has an inner space larger than that of the first embodiment. Further, the transparent protrusions 52aand 52bcan be tightly gripped by the upper and lower fixing portions 52aand 52bin order to decrease the gap between the light source housing and the upper and lower surfaces of the light pipe 54, preventing the bright line.

[0085] The transparent protrusions 54aand 54bcan be formed to be integrated with the light pipe 54 as shown in FIG. 8a. The transparent protrusions can be also formed by attaching a transparent material to the light pipe. Further, they can be formed by coating a transparent organic material on the light pipe and patterning by a photolithography process.

[0086] This embodiment features the transparent protrusions on the upper and lower surfaces of the light pipe. However, it is possible, as shown in FIG. 8b, that the light pipe 58 has only the lower transparent protrusion 54b, and the upper and lower parts of a light source housing 53bhave the bright line preventing structure. It is also possible that as shown in FIG. 8c, that the light pipe 58 has only the lower transparent protrusion 54b, and only the lower part of a light source housing 53chas the bright line preventing structure. Further, it is possible that as shown in FIG. 8d, that a light pipe 59 has only the upper transparent protrusion 54a; only the upper part of a light source housing 53dhas the bright line preventing structure; and an upper fixing portion of the light source housing 53dis shaped to encompass the upper transparent protrusion 54a. Furthermore, it is possible, as shown in FIG. 8e, that the light pipe 59 has only the upper transparent protrusion 54a; only the upper part of a light source housing 53ehas the bright line preventing structure; and an upper fixing portion of the light source housing 53dis shaped not to encompass the upper transparent protrusion 54a. In addition, it is possible, as shown in FIG. 8f, that the light pipe 59 has only the upper transparent protrusion 54a; a light source housing 53fdoes not have the bright line preventing structure like the above-mentioned embodiments; and an inner surface of the light source housing 53fincludes a portion facing and formed along the inclined surface of the upper transparent protrusion 54a.

[0087] Bright line preventing effects of the transparent protrusions 54aand 54bare explained as follows, referring to FIGS. 9a, 9b, and 9c. FIG. 9ais drawn based on the related art device as shown in FIG. 1; FIG. 9bis drawn based on the case of a device as shown in FIG. 8e; and FIG. 9cis drawn based on the case of a device as shown in FIG. 8f.

[0088] Because the light pipe has a refractive index higher than the air, light enters the light pipe with a refractive angle smaller than the incident angle, according to Snell's Law. Therefore, as shown in FIG. 9a, the bright line causing light in the prior art reaches far away from the light entering surface to be directed to the display region B. On the other hand, as shown in FIGS. 9band 9c, the width between the entering position and the exit position on the upper surface of the light pipe is reduced due to the transparent protrusion, so that the bright line causing light has a low possibility to reach the display region B.

[0089] FIG. 10 show a fourth embodiment. As shown in this Figure, this embodiment is different from the first embodiment in that the light source housing 63 has an inner space larger than the first embodiment.

[0090] In this embodiment, a concentrating portion 61 is divided into first and second concentrating portions 61aand 61b. Dotted lines shown in FIG. 10 indicate a boundary between the first and second concentrating portions. A light source 21 is located within the first concentrating portion 61a. In order to obtain a sufficient space to accommodate the light source, the first concentrating portion 61ahas an inner space larger than the second concentrating portion 61b. More strictly, vertical inner width of the first concentrating portion 61ais larger than vertical inner width of the second concentrating portion 61bwhich is defined as a distance between the upper and lower boundaries of the second concentrating portion 61band the fixing portion 62. Although the concentrating portions 61aand 61bare each shown to have square-shaped surfaces, they can also have a rounded surface.

[0091] The backlight of the present invention can be used as a collimated illuminating source for any other devices as well as the liquid crystal display device.

[0092] The embodiments described herein can provide the following advantages.

[0093] Referring to FIG. 4a, the upper and lower sides of the opening of the light concentrating portion 31, namely, the boundaries between inner surfaces of the light concentrating portion 31 and the fixing portions 32aand 32b, are located vertically between the upper and lower sides of the light pipe 24. Therefore, the light is prevented from entering the gap, and is reflected onto the inner surface of the light concentrating portion 31 to enter the light pipe 24 through the light entering surface 22, so that the entering light contributes to improve the brightness of the collimated light. Accordingly, the present invention overcomes the problems of the related art devices which remove the bright line by light scattering or absorption. Further, because each of the fixing portions 32aand 32beach have an inner surface opposed to the light entering surface 22, the light source 21 has a low possibility of being damaged by a shaken light pipe.

[0094] It will be apparent to those skilled in the art that various modifications and variation can be made in the backlight of the present invention without departing from the scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.