Prism sheet and backlight module using the same

[0001] This application is related to four co-pending U.S. patent applications, which are: application Ser. No. 11/938,307 and Ser. No. 11/938,308, filed on Nov. 12, 2007, and both entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”; application Ser. No. 11/940,328, filed on Nov. 15, 2007, and entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”, and application Ser. No. 11/246,867, entitled “PRISM SHEET AND BACKLIGHT MODULE USING THE SAME”. In the co-pending applications, the inventors are Tung-Ming Hsu and Shao-Han Chang. The co-pending applications have the same assignee as the present application.

[0002] 1. Field of the Invention

[0003] The present invention relates to prism sheets, particularly, to a prism used in a backlight module.

[0004] 2. Discussion of the Related Art

[0005] In a liquid crystal display device (LCD device), liquid crystal is a substance that does not itself illuminate light. Instead, the liquid crystal relies on light received from a light source to display information. In the case of a typical liquid crystal display device, a backlight module powered by electricity supplies the needed light.

[0006] FIG. 9 represents a typical direct type backlight module 100. The backlight module 100 includes a housing 11, a plurality of lamps 12 positioned above a base of the housing 11, and a light diffusion plate 13 and a prism sheet 10 stacked on top of the housing 11 in that order. Inside walls of the housing 11 are configured for reflecting certain of the light upwards. The light diffusion plate 13 includes a plurality of dispersion particles (not shown) therein. The dispersion particles are configured for scattering light, thus enhancing the uniformity of light exiting the light diffusion plate 13.

[0007] Referring to FIG. 10, the prism sheet 10 includes a base layer 101 and a prism layer 102 formed on the base layer 101. The prism layer 102 contains a plurality of parallel prism lenses 103 having a triangular cross section. The prism lenses 103 are configured for collimating received light to a certain extent. Typically, a method of manufacturing the prism sheet 10 includes the following steps: first, a melted ultraviolet(UV)-cured transparent resin is coated on the base layer 101 to form V-shaped lenses, then the melted ultraviolet-cured transparent resin is solidified to form the prism lenses 103.

[0008] In use, unscattered light from the lamps 12 enters the prism sheet 10 and becomes scattered. Thus, scattered light leaves the light diffusion plate 13 to the prism sheet 10. The scattered light then travels through the prism sheet 10 and is refracted out at the prism layer 102 of the prism lenses 103. Thus, the refracted light leaving the prism sheet 10 is concentrated at the prism layer 102 and a brightness (illumination) of the prism sheet 10 is increased. The refracted light then propagates into an LCD panel (not shown) positioned above the prism sheet 10.

[0009] When the light is scattered in the light diffusion plate 13, scattered light enters the prism sheet at different angles of incidence. Referring to FIG. 11, when scattered light generally travels through the prism sheet 10 at different angles of incidence, the scattered light generally travels through the prism sheet 10 along three light paths. In the first light path (such as a₁, a₂) the light enters the prism sheet 10 at small angles of incidence and refracts out of the prism lenses 103 with the refracted path closer to the normal to the surface of the base layer. In the second light path (such as a₃, a₄) the light enters the prism sheet 10 at angles of incidence larger than the first light path and refracts out of the prism lenses 103 with the refracted path being closer to the normal to the surface of the prism lenses 103. Both the first light path and the second light path contribute to the brightness of the LCD and the light utilization efficiency of the backlight model. However, in the case of the third light path (such as a₅, a₆), the light enters the prism sheets at angles greater than the second light path, such that when the refracted light in the third light path leaves the prism sheet 10 at the prism lenses 103, the refracted light impinges on the surface of adjacent prism lens 103 and reenters the prism sheet 10. Thus, light traveling along the third light path will eventually reenter the prism sheet 10 and may exit the prism sheet 10 on the same side the light entered. This third light path does not contribute to the light utilization efficiency of the backlight module 100. Further, the third light path may interfere with or inhibit other incident light, resulting in decreasing brightness of the backlight module 100.

[0010] What is needed, therefore, is a new prism sheet and a backlight module using the prism sheet that can overcome the above-mentioned shortcomings.

[0011] In one aspect, a prism sheet according to a preferred embodiment includes a transparent main body. The main body includes a first surface and a second surface. The first surface is opposite to the second surface. A plurality of elongated arc-shaped depressions are defined in the first surface. A plurality of micro-depressions are defined in the second surface. Each micro-depression is defined by at least three connecting sidewalls. A transverse width of each sidewall of each micro-depression progressively decreases with increasing distance from the second surface.

[0012] In another aspect, a backlight module according to a preferred embodiment includes a plurality of lamps, a light diffusion plate and a prism sheet. The light diffusion plate is positioned above the lamps and the prism sheet is stacked on the light diffusion plate. The prism sheet is same as described in a previous paragraph.

[0013] Other advantages and novel features will become more apparent from the following detailed description of various embodiments, when taken in conjunction with the accompanying drawings.

[0026] Reference will now be made to the drawings to describe preferred embodiments of the present prism sheet and backlight module, in detail.

[0027] Referring to FIG. 1, a backlight module 200 in accordance with a first preferred embodiment of the present invention is shown. The backlight module 200 includes a prism sheet 20, a plurality of lamps 22, a light diffusion plate 23, and a housing 21. The lamps 22 are regularly aligned above a base of the housing 21. The light diffusion plate 23 and the prism sheet 20 are stacked on the top of the housing 21 in that order.

[0028] Referring to FIGS. 2 through 4, the prism sheet 20 includes a transparent main body. The main body includes a first surface 201 and a second surface 203. The first surface 201 and the second surface 203 are on opposite sides of the main body. Furthermore, a plurality of elongated arc-shaped depressions 202 and a plurality of micro-depressions 204 are defined in the first surface 201 and the second surface 203 respectively. The prism sheet 20 is stacked on the light diffusion plate 23 in such manner that the first surface 201 is adjacent to the light diffusion plate 23, and the second surface 203 is away from the light diffusion plate 23. Each micro-depression 204 has a shape like an inverted pyramid formed by four triangular sidewalls. In the illustrated embodiment, each micro-depression 204 is a square pyramidal depression defined by four triangular sidewalls. A transverse width of each of the sidewalls progressively decreases with increasing distance from the second surface 203.

[0029] The elongated arc-shaped depressions 202 are configured for enabling the first surface to converge incident light from the lamps 22 entering the prism sheet 20 to a certain extent (hereafter first light convergence). The elongated arc-shaped depressions 202 are aligned side by side on the first surface 201 of prism sheet 20. An outline of each elongated arc-shaped depressions 202 taken along a plane perpendicular to the elongated arc-shaped depressions 202 is a circular arc. A pitch P₁ between adjacent elongated arc-shaped depressions 202 is configured to be in a range from about 0.025 millimeters to about 1.5 millimeters. A depth H of each elongated arc-shaped depression 202 is configured to satisfy a following expression: 0.01 millimeters≦H≦R. A radius R of a circular arc defined by taken along a plane perpendicular to the elongated arc-shaped depressions 202 is configured to satisfy a following formula: 0.01 millimeters≦R≦2P₁. In alternative embodiments, the elongated arc-shaped depressions 202 are aligned obliquely corresponding to edges of the prism sheet 20. In other alternative embodiments, adjacent elongated arc-shaped depressions 202 can be spaced apart from each other by a predetermined interval.

[0030] The micro-depressions 204 are configured for enabling the second surface 203 to converge light exiting the second surface 203 (hereafter second light convergence). The micro-depressions 204 are distributed on the second surface 203 in a matrix manner. Each micro-depression 204 is a square pyramidal depression formed by four triangular sidewalls. The rows and the columns of the micro-depressions 204 in the matrix are parallel to the edges of the prism sheet 20 (along the X-direction and a Z-direction) correspondingly. Sidewalls of adjacent micro-depressions 204 are joined, forming a crest correspondingly. The crest extends across the prism sheet 20 in the X-direction or Z-direction. A pitch P₂ between adjacent micro-depressions 204 along the X-axis direction is configured to be in a range from about 0.025 millimeters to about 1 millimeter. A pitch P₃ between adjacent micro-depressions 204 along the Z-axis direction is configured to be in a range from about 0.025 millimeters to about 1 millimeter. A dihedral angle (α or β) defined by sidewalls on opposite sides of each of the micro-depressions 204, is configured to be in a range from about 60 degrees to about 120 degrees.

[0031] A thickness of the prism sheet 20 is preferably in a range from about 0.5 millimeters to about 3 millimeters. The prism sheet 20 can be made of transparent material selected from the group consisting of polycarbonate (PC), polymethyl methacrylate (PMMA), polystyrene (PS), copolymer of methylmethacrylate and styrene (MS), and any suitable combination thereof.

[0032] Again referring to FIG. 1, the lamps 22 can be point light sources such as light emitting diodes or linear light sources such as cold cathode fluorescent lamps. In the illustrated embodiment, the lamps 22 are cold cathode fluorescent lamps. The interior of the housing 21 is configured to be highly reflective.

[0033] In the backlight module 200, when light enters the prism sheet 20 via the first surface 201, the light undergoes the first light convergence at the first surface 201. Then the light further undergoes the second convergence at the second surface 203 before exiting the prism sheet 20. Thus, a brightness of the backlight module 200 is increased. In addition, due to the micro-depressions 204, the light exiting the prism sheet 20 would mostly propagate along direction parallels close to the Y-direction. At the same time, few or less of the light would travel along direction parallels close to the X-direction, minimizing light energy loss. Thus, the light energy utilization rate of the backlight module 200 is high.

[0034] Moreover, in contrast to the conventional prism sheet the prism sheet 20 of the present invention is integrally formed by injection molding technology. Injection molding technology allows the prism sheet 20 to be easier to mass-produce than that of the conventional prism. Furthermore, because the prism lenses of the conventional prism sheet are formed by solidifying melted ultraviolet-cured transparent resin, in use, the prism lenses are easily damaged and/or scratched due to poor rigidity and mechanical strength. In contrast to the conventional prism sheet the present prism sheet has better rigidity and mechanical strength than that of the conventional prism sheet. Therefore, the present prism sheet 20 has a relatively high mechanical reliability.

[0035] Referring to FIG. 5, a prism sheet 30 in accordance with a second preferred embodiment of the present invention is shown. The prism sheet 30 is similar in principle to the prism sheet 20. A plurality of micro-depressions 304 are defined in the second surface 303 in a matrix manner. However, each micro-depression 304 is defined by four trapezium sidewalls. Shapes and sizes of sidewalls are same. Thereby, the four sidewalls and a base connecting to the four sidewalls cooperatively define a frustum of an inverted square pyramid. The micro-depressions 304 are aligned apart on second surface 303.

[0036] Referring to FIG. 6, a prism sheet 40 in accordance with a third preferred embodiment of the present invention is shown. The prism sheet 40 is similar in principle to the prism sheet 30. A plurality of micro-depressions 404 are defined in the second surface 403 in a matrix manner. However, four sidewalls and a base connecting to the four sidewalls cooperatively define a frustum of an inverted rectangular (non-square) pyramid. In the illustrated embodiment, each micro-depression 404 is substantially elongate in shape.

[0037] Referring to FIG. 7, a prism sheet according to a fourth embodiment is shown. The prism sheet 50 is similar in principle to the prism sheet 20. A plurality of micro-depressions 504 are defined in the second surface 503 in a matrix manner. However, each micro-recess 503 is an inverted triangular pyramid in shape. In an alternative embodiment, each micro-recess 531 can be a frustum of an inverted triangular pyramid in shape.

[0038] Referring to FIG. 8, a prism sheet according to a fifth embodiment is shown. The prism sheet 60 is similar in principle to the prism sheet 20. A plurality of elongated arc-shaped depressions 602 are formed in the first surface 601. However, an outline of each elongated arc-shaped depressions 602 taken along a plane perpendicular to the elongated arc-shaped depressions 602 is an elliptic arc.

[0039] Finally, while various embodiments have been described and illustrated, the invention is not to be construed as being limited thereto. Various modifications can be made to the embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.