Flexible backlight unit for key of input device

[0001] 1. Field of the Invention

[0002] The present invention relates to a backlight unit for a key of an input device provided for a wireless phone, a telephone, a portable digital assistant (PDA), a keyboard, and a calculator, and more particularly, to a backlight unit with a light guide plate having a characteristic in which bending is easy and having a light guide pattern portion in which a plurality of engraved or embossed patterns are formed in a plurality of requirement character shapes on a predetermined regular basis.

[0003] 2. Description of the Related Art

[0004] In general, a backlight unit which is used for a non-emissive flat panel display such as a liquid crystal display (LCD), keypad backlights of a telephone and a wireless phone, or light signboards is disclosed in Korean Patent Application Nos. 1997-73783, 1998-30060, 1999-47621, 2000-63111, and 2001-73468, and Korean Utility Model Application No. 1996-47524. In the disclosed backlight unit for a key of an input device, a plurality of light emitting diodes (LEDs) are directly disposed at each necessary part of bottom surfaces of keys to generate local light only at the necessary part, or a light emitting plate such as an organic electro luminescence light emitting diode (OLED) is installed.

[0005] In a method for directly disposing the plurality of LEDs at the bottom surfaces of the keys, the LEDs are required in number as many as the keys. Accordingly, this method has many drawbacks in cost and power consumption. In order to overcome these drawbacks, a method for suitably disposing LEDs in space between keys and lighting two or three keys by one LED without installing the LED at each key is used. However, this method has a lethal disadvantage in that constant luminance required at each key is not provided and luminance dispersion between the keys is greatly caused to thereby deteriorate product quality, and requires still many LEDs less than number of the keys.

[0006] Even in a method in which, instead of a LED, a surface light-emitting plate device such as an organic electro luminescence (EL) is disposed on a top surface of a point-contact switch, and a key is again disposed on a top surface of the resultant, there are several drawbacks such as a drawback of light efficiency of the device itself and a drawback of increase of a manufacture cost and endurance in a repeated operation.

[0007] In order to overcome the drawbacks, in a conventional backlight unit of a liquid crystal display (LCD) panel including a transparent light guide plate for transmitting light and a light emitting unit disposed on a side surface of the light guide plate, an edge light emitting method is widely used owing to its advantageous high efficiency and low price. However, in this method, since the light guide plate is of general plastic resin not bent, it is impossible that the light guide plate is used for a backlight unit of a key of an input device such as a telephone and a wireless phone. In other words, since a keypad used for a telephone and a wireless phone has a flexible bottom considering its function, when the key is pressed, the key should be pressed to the bottom to activate a point-contact switch and therefore, the light guide plate cannot be used in a conventional light guide plate method.

[0008] In the backlight unit for the key of the input device, since light emission is performed not over a total surface but over a local surface, there is a drawback in that a front light guide pattern method based on the conventional edge light emitting method is not performed at suitable efficiency.

[0009] Accordingly, in order to employ the edge light emitting method in the backlight unit of the key of the input device, it is required to use, instead of conventional solid material, flexible and transparent resin, such as relatively flexible silicone, as material of the light guide plate, and use a pattern using a particular method for enhancing light efficiency.

[0010] However, in this case, there are additional drawbacks in that it is impossible to employ a pattern installation method such as a conventional printing method due to a silicone adhesive property of an ink pattern, and it is impossible to process silicone in a scratch method due to softness of the silicone.

[0011] In a conventional method in which, in case where injection using a metal mold is applied to manufacture of the flexible light guide plate, minute protrusions are densely disposed on a surface of the light guide plate, there is a drawback in that thickness is relatively very small compared to a total area and it is difficult to obtain luminance enough to backlight the key of the input device that should concentrate light emission only on a specific character shape.

[0012] Meantime, as shown in FIG. 1, in a conventional backlight unit using an edge light emitting method, there is an advantage in that, when a pattern densely disposed over a whole surface of the light guide plate 10 is used, a backlight needing a general surface light source such as a liquid crystal display can equally obtain sufficient luminance from the whole surface. However, if such surface light emission is applied as it is to the backlight unit of the key of the input device such as the telephone and the wireless phone, light generated from other parts than a necessary light emitting part such as a character part on the key are all lost. In other words, there occurs a drawback in that effectively obtainable luminance is merely a very little compared to a total amount.

[0013] In order to overcome this drawback, as shown in FIG. 2, the pattern densely disposed over the whole surface of the light guide plate 10 can be grouped as a predetermined group not over the whole surface, and can be disposed in an island shape. However, in this case also, light emission can be reduced at a part other than a required position, but an amount of emitted light is not suddenly increased at a pattern part required for light emission. This is caused by the following reason. When incident light from a light emitting unit 20 passes through the light guide plate 10 and comes in contact with a light guide pattern, the incident light is diffused or scattered. In case where the light guide pattern is constructed in a plurality of island shapes forming a shape of a whole key terminal, though the light emission is generated from the shape of the whole key terminal, an amount of light capable of being actually recognized by a user through a character is limited by a shape and an area of the character.

[0014] In particular, in the light guide plate 10 for backlighting the key of the input device, a very little thickness of 500 μm or less in its product structure is required in comparison to a thickness of 1 mm or less of a conventional wireless phone light guide plate. In order to secure flexibility, the light guide plate 10 has a special property using silicone having a light transmittance of 90% or less even when compared to polymethyl methacrylate having a light transmittance of 98% or more which is material of the conventional light guide plate. Therefore, light is smoothly transmitted within the light guide plate 10. Accordingly, number of times the light emitted from the light emitting unit 20 is reciprocated within the light guide plate 10 is suddenly reduced while probability of absorbing the light within the light guide plate 10 is increased.

[0015] In other words, a design of a pattern shape allowing the light emitted from the light emitting unit 20 to pass through the opening portion 31 of a character shape at a possible high efficiency is required.

[0016] In such design direction, when the island shape is based on the character shape not a simple pattern of the key terminal shape, the probability in which the light emitted from the light emitting unit 20 passes through the opening portion 31 of the character is increased.

[0017] In case where the light guide pattern having the island shape is designed to have a size or a shape of a type accurately consistent with the shape of the opening portion 31 of the character, an amount of diffused and scattered light is suddenly reduced, thereby rather deteriorating total luminance.

[0018] In a product including the keys having a plurality of various characters, in case where the opening portion 31 area of the character is smaller than those of other keys, even though the opening portion 31 of the character itself provides the same level of luminance, there occurs a phenomenon in which, when a user recognizes the luminance at a predetermined distance with naked eyes, he/she feels darker than when a character portion having the small opening portion 31 is small. Accordingly, a method of luminance control inversely proportional to the opening portion 31 of the character is additionally required.

[0019] Accordingly, the present invention is directed to a backlight unit for a key of an input device that substantially overcomes one or more of the limitations and disadvantages of the conventional art.

[0020] One object of the present invention is to provide a backlight unit for a key of an input device, in which a thin-film type flexible transparent resin such as silicone and urethane can be applied to a light guide plate, thereby securing flexibility, and in which an engraved or embossed light guide pattern portion on a predetermined regular basis, which scatters and diffuses light, can be formed on one surface of the light guide plate to provide required luminance by a minimal number of light sources, thereby reducing a cost and minimizing consumption power.

[0021] Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims as well as the appended drawings.

[0022] To achieve the above and other objects and advantages, and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a flexible backlight unit for a key of an input device, the unit including: a light guide plate formed of flexible transparent resin and having a light guide pattern portion in which a plurality of engraved or embossed patterns are formed in a plurality of requirement character shapes so that light can be diffused and scattered with uniform luminance from a necessary light emitting portion; and at least one light emitting unit installed at a side wall of the light guide plate.

[0023] In the light guide pattern unit, the engraved or embossed pattern has 180 to 380% of an effective light-emitting character area depending on a distance remote from the light emitting unit.

[0024] It is to be understood that both the foregoing summary and the following detailed description of the present invention are merely exemplary and intended for explanatory purposes only.

[0034] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

[0035] FIG. 3 is a schematic sectional view illustrating a key of an input device of a telephone or a wireless phone having a backlight unit according to an embodiment of the present invention.

[0036] The backlight unit includes a light guide plate 10 having a light guide pattern portion 11 formed on its base surface, and diffusing and scattering light of uniform luminance at a necessary light emitting portion; at least one light emitting unit 20 installed at a side wall of the light guide plate 10 and irradiating light to the light guide plate 10; and a key assembly 30 having a character shaped opening portion 31 and positioned on a top surface of the light guide plate 10.

[0037] The light emitting unit 20 employs a cold cathode fluorescent lamp (CCFL) or a light emitting diode (LED). The light guide plate 10, resin having transparency and a characteristic of easy bending, can employ material such as silicone or urethane. A transparent resin 32 in FIGS. 2 and 3 is a part of keypad. A key attaching film 33 in FIGS. 2 and 3 is another part of the keypad, which is provided between neighboring keys and supports the keys.

[0038] The light guide pattern portion 11 is formed on a predetermined regular basis to have an island-shape group on the base surface of the light guide plate 10. The light guide pattern portion 11 has a plurality of engraved or embossed patterns having a dot shape, a straight-line type, a rectangular shape, a matrix shape, or a combination thereof. Further, the light guide pattern portion 11 having the plurality of engraved or embossed patterns has several requirement character shapes. Preferably, the light guide pattern portion 11 is formed using an injection molding or laser processing method.

[0039] In other words, the flexible and transparent resin light guide plate 10 is marked using a laser beam oscillated from a laser device, thereby forming the light guide pattern portion 11 having a predetermined pattern. Or, fixed amounts of curing agent and liquid-phase silicone are inputted and mixed, and a mixture thereof is injected into a metal mold having the light guide pattern portion 11 and is injection-molded in a heating and pressing method, thereby forming the light guide pattern portion 11 having the predetermined pattern.

[0040] Accordingly, drawbacks of disengagement, stain, thermal deformation, and luminance reduction of the light guide pattern portion, which are caused by a conventional printing or non-printing method, can be solved. Further, when the light guide pattern portion 11 is formed, a pattern depth of a suitable condition based on a distance from the light emitting unit 20 and a thickness of the light guide plate 10 is applied so that an amount of emitted light is concentrated at a local necessary position owing to the requirement character shape, thereby providing the backlight unit for the key of the input device having the most efficient light emission.

[0041] Further, the inventive backlight unit is based on design direction described in the above conventional art, that is, design direction of a pattern shape for allowing light emitted from the light emitting unit 20 to pass through and out of a character shaped opening portion 31 at possible high efficiency, and design direction considering phenomenon in which, in case where the character shaped opening portion 31 is smaller in area than those of other keys, the opening portion 31 of a character itself actually provides the same level of luminance and even so, when a user recognizes the luminance at a predetermined distance with naked eyes, he/she feels darker than in case where the opening portion 31 of the character is large. As a result, an island shape of the pattern is based on shapes of the opening portions 31 of a whole character and requires an optimal condition for its area. As shown in FIG. 4, the island shape is optimized when areas 11b, 11c, and 11d of the island shaped light guide pattern portion based on the shape of the opening portion 31 of the character are larger than an area 11a of a shape consistent with the shape of the opening portion 31 at more than a predetermined rate, and the pattern area is applied in inverse proportion to a rate of a sum of the areas of the opening portions of the whole character to an area of an opening portion of an individual character.

[0042] In other words, when the light guide pattern portion 111 is designed, luminance improvement and uniformity securing are considered. The luminance is decided in a process of optimizing the area where the pattern portion is formed on the light guide plate 10. The uniformity is decided at a ratio of maximal value to minimal value, on a surface of the opening portion 31 of each key character of the light guide plate 10. The luminance should be the maximum value in view of an average value. In general, a ratio of peripheral portion to central portion, or minimal luminance to maximal luminance is required to be about 70% to 80% or more. The uniformity can be controlled by control of a pattern density or area of the light guide pattern portion 11.

[0043] Preferably, the pattern density should be within a range of about 10% to 100% converted at percentage of the pattern area per square millimeter. A total area (S) of the pattern should be in a range of about 180% to 380% converted at percentage of the pattern area based on the character opening portion area (Sc).

[0044] FIG. 5 illustrates variation of relative luminance depending on a ratio of S/Sc. FIG. 5 illustrates an experimental result of the variation of the relative luminance depending on the ratio of S/Sc based on arbitrary experiment conditions (A, B, C, and D). In the experiment result, it can be appreciated that, when the ratio of S/Sc is less than a predetermined value, the luminance is relatively low, and as the ratio of S/Sc is gradually increased, normal luminance appears to be shown in a section, and when the ratio of S/Sc exceeds a predetermined value, the luminance is again reduced. When the normal luminance is relatively 90% or more, it can be appreciated that a corresponding normal section is 180% to 380%.

[0045] Further, when the light emitting unit 20 is minimized in size, for example, when the light emitting unit 20 is constituted of one or several LEDs, a separate design factor can be considered. Unlike a general light source, the LED has a characteristic of concentrating an amount of emitted light on a center of a light source. Therefore, accurate design cannot be accomplished by simple consideration of only a thickness (T) of the light guide pattern portion 11 and a distance (L) from the light emitting unit 20 employing the LED as shown in FIG. 6.

[0046] In other words, in case where the present invention considers the LED as the light emitting unit 20, in particular, in case where a minimal number of the light emitting units 20 is achieved using only one LED, when a light transmission path has a fan shape centering on an installation direction of the LED as shown in FIG. 7, a fan connected to a point where the light guide pattern portion 11 is positioned is influenced by an angle getting wider from a central line. As the angle gets larger, light received from the light emitting unit 20 gets smaller in amount. As a result, in a position widened by a predetermined angle left and right from the installation direction of the light emitting unit 20, a very small amount of light is supplied even at the same distance when compared to a central direction. Accordingly, in circumstance where other conditions are same, preferably, the pattern area and density of the light guide pattern portion 11 should be greater as an angle of the light guide pattern portion 11 remotely positioned from a central extension line of the light emitting unit 20 is increased.

[0047] Further, the design of the light guide pattern should be different also by mutual relationship between a position of the light emitting unit 20 and interferences of the respective light guide pattern portions 11. For example, in case where light goes straight and is transmitted to a plurality of light guide pattern portions 11 on the basis of the installation direction of the LED light source as shown in FIG. 8, it cannot be regarded that, when the light passing the plurality of light guide pattern portions 11 finally reaches the last light guide pattern portion 11, the same amount of light as that not passing other light guide pattern portions 11 reaches. In other words, an amount of light incident on the light guide pattern portion 11 at a position intended for the design according to the present invention depends on how many light guide pattern portions light passes in middle course. Accordingly, the pattern depth should be differently designed, depending on a rate of space area between the light guide pattern portion 11 and the light emitting unit 20 to areas of other light guide pattern portions 11 positioned within the space area, by drawing an imaginary line connecting from the light emitting unit 20 to the light guide pattern portion 11. For example, as the area rate of the other light guide pattern portions 11 provided in the space area between the light source and the light guide pattern portion 11, which is provided at the position intended for the design, is increased, the pattern area or density should be larger at the light guide pattern portion 11 of the design position.

[0048] Accordingly, in consideration of the above-described design factor according to the present invention, that is, in consideration of the area of the light guide pattern portion 11, the thickness of the light guide plate 10, the distance from the light emitting unit 20, the angle between the extension line of the central line of the light emitting unit and the imaginary line connecting from the light emitting unit to the light guide pattern portion 11, and the mutual relationship between the position of the light source and the interferences of the respective light guide pattern portions 11, the pattern density of the light guide pattern portion 11 at the design position can be defined as Equation below:
S·D=A·T·L·1/[EXP(−4.4·θ²)]·(S_t/S_C)·S₀/(S₀−S_P)

[0049] where,

[0050] S: Area of the light guide pattern portion at a position intended for design (mm²),

[0051] D: Pattern density,

[0052] A: Constant value,

[0053] T: Thickness of light guide plate (mm),

[0054] L: Distance from light emitting unit to pattern (mm),

[0055] θ: Angle between the extension line of the central line of the light emitting unit and the imaginary line connecting from the light emitting unit to the light guide pattern portion (radian value: 0 to π/2),

[0056] S_t: Total area sum of the character opening portions of the input device (mm²),

[0057] S_C: Opening portion area of the key character of the top surface of the light guide pattern portion (mm²),

[0058] S₀: Space area between center of the light emitting unit and two imaginary lines connecting from the light emitting unit to the light guide pattern portion (mm²), and

[0059] S_P: Areas of other light guide pattern portions provided within space between the center of the light emitting unit and the two imaginary lines connecting from the light emitting unit to the light guide pattern portion (mm²).

[0060] The “A” has a value of about 1.7E-7 to 2.0E-2.

[0061] The shape of the light guide pattern can be provided in various patterns so that the light irradiated from the light emitting unit can be reflected, diffused, and scattered. For example, the light guide pattern can be provided in the dot shape, the straight-line shape, and the combination thereof.

[0062] As described above, the inventive backlight unit using the light guide plate formed of the transparent resin having the flexibility such as silicone has a remarkable effect of providing a backlight applicable to a display requiring the flexibility.

[0063] Further, the inventive backlight unit can provide an effect of overcoming a drawback of reduction of light transmittance of the thin film typed flexible resin light guide plate, using an optimal design condition based on the density and the area of the light guide pattern portion, the distance from the light emitting unit, and the light emission area of the requirement character. By doing so, a direct type LED backlight structure applied to a product such as the backlight for the key of the input device of the telephone or the wireless phone, is changed into a side light guide type backlight structure. Accordingly, owing to reduction of number of the light sources, an effect of reduction of a manufacture cost and an effect of reducing consumption power can be accomplished at the same time.

[0064] While the present invention has been described with reference to exemplary embodiments thereof, it will be apparent to those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.