The present invention relates to a stacked-type backlight plate and, more particularly, to a stacked-type backlight plate which exhibits an excellent light mixing effect and is suited for performing the local area dimming control process.
Liquid crystal display (LCD) devices are now widely utilized in a variety of applications. An LCD device is advantageous over a traditional cathode ray tube (CRT) display device in many aspects, such as lightweight, compact, higher image resolution and less power consumption. All of these lead to an increasing trend towards replacement of traditional CRT display devices with LCD devices.
Meanwhile, light-emitting diodes (LED) lamps are increasingly adopted as a backlight source for LCD devices. The backlight plates that employ LEDs as a light source are normally of one of the following types:
(1) direct-lit backlight plates, in which a plurality of LEDs are arranged to constitute a surface light source emitting light directly towards an LCD panel; and
(2) edge-lit backlight plates, in which LED lamps are arranged in the form of a light bar capable of launching light into a side face of a light guide.
The two types of the backlight plates mentioned above have been shown to exhibit many advantages. For increasing the dynamic contrast ratio of an image to be displayed on a liquid crystal display panel, the technology used today is to partition the part of the LCD panel for displaying images into several areas and determine the chromaticity and brightness levels required for the respective areas. Such a technique, commonly referred to as the local-area dimming control technique, can perfectly apply to direct-lit backlight plates and achieve an enhanced dynamic contrast ratio of images and a reduced power consumption. However, some drawbacks may come along therewith, such as an increase in the panel thickness and a decrease in the light emission efficiency. In contrast, the so-called edge-lit backlight plates are advantageous in having high emission efficiency and being compact by employing a slim light guide to disperse and emit light, but have disadvantages of not suited for performing the local-area dimming control technique due to the fact that the light emission efficiency will have to be compromised as a result of a decrease in the overall light-emitting area after the partition of the LCD panel into areas.
Therefore, a gist of the invention is to provide a backlight plate having the advantages present in both of the direct-lit and edge-lit backlight plates, so as to reduce the overall volume of a display panel while making the backlight suited for being subjected to local-area dimming controlling.
In 1999, a related technique was proposed in U.S. Pat. No. 6,241,358 and R.O.C. Patent No. 412716 issued to Eizaburo Higuchi et al., entitled “Tandem Surface Light Source Device,” which involves modular manufacturing of a backlight. As shown in
As such, the light exiting faces of the light guide blocks 11, 12, 13 can together constitute a backlight with a large surface area. The light sources 14, 15, 16, which are respectively mounted correspondingly to the blocks 11, 12, 13, can be controlled individually. For instance, as long as the included angle between the microstructure surface 134 and the light exiting face 133 is small enough, the light entering at an angle into the light incident face 131 will be directed from the left side to the right side of the light block 13 and reflected between the light exiting face 133 and the microstructure surface 134 until some of the light reaches the scattering spots 1340 where the light is scattered and finally emitted through the light exiting face 133. The non-emitted light will eventually be reflected by the light stop face 132 and thus return back to the light guide block 13.
The design described above, however, is configured to adopt cold cathode fluorescent lamps (CCFLs) as a light source. While CCFLs are considered quite suitable for serving as a line light source in terms of their highly uniform brightness and chromaticity, they have a weakness of having a long response time and cannot be controlled by using the local-area dimming control technique. On the other hand, even if white-light LEDs or a combination of RGB LED lamps are used in place of CCFLs as the light source, it still has to take a considerable period of time for light mixing to achieve uniform light emission from the LED point light sources. In other words, the rest portions 110, 120, 130 have to be extended laterally a great distance to make the light mixing possible. Unfortunately, the success of the local-area dimming control technique is founded on a great number of areas partitioned with each area having a quite small coverage region. Therefore, in practice, the light guide blocks 11, 12, 13 are not allowed to extend extensively and, as a result, the extended lengths of the rest portions 110, 120, 130 are too limited to achieve a successful light mixing. The configuration of the prior art backlight plate, even if LEDs are used therein in place of CCFLs, still diverges from that to which the local-area dimming control technique is applicable.
In order to address the drawbacks described above, R.O.C. Patent No. I235803 issued to Osram Opto Semiconductors GmbH, entitled “Method to Producing a Lighting Device and Said Lighting Device,” proposed an improved light guide structure as shown in
After light passes through the start face 238 and enters the light exiting region 236, it is scattered by scattering spots 2340 formed on a microstructure surface 234, such that some of the light beams are emanated from the light exiting face 233. The start face 238 is exactly opposite to a light stop face 232. Both of the microstructure surface 234 and the light stop face 232 are provided with reflective plates (not shown) to prevent light leakage therefrom. As compared to the technique proposed in U.S. Pat. No. 6,241,358, the light guide devices 21, 22, 23 in this design are each additionally provided with a light mixing region 215, 225, 235 having a certain length along the light path and, thus, the emitted light is distributed more evenly when point light sources, such as LEDs, are used in the backlight.
In addition, US 20080205080 assigned to Luminance Devices Inc. discloses a light guide block configured differently from those described above, as shown in
In conclusion, the prior art light guide devices described above have a common structural feature in that the main portions of the light exiting regions each include a wedge-shaped structure. That is to say, a light exiting region gradually decreases in thickness along the light path, to thereby provide an accommodating space beneath the wedge-shaped structure. By this way, it would not only allow formation of a continuous light exiting face from the respective light exiting faces of the light guide blocks, but also reduce the overall thickness of the display device by installing light sources in the accommodating spaces beneath the wedge-shaped structures. In theory, when light is propagated in the TIR manner within the light guide device, the energy loss is insignificant until the light hits on the preformed scattering spots and is scattered to exit the light exiting face via a predetermined path. As such, it is possible to control the brightness distribution over the light exiting face by virtue of the dispersed intensity of the scattering spots.
In the case of the light guide device having a wedge-shaped structure, however, when light is propagated and totally internally reflected time and again within the wedge-shaped structure, the reflected angle does not remain fixed but instead gradually changes. After being totally internally reflected several times, a light beam may possibly strike on a surface of the wedge-shaped structure at an angle smaller than a particular critical angle and, as a consequence, the total internal reflection no longer occurs and the light beam escapes from the light guide device before it reaches the scattering spots. This causes a uncontrolled and therefore unevenly distributed light emission, as opposed to the original design in regard to the light exiting of the prior art backlight plate.
As shown in
Therefore, when the total internal reflection of the light beam is repeated n times, there gives an incident angle θ0+nθ and an included angle with respect to the norm, i.e., [90−(θ0+nθ)]. If the included angle [90−(θ0+nθ)] is smaller than a critical angle θc, the total internal reflection would no longer occur and the light beam is refracted at a position where the included angle with respect to the norm becomes smaller than the critical angle and escapes from the light guide device. It can be seen from the variable [90−(θ0+nθ)] that the larger the included angle θ of the wedge-shaped light guide device is, the smaller the number n of total internal reflection may take place. Given this, the angle θ of the wedge-shaped structure should be kept rather small, so as to obtain a sufficient number of total internal reflection.
However, when the so-called “local-area dimming control” technique is applied to a display panel, it requires that the display panel be partitioned into a significant number of areas with each area being small enough to maximize the advantageous effects of this technique on the dynamic display process. In general, the entire picture should be partitioned into at least 100 independently controlled areas. In the case of a 42-inch LCD television, each independently controlled area has a surface area of about 50 cm2, meaning that the light exiting region of a single light guide device has a dimension of only about 7 cm in length. If the light guide device is tailored to have an initial thickness of 5 mm, the wedge-shaped light exiting region should have an inclined angle θ of at least 4 degree to produce a suitable space for accommodating a light source.
Unfortunately, for a light beam that enters at an initial incident angle θ0=30°, it will only experience 5 times of total internal reflection to reach the critical angle θc=42° relative to the norm. In this case, the actual number n of the total internal reflection in the light guide device is only 5. The light beam, after experiencing 5 times of total internal reflection, can travel only a distance of scarcely 25 mm, meaning that the light beam will exit the light guide device by traveling less than one-third of the predetermined distance in the light exiting region. Referring to
When the field distribution of the LED pertains to a Lambertion distribution, 50% of the light energy emitted from the LED is distributed from θi=45° to θi=90° and will completely exit the light guide device before traveling a distance of 25 mm from the light incident face 41. The rest 50% of the light energy will exit from the final 45 mm of the light exiting region in an unevenly manner. Given the fact that the mean emission density of light from the light exiting region is approximately 2.0%/mm for the first 25 mm and approximately 1.1%/mm for the final 45 mm, the light emission from the light exiting region is highly uneven (nearly 1.8-time difference between the first 25 mm and the final 45 mm).
The aforesaid analysis does not consider the effects of scattering spots on the brightness distribution of the emitted light. If the uneven distribution of the scattering spots over the bottom face of the light exiting region is taken into account, the mean emission density of light from the light exiting region would be greater than 2.0%/mm for the first 25 mm and smaller than 1.1%/mm for the final 45 mm. This results in an even higher non-uniformity in light emission (the difference between the first 25 mm and the final 45 mm is greater than 1.8-time).
In particular, human eyes can easily distinguish between brightness and darkness in a short distance to an extent that a 2% difference between a ripple phase with alternate brightness/darkness and the brightness level can be recognized by human eyes. Moreover, since the light guide devices employed in the local area dimming control technique are intended to be assembled block-wise, the uniformity in brightness is prone to occur periodically block by block, which can be visually perceived even more easily. Due to a severe problem regarding repeated occurrence of the uniformity in brightness, the edge-lit backlight plates composed of wedge-shaped light guide devices as disclosed in the prior art above would not be able to satisfy the customers' needs and fail to be suited for performing the local area dimming control process.
Accordingly, a purpose of the present invention is to provide a backlight plate, in which front faces and back faces of light guide devices are arranged parallel to each other, such that the incident light beams are allowed to be propagated in a manner of total internal reflection without easily escaping from the light guide devices, thereby making the local area dimming control technique applicable to the backlight plate.
Another purpose of the invention is to provide a backlight plate, in which front faces and back faces of light guide devices are arranged parallel to each other, such that the incident light beams are propagated through a sufficient distance for light mixing without escaping from the light guide devices, thereby providing satisfactory uniformity in light emission.
It is still another purpose of the invention to provide a backlight plate, which is simple in structure and, therefore, has advantages of having an improved productivity and being cost effective and capable of being easily assembled, repaired and replaced.
It is still another purpose of the invention to provide a display device, in which front faces and back faces of light guide devices are arranged parallel to each other, such that the incident light beams are allowed to be propagated in a manner of total internal reflection without easily escaping from the light guide devices, thereby making the local area dimming control technique applicable to the backlight plate.
It is still another purpose of the invention to provide a display device, which is cost-effective and capable of being easily assembled and repaired.
The present invention therefore provides a stacked-type backlight plate for use in a display device. The backlight includes a plurality of light guide devices, each including a light incident face, as well as a front face and a back face. The front face and the back face are arranged opposite and parallel to each other and disposed adjacent to the light incident face. The front face has a light exiting region remote from the light incident face. The back face of at least one of the light guide devices is substantially parallel to the front face of an adjacent light guide device, so that the back face of the at least one light guide device is overlapped in part with the front face of the adjacent light guide device to expose the light exiting region of the front face of the adjacent light guide device. The backlight plate further comprises a plurality of light sources disposed in a manner corresponding to the light incident faces of the light guide devices.
The present invention further provides a display device incorporating a stacked-type backlight plate. The display device comprises a backlight plate. The backlight includes a plurality of light guide devices, each including a light incident face, as well as a front face and a back face. The front face and the back face are arranged opposite and parallel to each other and disposed adjacent to the light incident face. The front face has a light exiting region remote from the light incident face. The light incident faces of at least one of the light guide devices and an adjacent light guide device are parallel to each other, and the back face of the at least one of the light guide devices is substantially parallel to the front face of the adjacent light guide device, so that the back face of the at least one light guide device is overlapped in part with the front face of the adjacent light guide device to expose the light exiting region of the front face of the adjacent light guide device. The backlight further comprises a plurality of light sources disposed in a manner corresponding to the light incident faces of the light guide devices. A liquid crystal panel is disposed in front of the light exiting regions.
Since the light guide devices of the backlight plate disclosed herein are configured into a structure having opposite parallel faces, rather than a wedge-shaped structure, if a light beam strikes on the light incident face of the light guide device at an angle smaller than a critical angle above which the total internal reflection occurs, it could be propagated in a manner of total internal reflection within the light guide device. The invention allows the incident light beam to propagate through a sufficient distance for light mixing and prevents it from departing prematurely from the light guide device before arriving at the light exiting region. The uniformity of light emitted from the backlight plate thus approaches ideal. Especially, even if the entire picture is partitioned into a large number of areas, the respective light guide devices can still independently provide uniform light emission, whereby the local area dimming control technique is applicable to an edge-lit backlight plate and the slim profile of the edge-lit backlight plate is advantageously maintained.
Again, since the light guide devices disclosed herein are configured into a structure having opposite parallel faces, they are advantageous in having an improved productivity and being cost effective and capable of being easily assembled, repaired and replaced. The inventive backlight plate, as well as the display device incorporating the same, provide a market value that creates a win-win situation where both manufacturers and consumers will come satisfied. The purposes intended by the invention are achieved accordingly.
The above and other objects, features and effects of the invention will become apparent with reference to the following description of the preferred embodiments taken in conjunction with the accompanying drawings, in which:
A light guide device of a stacked-type backlight plate according to the invention for use in a display device is shown in
Referring together to
It can be readily appreciated by those skilled in the art that the light incident faces described above may not always be planar and can be in a convex form as shown in
between one of the light guide devices 51′, 52′, 53′ and the substrate 57′ is about 3 degree. The accommodating space described above is therefore configured to have a triangular cross-section with about 3 mm in height and 60 mm in length. Under the circumstance that a finished backlight assembly is normally about 3 d in thickness, an edge-emitting type LED which can be produced nowadays to have a height down to 1 mm, in combination with a conventional circuit board of about 1.2 mm in thickness and about 20 mm in width, can be readily installed within the accommodating space.
While the technical relationships described above are depicted in an exaggerated manner in the appended drawings, it is calculated from the equation above that the inclined angle α is in fact only about 3 degree and, as a result, the incident light will be guided to exit from the light exiting face in a direction directly toward a viewer. In this embodiment, the light mixing region covered by an adjacent light guide device has a length approximately equal to that of the light exiting region, so as to achieve a satisfactory light mixing effect to thereby obtain a light emission with excellent uniformity. According to this embodiment, a diffuser film 58′ is further provided in a manner facing the light exiting faces of the light guide devices, so as to uniform the light emission to a greater extent. As shown in
As such, the finished backlight assembly has an overall thickness
In this case, if it requires that the length λ2 of the light mixing region is 1.5 times longer than the length λ1 of the light exiting region, the overall thickness is about 3.5 d. In contrast, if it requires that the length λ2 of the light mixing region is 0.5 times longer than the length λ1 of the light exiting region, the overall thickness of the backlight assembly is only 2.5 d.
The assembling of the backlight module can be carried out in the manner shown in
Referring together to
It can be readily appreciated by those skilled in the art that there is no fixed technical relationship between adjacent rows. As shown in
In addition, there are also other options for the light incident faces of the light guide devices. As shown in
According to the aforesaid analysis, the inventive stacked-type backlight plate and the display device incorporating the same have the following advantages as compared to the prior art counterparts:
1. Since the inventive light guide device is configured into a structure having opposite parallel faces, an incident light beam is allowed to be propagated in a manner of total internal reflection along an ideal path without easily escaping from the light guide device. The invention makes the local area dimming control technique applicable to edge-lit backlight plates.
2. Owing to the opposite parallel faces that the inventive light guide device has, the length of the light mixing region can be readily adjusted to achieve the desired distance, to thereby effectively uniform the incident light beams. This is particularly true when LEDs of respective RGB colors are employed as light sources and the light mixing effect achieved thereby is far more superior over the prior art.
3. Since the inventive light guide device has a continuous profile without irregular corners, it is susceptible to optical design. The invention allows a complete control of light emission uniformity by adjusting the density of dispersed scattering spots and, thus, achieve a superior optical property over the prior art.
4. The inventive light guide device is simple in structure and, therefore, has advantages of having an improved productivity and being cost effective and capable of being easily assembled, repaired and replaced.
While the invention has been described with reference to the preferred embodiments above, it should be recognized that the preferred embodiments are given for the purpose of illustration only and are not intended to limit the scope of the present invention and that various modifications and changes, which will be apparent to those skilled in the relevant art, may be made without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
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98141155 | Dec 2009 | TW | national |