Backlight unit with an oxide compound-laminated optical layer

A backlight unit includes a substrate, a plurality of light sources arranged on the substrate in a spaced-apart relationship with one another for irradiating lights, a total reflection layer for upwardly reflecting the lights irradiated from the light sources, and an optical layer disposed above the light sources and having an oxide compound layer laminated thereon. The optical layer is adapted to allow a part of the lights to pass through the optical layer but reflect the remaining part of the lights downwardly toward the substrate, to thereby induce the lights from the light sources to optically resonate. The optical resonance caused by the oxide compound-laminated optical layer helps to reduce the half amplitude of a color-based spectrum histogram in a light-emitting spectrum, thereby enhancing the color purity.

FIELD OF THE INVENTION

The present invention relates to a backlight unit, and more specifically, to a backlight unit capable of increasing a luminance through the use of optical resonance in an optical layer.

BACKGROUND OF THE INVENTION

A cathode ray tube (“CRT”), one of typical display devices, has been extensively used in television sets or computer monitors, but fails to catch up with the recent trend of miniaturization and lightweight of electronic equipments.

Thus, a variety of technologies have been developed in an effort to replace the cathode ray tube with new display devices, examples of which include a liquid crystal display (“LCD”) using an electric field optical effect, a plasma display panel (“PDP”) using a plasma discharge and an electroluminescence display (“ELD”) using an electric field light-emitting effect.

Among these devices, the liquid crystal display, which features thin lightweight configuration and low electricity operability, is showing rapid expansion in its range of applications with the improvement of liquid crystal materials and the development of fine pixel processing techniques, and is widely used in household television sets, desktop computer monitors, notebook computer monitors, large-sized flat panel television sets and so forth.

Most of the liquid crystal displays require the use of a separate backlight unit that serves as a light-flatting element for regulating the quantity of an incoming light to display images.

As shown inFIG. 1, a liquid crystal display module1for use in typical liquid crystal displays is comprised of a liquid crystal display panel2filled with liquid crystal, polarizing plates4aand4bfor polarizing a light directed to the upper and lower surfaces of the liquid crystal display panel2, a backlight unit6for supplying an uniform light to the liquid crystal display panel2, a main support8afor maintaining an external configuration of the liquid crystal display module1, and a top case8b.

Unlike the cathode ray tube or the plasma display panel, the liquid crystal display panel2does not emit any light by itself but merely changes orientation or arrangement of the liquid crystal. This makes it necessary to provide, at the rear of the liquid crystal display panel2, the backlight unit6for evenly surface-irradiating the light on an information display surface.

In this regard, the backlight unit6is classified into an edge type and a direct type depending on the position of a light source. As illustrated inFIG. 2A, the edge type backlight unit includes a light source12disposed at one edge of a light guide plate14for surface-irradiating a light. In contrast, the direct type backlight unit is subdivided into a dot type wherein a plurality of dot-like light sources16aare mounted on a substrate30as shown inFIG. 2Band a line type wherein a plurality of linear light sources16bare mounted on a substrate30as shown inFIG. 2C. In such direct type backlight units, the light sources are substantially evenly distributed on the entire surface of the substrate.

Examples of the light source conventionally used include an electroluminescence (“EL”) element, a cold cathode fluorescent lamp (“CCFL”) and a hot cathode fluorescent lamp (“HCFL”). In recent years, extensive use is made of a light emitting diode (“LED”) that has a broad area of color reproduction and is environmentally friendly.

Research has been made to develop methods of using the light emitting diode as a light source in the backlight unit. Subjects of the research include a method of taking advantage of a blue color light emitting diode and an yttrium aluminum garnet (“YAG”) fluorescent body, a method of using an ultraviolet emitting diode in combination with fluorescent bodies of red, green and blue colors, and a method of employing red, green and blue light emitting diodes to admix the lights generated from them.

The method of taking advantage of a blue color light emitting diode and an yttrium aluminum garnet (“YAG”) fluorescent body is disadvantageous in that the light source thus produced has a reduced ability to express the red color and a low light emitting efficiency. Likewise, the method of using an ultraviolet emitting diode in combination with fluorescent bodies of red, green and blue colors poses a drawback in that it is difficult to develop the fluorescent bodies, with the resultant light source exhibiting a deteriorated thermal characteristic.

The method of employing red, green and blue light emitting diodes is effective in designing the light source to have a broadened range of color reproduction, thank to the increased intensity of red, green and blue lights emitted from the respective light emitting diodes. However, the method has a problem in that it is difficult to compose a combination of diodes for a white surface light source.

In the meantime, along with the recent trend of pursuing a large-sized and high image quality display device, a demand has existed for a backlight capable of outputting a high flux light. In order to comply with such a demand, there have been developed lenses for collecting lights emitted from light emitting diodes, semiconductor chips and diode materials.

A typical light emitting diode, one of solid semiconductor devices that convert electric energy to light energy, includes doping layers and an active layer. If a biasing voltage is applied to two oppositely positioned doping layers, electron holes and electrons are injected into the active layer and recombined to generate a light. The light generated in the active layer is emitted in all directions and escaped from the light emitting diode through every surface exposed to the outside. The light escaped from the light emitting diode is oriented to a desired direction by means of a backlight unit that incorporates the light emitting diode.

However, the light emitting diodes developed thus far cannot provide a sufficiently great light emitting efficiency, due to the light loss when penetrating a current diffusion layer and the light loss caused by a total reflection at a boundary surface.

Accordingly, in a backlight unit requiring an output of a high flux light, it is inevitable either to apply an increased current to a light emitting diode or to increase the number of light emitting diodes used.

In the case that an increased current is applied to a light emitting diode, a great deal of heat is generated from the light emitting diode, thus reducing the light emitting efficiency and making it necessary to add a heat radiation design to a substrate on which the light emitting diode is mounted. In the event that the number of light emitting diodes is increased, it becomes difficult to design the backlight unit, in addition to the increase of production cost of the backlight unit.

Although nitride semiconductor-based light emitting diodes and InGaAlP-based light emitting diodes have been developed for the purpose of enhancing the light emitting efficiency, they tend to emit a light whose flux is lower than that of a cold cathode fluorescent lamp and therefore are not suitable for use in a backlight unit.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a backlight unit capable of improving a light emitting efficiency and a color purity of a light through the use of optical resonance in an optical layer.

In accordance with the present invention, there is provided a backlight unit for use in a liquid crystal display having a liquid crystal panel, comprising: a substrate; a plurality of light sources arranged on the substrate in a spaced-apart relationship with one another for irradiating lights; a total reflection layer for reflecting the lights irradiated from the light sources in an upward direction; and an optical layer disposed above the light sources and having an oxide compound layer laminated thereon, the optical layer adapted to allow a part of the lights to pass through the optical layer but reflect the remaining part of the lights downwardly toward the substrate, whereby the backlight unit can induce the lights irradiated from the light sources to optically resonate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 3is a front elevational view depicting a part of a backlight unit in accordance with the present invention, andFIGS. 4A and 4Bare front elevational views showing laminated structures of an oxide compound in the optical layer shown inFIG. 3.

As shown inFIG. 3, a backlight unit100in accordance with the present invention includes a substrate30, a plurality of light emitting diodes40arranged on the substrate30in a spaced-apart relationship with one another, a total reflection layer50for reflecting a light irradiated from the light emitting diodes40in an upward direction, i.e., toward a liquid crystal layer, and an optical layer60provided above the light emitting diodes40for allowing a part of the light to be transmitted therethrough and reflecting the remaining part of the light toward the substrate30.

The substrate30, on which is formed a pattern, serves to support the light emitting diodes40and dissipate the heat generated by the light emitting diodes40. The light emitting diodes40are mounted on the substrate30at a generally equal spacing. The total reflection layer50is formed just below a light-emitting portion41of each of the light emitting diodes40.

Alternatively, the total reflection layer50may be directly bonded to the substrate30on which the light emitting diodes40are mounted. Such a total reflection layer50is formed by bonding a light-reflective film of high reflectance to an aluminum plate. It is preferred that the light-reflective film has a reflectance of no less than 80% but is low in its absorbency and transmittance.

The optical layer60is disposed above the light emitting diodes40and the total reflection layer50, with a predetermined optical distance “d” left from the total reflection layer50. The optical layer60allows a part of the light to be transmitted therethrough and reflects the remaining part of the light toward the substrate30. In order to ensure that the lights passed through the optical layer60can be subjected to constructive interference, the optical distance “d” between the total reflection layer50and the optical layer60is determined by the following equation:
Σ(n×t)=m×(λ/2)
where the “n” denotes a refractive index of the total reflection layer50or the optical layer60, the “t” means a geometrical thickness of the total reflection layer50or the optical layer60, the “λ” represents a peak wavelength of the light emitted from the light emitting diodes40, and the “m” is a integer of greater than 0.

In the case that the light emitting diodes40emit a red light, a green light or a blue light, the lights passed through the optical layer60can make constructive interference with each other and thus give rise to optical resonance, if one half of the peak wavelength of the lights multiplied by an integer is equal to the sum of the refractive index multiplied by the geometrical thickness of the total reflection layer50or the optical layer60.

In response to the optical resonance, the luminance of the light issuing from the backlight unit100is increased and the half amplitude of a color-based spectrum histogram in a light-emitting spectrum is reduced, thereby enhancing the color purity.

In this connection, it is a matter of course that a desired spectrum peak in the light-emitting spectrum can be obtained by adjusting the thickness of the total reflection layer50or the optical layer60.

The maximum transmittance Tmax, which results from the optical resonance caused by adjusting the optical distance “d” between the total reflection layer50and the optical layer60, is represented by the following Equation:
Tmax=[T1×T2×exp(−2β)]/[1−√{square root over (R1×R2)}×exp(−2β)]2

where the “T1” and “R1” denote a transmittance and a reflectance of the optical layer60, respectively, the “T2” and “R2” represent a transmittance and a reflectance of the total reflection layer50, respectively, the “k” means an extinction coefficient, the “t” refers to a geometrical thickness, the “θ” is an angle of the light advancing from the internal space between the total reflection layer50and the optical layer60to the outside, and the “λ” is a wavelength of the light emitted from the light emitting diodes40.

Once the maximum transmittance is calculated in this manner, it becomes possible to design an optical layer that has a reflectance corresponding to the maximum transmittance.

A part of the lights emitted from one of the light emitting diodes40mounted on the substrate30goes to the outside through the optical layer60. The remaining part of the lights is reflected by the optical layer60and then advances toward the total reflection layer50, at which time the lights are again reflected by the total reflection layer50toward the optical layer60.

As the lights are repeatedly subjected to transmission and reflection, the lights projected to the outside through the optical layer60undergoes constructive interference, thus ensuring that the lights emitted from the light emitting diodes40are amplified prior to irradiation to the outside through the optical layer60.

Furthermore, this helps to lengthen the travel path of the lights and thus makes it possible to efficiently mix the lights at the time when a white light is to be irradiated by mixing the lights emitted from the red, green and blue light emitting diodes.

Accordingly, it is preferred that the total reflection layer50has as great a reflectance as possible and as small a transmittance and absorbency as possible.

The optical layer60is formed by laminating oxide compounds or metallic materials. In case of laminating the oxide compounds, it is possible to independently or alternately laminate one or more layer of a low or medium refractive index oxide compound having a light refractive index “n” of smaller than 2.3 and a high refractive index oxide compound having a light refractive index “n” of equal to or greater than 2.3, as illustrated inFIG. 4A. As an alternative, it is also possible to independently or alternately laminate one or more layer of a medium refractive index oxide compound having a light refractive index “n” of greater than 1.5 but no greater than 2.3, a low refractive index oxide compound having a light refractive index “n” of 1.5 or less and a high refractive index oxide compound having a light refractive index “n” of greater than 2.3, as illustrated inFIG. 4B. In this case, the light absorbency is reduced and the luminance of the backlight unit is enhanced in proportion to the increase in the lamination thickness of the oxide compounds and the number of the layers laminated.

Laminating the layers of different refractive indexes improves the light reflecting property due to the inter-layer difference in the refractive indexes. The light absorbency is minimized and the light reflecting property is improved as the layers laminated grows in number.

SiO2is mainly used as the low refractive index oxide compound, Nb2O5is mainly used as the medium refractive index oxide compound, and TiO2, Ta2O3or Y2O3is used as the high refractive index oxide compound. Other oxide compounds may of course be used in due consideration of the reflective index.

Meanwhile, in case of forming the optical layer60with metallic materials, it is preferred that the metallic materials take the form of a thin film. Silver is suitable for the optical layer60and is preferably laminated to have a thickness of, e.g., 21 nm.

Referring toFIG. 5, there is shown a backlight unit provided with side-reflecting portions in accordance with one embodiment of the present invention. As shown inFIG. 5, the backlight unit100of the present invention further includes a pair of spaced-apart side-reflecting portions70each extending from the total reflection layer50up to the optical layer60at the opposite sides of the backlight unit100. The side-reflecting portions70are adapted to prevent the lights emitted by the light emitting diodes40from any leakage in a transverse direction through between the total reflection layer50and the optical layer60.

As with the total reflection layer50, it is preferred that the side-reflecting portions70has as great a reflectance as possible and as small a transmittance and an absorbency as possible. The side-reflecting portions70acts to reflect the lights incident thereon from the light emitting diodes40toward the center of the backlight unit100, thus increasing the quantity of the lights that penetrate the optical layer60.

FIGS. 6A and 6Billustrate a variety type of side-reflecting portions provided in the backlight units in accordance with the present invention. As shown these drawings, it is preferred that the side-reflecting portions70of the backlight unit100are formed to extend upwardly outwardly from the total reflection layer50toward the optical layer60. This is because the lights are irradiated to a target object through the optical layer60and therefore should preferably be reflected toward the optical layer60. The side-reflecting portions70may be of a flat shape as shown inFIG. 6A, or a gently curved shape as depicted inFIG. 6B.

Description will now be given to actual examples of forming the optical layer60to manufacture the backlight unit100in accordance with the present invention.

FIGS. 7A to 7Eare front elevational views showing laminated structures of an oxide compound in accordance with some test examples of the present invention.

Light emitting diodes of red, green and blue colors having an output power 1 W were prepared, the center wavelength of which is 627 nm in the red light emitting diode, 530 nm in the green light emitting diode, and 455 nm in the blue light emitting diode. Each of the light emitting diodes is of the type whose center wavelength varies within 5% depending on the driving current and the thermal characteristic. The driving current of the light emitting diodes is 200 mA.

The light emitting diodes thus prepared were grouped into a plurality of diode sets, each of which was mounted on a substrate at an equal spacing of 5-6 cm. Each of the diode sets consists of, e.g., one red light emitting diode, one green light emitting diode and one blue light emitting diode.

The combination of light emitting diodes in the respective diode sets may be modified arbitrarily and, likewise, the spacing between the diode sets may be changed depending on the number of light emitting diodes contained in each of the diode sets. For example, each of the diode sets may be comprised of two red light emitting diodes, two green light emitting diodes and one blue light emitting diode. Alternatively, each of the diode sets may be comprised of one red light emitting diode, two green light emitting diodes and one blue light emitting diode.

As illustrated inFIGS. 7A to 7E, five kinds of oxide compound-laminated structures (films) each containing an optical layer were produced. The optical layers of the respective structures have light transmittances of 40%, 50%, 60%, 70% and 80%, respectively, and are formed by laminating oxide compounds one atop above under the laminating conditions conforming to the boundary surface characteristics and the coating conditions of the oxide compound-laminated structures. The laminating conditions were derived through a computer simulation using an Essential McLeod program.

In the oxide compound-laminated structures, the number of layers should be increased in order to make greater the light reflectance. Taking this into account, the oxide compounds were laminated ten times to obtain the laminated structure having a light transmittance of 40%. The lamination was conducted eight times to acquire the laminated structures having light transmittances of 50%, 60% and 70%. The lamination was performed six times to produce the laminated structure having a light transmittance of 80%.

Backlight units were fabricated using the five kinds of oxide compound-laminated structures thus obtained and the characteristics of the backlight units were evaluated. In parallel, a backlight unit having no optical layer and reflecting plate was fabricated for the comparison of a light resonance effect. The characteristics of the inventive backlight units and the comparative backlight unit were analyzed.

SiO2was used as the low refractive index oxide compound, while TiO2was used as the high refractive index oxide compound.

FIG. 8graphically represents the correlation between a light reflectance and a luminance in the backlight units having oxide compound-laminated optical layer of the present invention and the comparative backlight unit. InFIG. 8, the term “bare” represents a test result for comparative backlight unit that provides no optical resonance effect.

As is apparent inFIG. 8, the backlight unit comprised of an oxide compound-laminated optical layer having a light transmittance of 80% (i.e., 20% reflectance) exhibits a luminance 14% greater than that of the comparative backlight unit. In case of the backlight units comprised of oxide compound-laminated optical layers whose light transmittances are 40%, 50%, 60% and 70% (i.e., 60%, 50%, 40% and 30% reflectance), it can be seen that the luminance is increased to a greater extent than the luminance of the comparative backlight unit.

As described in the foregoing, the backlight unit of the present invention provides an advantageous effect in that an oxide compound-laminated optical layer can induce optical resonance of the lights and improve luminance of the backlight unit, while allowing the lights to be mixed with each other efficiently.

This reduces electricity consumption in the backlight unit that requires a white color light of high luminance, thus prolonging the life span of the backlight unit. Further, the optical resonance caused by the oxide compound-laminated optical layer helps to reduce the half amplitude of a color-based spectrum histogram in a light-emitting spectrum, thereby enhancing the color purity. In addition, a desired spectrum peak in the light-emitting spectrum can be obtained merely by adjusting the thickness of a total reflection layer and/or an optical layer.