EP2235571A1 - Illumination apparatus using a light guide plate with a plurality of engraved grooves - Google Patents

Illumination apparatus using a light guide plate with a plurality of engraved grooves

Info

Publication number
EP2235571A1
EP2235571A1 EP07870585A EP07870585A EP2235571A1 EP 2235571 A1 EP2235571 A1 EP 2235571A1 EP 07870585 A EP07870585 A EP 07870585A EP 07870585 A EP07870585 A EP 07870585A EP 2235571 A1 EP2235571 A1 EP 2235571A1
Authority
EP
European Patent Office
Prior art keywords
guide plate
light guide
light
backlight apparatus
luminous radiation
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP07870585A
Other languages
German (de)
French (fr)
Inventor
Giovanni Barile
Paolo Vanni
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sirio Panel SpA
Original Assignee
Sirio Panel SpA
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sirio Panel SpA filed Critical Sirio Panel SpA
Publication of EP2235571A1 publication Critical patent/EP2235571A1/en
Ceased legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/0035—Means for improving the coupling-out of light from the light guide provided on the surface of the light guide or in the bulk of it
    • G02B6/0038—Linear indentations or grooves, e.g. arc-shaped grooves or meandering grooves, extending over the full length or width of the light guide
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/005—Means for improving the coupling-out of light from the light guide provided by one optical element, or plurality thereof, placed on the light output side of the light guide
    • G02B6/0055—Reflecting element, sheet or layer
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0033—Means for improving the coupling-out of light from the light guide
    • G02B6/0058—Means for improving the coupling-out of light from the light guide varying in density, size, shape or depth along the light guide
    • G02B6/0061—Means for improving the coupling-out of light from the light guide varying in density, size, shape or depth along the light guide to provide homogeneous light output intensity
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0066—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form characterised by the light source being coupled to the light guide
    • G02B6/0068—Arrangements of plural sources, e.g. multi-colour light sources
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/0001—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems
    • G02B6/0011—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form
    • G02B6/0066—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings specially adapted for lighting devices or systems the light guides being planar or of plate-like form characterised by the light source being coupled to the light guide
    • G02B6/0073—Light emitting diode [LED]

Definitions

  • the present invention ⁇ relates to an integrated light guide illumination apparatus, and more particularly to an integrated backlight apparatus for liquid crystal displays (LCD) .
  • LCD liquid crystal displays
  • backlight apparatuses strongly depends on the light guide plate used and on the working principle.
  • the most common typology of light guide plates employs a diffraction or scattering pattern that is located on one surface of the light guide plate (e.g., the bottom surface) and, during use, has the ability to diffract or scatter light beams, generated by a light source, toward an opposite transparent surface (e.g., the top surface), so that the light beams, exiting from the transparent surface of the light guide plate, can reach an LCD panel conveniently located parallel to the transparent surface .
  • a backlight apparatus of this kind is disclosed, for example, in US Patents 5,703,667 and 7,221,416.
  • a backlight apparatus 1 comprises a light guide plate 2 having a top surface 2a, acting as a light emitting surface, a first bottom surface 2b, on which a plurality of diffraction units 3 are pro ⁇ ided, a fluorescent tube 4,. facing a light incidence surface 2 ' c and generating light beams propagating practically along a reference x-axis, and a reflection plate 5 disposed -under the first bottom surface 2b and light shaping layers 6, 7 disposed over the first top surface -2a.
  • each diffraction unit 3 comprises a grating part 3a, including a series of grooves arranged side by side, and a non-grating part 3b, facing the reflection plate 5.
  • the proportion of the grating part width with respect to the non-grating part width in each diffraction unit 3 becomes larger as the diffraction unit 3 extends farther from the light incidence surface 2c, so that the amount of diffracted light increases proportionally as the quantity of light available from the fluorescent tube 4 decreases. Therefore, high, energy light rays close to the light incidence surface 2c undergo weaker diffraction with respect to low energy light rays far from the light incidence surface 2c.
  • each diffraction unit 3 must be precisely designed and developed, so that the light beams generated by the fluorescent tube 4 and incident on the grating part 3a are diffracted by the latter in such a way to maximize the light components that can exit from the top surface 2a, providing the illumination for- the LCD .
  • i indicates the incident angle
  • ⁇ indicates the diffraction angle
  • ⁇ indicates the wavelength of the light beam emitted by the fluorescent tube 4
  • m is an integer number indicating the diffraction order
  • d is a grating constant, i.e., the groove to groove distance of the grating part 3a of the diffraction unit 3.
  • the incident angle i and the grating constant d with respect to the wavelength ⁇ it is possible to predetermine the path, inside the light guide plate 2, of diffracted light beams of any diffraction order. For instance, it is possible to allow low order diffracted light beams
  • the incidence angle i required to have a particular diffraction angle ⁇ is obtained by inclining the bottom surface 2b at an angle varying from 0.5° to 5° over the x-axis. Since the light beam incident onto the diffraction unit 3 has a spectral distribution with peaks in red, green and blue parts of the spectrum, the diffracted light beam may exhibit RGB spectra.
  • a diffusion plate 6 and a collector prism sheet 7 are arranged over the top surface ' 2a, •parallel thereto, and have the function of mixing RGB spectra and generate white uniform light.
  • a backlight apparatus 10 includes a light guide plate 11 having a top surface 11a, acting as a light emitting surface, a bottom surface lib, a scattering pattern 12 including a series of grooves arranged side . by side and formed over the entire bottom surface lib and a plurality of monochromatic light emitting diodes (LEDs) 13 arranged on lateral sides 14a, 14b of the second light guide plate 11 and emitting .monochromatic red, green and blue light beams with upward and downward angles ⁇ .
  • LEDs monochromatic light emitting diodes
  • the upward and downward emission angles ⁇ of each monochromatic LED 13 can be adjusted by coupling the monochromatic LEDs 13 with appropriate lenses (not shown) in order to achieve modified upward and downward emission angles ⁇ ' , having an amplitude lower than the upward and downward emission angles ⁇ of the LEDs 13. ⁇
  • the above ⁇ described apparatus is therefore advantageous when the monochromatic LEDs 13 are used in combination with lenses that allow to achieve proper modified upward and downward emission angles ⁇ ' , so as to control the emission direction of the light beams
  • the groves of the scattering pattern 12 be precisely realized to obtain scattering of the incident light beams almost perpendicularly to the top surface 12, thus maximizing the light emitted by the second backlight apparatus 10 and reducing the internally reflected light beams.
  • the present backlight module for liquid crystal displays has a diffusion pattern for luminous radiation formed in the entire bottom surface of a light guide plate which does not impose limitations on either the LEDs angles of emission and the angles of incidence of the luminous radiation on the diffusion pattern.
  • a backlight apparatus as defined in claim 1.
  • the backlight module for LCDs comprises a light guide plate arranged parallel to an LCD panel to illuminate a back side of the LCD panel, the light guide plate having a diffusion pattern formed in a bottom surface, opposed and parallel to a top surface, of the light guide plate.
  • a first and a second arrays of light sources are aligned on opposite first and second lateral sides of the light guide plate to provide a luminous radiation between the bottom -and the top surface of the light guide plate;
  • a third array of light sources is aligned on a third lateral side of the light guide plate and is coupled with a night goggle vision (NGV) filter to cut the near infrared components of the luminous radiation during night vision.
  • the diffusion pattern formed in the bottom surface of the light guide plate comprises a plurality of diffusion elements, having variable width, defined by a plurality of engraving lines, having variable depth, adapted to diffuse the luminous radiation within the bottom surface .
  • a backlight reflector is placed below and parallel to the bottom surface and is adapted to reflect back into the light guide plate the luminous radiation diffused by the diffusion .pattern within the bottom surface.
  • the luminous radiation that interacts with the diffusion pattern is first diffused within the bottom surface of the light guide plate, then reflected back by the backlight reflector and finally re-diffused by the diffusion pattern into the light guide plate.
  • the luminous radiation undergoes at least a double diffusion before being emitted through the top surface of the light guide plate toward the LCD panel.
  • each engraving line of the diffusion pattern has, in a lateral view, a triangular shape so •that, in the direction of the light guide plate from the light sources, the depth of each engraving line progressively increases while the distance measured between a base vertex of an engraving line and the base vertex of the engraving ' line immediately neighboring thereon progressively decreases.
  • This geometry allows to diffuse the luminous radiation with maximum efficiency where the quantity of luminous radiation is lower (i.e., far from the light sources) .
  • the minimum base-to-base distance and maximum depth are attained at an intermediate distance between the first and second lateral sides.
  • the backlight apparatus further comprises at least one light shaping film, placed between the light guide plate and the LCD panel, to direct the emitted luminous radiation toward the LCD panel .
  • the first, second and third array of light sources includes white Light Emitting Diodes (LEDs)' " .
  • the first, second and third array of light sources may include an array of monochromatic RGB LEDs.
  • Figure 1 shows a lateral view of a first known backlight apparatus for liquid crystal displays
  • Figure 2 shows another lateral view of a second known backlight apparatus for liquid crystal displays
  • Figure 3 shows an exploded view of a backlight apparatus for liquid crystal displays according to an embodiment of the present invention
  • Figure 4 shows a top view, with a light shaping films removed, of -a diffusion pattern according to the embodiment of the present invention
  • Figure 5 shows a lateral view of the backlight apparatus for liquid crystal displays according to the embodiment of the present invention
  • Figure 6 shows a lateral enlarged view of a portion of the diffusion pattern during use according to the embodiment of the present invention
  • Figure 7 shows a light guide plate of a backlight apparatus for liquid crystal displays according to another embodiment of the present invention.
  • Figures 8a and 8b show a side view of a portion of the diffusion pattern according to other embodiments of the present invention..
  • Figure 3 shows an exploded view of a backlight module 20, including a light guide plate 21 having a diffusion pattern 22 formed continuously over the entire bottom surface 23 ; a first and a second array of light sources 24, 25, extending along an x-axis direction on a - S -
  • NUV night goggle vision
  • the light guide plate 21 may be made of optical grade plastic (e.g., .methacrylate, acrylic glass) or, generally, of optically clear, temperature-stable polymers; therefore the light guide plate 21 is cheap, has a high grade of purity and assures low degradation and dispersion of the luminous radiation emitted by the arrays of light sources 24, 25, 26. Other materials, such as glass, may be used to obtain analogous performances.
  • the light guide plate 21 preferably has a parallelepiped shape, having, for example, a shorter side of about 62 mm, a longer side of about 83 mm, and a thickness of 1.2 mm. It is advisable to precisely control the thickness of the light guide plate 21 during fabrication, because an uncontrolled thickness variation may cause an unpredictable uneven variation of the luminous radiation distribution inside the light guide plate 21 .
  • the first and the second, array of light sources 24, 25 are, in this embodiment, aligned along the shortest lateral sides of the light guide plate 21, while the third array of light sources 26 is aligned along one of the two longest lateral sides, with the interposition of the NGV filter 27.
  • the arrays of light sources 24, 25, 26 emit white luminous radiation (they may be, for example, arrays of white LEDs) .
  • near-infrared components of any luminous radiation interfere with the nighttime vision; for this reason, the luminous radiation generated by the third array of light sources 26 is filtered by the NGV filter 27 before entering the light guide plate 21, so that the near- infrared components are cut off .
  • the first and the second array of light sources 24, 25 are turned on and the third array of light sources 26 is off; for nighttime vision, the first and the second array of light sources 24, 25 are turned off and the third array of light sources 26 starts operating.
  • the color correction is provided by a color-correction film 38, for example made of polyester and 0.2 mm thick, placed parallel to the NGV filter 27.
  • the color-correction film 38 is used to shape the chromatic spectrum of the luminous radiation emitted by the third array of light sources 26, to obtain a substantially white luminous radiation.
  • the backlight reflector 28 formed by a commercially available, ultra-high reflecting, mirror- like optical enhancement film, having a thickness of, for example, 100 ⁇ m.
  • the reflectivity of the backlight reflector 28 is constant across the whole visible .spectrum, and does not produce unwanted color shifts.
  • Highly reflective backlight reflectors 28 are preferred to keep light loss at a minimum.
  • the backlight reflector 28 has no preferential reflecting direction, thus guaranteeing a uniform reflection for any light beam incident angle.
  • One or more light shaping films 29 are arranged above the light guide plate 21, and are of a kind commercially available.
  • the light shaping films 29 may be microstructured surfaces that employ refraction and/or reflection mechanisms for improving the backlight module efficiency and increasing the luminous radiation uniformity and concentration.
  • FIG 4 shows a top view, with light shaping films 29 removed, of the diffusion pattern 22 formed on the bottom surface 23 by engraving the bottom surface 23 along engraving grooves 35 parallel to those lateral sides of the light guide plate 21 facing a light source, so as to define the lateral sides of a plurality of diffusion elements 36.
  • Each diffusion element 36 has, e.g., a truncated pyramidal, oblique truncated pyramidal, parallelepipedal or generally prismatic shape, with lateral sides aligned along the x- and y- axes and defined by the engraving grooves 35.
  • the lateral sides of the diffusion elements 36 which extend along the x-axis, diffuse the luminous radiation emitted by the first and the second light source 24, 25 (i.e., for daytime vision)
  • the lateral sides of the diffusion elements 36 which extend along the y-axis diffuse the luminous radiation emitted by the third array of light sources 26 (i.e., for nighttime vision) .
  • the engraving grooves 35 may be formed by means of a laser engraving technique, using, for example, a speed of 1500 mm/s, a frequency ranging from 2 to 3 kHz and an engraving power ranging from 5 to 10 W for defining the engraving grooves 35 parallel to the x-axis, for daytime vision, and an engraving power ranging from 4 to 8 W for defining the engraving grooves 35 parallel to the y- axis, for nighttime vision.
  • each diffusion element 36 belonging to the diffusion pattern 22 has, in a top view, a quadrangular base with two sides parallel to the x-axis, and two sides parallel to the y-axis. As visible in Figure 4, the lengths of the sides parallel to the y- axis progressively decrease with increasing distance from the first and second array of light sources 24, 25 (toward the center of the diffusion pattern 22) , while the lengths of the sides parallel to the x-axis progressively decrease with increasing distance from the third array of light sources 26.
  • Figure 5 shows a cross-section view . of half of the backlight module during use, in which the lateral shape of engraving grooves 35 and diffusion elements 36 belonging to the diffusion pattern 22 can be appreciated.
  • the engraving grooves 35 are, in cross- section view, triangles with fixed base length b and variable depth h.
  • the depth h of parallel engraving grooves 35 increases and a base-to-base distance v between a base vertex of an engraving groove 35 and the base vertex of another engraving groove 35 immediately neighboring thereon decreases .
  • each diffusion element 36 has a shape defined by the engraving grooves 35 and by the bottom surface 23 of the light guide plate 21.
  • each diffusion element 36 has, in a cross-section view, a four-side convex polygon shape, one side being the ideal conjunction between top vertexes of neighboring engraving grooves 35.
  • the first array of light sources 24 emits a luminous radiation that is transmitted into the light guide plate 21 (for clarity, only a single light source
  • Part of the luminous radiation hits the diffusion pattern 22, respectively close to and far from the first light source 24, and are diffused by a respective diffusion element 36 below the diffusion pattern 22, -that is transparent to the luminous -radiation emitted by the arrays of light sources, as schematically shown in Figure S .
  • Each reflected light beam rl-r3 undergoes a second diffusion process that splits each reflected light beam rl-r3 into upward diffused light beams ul-u9, thus producing, at least, a double diffusion of the luminous radiation generated by the first array of light sources 24 of Figure 5.
  • the second diffusion process to which reflected light beams rl-r3 undergo, further increases the uniformity of luminous radiation inside the light guide plate 21 and, as a consequence, also the uniformity of the luminous radiation exiting from the top surface 30. Moreover, during the double diffusion process, monochromatic spectra that may be generated during the interaction between the white luminous radiation and the diffusion pattern 22 are mixed together, and white luminous radiation emission from the top surface " 30 is assured.
  • some of the light beams emitted by the light source 24 may undergo at least one internal reflection before impinging against the diffusion pattern 22.
  • some of the upward diffused light beams ul-u9 e.g. upward diffused light beam u6 in Figure 5
  • a fourth array of light sources 41 may be placed on the free side of the light guide plate 21, opposite to the third array- of light sources 26, thus rendering the light sources distribution symmetric.
  • the engraving grooves 35 have decreasing base-to-base distance v and increasing depth h moving from all the arrays of light sources 24-26, 41 toward the center of the diffusion pattern 22.
  • the backlight module 20 of Figure 7 is similar to the one described above with reference to Figures 3-6.
  • the engraving grooves. 35 have decreasing base-to-base distance v and increasing depth h moving from the arrays of light sources toward the opposite lateral side of the light guide plate 21. .
  • the optical guide 21 may have a shape other than rectangular, for example it may be square or rounded shaped, according to the shape of the LCD display and on the area that has to be illuminated.
  • the diffusion pattern 22 may have other shapes, for example those of the optical guide 21 and it may cover the entire surface of the light guide plate 21 or only some portions thereof, according to the needs.
  • Each engraving groove 35 may have a shape other o than the triangular shape in lateral view, such as rectangular of trapezoidal, as shown in Figures 8a, 8b, and the diffusion elements 36 may be arranged in a way other than the matrix-like pattern of Figure 4.
  • the first, second and third array of light sources 24, 25, 26 may also be realized with monochromatic light sources, e.g., red, green and blue LEDs. In this case, the angle of emission of each monochromatic light source is not important, since monochromatic radiations generated by each monochromatic light source will mix together, generating a ' white light, during the double diffusion process.

Abstract

A liquid crystal display (LCD) backlight apparatus having a light guide plate (21) with a first and a second main surfaces (23, 30) and a first lateral side; a reflection plate (28) facing the first main surface (23); a first light source (24) arranged laterally to the first lateral side of the light guide plate (21), configured to emit a luminous radiation (Ll, L2, L3 ); a transparent diffusion pattern (22) extending on the first main surface (23) and configured to receive the emitted luminous radiation (Ll, L2, L3), to generate a first diffused luminous radiation (d1, d2, d3) toward the reflection plate (28), to collect reflected luminous radiation (r1, r2, r3) from the reflection plate (28) and to generate a second diffused luminous radiation (u1-u9) toward the second main surface.

Description

INTEGRATED LIGHT GUIDE ILLUMINATION APPARATUS
FIELD OF THE INVENTION
The present invention ■ relates to an integrated light guide illumination apparatus, and more particularly to an integrated backlight apparatus for liquid crystal displays (LCD) .
BACKGROUND OF THE INVENTION As is known, • a typical liquid crystal display requires a backlight apparatus providing a uniform illumination over the whole display. The performance of
' backlight apparatuses strongly depends on the light guide plate used and on the working principle. The most common typology of light guide plates employs a diffraction or scattering pattern that is located on one surface of the light guide plate (e.g., the bottom surface) and, during use, has the ability to diffract or scatter light beams, generated by a light source, toward an opposite transparent surface (e.g., the top surface), so that the light beams, exiting from the transparent surface of the light guide plate, can reach an LCD panel conveniently located parallel to the transparent surface . A backlight apparatus of this kind is disclosed, for example, in US Patents 5,703,667 and 7,221,416.
In a known embodiment disclosed in US Pat. 5,703,667 and shown in Figure 1, a backlight apparatus 1 comprises a light guide plate 2 having a top surface 2a, acting as a light emitting surface, a first bottom surface 2b, on which a plurality of diffraction units 3 are proγided, a fluorescent tube 4,. facing a light incidence surface 2'c and generating light beams propagating practically along a reference x-axis, and a reflection plate 5 disposed -under the first bottom surface 2b and light shaping layers 6, 7 disposed over the first top surface -2a. In more details, each diffraction unit 3 comprises a grating part 3a, including a series of grooves arranged side by side, and a non-grating part 3b, facing the reflection plate 5. The proportion of the grating part width with respect to the non-grating part width in each diffraction unit 3 becomes larger as the diffraction unit 3 extends farther from the light incidence surface 2c, so that the amount of diffracted light increases proportionally as the quantity of light available from the fluorescent tube 4 decreases. Therefore, high, energy light rays close to the light incidence surface 2c undergo weaker diffraction with respect to low energy light rays far from the light incidence surface 2c.
For optimum performance of the apparatus described above, each diffraction unit 3 must be precisely designed and developed, so that the light beams generated by the fluorescent tube 4 and incident on the grating part 3a are diffracted by the latter in such a way to maximize the light components that can exit from the top surface 2a, providing the illumination for- the LCD .
The following equation (1) applies between the light beams incident onto the diffraction unit 3 and the corresponding diffracted light beams:
where i indicates the incident angle, θ indicates the diffraction angle, λ indicates the wavelength of the light beam emitted by the fluorescent tube 4, m is an integer number indicating the diffraction order and d is a grating constant, i.e., the groove to groove distance of the grating part 3a of the diffraction unit 3.
Therefore, by properly designing the incident angle i and the grating constant d with respect to the wavelength λ, it is possible to predetermine the path, inside the light guide plate 2, of diffracted light beams of any diffraction order. For instance, it is possible to allow low order diffracted light beams
(e.g., having in=l) to follow a specific path and reach the top surface 2a with an angle lower than a critical angle for which there would be total reflection. Since the light beams generated by the fluorescent tube 4 are emitted along the x-axis, the incidence angle i required to have a particular diffraction angle θ is obtained by inclining the bottom surface 2b at an angle varying from 0.5° to 5° over the x-axis. Since the light beam incident onto the diffraction unit 3 has a spectral distribution with peaks in red, green and blue parts of the spectrum, the diffracted light beam may exhibit RGB spectra. To collect diffracted light beams and emit white light out of the top surface 2a, a diffusion plate 6 and a collector prism sheet 7 are arranged over the top surface' 2a, •parallel thereto, and have the function of mixing RGB spectra and generate white uniform light.
Another known embodiment is disclosed in US Pat. 7,221,416 and shown in Figure 2.
Referring to Figure 2, a backlight apparatus 10 includes a light guide plate 11 having a top surface 11a, acting as a light emitting surface, a bottom surface lib, a scattering pattern 12 including a series of grooves arranged side . by side and formed over the entire bottom surface lib and a plurality of monochromatic light emitting diodes (LEDs) 13 arranged on lateral sides 14a, 14b of the second light guide plate 11 and emitting .monochromatic red, green and blue light beams with upward and downward angles β . In order to obtain a complete mixing of the monochromatic light beams, and thus having a white light emission from the second top surface 11a, it is important- to precisely control the upward and downward emission angle β of the monochromatic LEDs 13, so that the red, green and blue light beams can mix together, generating a white light beam before reaching the scattering pattern 12. In this way, only white light is scattered toward the second top surface 11a. To this end, the upward and downward emission angles β of each monochromatic LED 13 can be adjusted by coupling the monochromatic LEDs 13 with appropriate lenses (not shown) in order to achieve modified upward and downward emission angles β' , having an amplitude lower than the upward and downward emission angles β of the LEDs 13. ■ The above ■ described apparatus is therefore advantageous when the monochromatic LEDs 13 are used in combination with lenses that allow to achieve proper modified upward and downward emission angles β' , so as to control the emission direction of the light beams
■ before they hit the scattering pattern 12.
Moreover, it is important that the groves of the scattering pattern 12 be precisely realized to obtain scattering of the incident light beams almost perpendicularly to the top surface 12, thus maximizing the light emitted by the second backlight apparatus 10 and reducing the internally reflected light beams.
OBJECT AND SUMMARY OF THE INVENTION The aim of the present invention is to overcome the design constraints and drawbacks of the prior art. Advantageously, the present backlight module for liquid crystal displays (LCD) has a diffusion pattern for luminous radiation formed in the entire bottom surface of a light guide plate which does not impose limitations on either the LEDs angles of emission and the angles of incidence of the luminous radiation on the diffusion pattern.
According to. the present invention, there is provided a backlight apparatus, as defined in claim 1.
In one embodiment, the backlight module for LCDs comprises a light guide plate arranged parallel to an LCD panel to illuminate a back side of the LCD panel, the light guide plate having a diffusion pattern formed in a bottom surface, opposed and parallel to a top surface, of the light guide plate. A first and a second arrays of light sources are aligned on opposite first and second lateral sides of the light guide plate to provide a luminous radiation between the bottom -and the top surface of the light guide plate; a third array of light sources is aligned on a third lateral side of the light guide plate and is coupled with a night goggle vision (NGV) filter to cut the near infrared components of the luminous radiation during night vision. The diffusion pattern formed in the bottom surface of the light guide plate comprises a plurality of diffusion elements, having variable width, defined by a plurality of engraving lines, having variable depth, adapted to diffuse the luminous radiation within the bottom surface .
A backlight reflector is placed below and parallel to the bottom surface and is adapted to reflect back into the light guide plate the luminous radiation diffused by the diffusion .pattern within the bottom surface. Thus, the luminous radiation that interacts with the diffusion pattern is first diffused within the bottom surface of the light guide plate, then reflected back by the backlight reflector and finally re-diffused by the diffusion pattern into the light guide plate. According to this process, the luminous radiation undergoes at least a double diffusion before being emitted through the top surface of the light guide plate toward the LCD panel.
Preferably, each engraving line of the diffusion pattern has, in a lateral view, a triangular shape so •that, in the direction of the light guide plate from the light sources, the depth of each engraving line progressively increases while the distance measured between a base vertex of an engraving line and the base vertex of the engraving' line immediately neighboring thereon progressively decreases. This geometry allows to diffuse the luminous radiation with maximum efficiency where the quantity of luminous radiation is lower (i.e., far from the light sources) . When considering the case of first and second array of light sources arranged on opposite lateral sides of the light guide plate, the minimum base-to-base distance and maximum depth are attained at an intermediate distance between the first and second lateral sides. Preferably, the backlight apparatus further comprises at least one light shaping film, placed between the light guide plate and the LCD panel, to direct the emitted luminous radiation toward the LCD panel . Preferably, the first, second and third array of light sources includes white Light Emitting Diodes (LEDs)'". Alternatively, the first, second and third array of light sources may include an array of monochromatic RGB LEDs.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, an embodiment thereof, which are intended purely by way of example and are not to be construed as limiting, will now be described with reference to the attached drawings (all not to scale) , wherein:
• Figure 1 shows a lateral view of a first known backlight apparatus for liquid crystal displays;
• Figure 2 shows another lateral view of a second known backlight apparatus for liquid crystal displays,-
• Figure 3 shows an exploded view of a backlight apparatus for liquid crystal displays according to an embodiment of the present invention;
• Figure 4 shows a top view, with a light shaping films removed, of -a diffusion pattern according to the embodiment of the present invention;
• Figure 5 shows a lateral view of the backlight apparatus for liquid crystal displays according to the embodiment of the present invention; • Figure 6 shows a lateral enlarged view of a portion of the diffusion pattern during use according to the embodiment of the present invention;
• Figure 7 shows a light guide plate of a backlight apparatus for liquid crystal displays according to another embodiment of the present invention;
• Figures 8a and 8b show a side view of a portion of the diffusion pattern according to other embodiments of the present invention..
DETAILED DESCRIPTION OF PKBFEKElEiD EMBODIMENTS OF THE INVENTION
Figure 3 shows an exploded view of a backlight module 20, including a light guide plate 21 having a diffusion pattern 22 formed continuously over the entire bottom surface 23 ; a first and a second array of light sources 24, 25, extending along an x-axis direction on a - S -
first and a second lateral sides of the light guide plate 21, opposite one another, and emitting white luminous radiations insi'de the light guide plate 21; a third array of light sources 26, extending on a third lateral side of the light guide plate 21, along a y-axis direction, emitting a white luminous radiation, and coupled with a night goggle vision (NGV) filter 27; a backlight reflector 28, arranged under the bottom surface 23 of the light guide plate 21 and supported by a base layer 32; one or more light shaping films 29 (only one light shaping film 29 is shown in Figure 3), arranged between a top surface 30 of the light guide- plate 21 and a LCD panel 31 of a liquid crystal display (not shown) . The light guide plate 21 may be made of optical grade plastic (e.g., .methacrylate, acrylic glass) or, generally, of optically clear, temperature-stable polymers; therefore the light guide plate 21 is cheap, has a high grade of purity and assures low degradation and dispersion of the luminous radiation emitted by the arrays of light sources 24, 25, 26. Other materials, such as glass, may be used to obtain analogous performances. The light guide plate 21 preferably has a parallelepiped shape, having, for example, a shorter side of about 62 mm, a longer side of about 83 mm, and a thickness of 1.2 mm. It is advisable to precisely control the thickness of the light guide plate 21 during fabrication, because an uncontrolled thickness variation may cause an unpredictable uneven variation of the luminous radiation distribution inside the light guide plate 21 .
The first and the second, array of light sources 24, 25 are, in this embodiment, aligned along the shortest lateral sides of the light guide plate 21, while the third array of light sources 26 is aligned along one of the two longest lateral sides, with the interposition of the NGV filter 27. Preferably, the arrays of light sources 24, 25, 26 emit white luminous radiation (they may be, for example, arrays of white LEDs) . As known, near-infrared components of any luminous radiation interfere with the nighttime vision; for this reason, the luminous radiation generated by the third array of light sources 26 is filtered by the NGV filter 27 before entering the light guide plate 21, so that the near- infrared components are cut off .
For daytime vision, the first and the second array of light sources 24, 25 are turned on and the third array of light sources 26 is off; for nighttime vision, the first and the second array of light sources 24, 25 are turned off and the third array of light sources 26 starts operating.
The white luminous radiation generated by the third array of light sources 26, deprived of the near-infrared components by the NGV filter 27, assumes a slightly blue color. Consequently, a color correction of the luminous radiation filtered by the NGV filter 27 during nighttime vision is desirable, so as to maintain the color of the luminous radiation proximate to white. The color correction is provided by a color-correction film 38, for example made of polyester and 0.2 mm thick, placed parallel to the NGV filter 27. The color-correction film 38 is used to shape the chromatic spectrum of the luminous radiation emitted by the third array of light sources 26, to obtain a substantially white luminous radiation. under the light guide plate 21 is arranged the backlight reflector 28, formed by a commercially available, ultra-high reflecting, mirror- like optical enhancement film, having a thickness of, for example, 100 μm.
Preferably, the reflectivity of the backlight reflector 28 is constant across the whole visible .spectrum, and does not produce unwanted color shifts. Highly reflective backlight reflectors 28 are preferred to keep light loss at a minimum. Advantageously, the backlight reflector 28 has no preferential reflecting direction, thus guaranteeing a uniform reflection for any light beam incident angle.
One or more light shaping films 29 (only one shown in Figure 3) are arranged above the light guide plate 21, and are of a kind commercially available. The light shaping films 29 may be microstructured surfaces that employ refraction and/or reflection mechanisms for improving the backlight module efficiency and increasing the luminous radiation uniformity and concentration.
Figure 4 shows a top view, with light shaping films 29 removed, of the diffusion pattern 22 formed on the bottom surface 23 by engraving the bottom surface 23 along engraving grooves 35 parallel to those lateral sides of the light guide plate 21 facing a light source, so as to define the lateral sides of a plurality of diffusion elements 36. Each diffusion element 36 has, e.g., a truncated pyramidal, oblique truncated pyramidal, parallelepipedal or generally prismatic shape, with lateral sides aligned along the x- and y- axes and defined by the engraving grooves 35. The lateral sides of the diffusion elements 36, which extend along the x-axis, diffuse the luminous radiation emitted by the first and the second light source 24, 25 (i.e., for daytime vision) , while the lateral sides of the diffusion elements 36 which extend along the y-axis diffuse the luminous radiation emitted by the third array of light sources 26 (i.e., for nighttime vision) .
The engraving grooves 35 may be formed by means of a laser engraving technique, using, for example, a speed of 1500 mm/s, a frequency ranging from 2 to 3 kHz and an engraving power ranging from 5 to 10 W for defining the engraving grooves 35 parallel to the x-axis, for daytime vision, and an engraving power ranging from 4 to 8 W for defining the engraving grooves 35 parallel to the y- axis, for nighttime vision.
In the present embodiment, each diffusion element 36 belonging to the diffusion pattern 22 has, in a top view, a quadrangular base with two sides parallel to the x-axis, and two sides parallel to the y-axis. As visible in Figure 4, the lengths of the sides parallel to the y- axis progressively decrease with increasing distance from the first and second array of light sources 24, 25 (toward the center of the diffusion pattern 22) , while the lengths of the sides parallel to the x-axis progressively decrease with increasing distance from the third array of light sources 26.
Figure 5 (not in scale) shows a cross-section view . of half of the backlight module during use, in which the lateral shape of engraving grooves 35 and diffusion elements 36 belonging to the diffusion pattern 22 can be appreciated.
Here, the engraving grooves 35 are, in cross- section view, triangles with fixed base length b and variable depth h. In particular, moving from the first array of light sources 24 to the center of the diffusion pattern 22, the depth h of parallel engraving grooves 35 increases and a base-to-base distance v between a base vertex of an engraving groove 35 and the base vertex of another engraving groove 35 immediately neighboring thereon decreases .
Accordingly, the diffusion elements 36 have a shape defined by the engraving grooves 35 and by the bottom surface 23 of the light guide plate 21. For example, each diffusion element 36 has, in a cross-section view, a four-side convex polygon shape, one side being the ideal conjunction between top vertexes of neighboring engraving grooves 35.
In use, the first array of light sources 24 emits a luminous radiation that is transmitted into the light guide plate 21 (for clarity, only a single light source
39 belonging to the first array of light sources 24 is shown) .
Part of the luminous radiation, for instance emitted light beams Ll and L2 in Figure 5, hits the diffusion pattern 22, respectively close to and far from the first light source 24, and are diffused by a respective diffusion element 36 below the diffusion pattern 22, -that is transparent to the luminous -radiation emitted by the arrays of light sources, as schematically shown in Figure S .
In more detail, Figur/e 6, the emitted light beam Ll, impinging onto the diffusion pattern 22, is diffused uniformly under the diffusion pattern 22, as represented by downward diffused light beams dl, d2, d3 , ■ within a certain angle γ, i.e., the angle comprised between downward diffused light beams dl and d3.
Each downward diffused light beam dl-d3, schematically represented by arrows, reaches the backlight reflector 28 and is then reflected -back toward the diffusion pattern 22, as represented by reflected light beams rl, r2, r3. Each reflected light beam rl-r3 undergoes a second diffusion process that splits each reflected light beam rl-r3 into upward diffused light beams ul-u9, thus producing, at least, a double diffusion of the luminous radiation generated by the first array of light sources 24 of Figure 5.
The second diffusion process, to which reflected light beams rl-r3 undergo, further increases the uniformity of luminous radiation inside the light guide plate 21 and, as a consequence, also the uniformity of the luminous radiation exiting from the top surface 30. Moreover, during the double diffusion process, monochromatic spectra that may be generated during the interaction between the white luminous radiation and the diffusion pattern 22 are mixed together, and white luminous radiation emission from the top surface" 30 is assured.
As shown in Figure 5 , some of the light beams emitted by the light source 24 (e.g., L3) may undergo at least one internal reflection before impinging against the diffusion pattern 22. Moreover, some of the upward diffused light beams ul-u9 (e.g. upward diffused light beam u6 in Figure 5) may undergo further internal reflections, depending on the angle of incidence of each light beam ul-u9 with the top surface 30, thus being reflected back to the diffusion pattern 22 and
'undergoing again to the double diffusion process.
Other light beams (e.g., upward diffused light beams u4 and u5 in Figure 5) , impinging on the top surface 30 with an incident angle for which there is not total internal reflection, are emitted from the top surface 30.
Given the high' number of light beams traveling a substantially random path inside the light guide plate
21 and the variable geometry of the engraving grooves 35 and, thus, the diffusion elements 36, the -result is an extremely high uniformity of luminous radiation within the entire light guide plate 21. It is clear that numerous modifications and variants can be made to the present invention, all falling within the scope of the invention, as defined in the appended claims.
In particular, as shown in Figure 7, a fourth array of light sources 41, with a respective NGV filter 37 and a respective color-correction film 40, may be placed on the free side of the light guide plate 21, opposite to the third array- of light sources 26, thus rendering the light sources distribution symmetric. In this case, the engraving grooves 35 have decreasing base-to-base distance v and increasing depth h moving from all the arrays of light sources 24-26, 41 toward the center of the diffusion pattern 22. For the rest, the backlight module 20 of Figure 7 is similar to the one described above with reference to Figures 3-6.
It is also possible to use only two arrays of light sources, one for daytime vision, the other for nighttime vision, on a respective side of the light guide plate 21. In this case, the engraving grooves. 35 have decreasing base-to-base distance v and increasing depth h moving from the arrays of light sources toward the opposite lateral side of the light guide plate 21. .
Moreover, the optical guide 21 may have a shape other than rectangular, for example it may be square or rounded shaped, according to the shape of the LCD display and on the area that has to be illuminated.
The diffusion pattern 22 may have other shapes, for example those of the optical guide 21 and it may cover the entire surface of the light guide plate 21 or only some portions thereof, according to the needs.
Each engraving groove 35 may have a shape other o than the triangular shape in lateral view, such as rectangular of trapezoidal, as shown in Figures 8a, 8b, and the diffusion elements 36 may be arranged in a way other than the matrix-like pattern of Figure 4. Finally, the first, second and third array of light sources 24, 25, 26 may also be realized with monochromatic light sources, e.g., red, green and blue LEDs. In this case, the angle of emission of each monochromatic light source is not important, since monochromatic radiations generated by each monochromatic light source will mix together, generating a' white light, during the double diffusion process.

Claims

1. A liquid crystal display (LCD)- backlight apparatus, comprising: a light guide plate (21) having a first and a second main surfaces (23, 30) and a first lateral side; a reflection plate (28) facing the first main surface (23) ; a first light source (24) arranged laterally to the first lateral side of the light guide plate (21) , configured to emit a luminous radiation (Ll, L2, L3) ; characterized by further comprising a transparent diffusion pattern (22) extending on the first main surface (23) and configured to receive the emitted luminous radiation (Ll, L2, L3) , to generate a first diffused luminous radiation (dl, d2, d3) toward the reflection plate (28) , to collect reflected luminous radiation (rl, r2 , r3) from the reflection plate (28) and to generate a second diffused luminous radiation (ul-u9) toward the second main surface (30) .
2. The LCD backlight apparatus according to claim 1, further comprising a second light source (25) facing the second lateral side of the light guide plate (21) , opposite and parallel to the first lateral side.
3. The LCD backlight apparatus according to claim 1 or 2, further comprising a third light source (26) , facing a third side of the light guide plate (21) , perpendicularly to the first lateral side.
4. The LCD backlight apparatus according to claim 3, further comprising a fourth light source (41), facing a fourth lateral side of the light guide plate 21, opposite and parallel to the third lateral side.
5. The LCD backlight apparatus according to claim 3 or -4, further comprising a night vision filter (27) arranged between the third lateral side and the third light source (26) .
6. The LCD backlight apparatus according to claim 5, further comprising a color-correction film (38) arranged between the third lateral side and the night vision filter (27) .
7. The LCD backlight apparatus according to any of the preceding claims, wherein the diffusion pattern (22) comprises a plurality of diffusion elements (36) arranged in a matrix and separated from one another by engraving grooves (35) .
8. The LCD backlight apparatus according to claim 7, wherein depth (h) of the engraving grooves (35) increases and lateral spacing (v) between the engraving grooves (35) decreases moving away from the first light source (24) .
9. The LCD backlight apparatus according to claim 7 when depending on claim 2, wherein depth (h) of the engraving grooves (35) increases and lateral spacing (v) between the engraving grooves (35) decreases moving away from the first and the second light sources (24, 25), thereby the lateral spacing (v) is minimum and the depth (h) is maximum toward a first center line of the diffusion pattern (22) .
10. The LCD backlight apparatus according to any of claims 7-9, wherein depth (h) of the engraving grooves (35) increases and lateral spacing (v) between the engraving grooves (35) decreases moving away from the third light source (26) .
11. The LCD backlight apparatus according to claim 7 when depending on claim 4, wherein depth (h) of the engraving grooves (35) increases and lateral spacing (v) between the engraving grooves (35) decreases moving away from the third and the fourth light sources (26, 41), thereby the lateral spacing (v) is minimum and the depth (h) is maximum toward a second center line of the diffusion pattern (22) .
12. The LCD backlight apparatus according to any of claims 7-11, wherein each engraving groove (35) has, in lateral view, a shape selected among triangular, trapezoidal, rectangular.
13. The LCD backlight apparatus according to any of the preceding claims, further comprising a light shaping plate (29) facing the second main surface (30) of the light guide plate (21) .
14. The LCD backlight apparatus according to any of the preceding claims, wherein the light guide plate (21) is made of methacrylate, acrylic glass, or optically clear, temperature-stable polymers.
15. A method for generating white uniform light by virtue of a liquid crystal display (LCD) backlight apparatus comprising: receiving a luminous radiation (Ll, L2 , L3) on a lateral side of a light guide plate (21) ; generating, by a first main surface of the light guide plate, a first diffused luminous radiation (dl, d2 , d3 ) from the emitted luminous radiation; collecting a reflected luminous radiation by the first main surface; generating, by the first main surface-, a second diffused luminous radiation (til) ; and emitting the second diffused radiation from a second main surface of the light guide plate.
16. Use of the LCD backlight apparatus according to any of claims 1-14 in a civil and/or avionic environment as a white uniform light generator.
EP07870585A 2007-12-28 2007-12-28 Illumination apparatus using a light guide plate with a plurality of engraved grooves Ceased EP2235571A1 (en)

Applications Claiming Priority (1)

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PCT/IT2007/000929 WO2009084052A1 (en) 2007-12-28 2007-12-28 Illumination apparatus using a light guide plate with a plurality of engraved grooves

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US8702292B2 (en) 2010-09-22 2014-04-22 Terralux, Inc. Linear illumination devices having light guides and LED-based illumination modules
CN108983447B (en) * 2017-06-02 2021-07-20 群创光电股份有限公司 Display device
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