Patent Application
20110141395
The present invention relates to a backlight unit that supplies light to a liquid crystal display panel or the like, and also relates to a liquid crystal display device that incorporates such a backlight unit.
Conventionally, liquid crystal display devices incorporating a non-luminous liquid crystal display panel also incorporate a backlight unit that supplies light to the liquid crystal display panel. Such a backlight unit is expected to shine light as perpendicularly as possible into the liquid crystal display panel. The reason is that if too much light shines obliquely into the liquid crystal display panel, diminished or uneven brightness may result.
Typically, light from a light source is introduced into a single, plate-shaped light guide plate through an edge face thereof so that the light undergoes multiple reflection inside so as to eventually exit from the light guide plate through a top face thereof. In this case, inconveniently, it is difficult to make the light exit perpendicularly to the top face. Accordingly, it is difficult to make the light enter perpendicularly the liquid crystal display panel, which is disposed to cover the top face.
One modern solution is to use a light guide plate 111 that, as shown in
As a result, as shown in
To prevent backlight from being split in that way, in the backlight unit disclosed in Patent Document 1, as shown in
Specifically, as shown in
Backlight produced in this way by mixing together light in oppositely dispersed states is less unnecessarily colored than backlight obtained from a light guide plate 111 including a diffraction grating dg with no special measure taken.
Patent Document 1: JP-2006-120521 (paragraphs [0030], [0031]; FIG. 3)
Disadvantageously, a closer study on the backlight emitted from the backlight unit disclosed in Patent Document 1 reveals the following: as shown in
That is, the backlight from the backlight unit disclosed in Patent Document 1 contains violet- and green-tinged light, and thus cannot be said to be light with a satisfactorily high degree of whiteness.
The present invention has been made against this background, and an object of the invention is to provide a backlight unit that, even when comprising a light guide plate including a diffraction grating, produces light with a comparatively high degree of whiteness, and to provide a liquid crystal display device incorporating such a backlight unit.
According to one aspect of the invention, a backlight unit includes: a light source; and a light guide plate receiving light from the light source and making the light exit by subjecting the light to multiple reflection. The face of the light guide plate through which the light guide plate receives the light is called the light-receiving face, the face of the light guide plate through which the light exits is called the light-exit face, and the face of the light guide plate opposite from the light-exit face is called the bottom face.
On the light-exit face, a diffraction grating is formed that includes at least three grating ridge groups having grating ridges arranged with different periods respectively, and the three grating ridge groups correspond to light in different wavelength bands respectively. Moreover, the grating ridge groups diffraction-reflect, out of light in corresponding particular wavelength bands, only light incident thereon at incidence angles within a particular range such that the light returns to the side from which the light propagates. On the other hand, on the bottom face, a refractive optical element is formed that reflects toward the light-exit face the light thus diffraction-reflected so as to return.
With this structure, the three grating ridge groups so act that part of the light that has not been totally reflected on the light-exit face, that is, the light reaching them in corresponding particular wavelength bands at incidence angles within a particular range is diffraction-reflected in a particular direction (in such a way that the light returns to the side from which it propagates). Thus, the diffraction-reflected light in specific wavelength bands propagates while keeping comparatively high directivity; in addition, since the directivity here is uniform, the light mixes to a comparatively high degree.
Accordingly, when the light diffraction-reflected here is, for example, light in wavelength bands corresponding to the three primary colors of light, the mixed light is high-quality white light. To achieve that, it is preferable that, of the three grating ridge groups, one be a blue-light grating ridge group corresponding to a wavelength band of blue light, one be a green-light grating ridge group corresponding to a wavelength band of green light, and one be a red-light grating ridge group corresponding to a wavelength band of red light.
In addition, when diffraction-reflected light of different colors is reflected by the refractive optical element, for example, perpendicularly to the light-exit face, the light reaching the light-exit face then continues to exit perpendicularly to the light-exit face. This increase in the amount of light traveling perpendicularly to the light-exit face of the light guide plate eliminates the need for the backlight unit to include a lens sheet for condensing light.
It is preferable that the blue-, green-, and red-light grating ridge groups fulfill equation (M1) below:
d=λ/(2·nd.sin θ) Equation (M1)
where
It is preferable that the grating ridges have a height of 500 nm or more but 1000 nm or less.
It is preferable that, in addition, equations (C1) and (C2) below be fulfilled:
γ=θ±Δ Equation (C1)
γ+2·δA+2·δB=180° Equation (C2)
where
To maximize the amount of light exiting perpendicularly to the light-exit face, it is preferable that the backlight unit fulfill equation (C3) below:
δA<5° Condition (C3)
According to another aspect of the invention, a liquid crystal display device includes: a backlight unit as described above; and a liquid crystal display panel receiving light from the backlight unit.
According to the present invention, it is possible, by use of a diffraction grating formed on the light-exit face of a light guide plate and a refractive optical element formed on the bottom face of the light guide plate, to make high-quality white light exit perpendicularly to the light-exit face.
An embodiment of the present invention will be described below with reference to the accompanying drawings. For convenience' sake, hatching, reference signs, etc. do not necessarily appear in all relevant drawings, in which case reference is to be made to those drawings in which they appear. A solid black dot in a drawing indicates the direction perpendicular to the plane of the paper.
The liquid crystal display panel 59 is composed of an active matrix substrate 51, which includes switching elements such as TFTs (thin-film transistors), and a counter substrate 52, which faces the active matrix substrate 51, stuck together by a sealing member (not shown). The gap between the two substrates 51 and 52 is filled with liquid crystal (not shown). (The active matrix substrate 51 and the counter substrate 52 are sandwiched between polarizing films 53 and 53.)
The liquid crystal display panel 59 is of a non-luminous type, and achieves display by receiving light (backlight) from the backlight unit 49. Accordingly, illuminating the entire surface of the liquid crystal display panel 59 evenly with the light from the backlight unit 49 contributes to enhanced display quality on the liquid crystal display panel 59.
The backlight unit 49 includes an LED module (light source module) MJ, a light guide plate 11, and a reflective sheet 42.
The LED module MJ is a module that emits light; it includes a mount substrate 21 and an LED (light-emitting diode) 22, the latter being mounted on electrodes formed on a mounting surface of the former to receive electric current to emit light.
Preferably, to secure a necessary amount of light, the LED module MJ comprises a plurality of LEDs (point light sources) 22 as light-emitting elements. Preferably, these LEDs 22 are disposed in a row. For convenience' sake, only part of the LEDs 22 are shown in the drawing (in the following description, the direction of the row of the LEDs 22 is also referred to as J direction).
The light guide plate 11 is a plate-shaped member having edge faces 11S, a top face 11U, and a bottom face 11B, the latter two being so located as to sandwich the former. Of all the edge faces 11S, one (light-receiving face 11Sa) faces the light-emission face of the LED 22 to receive light therefrom. The light received undergoes multiple reflection inside the light guide plate 11 and eventually travels out of it, as planar light, through the top face (light-exit face) 11U. In the following description, the edge face 115 opposite from the light-receiving face 11Sa is referred to as the opposite face 11Sb, and the direction pointing from the light-receiving face 11Sa to the opposite face 11Sb is referred to as K direction (the light guide plate 11 will be described in more detail later).
The reflective sheet 42 is so located as to be covered by the light guide plate 11. The face of the reflective sheet 42 facing the bottom face 11B of the light guide plate 11 is a reflective surface. This reflective surface reflects the light from the LED 22 and the light propagating inside the light guide plate 11 back into the light guide plate 11 (through the bottom face 11B of the light guide plate 11) without letting it leak out.
In the backlight unit 49 described above, the reflective sheet 42 and the light guide plate 11 are stacked in this order (the direction in which they are stacked is referred to as L direction; it is preferable that J, K, and L directions be perpendicular to one another). The light from the LED 22 is turned by the light guide plate 11 into, and emanates therefrom as, planar light (backlight). The planar light reaches the liquid crystal display panel 59, and permits it to display an image.
Now, the light guide plate 11 in the backlight unit 49 will be described in detail with reference to
As shown in
n2·sin θ2=n1·sin θ1+m·λ/d (M0)
where
For a case where the incidence and emergence sides with respect to the top face 11U are both the light guide plate 11, equation (M0) can be given as equation (M0′) below.
n1·sin θ2=n1·sin θ1+m·λ/d (M0′)
Specifically, the diffraction grating DG so designed has, as shown in
For example, in a case where the light guide plate 11 is formed of polycarbonate (with a refractive index nd of 1.59), the distance from the base to the tip of the grating ridges 13, that is, the height (H) of the grating ridges 13, is 300 nm, and these grating ridges 13 are arranged with three different periods d (dB, dG, and dR=170 nm, 200 nm, and 230 nm respectively). The grating ridges 13 arranged with each period d (dB, dG, and dR) are densely located to form a grating ridge group 13gr (13gr.B, 13gr.G, and 13gr.R respectively), and a group of grating ridge groups 13gr.B, 13gr.G, and 13gr.R having grating ridges arranged with different periods forms one patch PH (see
When light comprising blue light (with a wavelength of about 470 nm), green light (with a wavelength of about 550 nm), and red light (with a wavelength of about 620 nm) is incident, at an incidence angle (θ1) of about 60°, on the top face 11U of the diffraction grating DG, where a number of such patches PH are arranged, the light is diffraction-reflected on the diffraction grating DG to become diffraction-reflected light having a reflection angle (θ2) equal to the incidence angle, that is, about 60°. Here, the diffraction-reflected light propagates in such a way as to return to the side from which the incident light propagates toward the diffraction grating DG. That is, the diffraction grating DG diffraction-reflects part of the light reaching it (light incident thereon at incidence angles within a particular range) in such a way as to return it to the side from which it propagates.
The results of the diffraction-reflection are shown in
From the above-discussed results shown in
nd=1.59 Condition (A1)
dB=170 nm Condition (A2)
dG=200 nm Condition (A3)
dR=230 nm Condition (A4)
H=300 nm Condition (A5)
where
In this way, the diffraction grating DG diffraction-reflects, into diffraction-reflected light of order −1, light (blue, green, and red light) in particular wavelength bands corresponding to the periods of the grating ridges 13 of the diffraction grating DG itself, and makes the diffraction-reflected light of different colors propagate all in the same direction. This makes it easy to mix blue, green, and red light. That is, blue, green, and red light with uniform directivity is mixed to produce high-quality white light.
The reflection angle of the light incident on the diffraction grating DG, which has been mentioned to be about 60°, is, in more specific numerical examples, 60°, 55°, and 65°, for instance. When light incident at these incidence angles is reflected as diffraction-reflected light of order −1, the reflection angle is as follows: for an incidence angle of 60°, a reflection angle of −60°; for an incidence angle of 55°, a reflection angle of −65.56°; and for an incidence angle of 65°, a reflection angle of −55.41°.
The phenomenon described above can be summarized as follows: diffraction efficiency is high when diffraction-reflected light of order −1 is reflected in the direction (reflection angle) opposite from the direction (incidence angle) from which the source light is incident on the diffraction grating GS. Accordingly, in equation (M0′), the following substitutions are possible: θ1=−θ2=θ (θ will be described later); and m=−1. Thus, equation (M1) below is derived.
Moreover, the grating periods (nm) of the grating ridges 13 that diffract light in the grating ridge groups 13gr.B, 13gr.G, and 13gr.R are about half the wavelengths of visible light in the corresponding wavelength bands. Moreover, the height (H) of the grating ridges 13 is determined based on its correlation with the diffraction efficiency found by an RCWA (rigorous coupled wave analysis) method (the height of the grating ridges 13 is typically 50 nm or more but 1000 nm or less).
d=λ/(2·nd·sin θ) Equation (M1)
where
As shown in
This diffraction-reflected light of order −1 (the light diffraction-reflected backward on the diffraction grating DG) then needs to be directed to the top face 11U, and for this purpose a prism 15 (refractive optical element) is formed on the bottom face 11B of the light guide plate 11. The prism 15 is a triangular prism; as shown in
Of these two prism faces, the one closer to the opposite face 11Sb of the light guide plate 11 (farther away from the LED 22), that is, the front prism face 15Sf, is so located as to receive the diffraction-reflected light of order −1 from the diffraction grating DG. Moreover, the front prism face 15Sf is so inclined as to reflect the received diffraction-reflected light of order −1 toward the rear prism face 15Sr, that is, the other of the two prism faces which is closer to the light-receiving face 11Sa of the light guide plate 11 (closer to the LED 22).
The rear prism face 15Sr is so located as to receive the diffraction-reflected light of order −1 from the front prism face 15Sf. Moreover, the rear prism face 15Sr is so inclined as to reflect the received diffraction-reflected light of order −1 toward the top face 11U.
Preferably, the rear prism face 15Sr is so inclined as to reflect the diffraction-reflected light of order −1 perpendicularly to the top face 11U. To achieve that, it is preferable that the prism 15 be formed so as to fulfill equations (C1) and (C2) below.
γ=θ±Δ Equation (C1)
γ+2·δA+2·δB=180° Equation (C2)
where
These equations (C1) and (C2) will now be described with reference to an enlarged sectional view in
The diffraction-reflected light of order −1 traveling toward the prism 15 has a reflection angle of “γ.” Consider a first imaginary triangle which has a first side along the diffraction-reflected light of order −1 until reaching the prism 15, a second side along a line N normal to the bottom face 11B (and the top face 11U), and a third side along a first extension plane E1 which is an extension of the bottom face 11B into the prism 15. The first imaginary triangle then has angles of “γ” and 90°. The third angle thus equals “90°−γ.” This third angle is vertically opposite to the angle formed between the first extension plane E1 and the diffraction-reflected light of order −1. Thus, the angle formed between the first extension plane E1 and the diffraction-reflected light of order −1 also equals “90°−γ.”
Consider a second imaginary triangle which has a first side along the front prism face 15Sf, a second side along the diffraction-reflected light of order −1 traveling toward the front prism face 15Sf, and a third side along the first extension plane E1. In this second imaginary triangle, the angle formed between the front prism face 15Sf and the diffraction-reflected light of order −1 equals “δA” subtracted from the angle formed between the first extension plane E1 and the diffraction-reflected light of order −1, namely “90°−γ” (that is, “90°−γ−δA”).
Assume that the diffraction-reflected light of order −1 incident on the front prism face 15Sf is totally reflected, and consider a third imaginary triangle which has a first side along the totally reflected diffraction-reflected light of order −1, a second side along the front prism face 15Sf, and a third side along the rear prism face 15Sb. In this third imaginary triangle, the angle formed between the totally reflected diffraction-reflected light of order −1 and the front prism face 15Sf also equals “90°−γ−δA.”
Moreover, in the third imaginary triangle, the angle formed between the front prism face 15Sf and the rear prism face 15Sb equals, as dictated by the shape of the triangular prism, “180°−(δA+δB).” Then, the third angle in the third imaginary triangle, that is, the angle formed between the totally reflected diffraction-reflected light of order −1 and the rear prism face 15Sb, equals “γ+2·δA+δB−90°.”
When the diffraction-reflected light of order −1 propagating from the front prism face 15Sf is totally reflected on the rear prism face 15Sb, the angle formed between the diffraction-reflected light of order −1 that has thus been totally reflected for the second time and the rear prism face 15Sb also equals “γ+2·δA+δB−90°.” Moreover, of the angles formed between a second extension plane E2 which is an extension from the rear prism face 15Sb and the bottom face 11B, the one vertically opposite to the angle “δB” in the prism 15 equals “δB.”
Then, the sum of the angle formed between the second extension plane E2 and the bottom face 11B and the angle formed between the diffraction-reflected light of order −1 that has been totally reflected for the second time and the rear prism face 15Sb (“γ+2·δA+2·δB−90°”) is the reflection angle of the diffraction-reflected light of order −1 that has been totally reflected for the second time with respect to the bottom face 11B (hence the top face 11U). Accordingly, when this sum “γ+2·δA+2·δB−90°” equals 90°, the diffraction-reflected light of order −1 from the diffraction grating DG exits perpendicularly to the top face 11U.
That is, when the prism 15 is designed to fulfill equation (C2), “γ+2·δA+2·δB=180°,” derived from “γ+2·δA+2·δB−90°=90°,” the diffraction-reflected light of order −1 from the diffraction grating DG exits perpendicularly to the top face 11U.
With this structure, the diffraction-reflected light of order −1, containing blue, green, and red light, from the diffraction grating DG reaches the prism 15 in a state mixed to a comparatively high degree, and is then guided by the prism 15 to travel and exit perpendicularly to the top face 11U. Thus, the backlight unit 49 no longer requires a lens sheet for condensing light, and this helps reduce cost.
In a specific numerical example of the prism 15, the relevant parameters have the following values:
δA=4°;
δB=58.5°;
F=10 μm
where
If the angle δA is equal to or greater than 5°, part of the diffraction-reflected light of order −1 that propagates in such a way as to return toward the prism 15, in particular light having comparatively small reflection angles (θ2), is less likely, after being reflected on the front prism face 15Sf, to travel toward the rear prism face 15Sb. Rather, light reaching the front prism face 15Sf at comparatively small reflection angles (θ2) is reflected to travel, not toward the rear prism face 15Sb, but toward the bottom face 11B.
An increase in the amount of such light results in a decrease in the amount of light reaching the rear prism face 15Sb, and hence a decrease in the amount of light exiting upright through the top face 11U. For this reason, it is preferable that condition (C3) below be fulfilled.
δA<5° Condition (C3)
Even if part of the diffraction-reflected light of order −1 happens to be transmitted through the prism 15, it is reflected by the reflective sheet 42 back to the bottom face 11B of the light guide plate 11.
It should be understood that the present invention may be carried out in any other manners than specifically described by way of an embodiment above and allows for many modifications and variations without departing from the spirit of the invention.
For example, although the foregoing description mentions, as an example of the material of the light guide plate 11, polycarbonate fulfilling conditions (A1) to (A5) and equation (M1) noted above, this is not meant to be any limitation. The light guide plate 11 may instead be formed of, for example, silicone resin. Even in that case, in particular when the light guide plate 11 fulfills conditions (B1) to (B5) below, it permits light to behave as shown in
nd=1.3 Condition (B1)
dB=210 nm Condition (B2)
dG=245 nm Condition (B3)
dR=270 nm Condition (B4)
H=300 nm Condition (B5)
Also with this light guide plate 11 formed of silicone resin, the grating ridge groups 13gr.B, 13gr.G, and 13gr.R so act that the light reaching them in corresponding particular wavelength bands at incidence angles within a particular range (about 60°) is diffraction-reflected in a particular direction, that is, at a reflection angle of about 60° (in such a way that the light returns to the side from which it propagates).
Thus, the diffraction-reflected light in specific wavelength bands propagates while keeping comparatively high directivity; in addition, since the directivity here is uniform, the light mixes to a comparatively high degree. Accordingly, when the light diffraction-reflected here is light in wavelength bands corresponding to the three primary colors of light, the mixed light is high-quality white light. In this way, the same effect is obtained as with the light guide plate 11 of Embodiment 1 which is formed of polycarbonate and includes the diffraction grating DG; that is, high-quality white light is produced.
Also with this light guide plate 11 formed of silicone resin, a specific numerical example in which the incidence angle of light incident on the diffraction grating DG is about 60° is similar to one involving the light guide plate 11 of polycarbonate. Specifically, when the incidence angle of light incident on the diffraction grating DG is 60°, the reflection angle of the diffraction-reflected light of order −1 is −60°; when the incidence angle is 55°, the reflection angle is −65.56°; and when the incidence angle is 65°, the reflection angle is −55.41°.
Also with this light guide plate 11 formed of silicone resin, when equations (C1) and (C2) are fulfilled, the diffraction-reflected light of order −1 from the diffraction grating DG exits perpendicularly to the top face 11U. Thus, the diffraction-reflected light of order −1, containing blue, green, and red light, from the diffraction grating DG reaches the prism 15 in a state mixed to a comparatively high degree, and is then guided by the prism 15 to travel and exit perpendicularly to the top face 11U.
In this way, as a result of diffraction-reflected light of different colors being reflected by the prism 15 so as to travel perpendicularly to the top face 11U, the light exiting from the light guide plate 11 has a directivity perpendicular to the light guide plate 11. Thus, even when incorporating such a light guide plate 11 formed of silicone, the backlight unit 49 does not require a lens sheet for condensing light, and this helps reduce cost.
In summary, the light guide plate 11 has, formed on its top face 11U, the diffraction grating DG which returns the light reaching the face to the side from which the light propagates; moreover the light guide plate 11 has, formed on its bottom face 11B, the prism 15 which reflects the thus backward diffraction-reflected light toward the top face 11U. So long as these requirements are met, no specific conditions matter.
Accordingly, there are no particular limitations on the refractive indices of the materials of the light guide plate 11, the diffraction grating DG, and the prism 15, and the grating ridges 13 may be, instead of in the shape of parallelepipeds, cylindrical, conical, etc. The grating periods of the grating ridges 13 may be other than about half the wavelengths of visible light in specific wavelength bands. Needless to say, the height of the grating ridges 13 is not limited to 300 nm, which is mentioned above as a mere example.
In a specific numerical example of the above-described prism 15 formed of silicone resin, the relevant parameters have the following values:
δA=3°;
δB=59.5°;
F=10 μm.
It is here preferable that, instead of condition (C3) noted previously, condition (C4) below be fulfilled. Fulfilling this condition (C4) gives an effect similar to that obtained by fulfilling condition (C3).
δA<4° Condition (C4)
From the numerical examples of the prism 15 formed of polycarbonate and that formed of silicone resin, equation (C5) below is also derived. Specifically, when this condition (C5) holds, the prism 15 reflects the diffraction-reflected light of order −1 propagating from the diffraction grating DG such that it exits perpendicularly to the top face 11U.
δA+δB=62.5° Condition (C5)
Although the above description takes up an LED 22 as a light source, this is not meant to be any limitation. Instead, it is possible to use a linear light source such as a fluorescent lamp, or a light source based on a self-luminous material such as one producing organic or inorganic EL (electro-luminescence).
Although the above description deals with a case where the diffraction grating DG includes three grating ridge groups 13gr, it may instead include more grating ridge groups 13gr. In a case where white light is produced by mixing light in four or more specific wavelength bands, the diffraction grating DG may include four or more grating ridge groups 13gr.
Although the above description takes up a prism 15 as an optical element for guiding the diffraction-reflected light of order −1 to the top face 11U, this is not meant to be any limitation. Instead, it is possible to use a mirror.
11 Light guide plate
11B Bottom face of the light guide plate
11U Top face of the light guide plate (light-exit face)
11S Side face of the light guide plate
11Sa Light-receiving face of the light guide plate
11Sb Side face of the light guide plate opposite from the light-receiving face, that is, opposite face
13 Grating ridges
13
gr.B Grating ridge group corresponding to blue light (blue-light grating ridge group)
13
gr.G Grating ridge group corresponding to green light (green-light grating ridge group)
13
gr.R Grating ridge group corresponding to red light (red-light grating ridge group)
PH Diffraction grating patch
DG Diffraction grating
15 Prism (refractive optical element)
15S Face of the prism
15Sf Front prism face (prism face farther away from the light source)
15Sr Rear prism face (prism face closer to the light source)
21 Mount substrate
22 LED (light source)
42 Reflective sheet
49 Backlight unit
59 Liquid crystal display panel
69 Liquid crystal display device
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-188249 | Jul 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/058611 | 5/7/2009 | WO | 00 | 1/11/2011 |
