JPH10233113A - Surface light source device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve emission efficiency without increasing device thickness and cost. SOLUTION: In a surface light source device, in which a reflector-equipped light emitter 20 is placed on the edge of a light guide body 21, a cholesteric film 23 and a λ/4 plate 24 are interposed between the light emitter and the edge 21 a. Light reflected by the cholesteric film is reflected by the reflecting plate of the light emitter 20, and this reflected light becomes counterclockwise circularly polarized light, so that it is transmitted through the cholesteric film 23 without hindrance. Therefore, because light going out of the light emitter 20 is converted into linearly polarized light by the λ/4 plate 24 and introduced into the light conductor 21 without losses, emission efficiency is improved. Also, because no extra laminate exists on the light conductor 21, the overall thickness is reduced to contribute to thinning. Further, because the sizes of the cholesteric film 23 and the λ/4 plate 24 can be as small as the size of the edge surface of the light conductor 21, a great cost reduction can be achieved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a surface light source device, and more particularly, to a surface light source device suitable for use in a backlight system of a liquid crystal display. In general, backlight systems for liquid crystal displays are required to have high luminous efficiency, low power consumption, thinness and light weight, but especially for portable liquid crystal display applied equipment, In many cases, a surface light source device using a cold cathode fluorescent lamp as a light emitter and a light guide system (also called an edge light system) as an illumination optical system is used.

[0002]

2. Description of the Related Art FIG. 3 shows an example of a conventional surface light source device. 1
Is a laminated light guide formed by sandwiching an acrylic resin or the like between the diffusion plate 1a and the reflection plate 1b; 2 is a cold cathode fluorescent lamp arranged along the edge of the light guide 1; It is a panel. Light emitted from the fluorescent lamp 2 is guided toward the diffusion plate 1a by multiple reflection of light on the inner surface and the inner surface of the light guide 1, and uniformly irradiates the lower surface of the liquid crystal panel 3.

The cold cathode fluorescent lamp 2 consumes less power than a hot cathode fluorescent lamp requiring a heater, and the light guide type (edge light type) is a direct type or a reflector type. This is particularly advantageous for portable liquid crystal display-applied devices because it can be made thinner than (a type that collects and reflects the light of a fluorescent lamp arranged on the back side of a diffusion plate with a concave mirror and reflects it). Each of the light guide methods has a disadvantage that the luminous efficiency is inferior.

Therefore, as a device for transmitting light emitted from the fluorescent lamp 2 to the liquid crystal panel 3 without waste, a device having a structure shown in FIG. 4 has been proposed (SID96 A4.3 High Perfor- mation).
mance Wide-Bandwidth Reflective Cholesteric Polari
zers, Coates et al.). 4, reference numeral 10 denotes a fluorescent lamp, 11 denotes a light guide, 12 denotes a reflection plate provided on the lower surface of the light guide 11, and 13 denotes a film provided on the upper surface of the light guide 11 (a counterclockwise circularly polarized light). A film having the property of transmitting and reflecting clockwise circularly polarized light: a film made of cholesteric liquid crystal, commonly called a cholesteric film), 14
Is λ / provided on the cholesteric film 13.
4 plates (having the property of converting circularly polarized light into linearly polarized light and extracting it).

According to this, of the light A emitted from the light guide 11, the left-handed circularly polarized light B is transmitted through the cholesteric film 13 as it is, and the right-handed circularly polarized light C is transmitted through the cholesteric film.
Reflector 1 reflected on film 13 on the lower surface of light guide 11
As a result of being re-reflected at 2, the light becomes counterclockwise, that is, counterclockwise circularly polarized light D, and passes through the cholesteric film 13. Therefore, theoretically, all light can be converted into linearly polarized light by the λ / 4 plate 14, and the luminous efficiency can be improved.

[0006]

However, in such a conventional surface light source device, the cholesteric film 13 and the λ / 4 plate 14 are stacked on the upper surface of the light guide 11, so that the thickness of the device is reduced. (Note 1), and the cholesteric film 13 and the λ / 4 plate 14 having a large size (same size as the light guide 11) are required, which leads to an increase in cost. Note 1: The thickness of the cholesteric film 13 is about 1.5 m
The thickness of the m, .lambda. / 4 plate 14 is about 0.5 mm, which is only about 2 mm at most, but a recent thin design can only be realized by patiently collecting these small numerical values. It is a thickness that cannot be ignored even though it is about 2 mm.

Accordingly, an object of the present invention is to improve the luminous efficiency without increasing the thickness of the device and without increasing the cost.

[0008]

According to the present invention, there is provided a surface light source device in which a light-emitting member having a reflector is arranged at an edge of a light guide.
A cholesteric film and a λ / 4 plate are interposed between the luminous body and the edge, or a λ / 4 plate and natural light are provided between the luminous body and the edge. An optical medium having the property of reflecting linearly polarized light in a predetermined direction and transmitting polarized light perpendicular to the predetermined direction is interposed.

In the present invention, (a) the light reflected by the cholesteric film (or the optical medium) is reflected by the reflector of the luminous body, and the reflected light is reversely circularly polarized light (or orthogonal to a predetermined direction). (Polarized light) and therefore pass through the cholesteric film (or optical medium) without hindrance.
Therefore, the light emitted from the light emitter is converted to linearly polarized light by the λ / 4 plate without waste and introduced into the light guide, so that the luminous efficiency is improved. (B) Since there is no unnecessary laminated body on the light guide, the overall thickness is suppressed, which contributes to a thin design. Further, (c) the size of the cholesteric film (or the optical medium) and the size of the λ / 4 plate need only be the size of the edge surface of the light guide.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a view showing one embodiment of the surface light source device according to the present invention. In FIG. 1, reference numeral 20 denotes a fluorescent lamp as a light-emitting body. The fluorescent lamp 20 may have any shape as long as it is not a flat plate type (straight tube type, U-shaped, W-type, etc.). Considering application to applied equipment, a straight-tube cold-cathode type would be desirable. Also, needless to say, a reflector for condensing light is essential. In addition, “○” of the fluorescent lamp 20 represents a tube cross section,
“<” In the above indicates the light extraction direction.

Reference numeral 21 denotes a light guide. The light guide 21 is formed by attaching a reflection plate (omitted) to the lower surface of a transparent plate (single plate or laminated plate) made of acrylic resin or the like, and uses the multiple reflection of light to reduce the light introduced from the edge 21a. It is taken out from one surface (the surface opposite to the mounting surface of the reflection plate: the upper surface in the figure). Generally, in order to increase the uniformity of the extracted light (plane light), a reflection pattern is printed on the interface with the reflection plate, and further, in order to align the optical axis of the plane light, a large number of lenses 22 ( Or prisms).

The feature of this embodiment is that the fluorescent lamp 2
0 and edge 21a. That is, the fluorescent lamp 2
It is characterized in that a cholesteric film 23 and a λ / 4 plate 24 are interposed in order from the fluorescent lamp 20 side between the 0 and the edge 21a. The cholesteric film 23 has a property of transmitting left-handed circularly polarized light and reflecting right-handed circularly polarized light, as described in the description of the prior art (FIG. 4). It has the property of converting circularly polarized light into linearly polarized light and extracting it.

The cholesteric film 23 has
Reveo or Merck products can be used, and the λ / 4 plate 24 is manufactured by Nitto Denko Corporation (for example, with a film thickness of 140 nm).
NRZ film) can be used.
d Signal products (film products) can be used. In such a configuration, the left-handed circularly polarized light of the light emitted from the fluorescent lamp 20 is cholesteric film 2
3, the right-handed circularly polarized light is reflected by the film 23 and re-reflected by the reflector of the fluorescent lamp 20,
The light becomes counterclockwise circularly polarized light and passes through the cholesteric film 23. Therefore, since most of the light that has exited the fluorescent lamp 20 reaches the λ / 4 plate 24, the loss of light introduced into the edge 21a of the light guide 21 can be limited, and the light emission can be limited as in the prior art (FIG. 4). Efficiency can be improved.

Moreover, in this embodiment, cholesteric
Since the film 23 and the λ / 4 plate 24 are interposed between the fluorescent lamp 20 and the edge 21a, the thickness of the device can be reduced, and the cholesteric film 23 and the λ / 4 plate 2 can be reduced.
4 is only required to be about the area of the edge 21a, and a significant cost reduction can be achieved.

FIG. 2 is a view showing another embodiment of the surface light source device according to the present invention. Elements common to those in FIG. 1 are denoted by the same reference numerals and description thereof is omitted. In FIG. 2, reference numeral 30 denotes an optical medium. This optical medium 30
It is a medium having the property of reflecting linearly polarized light in a predetermined direction of natural light and transmitting polarized light orthogonal to the predetermined direction. For example, a DBEF film manufactured by Sumitomo 3M Limited can be used.

The characteristic configuration of this embodiment is also between the fluorescent lamp 20 and the edge 21a, as in the above-described embodiment, but in this embodiment, the λ / 4 plate 24 is sequentially arranged from the fluorescent lamp 20 side.
And that the optical medium 30 is interposed. That is, cholesteric films are not used. In such a configuration, the light transmitted through the optical medium 30 is only polarized light perpendicular to the linearly polarized light in a predetermined direction, and the linearly polarized light in the predetermined direction is reflected by the optical medium 30. After being re-reflected by the reflector of the lamp 20, the light becomes perpendicularly polarized light and reaches the optical medium 30 again, so that most of the light emitted from the fluorescent lamp 20 can be introduced to the edge 21a as in the above-described embodiment. Light emission efficiency can be improved by limiting light loss. Moreover, also in this embodiment, the thickness of the apparatus can be reduced, and the required size of the λ / 4 plate 24 and the optical medium 30 can be reduced to about the area of the edge 21a, so that the cost can be significantly reduced.

[0017]

According to the present invention, the luminous efficiency can be improved, the overall thickness can be suppressed, and a thin design can be achieved, and the required size of the cholesteric film (or optical medium) and the λ / 4 plate can be reduced. By reducing the size, a significant cost reduction can be achieved. Therefore, it is possible to provide a technique that is useful when applied to a backlight system for a liquid crystal display, particularly, a portable liquid crystal display application device.

[Brief description of the drawings]

FIG. 1 is a schematic sectional structural view of one embodiment.

FIG. 2 is a schematic sectional structural view of another embodiment.

FIG. 3 is a schematic sectional structural view of a conventional example.

FIG. 4 is a schematic sectional structural view of an improved conventional example.

[Explanation of symbols]

 20: Fluorescent lamp (luminous body) 21: Light guide 21a: Edge 23: Cholesteric film 24: λ / 4 plate 30: Optical medium

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yukio Iigahama 4-1-1, Kamidadanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Takeshi Takeshi 4 Ueodanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Chome 1-1 Fujitsu Limited

Claims (2)

[Claims] 1. A surface light source device in which a light-emitting body with a reflector is arranged at an edge of a light guide, wherein a cholesteric film and a λ / 4 plate are interposed between the light-emitting body and the edge. Characteristic surface light source device. 2. A surface light source device in which a luminous body with a reflector is disposed at an edge of a light guide, wherein between the luminous body and the edge, a λ / 4 plate and linearly polarized light in a predetermined direction out of natural light. A surface light source device comprising an optical medium having a property of reflecting and transmitting polarized light orthogonal to the predetermined direction.