JPH0530280Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial field of application] The present invention relates to light source devices used in various types of office automation equipment, etc., and in particular relates to the improvement of surface light source devices that emit uniform illumination light over a surface. be.

[Conventional technology]

Conventionally, such a surface light source device has been constructed as shown in FIG.

FIG. 2 is a schematic cross-sectional view showing an example of a surface light source device for back illuminating a transmissive liquid crystal display device.

In the figure, a surface light source device 1 includes light sources 2a, 2b, 2c formed by arranging a plurality of cold cathode tubes, for example,
It consists of a light diffusing plate 3 disposed in front of this light source, and in the illustrated case, a liquid crystal cell 4 having a known configuration is disposed in front of the light diffusing plate 3.

In such a surface light source device 2, the light emitted from each light source enters the light diffusing plate 3, is uniformly diffused, and back-illuminates the liquid crystal cell 4 from below. As a result, the pattern displayed by the liquid crystal cell 4 is brought to light through appropriate drive control.

[The problem that the idea aims to solve]

By the way, in the surface light source device 1 having the above configuration, for example, L ₁ of the light emitted from the light source 2a
The optical path of the illumination light to the liquid crystal cell 4 is shorter than that of other illumination lights, and it is not diffused very much, so there is little loss in the amount of light, so it passes through the light diffusion plate 3 and changes the planar shape. When the illumination light reaches the liquid crystal cell 4 after being diffused into a uniform illumination light with a spread of

On the other hand, when looking at L ₂ , which has a relatively long overall optical path, the light passes through a longer optical path than L ₁ before entering the diffuser plate 3, so the light is diffused, and Since the distance is long even when the light is passing through the interior, the light is further diffused and the amount of light decreases significantly, and when it reaches the liquid crystal cell 4, the illuminance is at a low level as shown in the graph.

In this way, regarding the other light sources 2b and 2c,
Since the illumination light emitted from these has a different reduction rate in light intensity depending on the distance of its optical path, as a result, the back illumination by the surface light source device 1 becomes uneven as a whole, and the display accuracy of the liquid crystal cell 4 decreases. do.

The present invention was devised in view of the above-mentioned problems, and aims to provide a surface light source device that can illuminate uniformly with sufficient illuminance over the entire illumination range that spreads in a planar manner. purpose.

[Means to solve the problem]

According to the present invention, the above object is achieved by providing a light source for illumination;
In this surface light source device comprising a light diffusion plate provided in front of the illumination light source, the thickness of the diffusion plate is thick in the area of the light diffusion plate close to the illumination light source, and in the area spaced apart from the light source. This is achieved by using a surface light source device that is formed to have a thin plate.

[Effect]

According to the above configuration, in the area of the light diffusing plate disposed in front of the illumination light source close to the light source,
Since the thickness of the diffuser plate is thick, the optical path of the illumination light emitted from the illumination light source until it enters the light diffuser plate is short, and even if the light is not diffused much during that time, the illumination light will not be diffused. When the light passes through the thick part of the light diffusing plate, it is sufficiently diffused, resulting in uniform illumination light across the surface.

In addition, since the thickness of the light diffusion plate is made thinner in areas away from the illumination light source, the optical path for the illumination light from the light source to enter the light diffusion plate is long. Even if a lot of light is diffused during that time and the amount of light decreases, when passing through the thin part of the light diffusing plate, the diffusion rate will decrease by the shorter distance through the diffusing plate. Therefore, the illuminance will not be lowered more than necessary.

〔Example〕

Next, a preferred embodiment of the present invention will be described in detail with reference to FIG.

FIG. 1 is a cross-sectional view schematically showing an embodiment of a surface light source device according to the present invention, and the upper part of FIG. 1 shows a graph showing the illuminance characteristics of the light source device.

In the figure, a surface light source device 10 includes a plurality of, for example, three, illumination light sources 12a, 12b, and 12c, and a light diffusing plate 13 disposed in front of these light sources.

In this embodiment, the surface light source device 10 includes:
It is used for back illuminating a liquid crystal display device, and for this purpose, a liquid crystal cell 14 having a known structure is arranged in front of the light diffusing plate 13.

In this embodiment, a plurality of cold cathode tubes are used as the illumination light sources 12a, 12b, 12c,
Three of them are arranged at approximately equal intervals with their longitudinal direction along the direction perpendicular to the drawing, and are mounted on a substrate (not shown), and these are housed in the case 15.

The light diffusing plate 13 includes each light source 12a, 12b,
These light sources 12
It is arranged on the front side of a, 12b, and 12c.

The material constituting the light diffusing plate 13 is a known one, such as a plate-shaped diffuser that has a milky white color as a whole called a so-called opalescent plate, or a predetermined synthetic resin material mixed with a fine scattering agent and solidified. Generally, it is possible to use a plate-shaped body, and in rare cases, a smoked plate can be used, and any type of plate in which light is diffused by being diffusely reflected during the process of passing through it can be used.

Here, each light source 12a, 12b of the diffuser plate 13,
The plate thickness of the regions 13a, 13a, 13a close to the diffusion plate 12c is configured to be thicker than the other regions of the diffusion plate 13. In this embodiment, the other regions 13b, 13b of the diffusion plate 13,
13b is cut away to form a stepped portion, so that the illumination light passing therethrough is not diffused more than necessary. It is preferable that the other regions 13b be formed so that the plate thickness gradually becomes thinner as the distance from each light source increases, but even if the other regions 13b are formed as shown in the figure, the It has been confirmed that as a result of most of the reduction in the amount of illumination light, it is possible to obtain substantially uniform illumination light within an allowable range for the use of the surface light source device.

Note that the method for determining these plate thicknesses will be described later.

The present embodiment is configured as described above, and its operation will be explained next.

In FIG. 1, light is emitted from each cold cathode tube 12a which is a light source for illumination, and a light source 12a of a light diffusing plate 13 is illuminated.
The illumination light L ₁ incident on the region 13a close to
The optical path to the light diffusing plate 13 is relatively short as shown. Therefore, the amount of light lost due to diffusion during this period is small, and the light enters the light diffusing plate 13 while maintaining sufficient illuminance. Here the illumination light L ₁
Since the distance through which the illumination light L1 passes through the light diffusing plate 13 is relatively long, the illumination light _L1 is sufficiently diffused in a planar shape. Thus, the illumination light _L1 can illuminate the liquid crystal cell 14 from behind with a high illumination intensity of h level, as shown in the graph of FIG.

On the other hand, the illumination light _L2 emitted from the cold cathode tube 12a and incident on another region 13b of the light diffusing plate 13 that is distant from the light source 12a has a relatively long optical path until it enters the light diffusing plate 13. The amount of light lost due to diffusion while passing through this optical path is
More than L ₁ . However, after entering the light diffusing plate 13,
Since the distance through which the light passes is relatively short, the light does not diffuse more than necessary, so there is no significant loss of light quantity. Therefore, the illumination light L ₂ is also the first
As shown in the graph of the figure, the liquid crystal cell 14 is back-illuminated with high illuminance at the h level, and the same is true for the other light sources 12b and 12c.

In this way, the surface light source device 10 can back-illuminate the liquid crystal cell 14 from below uniformly and with sufficient illuminance, thereby accurately displaying the pattern displayed by the liquid crystal cell 14 through appropriate drive control. It can be made to float.

Here, each region 13a, 13b of the light diffusing plate 13
To determine the thickness of each plate, for example, the following method may be used.

In the figure, in order for the illumination light L ₁ to be sufficiently diffused before reaching the liquid crystal cell 14, it is necessary that the region 13a of the light diffusion plate 13 has a predetermined thickness. A light diffusing plate 13 having a thickness of 50% is prepared. And this light diffusing plate 1
The thickness of the other region 13b away from the light source No. 3 may be removed gradually so that it becomes thinner.

At this time, if the original thickness of the light diffusing plate 13 set so that the transmittance is 50% is t ₀ , then the required thickness t of the other area 13b of the diffusing plate 13 is
The relationship between t and its transmittance A is t=t ₀ logA/log0.5, and in the end, if the corresponding part 13b of the light diffusing plate 13 is removed from the original thickness by t ₀ -t, then good.

In addition, if the light intensity of each light source 12a, 12b, 12c is improved, the above-mentioned other area 13b of the light diffusing plate 13
Instead of the above method of reducing the thickness of the plate to be less than the original plate thickness, the plate thickness of the area 13a may be made thicker than the original plate thickness, and thereby the same effect as above can be expected. Of course.

[Effect of idea]

As described above, according to the present invention, it is possible to provide sufficient illuminance over the entire illumination range that spreads over a planar shape.
It is possible to provide an excellent surface light source device that can provide uniform and accurate illumination.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view showing the whole of a preferred embodiment of the surface light source device according to the present invention, and the chart at the top shows the illuminance distribution of the illumination light. FIG. 2 is a schematic cross-sectional view showing the entire conventional surface light source device, and the chart at the top shows the illuminance distribution of the illumination light. 10... Surface light source device, 12a, 12b, 12c
...Light source for illumination, 13...Light diffusion plate, 13a...
Proximity region, 13b... Distant region, 14... Liquid crystal cell, 15... Case.

Claims (1)

[Scope of utility model registration request] In a surface light source device comprising an illumination light source and a light diffusion plate provided in front of the illumination light source, the thickness of the diffusion plate is thick in an area where the light diffusion plate is close to the illumination light source. A surface light source device, characterized in that the plate is formed to have a thinner plate thickness in a region spaced apart from the light source.