1. Field of the Invention
The present invention relates to an optical adjusting member that controls the traveling direction of incident light and an illumination device and a liquid crystal display device including the same.
2. Description of the Background Art
Various conventional illumination devices such as a backlight unit for a liquid crystal display include mechanisms for adjusting the diffusion and brightness of light from a light source. Most illumination devices include a sheet used to control the directivity of light in its optical path or at the outlet of a light source housing. The optical adjusting member has optical transparency and is capable of collimating incident light in a predetermined direction or diffusing incident light.
A prism sheet is a typical example of the optical adjusting member having the capability of collimating incident light in a predetermined direction, in other words, the capability of controlling its optical directivity (see for example JP 10-506500 A). In general, the prism sheet includes a plurality of optical members that extend in a predetermined direction and each has a triangular section orthogonal to its lengthwise direction (hereinafter also referred to as “prism member”) or a plurality of optical members having a semi-circular (semi-elliptical) section (hereinafter also referred to as “lens type member”) continuously arranged on a sheet type base member. The traveling direction of light is controlled by taking advantage of the prism effect or lens effect provided by these optical members formed on the base member.
In a backlight unit for a liquid crystal display device, using two prism sheets including a plurality of prism members as described above provided on a base member, the lengthwise directions of the prism members of the prism sheets are orthogonal to each other (see for example JP 10-506500 A). A general structure of the backlight unit for such a liquid crystal display device is shown in FIG. 1. A general structure of the prism sheet is shown in FIG. 2. As shown in FIG. 1, a backlight unit 501 for a liquid crystal display device includes a light source 503, a light guide plate 504 that changes light 510 radiated from the light source 503 into a surface light source, a reflection sheet 505 provided under the light guide plate 604 (on the opposite side to a liquid crystal display panel 502), and a plurality of functional optical sheet groups 506 to 508 provided on the light guide plate 504 (on the side of the liquid crystal display panel 502). The functional optical sheet groups include the lower diffusion sheet 506, the prism sheet group 507 and the upper diffusion sheet 508.
The backlight unit as shown in FIG. 1 is a so-called edge light (side light) type illumination device having the light source 503 arranged on a side part of the light guide plate 504. The light 510 radiated from the light source 503 is incident to the side part of the light guide plate 504 and the incident light is output from the surface 504a of the light guide plate 504. At the time, the outgoing light 511 from the light guide plate 504 is collimated in the directivity to some extent, and its direction is inclined at a predetermined angle with respect to the normal direction of the output surface 504a. In the inclination direction, the luminance of the outgoing light 511 is maximized. In the following description, the light component of the outgoing light 511 traveling in the inclination direction in which the luminance is maximized will be referred to as “luminance peak light.” Note that in FIG. 1, the members are shown as they are apart from one another for the ease of illustrating the structure of the liquid crystal display device 500, while in practice, the members are stacked in contact with one another.
The prism sheet group 507 includes two prism sheets 507a and 607b. As shown in FIG. 2, in each of the prism sheets, prism members 507d that extend in a predetermined direction and each have a triangular section orthogonal to the lengthwise direction are arranged in parallel on a sheet base member 507c. In the backlight unit 501, the lengthwise directions of the prism members 507d of the prism sheets 507a and 507b are arranged orthogonal to each other.
As described above, in a conventional backlight unit for a liquid crystal display device, a prism sheet (optical adjusting member) having a plurality of optical members (in a triangular prism shape) as shown in FIG. 2 is used so that light output from the light guide plate is collected and efficiently irradiated upon the liquid crystal display panel. The conventional prism sheet has good light collecting capability but the use of a single prism sheet causes the color separation of light output from the prism sheet. Consequently, when an object is illuminated with an illumination device using the prism sheet, the edge part of the shadow of the object could be blurred with color or when the prism sheet is used for a backlight unit for a liquid crystal display device, a color may look different between when it is viewed at a certain angle and when it is viewed from the front. Besides, the use of only a single prism sheet does not provide sufficient luminance.
The above-described color separation will be described more specifically with FIG. 4. FIG. 4 is an enlarged sectional view of the prism member 507d of the prism sheet 507a shown in FIG. 2 showing how light 512 is refracted at the prism member 507d as the light 512 is incident to the prism sheet 507a at a predetermined incident angle. Note that FIG. 4 shows the case in which a conventional prism sheet is directly provided on the output surface of the optical guide plate, in other words, how light is refracted in an edge light type backlight unit having the structure as shown in FIG. 3 for the ease of illustrating the problem of the color separation. The light 512 in FIG. 4 represents the light component of light 511 incident to the prism sheet 507a that advances in the direction in which the luminance is maximized, i.e., luminance peak light.
The luminance peak light 512 incident to the prism member 607d is refracted at the surface 507e of the prism member 507d on the side of the light traveling direction as shown in FIG. 4 and output in the thickness-wise direction of the prism sheet 507a. At the time, the refractive index of the material forming the prism member 507d (prism sheet 507a) varies depending on the wavelength of the light, and therefore the amount of refraction at the surface 507e of the prism member 507d varies depending on the waveform component included in the luminance peak light 512. As a result, as shown in FIG. 4, the refraction direction of the refracted light at the surface 507e changes depending on the waveform and color separation is generated in a prescribed pattern in the output light 513 from the prism sheet 507a. Note that in FIG. 4, separation of only two waveform components is shown for the ease of description.
Besides the problem of the color separation described above, sufficient luminance does not result using only a single prism sheet. A backlight unit for use in a conventional liquid crystal display device, especially in an edge light type backlight unit, a plurality of optical sheet groups (prism sheets 507a and 507b and diffusion sheets 506 and 508) are normally used as shown in FIG. 1 in order to solve the problems of the color separation and insufficient luminance described above.
In an illumination device and a liquid crystal display device having the structure as shown in FIG. 1, however, a large number of optical sheet groups (the two prism sheets and two diffusion sheets in the example in FIG. 14) are necessary, which restricts reduction in the thickness and the cost of the illumination device and the liquid crystal display device.