Light source devices utilizing a light scattering guide plate and a prism sheet have been proposed and put in use for purposes such as back-lighting for liquid crystal displays. A conventional prism sheet is constituted by a plate-like member made of an optical material having a prism surface formed with repetitive rows of V-shaped irregularity. It is known that such a prism sheet has a function of modifying the directional propagation characteristics of a beam.
FIG. 1 shows a general arrangement of a conventional surface light source device utilizing a light scattering guide plate and a prism sheet. A light scattering guide plate 1 having a wedge-like section comprises a matrix made of polymethyl methacrylate (PMMA) and a material of a different refractive index mixed and dispersed uniformly in the matrix. The term "material of a different refractive index" means a material having a refractive index which is substantially different from the refractive index of the matrix.
One of the end faces that define the larger thickness of the light scattering guide plate 1 serves as an incidence surface 2, and a light source device (fluorescent lamp) L is arranged in the vicinity of the same.
A reflector 3 is arranged along one surface (rear surface 6) of the light scattering guide plate 1. The reflector 3 is constituted by a sheet of silver foil having regular reflectivity or a white sheet having diffuse reflectivity. Illuminating light is obtained at an exiting surface 5 on the other side of the light scattering guide plate 1. The prism sheet 4 is arranged outside the exiting surface 5.
For convenience in illustration, the interval between the light scattering guide plate 1 and the rows of the prism elements and the pitch and depth of the rows of prism elements are exaggerated. The inner surface of the prism sheet 4 is a prism surface formed by repetitive symmetric V-shaped slopes 4a and 4b. The outer surface of the prism sheet 4 constitutes a flat surface (luminous surface) 4e from which an illuminating beam 4f emits. A back-lighting configuration for a liquid crystal display can be arranged by disposing a well-known liquid crystal display panel outside the prism sheet 4.
Thickness of the light scattering guide plate 1 decreases as distance from the incidence surface 2 increases. Thus, repetitive reflection occurs in an effective manner within the light scattering guide plate 1. As a result, the surface light source device has excellent light utilization efficiency and uniformity of luminance.
Light introduced from the light source device L into the light scattering guide plate 1 is guided toward an end face 7 that defines the larger thickness thereof while being subjected to scattering and reflection in the light scattering guide plate 1. During this process, illuminating light is gradually emitted from the exiting surface 5.
The light emitted from the exiting surface 5 has a preferred propagating direction that depends on the particle diameter of particles of a different refractive index dispersed in the light scattering guide plate 1 (in general terms, correlation distance associated with the structure having a non-uniform refractive index). In other words, the exiting surface 5 emits a beam which has been strongly or weakly collimated.
The greater the diameter of the particles of a different refractive index (in general terms, the greater the correlation distance associated with structure of non-uniform refractive index), the stronger the light emitted from the exiting surface 5 is collimated. The preferred propagating direction (the primary propagating direction of the illuminating beam) is normally a direction that defines an upward angle of about 25.degree. to 30.degree. with the exiting surface as viewed from the incidence surface 2.
Such a fact relates to the function of the prism sheet 4 of modifying directional propagation characteristics.
FIG. 2 is a view that illustrates the behavior of light in a section taken in the longitudinal direction of the configuration shown in FIG. 1. The term "longitudinal direction" refers to a direction which is parallel with the direction in which light is supplied to the light scattering guide plate 1 (i.e., perpendicular to the incidence surface 2). On the other hand, a direction which is perpendicular to the direction in which light is supplied to the light scattering guide plate 1 (i.e., parallel with the incidence surface 2) is referred to as "transverse direction".
Referring to FIG. 2, the prism sheet 4 is disposed along the exiting surface 5 of the light scattering guide plate 1 with the prism surface facing inwardly. A preferred angle .phi.3 of the apex angle of each of the prism elements that form the row of prism elements is about 60.degree..
When the direction of incidence is expressed by the arrow L', the preferred propagating direction of the beam emitted from exiting surface 5 defines an angle .phi.2=approximately 60.degree. with the normal to the exiting surface 5.
If it is assumed to use PMMA (one of typical matrix materials having a refractive index n=1.492) as the matrix of the light scattering guide plate 1, an angle of incidence .phi.1=approximately 35.degree. on the exiting surface 5 satisfies .phi.2=approximately 60.degree..
A beam that corresponds to such a preferred propagating direction is referred to as "representative beam". Here, the representative beam is indicated by the reference symbol B1.
The representative beam B1 emitted from the exiting surface 5 straightly travels a layer of air AR (having a refractive index n0=approximately 1.0) and thereafter impinges on one slope 4a of the prism sheet 4 at an angle close to perpendicularity (.phi.3=approximately 60.degree.). The percentage of such beams incident upon another slope 4b is relatively low.
Next, the representative beam B1 substantially straightly travels in the prism sheet 4 up to the opposite slope 4b to be subjected to regular reflection. The beam which has been subjected to regular reflection impinges upon the flat surface 4e of the prism sheet 4 at an angle close to perpendicularity and exits from the prism sheet 4. This process modifies the preferred propagating direction of the beam emitted from the exiting surface 5 into a direction which is substantially perpendicular to the exiting surface 5.
However, the preferred propagating direction after the modification may be shifted from the direction perpendicular to the exiting surface 5. The shifting angle from the perpendicular direction can be adjusted to some degree depending on the apex angle .phi.3 of the prism sheet 4, the material (refractive index) of the prism sheet 4, and the material (refractive index) of the light scattering guide plate 1.
FIG. 3 shows another configuration of the prism sheet 4 and the behavior of light. In this configuration, the prism surface faces outwardly. For example, the apex angle .phi.4 of each prism element on the prism surface is approximately 70.degree..
In this configuration, the range of the apex angle that provides preferable results is wider than that in the above-described configuration wherein the prism surface faces inwardly.
If it is assumed that the direction of incidence is the direction indicated by the arrow L', a representative beam B2 corresponding to the preferred propagating direction is incident upon the exiting surface 5 at an angle .phi.1=approximately 35.degree.. The beam is mostly emitted into the layer of air AR (having a refractive index n0=1.0). The emitting angle .phi.2 in this case is approximately 60.degree..
The representative beam B2 straightly travels through the layer of air AR and thereafter impinges upon the flat surface 4e of the prism sheet 4 at an angle. It follows a refraction path as illustrated and is emitted from one surface 4c of the prism sheet 4 at an angle close to perpendicularity to the exiting surface 5. The percentage of such beams that are emitted from another surface 4d is relatively low.
The path of the beam after the incidence upon the flat surface 4e of the prism sheet 4 varies depending on the refractive index n2 of the prism sheet 4 and the apex angle .phi.4 of the prism. Therefore, the preferred propagating direction can be adjusted by adjusting those parameters.
However, the conventional surface light source device described above does not satisfy all of requirements for the level and uniformity of the brightness of the luminous surface (the upper surface of the prism sheet) as viewed by naked eyes and sense of softness that the illumination gives.
In other words, the prior art has not been successful in providing a luminous surface that has fineness, gives no sense of glitter, and has sufficient whiteness. One significant problem is that the so-called reflective projection (the appearance of bright and dark regions originating from reflection). For example, such reflective projection adversely affects the display quality of a liquid crystal display.
This is assumed to be attributable to the following reason. The light scattering guide plate 1 in the configuration shown in FIG. 1 has not so strong scattering power from a visual point of view. This tendency becomes more significant for larger luminous surfaces for which weaker scattering power is chosen. As a result, a considerable amount of light is reflected by the reflector 3 disposed along the rear surface of the light scattering guide plate 1 to be incident upon the eyes of an observer without being dispersed sufficiently.
When a sheet having regular reflective properties such as a sheet of silver foil and a sheet of aluminum foil is used as the reflector 3, it gives a visual sense that is unique to a surface having regular reflection. Such a visual sense is accompanied by the so-called "lack of whiteness" and "lack of softness or a sense of glitter". Wrinkles or grooves are sometimes seen through the prism sheet.
It is assumed that such phenomena and the sense given by them to an observer are related not only to the level of light amount but also to composite factors such as color temperature and directional propagation characteristics of the illuminating beam.
The problem of whiteness can be mitigated to some extent by employing a white sheet having diffuse reflective properties as the reflector 3. However, this results in a reduction in the uniformity of brightness and the level of light amount of the luminous surface as a whole. Regardless of whether the reflector used has regular reflective properties or diffuse reflective properties, any unevenness (e.g., local wrinkles or irregularity) on the surface of the reflector 3 can cause appear as visible unevenness.
The inventor has made two proposals as follows in order to solve the above-described problems.
(1) A separate prism sheet is arranged along the rear surface of the light scattering guide plate. This prism sheet is oriented such that the rows of prism elements extend in parallel with the direction in which light is supplied (Japanese Patent Application No. H7-74671). PA1 (2) Double-sided prism sheet is used as the prism sheet arranged on the side of the exiting surface of the light scattering guide plate (Japanese Patent Application No. H7-213964). This double-sided prism sheet is formed with rows of prism elements on both sides which are orthogonal to each other. PA1 (1) Further improvement of the level of brightness of a display when viewed in a primary viewing direction (substantially forward direction). PA1 (2) There is a demand for characteristics such that the level of brightness smoothly decreases with increase in an angular deviation from the primary viewing direction (substantially forward direction) and such that the output of illuminating light is suppressed as much as possible in viewing directions in which viewing is less likely to take place in order to avoid useless illumination. To satisfy such a demand, optical output must be suppressed as much as possible in directions which deviate from the forward direction by 30 deg. or more. PA1 (1) The prism sheet includes rows of prism elements on a first surface (inner surface) thereof and rows of prism elements or lens elements on a second surface (outer surface) thereof. PA1 (2) The rows of prism elements on the inner surface and the rows of prism elements or lens elements on the outer surface are aligned in directions (first and second directions) orthogonal to each other. PA1 (3) The prism sheet is oriented such that the direction in which the rows of prism elements on the inner surface are aligned (the first direction) is in parallel with the incidence surface of the light scattering guide plate (perpendicular to the direction in which light is supplied). Under such a condition for orientation, the direction in which the rows of prism elements or lens elements on the outer surface are aligned (the second direction) is obviously perpendicular to the incidence surface of the light scattering guide plate (parallel with the direction in which light is supplied). PA1 (4) The rows of prism elements or lens elements on the outer surface are formed such that light which has been guided in the forward direction within the prism sheet is shifted in a direction substantially perpendicular to the direction in which the light is supplied and then is returned toward the inner surface. PA1 (5) When the outer surface is a prism surface, the rows of prism elements preferably have a prism apex angle in the range from 70.degree. to 130.degree.. The most preferable value of this angle is, for example, 96.degree.. PA1 (6) The rows of prism elements formed on the inner prism surface are constituted by repetitive patterns of a slope having a first relatively small angle of inclination and a slope having a second relatively large angle of inclination which are alternately arranged. Thus, the inner surface is a surface on which a multiplicity of asymmetric prism grooves aligned and formed. PA1 (7) The slopes having the second angle of inclination of the rows of prism elements formed on the inner surface are directed toward the incidence surface. Under such a condition, the slopes having the first angle of inclination is obviously directed opposite with the incidence surface. PA1 (8) The first relatively small angle of inclination is approximately 15.degree., and a preferable value of the second relatively large angle of inclination is approximately 32.5.degree.. PA1 (9) Preferably, thickness of the light scattering guide plate decreases as distance from the incidence surface increases. Typically, the light scattering guide plate has a wedge-shaped sectional configuration. A light supplying means is arranged along one of the end faces that define the larger thickness of the light scattering guide plate.
This first proposal solves the above-described problems. However, it requires two prism sheets, and improvement is still needed to achieve a compact structure and a lower manufacturing cost.
The prism sheet is oriented such that the rows of prism elements on the inner prism surface extend in parallel with the incidence surface and such that the rows of prism elements on the outer prism surface extends perpendicularly to the incidence surface.
The apex angle of the prisms on the inner prism surface is designed as an angle such that light propagating from the light scattering guide plate in the preferred propagating direction is guided in a substantially forward direction within the prism sheet.
On the contrary, the apex angle of the prisms on the outer prism surface is designed so that the light guided in a substantially forward direction within the prism sheet is shifted into a direction substantially perpendicular to the light supplying direction and is then reversed (returned). A typical value of such a prism apex angle is 90.degree., and the actual range for this angle is from about 70.degree. to about 130.degree..
This second proposal has improved the level and uniformity of brightness, suppression of reflective reflection, and visual appearance (whiteness and softness) of the luminous surface while solving the problems which are not solved by the above-described first proposal.
However, increasingly higher quality is required for illuminating light used for purposes such as back-lighting of a liquid crystal display. Such demands include the following requirements that have recently arisen.
A surface light source device according to the above-described proposal for improvement is unsatisfactory in consideration to such demands. Particularly, it does not have performance that satisfies the demand as mentioned in the above item (2) as will be obvious from an embodiment for comparison to be described later.
The fact that useless illuminating light is output in directions that greatly deviate from the forward direction also indicates that there is still a need for improvement on the point described in the item (1).