In general, an optical device for a display such as a projection TV, a screen for rear-projection projectors, a TFT-LCD, a PDT TV, or a CRT monitor has a fine lens structure or a waveguide structure in order to broaden the viewing angle thereof. The present invention relates to an optical device for a display having a waveguide structure and a method of manufacturing such devices.
Conventional optical devices for displays will be explained in brief, referring to FIGS. 1 and 2.
FIG. 1 is an enlarged view of the screen of a conventional optical device for a display. FIG. 2 is an enlarged sectional view of a conventional optical device for a display.
As shown in FIGS. 1 and 2, the conventional optical device for a display 10 is provided with a light emitting region 12 for diffusing imaging light rays incident thereon and a light-absorbing region 14 for absorbing external lights and reducing reflection. As depicted in FIG. 2, the imaging light rays are diverged through the light emitting region 12, i.e., the imaging light rays are inputted into the optical device 10, and then their travelling paths are diverted through a fine lens 16 such that the light rays can be diverged. As such, the optical device of FIG. 2 requires numerous fine lenses on the surface thereof, which leads to a cumbersome manufacturing process.
U.S. Pat. Nos. 3,279,314 and 5,462,700 disclose an optical device for a display, in which fine tapered waveguides are uniformly distributed, instead of the above lenses.
FIG. 3 is a sectional view of an optical device for a display using a tapered waveguide;
As shown in FIG. 3, the optical device 20 having a tapered waveguide does not cause the light refraction, dissimilar to the optical device of FIG. 2 employing a lens 16.
In contrast, the optical device 20 of FIG. 3 is structured such that light rays are reflected on the inclined sidewall 23 of the waveguide 22 and guided so as to have an increased angle with respect to the sidewall 23 thereof when passing the waveguide 22, thereby providing image diffusion effect.
Sunlight or surrounding illumination may be reflected on the front face of the optical device 20, so that the observed image may be relatively dark or blocked by means of unnecessary light rays. In order to minimize this phenomenon, the light-absorbing region 24 of FIG. 3 is structured in such a way that a black material is coated or filled in the area excepting the light-diverging region of the optical device 20, thereby absorbing the surrounding light rays.
The above-mentioned waveguide 22 employs light reflection on the interface between two mediums having different refraction indices. The incident light rays are totally reflected, or partially reflected and partially passes through the interface, depending upon the magnitude of the incident angle. In order to improve the efficiency of reflection occurring inside the tapered waveguide 22, the larger the difference in the refraction indices between two mediums is the more effective.
In U.S. Pat. No. 5,462,700, the waveguide is formed using ultraviolet rays and thus usable medium is limited and the selection range for mediums having a lower refraction index is more narrowed. In addition, in U.S. Pat. No. 6,538,813, a metallic coating is further provided in the reflection wall (interface between mediums), or after providing the metallic coating and removing the waveguide, the remaining metallic coating is used for reflection medium, thereby maximizing reflection effect. Due to the trend of pursuing a large-scale screen, these patented inventions cause an increase in the number of process steps and the materials cost, and thus fail to meet the current or future requirements therefor. The material disclosed in U.S. Pat. No. 5,462,700 has a maximum refraction index of 1.6 and the lower index resin filled in the surroundings is limited to around 1.3. Thus, a difference of about 0.3 in refraction indices between two mediums causes a difficulty and limitation in the design of a waveguide. In addition, at present, the screen for a projection TV is manufactured up to 61 inches and, in case of PDP, up to 80 inches. In case of such large-scale screens, provision of a metallic coating to the front face thereof causes many difficulties and problems, in terms of quality, productivity, facility or the like. In particular, it is difficult to selectively coat only the sidewall of a waveguide excepting the light output surface. An expensive facility is required for depositing a large screen in a short period of time. In terms of productivity related to mass production, the competitive force thereof may be lowered.
In addition, the light-absorbing region is formed of a resin having a low refraction index relative to the waveguide and dispersed with fine carbon black particles or a black colorant. In particular, U.S. Pat. No. 5,462,700 employs the former method, and mentions that the carbon black particles must be controlled not to be contacted with the sidewall of the waveguide.
The light-absorbing material contains fine carbon particles as its main constituent, and absorbs lights flowing in the screen from the outside so as not to be reflected on the front face of the screen. Thus, it provides a more distinct and clear screen image to the viewers.
In the conventional screen structure, however, since the light-absorbing material is filled in the inverted-conical spaces between the waveguides, the light projected through the waveguide is absorbed in the light-absorbing material while reflecting inside the waveguide, thereby causing light loss. Thus, distinctness of images is deteriorated and consequently screen resolution is degraded.
Referring to FIG. 4, the above problems in the art will be explained below.
FIG. 4 is a partial enlarged view of a light-absorbing region filled with carbon black particles.
As shown in FIG. 4, the light rays incident into the waveguide 20 are reflected on the inclined sidewall 23 of the waveguide 20. At this time, if the carbon black particles 25 are contacted with the sidewall 23, the incident imaging light is not reflected, but absorbed in the carbon black particles 25 by means of the black body effect, thus consequently leading to loss in the light quantity to be diverged in the light output surface.
Some patents, for example, U.S. Pat. No. 6,417,966, propose a process for black-treating the front exposed portion only of the resin, instead of dispersing carbon-black particles. However, the process itself lacks practicality and cannot be easily applied to a practical manufacturing process.
In addition, U.S. Patent Application Publication No. 2002/0080484 discloses a waveguide structure to solve the problems in the art, in which the space between the waveguides remains empty, and a black film is covered on top of the empty space. However, this publication fails to propose a suitable process for forming the structure. The above black film disposed between the waveguides without any support structure, due to its instability, cannot become a complete solution to the prior art problems.