1. Field of the Invention
The present general inventive concept relates to an illumination unit to illuminate a light beam and an image projecting apparatus, and more particularly, to an illumination unit having an improved prism to synthesize light beams emitted from a compact light source and an image projecting apparatus employing the same.
2. Description of the Related Art
In general, an illumination unit includes a light source emitting a light beam in one direction and an illumination optical system transmitting the light beam. The illumination unit is widely used for an image projecting apparatus that generates an image using an image forming device, such as an LCD device or a digital micromirror device, which cannot generate a light beam by itself.
An illumination unit and an image projecting apparatus adopting a compact light emitting device, such as a light emitting diode (LED) or a laser diode, as a light source have been developed. The compact light emitting device, which can emit each of light beams having red, blue, and green wavelengths, is advantageous in that it does not require an additional color wheel device mechanism to generate a color image in a panel type color image projecting apparatus. In order to emit the respective color light beams, a plurality of compact light emitting devices and a mechanism to synthesize the respective color light beams are needed.
Referring to FIG. 1, an illumination unit emits light beams having red, blue, and green wavelengths and includes first through third LED light sources 11, 12, and 13 arranged at different positions and a trichroic prism 20 which makes light beams emitted from the first through third LED light sources 11, 12, and 13 proceed in the same path.
The trichroic prism 20 includes first through third prisms P1, P2, and P3 that are three individual prisms combined together, a first color filter 21 provided between the first prism P1 and the third prism P3, and a second color filter 25 provided between the second prism P2 and the third prism P3. The first and second color filters 21 and 25 selectively transmit or reflect incident light beams according to the wavelengths thereof. For example, the first color filter 21 reflects a first light beam R having a red wavelength and transmits second and third light beams G and B having green and blue wavelengths, respectively. The second color filter 25 reflects the third light beam B and transmits the second light beam G.
The first light beam R input to the first prism P1 is totally reflected by an exit surface 20a of the first prism P1 according to a critical angle total reflection principle, and then travels toward the first color filter 21. The first light beam R is reflected by the first color filter 21 and passes through the exit surface 20a of the first prism P1 to travel along a path. The second light beam G sequentially passes through the second and first color filters 25 and 21 and proceeds on the same path as the first light beam R. The third light beam B is totally reflected by a surface 20b of the third prism P3 facing the first prism P1 according to the critical angle total reflection principle, travels toward the second color filter 25, is reflected by the second color filter 25, sequentially passes through the third and first prisms P3 and P1, and proceeds on the same path as the first and second light beams R and G. Thus, the first through third light beams R, G, and B emitted from the first through third LED light sources 11, 12, and 13 are synthesized to proceed on the same path with respect to each other.
In the illumination unit configured as described above, to have the third light beam B totally reflected by the surface 20b of the third prism P3, the first prism P1 and the third prism P3 must be separated as much as a predetermined gap Gair during the optical arrangement thereof. This is because, when the critical angle total reflection principle is used, there must be not only the angle made between the surface 20b of the third prism P3 and the third light beam B, but also a difference between a refractive index of the third prism P3 and a refractive index therearound. Thus, when the light beams emitted from the plurality of light sources are to be synthesized by using the trichroic prism 20, it is a problem that the optical arrangement of the trichroic prism 20 is difficult.
Also, when the light beams are synthesized by the trichroic prism 20, it may be difficult to install an additional light source emitting a light beam, for example, yellow, magenta, or cyan, in addition to the first through third light sources emitting light beams having different wavelengths such as red, blue, and green, due to the limit in the optical arrangement thereof. Thus, a range of color to be realized may be more limited than in an illuminating unit using a four color light source.