Image forming apparatus and projector using the same

Abstract: An image forming apparatus of this invention relates to an image forming apparatus for forming a color image and, more particularly, to a projector using the image forming apparatus. The image forming apparatus includes a color light illuminating system having first and second light sources for emitting different color light components, and first, second, and third imaging units or liquid crystal light valves for modulating the color light components to form color images. The first light source is a light source for mainly emitting two color components corresponding to the images formed by the first and second imaging units, and the second light source is a color light source for mainly emitting a color component corresponding to the image formed by the third imaging unit.

Description: The present invention relates to an image forming apparatus according to the preamble of claim 1 A conventional image forming apparatus is exemplified by a projector shown in Fig. 1. Light emitted from a light source 30 directly or through a reflector 5 is color-separated into red, green, and blue light components by a dichroic mirror 7 for reflecting the red light component and transmitting the green and blue light components therethrough and a dichroic mirror 8 for reflecting the blue light component and transmitting the green light component therethrough and a total reflection mirror 20. The color-separated light components pass through liquid crystal light valves 1, 2, and 3 and are synthesized through a dichroic mirror 9 for reflecting the blue light component and transmitting the red light component therethrough and a dichroic mirror 10 for reflecting the green light component and transmitting the red and blue light components therethrough. The synthesized light is projected through a projection lens 6. A metal halide lamp having spectral characteristics in the entire visible light range, as shown in Fig. 2 is used as the light source 30. In the conventional apparatus, a color-separating optical system is located between the light source and the liquid crystal light valves, and optical paths between the light source and the liquid crystal light valves must have predetermined lengths. Illumination light from the light source cannot be perfect telecentric light and contains convergent and divergent components. For this reason, the illumination light has a considerably large loss by divergence during propagation along the optical paths having the predetermined lengths. In addition, the color-separating optical system causes an increase in the size of the apparatus as a whole, thereby degrading portability of the apparatus. As one method of solving these conventional problems, there may be proposed a single light valve type liquid crystal projector which has one white light source and one liquid crystal light valve having an RGB mosaic color filter. This arrangement allows formation of a projector by one liquid crystal light valve, and the color-separating optical system and the color-synthesizing optical system can be omitted, thereby realizing a very compact apparatus. However, the following problems are still posed by this apparatus. As the color filter is generally of an absorption type, the amount of white light passing through the color filter is reduced to about 1/3. For this reason, the transmittance of the liquid crystal light valve is reduced, and hence a sufficiently bright display image cannot be obtained. It is an object of the present invention to provide a compact image forming apparatus having a high light utilization efficiency and a high resolving power and a projector using the same. The above-stated object is achieved by means of the combination of features as defined in claim 1. Preferred embodiments of the image forming apparatus according to claim 1 are set forth in the dependent claims 2 and 3, respectively. According to a further aspect of the present invention, there is provided an image forming apparatus having a plurality of imaging means for forming color-separated images and color-synthesizing means for synthesizing the image light components from the plurality of imaging means, characterized by comprising a plurality of light sources, wherein the plurality of imaging means are illuminated with different color light components. In the following the invention is further illustrated by embodiments with reference to the attached figures. Fig. 1 is a schematic view showing an arrangement of a conventional projector; Fig. 2 is a graph showing the spectral characteristics of a white light source used in the conventional projector; Fig. 3 is a schematic view showing an arrangement of an embodiment of the present invention; Fig. 4 is a schematic view showing an arrangement of another embodiment of the present invention; Fig. 5 is a graph showing the spectral characteristics of a color light source used in the embodiments of the present invention; Fig. 6 is a schematic view showing an arrangement of still another embodiment of the present invention; Fig. 3 is a schematic view showing an arrangement of an embodiment of the present invention. Light valves 1, 2, and 3 as imaging means form color images corresponding to the blue, green, and red light components, respectively. An apparatus in Fig. 3 also includes a projection lens 6, dichroic mirrors 8, 9, and 11, a total reflection mirror 20, and color light sources 31 and 32 having spectral characteristics represented by B and G+R, respectively, as shown in Fig. 5. More specifically, the light source 31 is a blue light source, and the light source 32 is a yellow light source having green and red spectra. These components of the apparatus have a layout, as shown in Fig. 3. The light source 32 may be constituted by green and red light sources which are arranged adjacent to each other. The blue light source 31 is located immediately behind the liquid crystal light valve 1. For this reason, the length of the illumination optical path can be reduced to 1/4 or less of the conventional apparatus shown in Fig. 1. Illumination light emitted from the yellow light source 32 is color-separated into red and green light components by the dichroic mirror 8. The red and green light components are radiated on the liquid crystal light valves 2 and 3, respectively. The length of the optical path can be reduced to about 1/2 the conventional apparatus shown in Fig. 1 in which light components reach the liquid crystal light valves 2 and 3 through two dichroic mirrors. The loss of the divergent illumination light can be greatly reduced, and a brighter image forming apparatus can be arranged. Note that combinations of the color filters and a white light source may be used in place of the blue and yellow light sources described above. After the light components pass through the liquid crystal light valves 1, 2, and 3, they are synthesized again through the total reflection mirror 20, the dichroic mirror 9 for transmitting the blue light component and reflecting the green component, and the dichroic mirror 11 for transmitting the red light component and reflecting the green and blue light components. The synthesized light is projected through the projection lens 6. In this embodiment, if the dichroic mirrors 8 and 11 are designed to transmit the red light component and reflect the blue and green light components, the dichroic mirrors 8 and 11 can be identical to each other, thus resulting in an economical advantage. At this time, if the dichroic mirrors 8 and 11 are constituted by a single dichroic mirror, the number of optical elements and the number of positioning operations can be reduced. Man's eye is most sensitive to the green light component among the components in the visible range. On the other hand, when light is obliquely incident on parallel flat plates, as in this embodiment, an astigmatism occurs. Judging from these two factors, it is found that the number of times of passing of the green light component through a dichroic mirror can be reduced to obtain a high resolving power. In this embodiment, since the green light component does not pass through a dichroic mirror, a display image having a high resolving power can be obtained. Fig. 4 is a schematic view showing an arrangement of another embodiment according to the present invention. Light valves 1, 2, and 3 as imaging means form color images corresponding to the blue, green, and red light components, respectively. An apparatus in Fig. 4 also includes a projection lens 6, dichroic mirrors 8, 9, and 10, a total reflection mirror 20, and color light sources 31 and 32 having spectral characteristics represented by B and G+R, respectively, as shown in Fig. 5. More specifically, the light source 31 is a blue light source, and the light source 32 is a yellow light source having green and red spectra. These components of the apparatus have a layout, as shown in Fig. 4. The blue light source 31 is located immediately behind the liquid crystal light valve 1. For this reason, the length of the illumination optical path can be reduced to 1/4 or less of the conventional apparatus shown in Fig. 1. Illumination light emitted from the yellow light source 32 is color-separated into red and green light components by the dichroic mirror 8. The red and green light components are radiated on the liquid crystal light valves 2 and 3, respectively. The length of the optical path can be reduced to about 1/2 the conventional apparatus shown in Fig. 1 in which light components reach the liquid crystal light valves 2 and 3 through two dichroic mirrors. The loss of the divergent illumination light can be greatly reduced, and a brighter image forming apparatus can be arranged. After the light components pass through the liquid crystal light valves 1, 2, and 3, they are synthesized again through the total reflection mirror 20, the dichroic mirror 9 for transmitting the blue light component and reflecting the green component, and the dichroic mirror 10 for reflecting the red light component and transmitting the green and blue light components. The synthesized light is projected through the projection lens 6. With this arrangement, the direction of a projection image can be shifted from that of the embodiment of Fig. 3 by 90°. The white light source 30 of the conventional projector shown in Fig. 1 often comprises a metal halide lamp. Light emitted from a metal halide lamp generally has a shortage of the blue light component. In addition, when light passes through or reflected by a dichroic mirror, some light components having a wavelength of 400 nm or less are absorbed by the dichroic mirror. When the above factors are taken into consideration, a light source for generating a blue light component which tends to be short is independently prepared, and a light source for emitting the remaining yellow light component (i.e., the green and red light components) is prepared to obtain an excellent color balance, as in the embodiments of Figs. 3 and 4. Alternatively, the length of the optical path between the light source 31 for emitting the blue light component and the liquid crystal light valve 1 may be intentionally increased to obtain a length equal to that between the light source 32 for emitting the yellow light component and the liquid crystal light valves 2 and 3. This arrangement much facilitates white balance control. Fig. 6 is a schematic view showing still another embodiment of the present invention. Light valves 1, 2, and 3 as imaging means form color images corresponding to the blue, green, and red light components, respectively. An apparatus in Fig. 6 also includes a projection lens 6, dichroic mirrors 11, 12, and 13, a total reflection mirror 20, and color light sources 33 and 34 having spectral characteristics for emitting a green color component and red and blue light components, respectively. These components of the apparatus have a layout, as shown in Fig. 6. The green light source 33 is located immediately behind the liquid crystal light valve 2. Illumination light emitted from the color light source 34 is color-separated into red and blue light components by the dichroic mirror 12. The red and blue light components are radiated on the liquid crystal light valves 3 and 1, respectively. As in the previous embodiments described above, the loss of the divergent illumination light can be greatly reduced, and a brighter image forming apparatus can be arranged. The respective light components pass through the liquid crystal light valves 1, 2, and 3 and are synthesized again through the total reflection mirror 20, the dichroic mirror 13 for reflecting the blue light component and transmitting the green light component, and the dichroic mirror for transmitting the red light component and reflecting the blue light component. The synthesized light is projected through the projection lens 6. In this embodiment, if the dichroic mirrors 11 and 12 are designed to transmit the red light component and reflect the blue and green light components, the dichroic mirrors 11 and 12 can be identical to each other, thus resulting in an economical advantage. At this time, if the dichroic mirrors 11 and 12 are constituted by a single dichroic mirror, the number of optical elements and the number of positioning operations can be reduced. Light passing through a dichroic mirror generally has a larger light amount loss than light reflected thereby. In this embodiment, the blue light component does not pass through a dichroic mirror, the loss of the blue light component can be minimized. Alternatively, the length of the optical path between the light source 33 for emitting the green light component and the liquid crystal light valve 2 may be intentionally increased to obtain a length equal to that between the light source 34 for emitting the red and blue light components and the liquid crystal light valves 1 and 3. This arrangement much facilitates white balance control. If the dichroic mirror 11 is replaced with the dichroic mirror 10 for reflecting the red light component and transmitting the green and blue light components, the projection direction can be shifted by 90° as in the embodiment shown in Fig. 4.

Claims: An image forming apparatus comprising a first, second and third imaging means (1, 2, 3) for modulating color light components to form color images, a first light source (31) for emitting a first color light component directly to the first imaging means (1) for forming a first color image, said first light source (31) being a color light source for emitting a color light mainly containing a blue light component, a second light source (32) for emitting second and third color light components, said second light source (32) being a color light source for emitting color light mainly containing green and red light components, a separating means (8) for separating said second and third color light components from each other before each of said second and third color light components is transmitted to the second and third imaging means (2, 3), respectively, for forming second and third color images and a color synthesizing means (9, 11, 20; 9, 10, 20) for synthesizing said first, second and third color images, characterized in that said second light source (32) emitting mainly green and red light components is constituted of a metal halide lamp. An apparatus according to claim 1, wherein said color synthesizing means (9, 11, 20; 9, 10, 20) comprises a plurality of dichroic mirrors, and said plurality of dichroic mirrors and said color separating means (8) have functions of reflecting the green light components, respectively. An apparatus according to claim 1 or 2, further comprising light amount adjusting means arranged in optical paths from said first, second, and third light sources, respectively.

Inventors: ABE TAKESHI, KUREMATSU KATSUMI, TANAKA TSUNEFUMI, ABE, TAKESHI, KUREMATSU, KATSUMI, TANAKA, TSUNEFUMI, Abe, Takeshi, c/o Canon Kabushiki Kaisha, KUREMATSU, KATSUMI, C/O CANON KABUSHIKI KAISHA, TANAKA, TSUNEFUMI, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G03B27
IPC codes: G02F1, H04N9, G03B33