1. Field of the Invention
The present invention relates to an image projection system designed to illuminate and then project an image displayed on a light valve onto a screen on an enlarged scale through a projection lens assembly.
2. Description of the Prior Art
To obtain an enlarged picture on a screen, a method is well known in which an image formed on a miniature light valve in response to a video signal applied thereto is illuminated by rays of light from a light source with the imagewise rays of light being subsequently projected through a projection lens assembly. FIG. 7 illustrates an image projection apparatus such as disclosed in, for example, the Japanese Laid-open Patent Publication No. 2-250015, in which liquid crystal panels are employed as light valves.
Referring to FIG. 7 for the discussion of the prior art image projection apparatus, the apparatus shown therein comprises a lamp 1, a converging optical apparatus 2 for converging rays of light from the lamp 1, a cut-off filter 3 for reflecting both ultraviolet rays of light and heat waves, a color separating optical means 7 including three dichroic mirrors 4, 5 and 6, three field lenses 8, 9 and 10, three liquid crystal panels 11, 12 and 13 of active matrix type, three projection lenses 14, 15 and 16, and a screen 17. White rays of light emitted from the lamp 1 are transformed by the converging optical apparatus 2 into a substantially parallel beam which subsequently enters the color separating optical means 7 after the ultraviolet rays of light and the heat waves have been reflected by the filter 3.
When the parallel beam enters the color separating optical means 7 in the manner described above, the beam is separated by the dichroic mirrors 4, 5 and 6 into blue, green and red light components which are, after having subsequently been passed through the respective field lenses 8, 9 and 10, transmitted to the liquid crystal panels 11, 12 and 13. The blue, green and red light components entering the liquid crystal panels 11, 12 and 13 are modulated by the liquid crystal panels 11, 12 and 13 in response to the applied video signal to form respective images which are subsequently transmitted as blue, green and red imagewise beams to the projection lenses 14, 15 and 16. These projection lenses 14, 15 and 16 then project the imagewise beams onto the screen 17 to effect a color image display.
In this prior art image projection apparatus, since the three projection lenses are employed, not only is the use of a color combining optical means required, but a lens having a relatively small focal length can be employed for each of the projection lenses, making it possible to manufacture the image projection apparatus as a whole in a compact size.
The illuminance E of the image projected on the screen can be given by the following equation: EQU E=E.sub.o .multidot.K.multidot.cos.sup.4 .omega. (1)
wherein E.sub.o represent the illuminance at the center of the projected image in alignment with an optical axis of the projection lens, K represents the vignetting factor of the projection lens, and .omega. represents the field angle of the projection lens. Thus, the illuminance E at the screen is proportional to the product of the vignetting factor K multiplied by cos.sup.4 .omega..
In the prior art system shown in FIG. 7, since the three projection lenses have their own optical axes positioned at different locations, these projection lenses for projecting the blue, green and red imagewise beams, respectively have different field angles with respect to an arbitrarily chosen point on the screen and, therefore, the screen is illuminated by the blue, green and red imagewise beams in varying patterns of distribution of illuminance. FIG. 8 illustrates a graph showing a pattern of distribution of illuminance of each image, formed on the screen by the respective blue, green and red imagewise beams projected by the associated projection lens, with respect to the fourth power of the cosine of the field angle (cos.sup.4 .omega.) of the associated projection lens.
As can be understood from the graph of FIG. 8, the prior art system shown in FIG. 7 has a problem in that the composite color image formed on the screen by combining the blue, green and red imagewise beams has an even distribution of color over the screen.
This color non-uniformity may be corrected either by modulating the video signals to be applied to the liquid crystal panels with luminance or by the use of a light attenuating filter. Any of these methods is such that the illuminance of one or two of the color imagewise beams projected on the screen is reduced to coordinate with that of the remaining imagewise beam or beams projected on the same screen, followed by an adjustment of the white balance thereby to compensate for the color non-uniformity, and therefore poses a problem in that the luminance of the image projected on the screen tends to be lowered.
Where the light attenuating filters are employed to make the illuminance of the blue, green and red beams projected on the screen substantially uniform over the screen, those filters must be have light transmittances sufficient to accommodate different cos.sup.4 .omega. exhibited by the respective projection lenses. The use of such filters in turn brings about an increase in cost of the optical system.