Optics: image projectors – Plural
The present first invention relates to a projection display system that displays images by driving one light valve in response to light signals that present different colors temporally (time-division driving).
The present second invention relates to a projection display system that magnifies and projects images generated by a reflection-type light valve without relying on a polarizing beam splitter (hereinafter, referred to as PBS).
The market for large-scale image display systems that are used primarily for presentation is growing rapidly today. A wide range of applications, from portable displays to extremely large screens used in halls or the like, is included in this market. The common requirements to be met by the individual display systems for such applications are high brightness, low cost, and miniaturization. There are two types of projection display systems: three-plate and single-plate. The three-plate type is provided with light valves, one each for R (red), G (green), and B (blue). The single-plate type is provided with one light valve for displaying color images. To meet the above-described requirements, in particular, to achieve cost reduction, the projection display systems of the single-plate type have been used increasingly in recent years.
The single-plate type also can be classified broadly into two systems: one is a system using a light valve provided with pixels corresponding to each of the RGB colors; the other is a system using a light valve that displays images by changing modulation factors temporally in response to each of the RGB signals with the same pixel.
The first system can have a simple configuration. However, the quality of projection images is poor, strictly speaking, as the RGB in those images are displaced. On the other hand, the second system can provide good image quality without displacement of the RGB. However, its configuration is more complex than the first system.
The present invention is intended to improve the second system.
Hereinafter, the second system, i.e., a single-plate display system employing time-division driving, will be described. In this display system, a light valve is driven at a speed that is three times as fast as the conventional one in response to each of the input signals of RBG. It is necessary that the incident light on the light valve also be switched correspondingly.
A lighting system that illuminates a subject by switching white light sequentially to the RBG colors of light is disclosed in, e.g., JP 2-119005 A.
shows a schematic configuration of the lighting system. The light from a light source
, which emits white light, passes through a condenser lens
and a color wheel
into the incident end of a light guide
. Then, the light is projected onto a subject for observation from the exit end of the light guide. In this case, the color wheel
is a rotating disk formed of three fan-shaped filters. The three filters are as follows: a red-transmission filter for passing only light in the wavelength range of red, a green-transmission filter for passing only light in the wavelength range of green, and a blue-transmission filter for passing only light in the wavelength range of blue. The color wheel
is rotated by a motor
. The rotation of the color wheel
allows the subject to be illuminated with red, green, and blue light that is switched sequentially.
The above lighting system is applied to a projection display system, which is disclosed in, e.g., JP 9-185902 A.
shows a schematic configuration of the projection display system. The light emitted from a light source
is reflected from a reflecting mirror
toward the opening thereof. Then, only visible light is reflected from a reflecting mirror
provided with a filter for rejecting ultraviolet and infrared rays, and its optical path is deflected by 90 degrees. The reflected visible light passes through a brightness-modulation filter
and color-modulation filters
in this order, so that the entire brightness of the light is modulated. Then, the light enters a color wheel
. The color wheel
is provided with a tri-color filter including: a filter for passing only light in the wavelength range of red; a filter for passing only light in the wavelength range of green, and a filter for passing only light in the wavelength range of blue. By rotating the color wheel
, the color of the light passing through the color wheel can be selected sequentially. The transmitted light is collimated by a condenser lens
into parallel light, reflected from a mirror
, and enters a projection gate
. Then, the light is modulated and emitted from the projection gate
and directed through a relay lens
and a stop
to a projection lens
. Thus, an image on the projection gate
is magnified and projected onto a screen (not shown). At this time, a color signal that drives the projection gate
and a color of the light passing through the color wheel
are synchronized, so that modulation can be performed in accordance with a color of the light entering the projection gate
. This makes it possible to display color images with a single light valve. In the above configuration, the light from the light source
is condensed on the color wheel
or its vicinity. This is because the size of the color wheel
is reduced and a period of color mixture is minimized; the color mixture occurs when the incident light on the color wheel
passes through two different adjacent color selecting filters at the same time.
As described above, when a rotating color wheel is used for time-division driving, it is preferable that an image of the light source is small, which is condensed and formed on a color selecting filter of the color wheel or its vicinity. On the other hand, since the time-division driving basically reduces the optical output of a system to one-third, a light source that can provide high brightness is necessary. However, a discharge tube is used generally as the light source of a projection display system. Therefore, to achieve high brightness as well as practical lifetime, the distance between electrodes is increased and a light-emitting portion becomes large. When such a light source with high-brightness and a large light-emitting portion is used, the image of the light source that is condensed and formed on a color selecting filter of the color wheel or its vicinity also becomes large. This causes an increase in the size of the rotating color wheel, the degradation of projection images because of color mixture, or the like.
Thus, for a conventional projection display system that performs time-division driving with a rotating color wheel, it has been difficult to achieve high brightness.
The market for large-scale image display systems that are used primarily for presentation is growing rapidly today. A wide range of applications, from portable displays to extremely large screens used in halls or the like, is included in this market. The common requirements to be met by the individual display systems for such applications are high brightness, high resolution, low cost, and miniaturization. It should go without saying that the selection of a light source suitable for each device size and the optimization of optical systems are needed to satisfy these requirements.
Hereinafter, an example of a configuration of a conventional projection display system employing a reflection-type light valve will be described.
A first conventional technique that is disclosed in JP 5-150213 A will be described. As shown in
, among the light from a light source
, the light reflected from a reflector
passes through a polarizing plate
and enters a reflection-type liquid crystal panel
. The reflection-type liquid crystal panel
modulates the incident polarized light to image light corresponding to an im
Esplin D. Ben
Merchant & Gould P.C.
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