JPH0593887A - Increase of contrast in reflected light valve system and elimination of ghost - Google Patents

Abstract

PURPOSE: To remove a ghost image due to high luminance light radiated from a light source and reflected by the surface of a projection lens in a reflection liquid crystal(LC) light bulb projection system and to improve contrast. CONSTITUTION: In the reflection LC light bulb projection system for reflecting light from the light source 10 to an LC light bulb 34 by a polarized beam splitter 16, controlling the reflection of the bulb 34 by an image generator 36, reflecting light P of a 1st polarized state from the bulb 34, transmitting the light P to a projection lens 38 for projecting the reflected light to a screen through a polarized beam splitter 16, reflecting a part of the light transmitted to the lens 38 from the surface of the lens 38 to the bulb 34, and forming a visible ghost image, a 1/4 wavelength plate is close inserted as a polarization control means between the lens 38 and the splitter 16 to suppress light transmitted to the bulb 34 and reflected from the surface of the lens 38.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to reflective light valve systems, and more particularly to increasing contrast and eliminating ghosts in liquid crystal light valve projection systems.

[0002]

2. Description of the Related Art A liquid crystal light valve (LCLV) is a multi-layered thin film formed of a liquid crystal layer sandwiched between two transparent electrodes, a dielectric mirror, a light blocking layer, and a light sensitive layer. It is a structure. In an LCLV projection system, a polarized projection beam is directed through a liquid crystal layer and onto a dielectric mirror. An input image of low intensity light, such as that produced by a cathode ray tube, is provided to the photosensitive layer, thereby switching the electric field across the electrodes from the photosensitive layer on the liquid crystal layer that drives the liquid crystal. The linearly polarized projection light from the high power light source passes through the liquid crystal layer and is reflected from a dielectric mirror that is polarization modulated according to the light information incident on the photosensitive layer. Therefore, if a complex distribution of light, for example a high resolution input image, is focused on the photoconductor surface, the device will magnify and project a high intensity image onto the screen to produce a reflective replica image. Convert the image to. This type of projection system is described by Koda et al. In U.S. Pat. No. 4,650,286, Bl.
U.S. Pat. No. 4,343,535 by eha, Jr., Jaco
US Pat. No. 4,127,322 by bsen et al., and Hong
They are described in several US patents, including US Pat. No. 4,191,456.

[0003]

The high brightness of the high power light sources used in such systems causes the problem of reducing the contrast of the projected image and providing a ghost image display of the light source. The lens component of the projection lens is provided with an anti-reflection coating, so that even if the incident light has a very low reflection rate, this low proportion of light reflected from the high-intensity light source causes the light valve behind the projection screen to Is fully reflected by the off state of. This is an unfavorable image of the arc lamp source and the reflector of the projection screen, producing a more noticeable ghost in the black areas of the screen when only part of the screen is white. The undesired ghost is the portion of the light that is reflected from the air / glass interface of the lens component, forming a focused image of the arc lamp behind the light valve surface. This image is reflected by the liquid crystal light valve and fed through the lens to the screen. This problem is associated with all lens lens component surfaces that form an arc lamp image on the bulb mirror and is projected by a projection lens onto the screen to produce the image.

The intensity of the ghost image is reduced by a highly efficient antireflection coating on the lens component. However, with a very bright arc lamp source, even 1% of it is very noticeable when projected into the dark areas of the screen, resulting in a loss of overall contrast and disturbing the image of the lamp and reflector on the screen.

Accordingly, it is an object of the present invention to provide a reflective liquid crystal light valve projection system that avoids or minimizes the above problems.

[0006]

In practicing the principles of the present invention in accordance with the preferred embodiment, light reflected from the surfaces of all components of the projection lens is protected from transmission back to the liquid crystal light valve. This is accomplished by changing the polarization state of the light so as to prevent it from retransmitting back through the system's polarizing beam splitter. More specifically, a quarter wave plate is positioned between the polarizing beam splitter and the projection lens so that the light transmitted from the polarizing beam splitter to the lens is polarized by the quarter wave plate and rotated by 45 °. Light reflected from the lens having a polarization beam splitter back towards the polarizing beam splitter additionally has a polarization which is rotated by 45 °, thereby causing the polarizing state by the polarizing beam splitter to be reflected from the light valve.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows U.S. Pat.
Figure 4 shows a liquid crystal light valve projection system of the type as shown in 43,535 and Koda et al. 4,650,286. The projection system includes a high power light source, such as a high intensity arc lamp 10, that emits unpolarized light and directs a beam of light 14 onto a polarizing beam splitter 18, shown as a buried MacNeille prism, a collimating lens 12
Be transmitted through. MacNeill Prism, MacNeill
A polarizing beamsplitter that achieves selective polarization as described in US Pat. No. 2,403,731 to Mr. E. The buried prism 18 represented schematically in FIG.
It comprises parallel flat sided transparent prism plates 16 covered with a plurality of thin dielectric layers arranged in a prism fluid 20 as described in cNeille U.S. Pat.No. 2,403,731, which at 22 It is located in the liquid-tight housing shown and has a transparent front window 24 and a transparent outlet window 26. ..

Polarizing beam splitter 18 is an input window 28 that receives irregularly polarized light from arc lamp source 10.
including. Typically, a beam splitter will transmit light of one polarization state, such as the "P" polarization state, and reflect light of another polarization state, such as the "S" polarization state.

The reflected S-polarized light is transmitted along the reflected beam 32 to a liquid crystal light valve 34 which is modulated by an imaging source such as a cathode ray tube 36. Cathode ray tube 36
Light bulb 34 in dark places where the screen does not emit fluorescent light
The corresponding region of the is left in the off or dark state, and light is reflected from the dark portion of the light valve 34 back to the polarizing beam splitter without changing the polarization state. Since the polarization of the light does not change from the original S state, the light is reflected again from the beam splitter prism plate and returns to the light source 10. This S-polarized light is projected from the light valve 34 to the projection lens.
Neither is transmitted by the beam splitter to 38, so the corresponding area of the image remains dark due to the projection lens 38. For the bright areas of the screen of the cathode ray tube 36, the corresponding areas of the liquid crystal light valve are on or bright. The light reflected from the bright areas of the light valve 34 is partially or wholly rotated from the S-polarized state to the P-polarized state to obtain an intensity proportional to the intensity of the light from the cathode ray tube screen. Light reflected in the opposite direction of the polarization state P is transmitted from the liquid crystal light valve through the polarization beam splitter 18 to form a bright image on the exit window 26 of the beam splitter and the projection screen (not shown). Through the projection lens 38.

FIG. 2 is a schematic diagram of the system of FIG. 1 arranged schematically to show transmission and reflection of light in various polarized states and to represent some aspect of the method.
A ghost image or a reduction in contrast occurs in this reflective liquid crystal light valve projection system.

In FIG. 2, rather than attempting to indicate the angle of reflection of each light, the arrows designated S or P with the appropriate subscripts indicate the transmission and reflection of light in each propagation state S and P. Used for.

As shown in FIG. 2, high intensity arc lamp 10 transmits to polarizing beam splitter 16 unpolarized light or irregularly polarized light, which is indicated by lights S ₀ and P ₀ . Polarization beam splitter 16 transmits light of polarization state P ₀ as indicated by arrow P ₀ , and arrow S ₀
S-state polarized light as shown by
When the light S _{0 in the} S state is incident on a dark area (corresponding to the dark area of the cathode ray tube) such as the area 42 of the liquid crystal light valve 34, it is reflected without any change in polarization as indicated by the S ₁ component. Which is transmitted to the beam splitter and reflected from it in the manner of an arc lamp. The light of polarization state S is a liquid crystal light valve (cathode ray tube) as indicated by S ₀ .
When incident on a bright region 44 (corresponding to 36 bright regions), it is reflected by the beam splitter in polarization state P ₁ . The latter transmits the light P ₁ of polarization state P to the lens 38. This is the light projected on the screen by the lens system. That is, the light reflected in polarization state P from the bright region of the liquid crystal light valve is transmitted through the lens to the screen. However, as mentioned above,
A small amount of light incident on the surface of the lens component is reflected back towards the beam splitter as indicated by the component P ₂ . Light of this polarization state P is transmitted through the beam splitter and can enter different regions of the liquid crystal light valve depending on the curvature of the lens and the different light angles received by the lens.

Light of polarization state P reflected from the projection lens as indicated by arrow P ₂ is incident upon a dark region, such as the dark region 46 of the liquid crystal light valve, from FIG.
Is reflected back without any change in the polarization state, such as the light component indicated by P ₃ in FIG. The component of the light P ₃ re-reflected from the “dark” areas of the liquid crystal light valve passes through the beam splitter towards the lens system and then through the lens system (except for very low reflection from the surface of the lens component). It is transmitted to the screen where it forms a ghost image of the arc lamp, which increases the illumination intensity in the ideally dark areas of the projection screen and thereby reduces the contrast.

The following discussion is for understanding the problems due to the reflection of lens components. A typical screen image consists of light emitted from an arc lamp that illuminates the light valve surface and is selectively reflected by the screen through a projection lens. The objectionable ghost associated with the present invention is an unwanted image of arc lamps and reflectors on the projection screen, more noticeable in the black areas of the screen when only part of the screen is white.

Unlike the standard ghost image, which is an undesired image of a star-shaped point target object, this ghost image is an image of the light source spread over the screen. The undesired ghost is the portion of the light that is reflected back from the air-glass interface on the surface of the lens component, forming an arc lamp focused image on the light valve surface. This image is reflected by the bulb through the lens and sent to the screen. Although the broadband anti-reflective coating helps control the ghost image, the arc lamp brightness is very bright, which is noticeable on the screen even at low reflectance.

According to the present invention, the above problem is solved by the method of FIG.
1 between the beam splitter and the lens 38 as shown in
It is solved by inserting the quarter-wave plate 50. Plate 50 is a broadband quarter wave plate that transmits light with a 45 ° polarization shift for each transmission. It works with a wide range of visible frequencies. The P-state polarized light indicated by the component P ₁ is reflected again by the changed polarization state of the S-polarized state light S ₀ incident on the bright region 44 of the liquid crystal light valve 34. These components P ₁ as in the arrangement of FIG. 2 pass through the beam splitter and through the quarter wave plate 50 (FIG. 3). The quarter-wave plate is shown in Figure 3
Rotate the light in the P polarization state by 45 ° to provide light represented as ₁ + 45 °. The small portion of the light having the polarization state P ₁ + 45 ° has the same polarization and the lens 38
Reflected from the rear surface. This is the component P ₂ + 45 °
As shown in FIG. Component P ₂ reflected from the lens
The + 45 ° passes through the quarter-wave plate 50 a second time and is additionally rotated by 45 °, effectively changing the polarization state to S as shown in the figure by the component S ₂ . The components of the polarization state S ₂ are not transmitted through the beam splitter, but, on the contrary, are reflected out of the liquid crystal valve and the system. In this way, changing the polarization state of the light from the liquid crystal light valve after it has passed through the beam splitter prevents the liquid crystal light valve from reaching even these weak reflections from the surface of the lens component. The polarization state of the light reflected from the lens is changed so that it is not transmitted through the beam splitter. Therefore, the cause of this ghost image and the reduction in contrast are neglected.

In tests performed with different lenses, photometric measurements were taken at various locations to measure the intensity of light incident on the projection screen. The measurements were taken with or without the quarter wave plate in the position and compared with the measurements on the white side of the screen without ghost images or contrast reduction. Testing was done with a simulated half-on / half-off light valve.

The contrast ratio, which is the intensity of light incident on the bright areas of the screen relative to the intensity of light incident on the dark areas of the screen, is a value of 36: 1 without a quarter wave plate and is correct for the lens. 1 /
Increased to a value of 105: 1 with four wave plates. In another screen it has a value of 43: 1 without a quarter wave plate to a quarter wave plate 11
Increased to a value of 0: 1. The contrast ratio for the second lens is increased from 38: 1 to 54: 1. The loss of light for the preferred polarization is less than 11% due to the use of quarter wave plates without antireflection coatings. 1
It is preferred to use a quarter wave plate with an antireflective coating which significantly reduces the losses in the presence of the quarter wave plate.

[Brief description of drawings]

FIG. 1 shows components of a conventional liquid crystal light valve projection system.

2 is a schematic diagram of the system depicted in FIG. 1 showing various polarization states of light reflected between system components.

FIG. 3 shows a liquid crystal projection system having a polarizing plate sandwiched between a polarizing beam splitter and a projection lens for increasing the contrast of the projection system lens and eliminating ghost images.

[Description of symbols] 10 ... Arc lamp, 16 ... Polarization beam splitter, 34 ... Liquid crystal light valve, 36 ... Image generator, 38 ... Projection lens.

Claims (8)

[Claims] 1. Light from a light source is reflected from a polarizing beamsplitter to a liquid crystal light valve that is transmitted to and controlled by an image generator that modulates the light reflected from the first light polarization state. Light having a value of 1 is reflected from the light valve and transmitted through the polarization beam splitter to the projection lens for projection onto the screen, and a portion of the light transmitted to the lens is reflected from the lens surface to the light valve and reflected back to the lens. In a reflective liquid crystal light valve projection system that reduces the screen contrast and produces a visible ghost image, between the lens and the polarizing beam splitter to block light reflected from the lens surface transmitted to the liquid crystal light valve. A reflective liquid crystal light valve projection system, characterized in that it comprises an interpolated polarization control means. 2. The projector according to claim 1, wherein the polarization control means includes means for changing the polarization of the light reflected from the lens into a polarization state in which the light is reflected from the beam splitter. 3. The polarization control means comprises means for rotating the polarization of the light transmitted from the polarization beam splitter to the lens and rotating the polarization of the light reflected from the lens to the polarization beam splitter. Item 2. The projection system according to item 1. 4. The polarization control means comprises a quarter-wave plate positioned between the polarization beam splitter and the lens, and the quarter-wave plate transmits the light transmitted from the polarization beam splitter to the lens by 45 °. The light that is rotated and reflected from the lens to the polarization beam splitter has an additional polarization that is rotated by 45 °, causing a polarization state to be reflected from the polarization beam splitter in the light reflected from the lens to the polarization beam splitter. Item 2. The projection system according to item 1. 5. A light source, a reflective liquid crystal light valve, image generator means for modulating the light transmitted to said liquid crystal light valve, a projection lens, between said light valve and said projection lens and said light The projection lens is disposed between the bulb and the light source, reflects the light of the first polarization state from the light source to the liquid crystal light valve, and reflects the light of the second polarization state reflected from the light valve. And a polarizing beam splitter means for transmitting to
The projection lens has at least one surface that reflects a portion of the light received from the light valve back to the light valve, and further comprises: a light source for blocking the transmission of light from the projection lens to the liquid crystal light valve. A projection system comprising: a polarizing beam splitter means and means interposed between the projection lens. 6. The projection system according to claim 5, wherein said blocking means comprises a polarization beam splitter means and a polarization rotation device sandwiched between said beam splitter means and a lens. 7. The means for blocking causes the light incident on the projection lens from which it is reflected to the beam splitter means to produce light having a polarization state reflected from the polarizing beam splitter means. 6. The projection system according to claim 5, which comprises: 8. The blocking means is interposed between the polarizing beam splitter means and the projection lens.
The projection system of claim 5, comprising a quarter wave plate.