KR20010089697A - Optical assembly for reflective light valves - Google Patents

Abstract

The present invention relates to a compact polarization unreacted prism assembly for a projection display system using a reflected light valve.

The system includes a tricolor prism assembly having three coated prisms, including a trapezoidal shape in a specific shape, in one embodiment, and a beam splitter.

The angle of the prism is optimized for best color separation and recombination with minimal polarization sensitivity.

The assembly forms a compact and efficient optical arrangement for a reflective LCD projector.

Description

[0001] Optical assembly for reflective light valves [0002]

Conventional projection displays employ mainly transmissive light valves such as active matrix LCD panels.

On the other hand, reflective mode light valves offer advantages such as high aperture ratio, high light efficiency and good quality projection images.

Laser-scale projection systems based on Liquid Crystal Light Valves (LCLV) have been successfully developed and manufactured.

In recent years, crystalline silicon based on a CMOS liquid crystal light valve is preferably used. This crystalline silicon offers the advantages of sufficient integration and mass production of CMOS circuits on the chip.

Thus, a compact optical system is required for a new-class light valve.

In a reflective projection system, a collimated light source is divided into three primary colors by two dichroic color filters (a first blue filter and a second red filter).

Next, the light beams are directed to the light valves aligned along different optical paths.

The reflected light beam is recombined using two dichroic filters, with the changed polarization. These filters are the same color separation filters or different filters.

The reflected light is separated from the incident light using PBS and finally projected onto the screen.

U.S. Patent No. 4,687,301 discloses a color separation-recombination optical assembly.

The dichroic filter contained in the refractive index matching fluid is used for both color separation and recombination. The incident angle on the blue filter is 24 占 and the incident angle on the red filter is 12 占.

U.S. Patent 4,969,730 describes a tri-color-prism assembly known as a color splitting prism. The prism functions as both a color separator and a color re-combiner.

The principle disclosed in U.S. Patent 4,687,301 is identical to the present invention, but greatly improves with respect to ease of manufacture. A blue filter and a red filter are coated on the surface of the prism. The incident angles are all 30 degrees, and the PBS is used to separate the incident beam from the polarization-modulated reflected beam.

U.S. Patent No. 5,644,432 discloses a projection system having a color separator and a recombiner comprised of the same tri-color prism assembly.

PBS is used to separate the reflected light beam and the incident light beam. In this case, since there is no air gap in the blue filter, the tri-color prisms can stick to each other.

The blue and red dichroic filters have a large incidence angle of 30 ° to maintain a short rear working distance for the reflective lens.

Full Color Reflective Mode The optical system of the LCLV projection must conform to the following features. (1) large system optical invariance, or (2) large output light flux, meaning that the system with LCLV signifies the etendue, (2) the use of double-polarized light, (3) (4) Inverted focus for compact, small reflective lenses.

Conventional color separation and recombination prisms do not require these requirements.

Accordingly, it is an object of the present invention to provide a color splitting recombination prism having a small angle of incidence towards a dichroic coating.

Currently, liquid crystal projection displays are image generators primarily based on transmissive LCD light valves. A drawback of this type of LCD projector is that the aperture ratio (AR) of the LCD panel is small. As the resolution of the light valve increases, the aperture ratio becomes smaller.

For example, the aperture ratio (AR) for the SVGA display is about 0.67 and the aperture ratio for the XGA LCD panel is about 0.5.

In addition to low optical efficiency, a low aperture ratio (AR) requires a black matrix to be able to hide the transistors that cause pixelation.

Therefore, depixelization is required, and optical system design is complicated.

In such a transmissive projector, different color filter columns are used to separate input light into the three primary colors, and are used to recombine after passing the LCD light valve.

The color recombiner is usually performed in an X-cube.

The reflective mode silicon CMOS liquid crystal light valve can overcome the disadvantage of low aperture ratio (AR) of the transmissive LCD panel.

The aperture ratio (AR) of silicon based on CMOS LCD is as high as 92% regardless of resolution. This is because the transistor is hidden underneath the reflective mirror on the pixel.

Thus, the optical efficiency and the quality of the projected image are greatly improved. The projector optics of the reflective light valve are more complex than the transmissive optics. One consideration is the variation of S and P polarization in the optical path.

A full color projector is either a time-sequential type employing a reflective LCD panel or a three-panel type having three main colors at the same time. This applies in relation to the 3-panel type.

Generally, the projector includes an optical sub-system for separating the main color from an input white light source (arc lamp), and a separate sub-system for recombining the three main colors after being modulated by the reflected light valve need.

The color separator and the color combiner are the same optical item or physically different. Physically different from a compact projection system is desirable.

There are several designs of optical subassemblies for reflective light valves based on color projectors. The basic element for a reflected light valve is a polarizing beam splitter (PBS) that reflects S-polarized light and transmits P-polarized light.

For most designs, 3PBS can be used with three main color panels.

Color separation and color recombination will separate a set filter similar to a transmissive projector. Figure 1 shows a basic configuration. Dichroic reflectors are used to reflect red and blue light.

The R, G, and B channels are sent to three PBSs and three light valves. Next, the image-modulated reflected light is directed to an X-prism for color recombination for projection.

Because the reflective coating of the X-prism works best for S-polarizations, the half-wave plate requires red and blue channels to rotate the P-polarized light from the reflective LCD. Thus, such a system is quite complex.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for a reflection mode light valve of a full color display and more particularly to a trichroic prism assembly (TPA) having a compact and high optical efficiency To a polarizing beam splitter (PBS).

The combination of TPA and PBS is arranged between an objective lens and an LCD panel or other type of reflected light valve to form a compact projection system.

FIG. 1 is a graph showing the sensitivity of a dichroic coating by polarized light. The spectral reflectances varying as the angle of incidence is changed are as shown in (a), (b) and (c).

2 is a schematic diagram illustrating an exemplary projection system;

Figure 3 details the color splitting / recombination assembly.

Figure 4 shows a non-cubic polarizing beam splitter.

5 shows an optical system for a reflective color projector using a conventional arrangement.

Figure 6 shows an optical system for a reflective color projection display employing a tricolor prism assembly.

Figure 7 shows the spectral reflectance measured for the red and blue edge reflective coatings according to (a) 16 °, (b) 30 °, (c) 45 ° incident angle, solid lines indicate P- Polarized light.

Figure 8 shows a new tricolor prism assembly with a new non-cubic PBS.

According to a first aspect of the present invention, there is provided a polarizing beam splitter having a non-cubic arrangement, the arrangement of which ensures that the angle of incidence for the dichroic coating or filter is kept small.

The divider has a trapezoidal configuration.

According to a second aspect of the present invention, there is provided a prism assembly employed as a color separator and a color re-combiner, and an apparatus for a full color projection display comprising a plurality of reflective light valves.

The prism assembly is tricolor.

The following equation describes the present invention to provide an optical sub-system in which the angle of incidence of the light to the dichroic coating is kept as small as possible. This is due to the polarization separation phenomenon.

The dielectric multi-layer optical coating material used to form the dichroic filter always consists of a periodic laminate of a high refractive index layer (H) and a low refractive index layer (L).

In the case of,

The reflected light is expressed by the following equation.

(I)

here Represents the bandwidth of the reflected light, Wow Represent the effective admittance of the H and L layers, respectively, and Wow Represents the action of the incident angle and the polarization state of light.

For S-polarization 0 And P-polarization to be.

Where I = H or L Is the refraction angle in each film.

remind And the incident angle [theta] .

Hereinafter, a prism assembly and a non-polarizing beam splitter embodying the present invention will be described by way of example with reference to the accompanying drawings.

The effective admittance? Of the thin film varies with the change of the incident angle?.

The difference between the spectral reflectance of S-polarized light and the spectral reflectance of P-polarized light becomes larger as the incident angle increases.

Thus, a large incidence angle leads to a large separation between the spectral reflectance band of S-polarized light and the spectral reflectance band of P-polarized light.

Figure 1 shows the spectral reflectance of a red edge filter and a blue edge filter that are schematically illustrated.

These coatings are used in tricolor prism assemblies to separate red, green, and blue ternary colors.

In the measurement, the incident angles of 16 DEG (a), 30 DEG (b), and 45 DEG (c) (FIG.

The solid curved line belongs to P-polarized light and the dotted curved line is for S-polarized light. The coating is taken between air and glass (n = 1.5163).

As can be clearly seen in FIG. 1, as the angle of incidence increases, the spectral reflectance for S-polarized light and P-polarized light becomes increasingly different.

This phenomenon is highly undesirable because a reflective light valve acts on the polarization modulation.

A change in reflectance based on a change in polarization causes loss of reflected signals on the projected screen, variation in color combination, and reduced color fidelity.

The 45 ° curve (c) corresponds to the case of the X-cube color separator or recombiner.

It is clear that the X-cube can not be used as a color separator and as a color remover.

On the other hand, as shown in Fig. 1, the 16 [deg.] (A) incident angle is acceptable, which is the incident angle achieved in the present invention.

Figure 2 shows a preferred embodiment of the present invention. This shows a projection system employing a reflective light valve.

Three primary colors of red, green, and blue are used for the three light valves 13, 14, and 15, respectively.

The light source 22 is first collimated by a reflector and lens combination 19,21 having its own light source.

The cold mirror 20 is used to remove unwanted infrared radiation that causes excessive heating of the light valve.

Next, the light beam is injected into a non-cube, in one embodiment a trapezoidal PBS 17.

The coating material on the inner surface 17b of the PBS 17 transmits P-polarized light and reflects S-polarized light. Next, the P-polarized reflected light is projected onto the tricolor prism assemblies (10, 11, 12).

FIG. 3 is a detailed view of a tricolor prism assembly including a tricolor prism.

The prism 10 has an entrance face 10a perpendicular to the optical axis, and the preceding case generally forms an angle of 90 - ₁₀ with respect to the optical axis.

This is to allow the incident angle to be accommodated on the small-formed surface 10b.

The first dichroic coating is coated on the surface 10b of the prism 10. This coating is coated to reflect one main color.

In one implementation, blue is reflected, and in another embodiment, red may be reflected.

At this time, it is important that the color reflected on the inside of the prism at an angle larger than the critical angle for total internal reflection penetrates onto the surface 10a.

Next, the entire internally reflected light is emitted through the surface 10c so as to be incident perpendicularly on the reflected light valve 13. [ The reflected light from the light valve 13 is modulated into polarized light.

The reflected light passes through the same path and penetrates the surface coating 17b of the PBS. Reflected or transmitted behind the light source, and projected on the screen in accordance with the action of the light valve 13. [

Thus, the reflected light induces the image to the screen through polarization modulation.

At the surface 10b, the transmitted light enters the prism 11 and penetrates onto the surface 11b. This surface is usually coated with a dichroic filter in which one color component in red is reflected or directed to the air gap 11a.

The angle of incidence of light in the air gap causes total internal reflection. The total internal reflected light is sent to the light valve 15. The exit surface 11c is substantially perpendicular to the light beam.

The reflected light valve modulates the polarization of the light beam.

As the reflection is restored to its original optical path, the modulated light is impinged on the surface 17b of the PBS.

Again, the light beam will either be reflected or transmitted for projection in response to the action of the light valve.

The last color component of the light is usually green, transmitted through the prism 12, and directed toward the light valve 14.

The reflected light from the light valve 14 will follow the back of the main optical axis.

Similar to other components, the reflected light will form an image on the screen based on polarization modulation by the light valve 14.

The outer surfaces of all the air / glass 10a, 10c, 11a, and 11c and the prism 12 are coated with the AR coating material.

The AR coating of the surfaces 11a, 10a is a special broadband AR coating. At this time, the phase change is induced by the dichroic coating and the total internal reflection.

There is also an air gap between the prisms 10 and 11 and between the prism 10 and the PBS 17. [

These air gaps are needed for full internal use. As another embodiment, a special coating may be used with the aid of various color components without air gaps.

The angle of the tri-color prism is very important. The incident angle should be minimized, and the total internal reflection must be achieved.

Therefore, the incidence angles 10b and 11b of the two dichroic coating materials are the same, indicated by?, And can be expressed by the following equations.

(2)

Where n is the refractive index of the prism and NA is the numerical aperture of the projection system.

Therefore, if the refractive index of the prism is 1.52 and the number of systems F is 3.5, then the angle of incidence from the equation (2) can be 16 degrees. This is the smallest possible angle. If the dichroic filters have different incident angles, another value can be obtained.

As the other angles of the prism assembly from the following equation,? ₂ = 32 占 and? ₃ = 74 占. Further,? ₁ must satisfy the total internal reflection at the prism 10.

(3)

If φ ₁ is 32 °, the angles of the prisms 10 and 11 will be the same.

This facilitates the manufacture of the prism assembly.

The size of the prism is determined by the path length of the three color light beams inside the prism assembly and the size of the reflected light valve.

In addition, the incident angle should depend on the refractive index of the prism.

If a prism having a higher refractive index is used, the incident angle of the dichroic coating material can be further reduced.

In this embodiment, the two dichroic coatings have the same incident angle. It is suitable for mass production purposes. However, this is not an essential condition.

It is apparent from Fig. 2 that the influence of the polarization on the spectral reflectance is very small.

Also, the small incident angle reduces the dependence of the reflectance upon the change of the incident angle (receiving angle)

The large receiving angle of the dichroic coating means that the prism structure improves the numerical aperture of the optical system.

Figure 4 shows a polarization beam splitter of a non-cubic shape.

The PBS 17 is composed of the same right angle prism as the conventional PBS and other components having a special angle of? = 73 degrees. This is necessary so that the surface 10a of the prism 10 is matched. The surfaces 10a, 17a are coated with an AR coating.

In a preferred embodiment, the material of PBS 17 is BK7 glass, and the PBS has low stress, high transparency at short wavelengths, and high polarization efficiency.

Thus, using the present invention described with reference to the accompanying drawings, there is provided an optical sub-system for a full color projection display employing a reflective light valve.

The sub-system consists of a tri-color prism assembly that acts both on a non-cubic polarizing beam splitter, a color separator for three-color color, and a color re-coupler.

The tricolor prism assembly includes two dichroic edge filter coatings separating the three main colors, the three primary colors of red, green, and blue into three coherent channels.

The two dichroic coatings are held inside the prism assembly and are utilized for small incident angles and low polarization dependence.

The same tri-color prism assembly is used for color recombination. The three color channels are modulated with polarized light by a reflected light valve.

Although the rays have different polarizations, they are inherently restored to their original optical path, and pass through the same dichroic filter. The dichroic filter has no response to polarization.

In order to keep the angle of incidence of the tricolor prism on the dichroic filter small, a non-cubic polarizing beam splitter (PBS) is provided,

By making the PBS have a trapezoidal shape, the incident angle to the dichroic filter can be kept small.

Preferably, the color projector using the reflection light valve can be performed with an optical coating material having the same color division and color recombination so as to be compact.

Conventional optical assemblies for performing these functions are called so-called Philips prisms.

The tricolor prism assembly (TPA) is operated in association with only one PBS to form the core of the compact color reflector. This system is shown in FIG.

The PBS first sends P or S polarized light to TPA. The choice of S or P polarization depends on the position of the arc lamp and the projection lens. The first two prisms are 30 ° prisms. The coating on the discharge surface is used to separate the blue and red colors.

The air gap is provided so that the entire internal reflection is reflected to reflect the red and blue beams.

Three color channels pass through the TPA and enter the reflective LCD light valve.

The action of the nematic liquid crystal is to control the polarization state of the reflected light.

Many optical modes have been proposed for LCD panels.

In all of these cases, when the pixel is selected, the reflected light is rotated by 90 °. This light beam is restored to the same optical path as the incident beam and passes through the PBS.

High reflectance coatings for blue and red light should act for S and P polarizations. This is a difficult condition for coating material design.

Despite the zero incident angle, it is known that it is impossible for a dichromatic coating to have polarization dependence.

Due to the conditions of total internal reflection for the red and blue beams, the angle of incidence on the blue and red color filter surfaces is limited to at least 30 degrees. Therefore, the conventional TPA has severe polarization dependency.

This causes the so-called S-P polarization splitting to cause loss of light efficiency and decrease color fidelity of the display.

Figure 7 measures the spectral reflectance for the red and blue coatings for three different angles of incidence of 16 [deg.], 30 [deg.] And 45 [deg.].

The red and blue edge filter spectra are shown in almost the same figure showing three color separations. The solid line indicates P-polarized light and the dotted line indicates S-polarized light.

The coating material is placed between glasses having an exponent n3 of exponent 1.5163. As the incident angle increases, the spectral separation rate between S-polarized light and P-polarized light increases significantly.

This causes the so-called S-P polarization splitting to lose the intensity of the reflected light. The color coordinates of the separated and recombined light fluctuate.

The present invention is intended to alleviate this disadvantage.

The S-P polarization splitting for the optical coating material leads to the fact that the specific admittance of light is different from S-polarization and P-polarization.

The admittance is an action of an incident angle and a polarization state of light, and is expressed by the following equation.

(One)

(2)

(1), (2) In the equation, subscript i represents the number of layers stacked in the coating layer.

The effective admittance? Of the thin film changes with the incident angle of light being changed.

The S and P polarization spectra become larger as the incident angle increases.

Thus, a large angle of incidence leads to a large separation between the spectral reflectance of S-polarized light and the spectral reflectance of P-polarized light. This is shown in the measured spectra in FIG.

The main point of optimizing the tri-color splitting / recombining prism is to reduce the incident angle of light on the dichroic coating.

The incident angle is limited by the conditions of total internal reflection for the blue and red channels.

Thus, a tricolor color separating / recombining prism assembly with an incident angle to the dichroic coating reduced to 16 is provided. An implementation of the prism is shown in FIG.

The main idea is that PBS has a non-orthogonal shape, not a conventional cube shape. This relaxes the design conditions and allows for smaller angles of incidence.

The blue high reflectivity coating is applied to the surface BC and the red high reflectivity coating is applied to the surface CD.

The incident angles of the two dichroic coatings are the same and are indicated by?. Here, the conditions of the total internal reflectance of the two inner surfaces (AB and BC) must be maintained. It is also preferable that the surfaces AB and CD are parallel.

The relation between the various angles of the TPA should satisfy the following conditions.

(3)

(4)

(5)

(6)

(7)

In these equations, n represents the refractive index of the prism material, and NA represents the numerical aperture of the projection system.

Therefore, if the refractive index of the prism is 1.52, the F-number of the optical is 4, and the smallest allowable incident angle from Eq. (7) is 16 占.

Additionally, φ = 16 ° and φ ₃ = 74 °. The size of the prism of each course depends on the size of the LCD light valve.

Thus, a very simple prism assembly is provided in which the two dichroic coatings have the same incident angle.

Prism ABC and BCD are the same and therefore mass production is easy.

The TPA has a small and acceptable polarization effect. Also, a small incident angle means a small scattering at the incident angle.

Thus, the dichroic coating has a large acceptance angle or a large tendency

Such a new prism structure can greatly improve the numerical aperture of the overall optical system and at the same time maintain excellent color separation characteristics.

Conventional TPA and new TPA are configured for comparison. The dichroic coating material is optimally applied for red and blue light separation.

Reflectance for various polarizing and color coating materials has been standardized for control. The data was obtained using a PR650 spectrometer. The three peaks for the conventional TPA coincide with the red, green, and blue channels of the output type of the TPA.

There is a change in spectra for S and P-polarized light. The P-polarized reflectance is changed to blue by 10 nm.

In the case of the new spectral reflectance for TPA having an incident angle of 16 [deg.], The SP polarization splitting is negligible.

These results coincide with the numerical results as shown in Fig. The spectra for S-polarized light and P-polarized light are very sharp and identical. The TPA is used in a compact color projector as shown in FIG.

In summary, the main limitation for the design of compact color projectors using reflective LCD light valves is the design of color separators and recombiners.

By modifying the design of the existing TPA, the S-P polarization splitting problem can be improved. Thus, using the present invention, color separation and recombination can be achieved as a single optical element with high fidelity. The incident angle of the dichroic coating material is preferably about 16 degrees.

As described above, the prism has a low S and P polarization dependence on the spectral reflectance. Therefore, it can be used for color separation and color recombination together with polarization change.

The tricolor prism assembly can be used in a compact color projector employing a reflective liquid crystal light valve.

Optical TUNEU and spectral / polarization effects are two main characteristics of optical projection systems required for optimization. This is useful for reflective or transmissive mode projectors.

Optical coatings for reflective projectors are the main limitation of the system to tandu.

The polarizing splitting effect on the tricolor prism assembly causes deterioration of optical system performance. Here, the optical system provides an improvement over the existing design. The new TPA will be mainly applicable to compact projectors such as desktop monitors and flat panel TVs.

Claims (23)

And has an arrangement such that the incident angle to the dichroic coating or filter is kept at a small value while having a non-cubic shape. The polarizing beam splitter of claim 1, wherein the non-cube shape is trapezoidal. Wherein the device for full color projection display comprises a prism assembly that acts as a color separator, a color re-combination device, and a plurality of reflective light valves. 4. The apparatus of claim 3, wherein the prism assembly is tri-color. The projection system for full color image display is: a) a light source for collimating and guiding a white light beam and a light source b) a polarization beam splitter having a non-cubic shape to reflect a specific polarization of input light; c) a tricolor prism assembly used to separate the three primary colors from the white light beam and direct them to each of the three reflected light valves d) generating a plurality of color image signals respectively corresponding to the plurality of reflective polarizations e) a tricolor prism, similar to that described in c), which is used to recombine the reflected light, f) the same polarizing beam splitter as in b) used to direct the light beam towards the screen or to be restored to the light source, g) Reflective lens system that illuminates the screen with light and forms a full color image on the screen And a projection system for a full color image display. 6. The projection system of claim 5, further comprising a right angled prism and a trapezoid prism having a polarizing coating material therebetween. 7. The projection system of claim 6, wherein the angle of the trapezoidal prism is 74 +/- 5 degrees. The full color image display device according to any one of claims 5 to 7, wherein the three-color prism assembly is composed of three prisms, two of which are triangular prisms and the other is a trapezoidal prism Projection system. 9. The projection system of claim 8, wherein a dichroic coating is coated on the surface between the prisms. 10. The projection system for full color image display according to any one of claims 5 to 9, wherein the angles of the triangular prisms are 32 占 5 占 48 占 5 占 100 占 5 占 respectively. 11. The projection system of claim 9 or 10, wherein the two dichroic coatings have the same incident angle. 12. The projection system of claim 11, wherein the incident angle is 16 +/- 5 degrees. 13. The full color image display of claim 11 or 12, wherein the dichroic coating on one surface of the tri-color prism assembly is an edge filter having a high reflectance for a blue color and a transmissive for remaining colors. Projection system. 13. The full color image display as claimed in claim 11 or 12, wherein the dichroic coating on one surface of the tri-color prism assembly is an edge filter having a high reflectance for a red color, Projection system. 15. The projection system of claim 13 or 14, wherein the spectral reflectance of the dichroic coherent material has a spectral variation between S-polarized light and P-polarized light of 5 nm or less. The projection system of any one of claims 5 to 15, wherein the reflective light valve comprises a liquid crystal light valve. 17. The projection system of claim 16, wherein the liquid crystal light valve is a silicon backplane active matrix driven liquid crystal cell. 18. A projection system for full color image display according to any one of claims 5 to 17, characterized in that the transmitted light of the polarizing beam splitter reflects S-polarized light. 18. A projection system for full color image display according to any one of claims 5 to 17, characterized in that the transmitted light of the polarizing beam reflects P-polarized light. 20. A projection system for full color image display according to any one of claims 5 to 19, characterized in that the light source enters the polarizing beam splitter along the main axis of the tricolor prism assembly. 20. A projection system for full color image display according to any one of claims 5 to 19, characterized in that the light source enters a polarization beam splitter perpendicular to the main axis of the tricolor prism assembly. The projection system of any one of claims 5 to 21, wherein the tri-color prism assembly has an air gap between the triangular prisms. The projection system of any one of claims 5 to 21, wherein the polarizing beam splitter and the tri-color prism have an air gap between the polarizing beam splitter and the tri-color prism.