WO2008087575A1

WO2008087575A1 - Speckle reduction in a projection system

Info

Publication number: WO2008087575A1
Application number: PCT/IB2008/050108
Authority: WO
Inventors: Rifat A. M. Hikmet; Johannes P. M. Ansems; Ties Van Bommel
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-01-19
Filing date: 2008-01-14
Publication date: 2008-07-24
Also published as: JP2010517070A; TW200846808A

Abstract

A projection system (1) comprising at least one coherent light source (2), a modulator (4) for modulating light from said light source, and a lens (8) for projecting modulated light from the modulator onto a screen (9). The system further has a fast switching liquid crystal (LC) element (6) positioned in the optical path between said at least one coherent light source and said lens (8), and adapted to scatter light passing said LC element; and a driver for varying the scattering of said LC element. By controlling the scattering of the LC element to be continuously changing whenever light is projected onto the screen, the coherency of the light is removed which reduces the speckle in the displayed image.

Description

Speckle reduction in a projection system

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a projection system comprising at least one coherent light source, a modulator for modulating light from said light source, and a lens for projecting modulated light from the modulator onto a screen.

The invention also relates to a method for reducing speckle in such a system.

BACKGROUND ART

The use of laser light sources in projection systems is desirable, seeing the many advantages the laser source provides in comparison to conventional white light sources, which waste a considerable amount of the light energy. In favor of the laser is, for instance, the laser only generating light based on the wavelength of the laser source, its small etendue, and its high efficiency along with a long lifetime.

The sources expected to realize the desired laser projection systems, are compact and high-power laser light sources. Heretofore, high-power red laser diodes (LDs) have been used for rewritable optical disk systems and green high-power second harmonic generation (SHG) green lasers have been realized. In the blue region, GaN semiconductor lasers and SHG lasers, using quasi-phase-matched (QPM) devices, have been developed. These developments in compact visible lasers based on semiconductor lasers, indicate possibilities of laser displays.

However, one of the important problems, which need to be solved to realize laser displays, is the suppression of interference and speckle noises super-positioned on projected images, due to the coherency of light sources. Speckle arises when coherent light scattered from a rough surface, such as a screen, is detected by a square-law detector with a finite aperture, such as an observer's eye. In theory, speckle is defined as the random intensity variation that is caused by the random interference of the light. An object with a rough surface, such as a screen, when illuminated with coherent light from a laser, exhibits a speckled appearance. Since variations in the surface are greater than the wavelength, coherent light scattered by the individual elements of the surface interferes to form a stationary pattern. The speckle appears to scintillate or sparkle when there is any relative movement of the surface and the observer. These intensity fluctuations, i.e. speckle, can to the eye of the observer be experienced as the perception of mainly dark areas with bright islands.

Suppression of described speckle can be obtained by diversification of light parameters, of which one parameter is the light scattering angle. Varying the scattering angle quickly and frequently over time will correspondingly cause the speckle pattern to change quickly and frequently over time. If the pattern is altered quickly enough, the eye of the observer will experience the speckle pattern smoothed out, and subsequently no speckle will be observed.

One common approach to obtain angle diversity is by employing a time varying diffuser, which is an optical element designed to produce multiple scattering. Accordingly, a solution to the problem of speckle in displays is to vibrate the screen at a high frequency, thereby causing the speckle pattern to change quickly and be smoothed out in such a way that the eye of the observer does not see it. This type of display requires the mechanical movement of the display screen, thus complex and costly to operate.

US 6 122 023, for instance, discloses a laser projection system where the display screen itself acts as a diffuser. The screen is a liquid crystal projection display screen constructed in a highly scattering state. A voltage is applied to the liquid crystal molecules, causing them to vibrate slightly and thereby reduce any speckle typically observed.

However, to have the screen itself form the diffuser, gives rise to restrictions. To use a liquid crystal element as display screen, results in the screen being expensive, especially for large screens.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to mitigate some of the problems with prior art solutions and to provide an improved speckle suppression of a laser source image, through angular diversity, which suppression is not restricted to the material composition of the projected display.

According to a first aspect of the invention, this and other objects are achieved by a projection system comprising a fast switching liquid crystal (LC) element positioned in the optical path between said at least one coherent light source and said lens, and adapted to scatter light passing said LC element; and a driver for varying the scattering of said LC element, so as to introduce angular diversity and thereby suppress speckle in an image on said screen. The term coherent light source is intended to include any light source providing light that is sufficiently coherent to cause speckle effects. Typically, the light source is some kind of laser, e.g. a diode laser, but the invention is not limited to this implementation.

According to the present invention, a diffuser in the form of a fast switching liquid crystal element scatters the light from the light source. By controlling the scattering of the LC element to be continuously changing whenever light is projected onto the screen, the coherency of the light is removed which reduces the speckle in the displayed image. Thus, the need for unreliable mechanics needed to move the diffuser, which traditionally is used to suppress speckle, is eliminated.

The diffuser of the present invention is positioned in the optical path between the light source and the projection lens, preferably at a focal point of the optics, such as in an intermediate image plane. The positioning differs from the system disclosed in above- mentioned US 6 122 023, in which the diffuser is integrated with the display, i.e. the screen itself forms the diffuser. By instead positioning the diffuser in the optical path, in accordance with the present invention, the screen onto which the system projects images is relieved from forming the element in which suppression of speckle is performed. Excluding the screen from bringing on the reduction additionally lightens the restrictions on the screen onto which the images are projected. For instance, the screen does not need to comprise material with diffusing characteristics, neither does it need to be applied with voltage. Subsequently, this facilitates the complexity of the screen, or area, onto which the images are projected, thus resulting in cost reductions.

The liquid crystal element is preferably integrated with the main unit of the projector. This facilitates the handling of the projection system, as no separate part outside the projector is required to accomplish suppression of speckle. Additionally, positioning the diffuser between the laser and the lens of the projector implies that the lens is the element most immediate to the screen. Arranging the lens closest to the screen is favorable, as the diffuser will then not interfere with the lens focusing the light beam onto the screen.

The liquid crystal element can be arranged to scatter the light within a focusing area of collecting optics following said LC element. Typically, this collecting optics is the projection lens of the system. Preferably, the beam profile FWHM, i.e. full beam width at half maximum after passing through the diffuser, is less than 45°, and even more preferably less than 20°. Arranging the diffuser so as to scatter the light in the forward direction, within the boundaries of the collecting optics, ensures that light losses are minimal.

If the scattering variation can be accomplished without introducing intensity variations, the variations will not be detected by an observer, and the rate of variation is not critical. As long as the scattering does not lead to observable intensity variation on the screen it can be very slowly changing.

If the time varying scattering creates varying intensity of the image, such variations may be detected by the eye of an observer. Therefore, in such cases, the switching of the liquid crystal element is preferably faster than the reaction of the human eye, i.e. with a frequency greater than around 50 Hz.

In addition to this, it is also advantageous if any impact on the average intensity caused by the scattering is constant. In other words the average intensity of any fully open pixel (letting through maximum light) in each image frame is held constant. Otherwise, this also could be detected by the eye of an observer.

The projection system can comprise deflection optics for scanning the modulated light across the screen. Deflection optics are required to form a complete image if the modulator is only adapted to produce a portion of the image which is to be projected.

The liquid crystal element can be pixilated, i.e. the diffuser cells can be divided into different controllable switching areas so as to form a patterned morphology. Such controllable areas can be created by structured electrodes. Pixelating the diffuser presents an additional manner in which the diffuser can be arranged to introduce time varying scattering or diffraction. This is for instance relevant for a diffuser comprising a homogenous liquid crystal, such as a ferroelectric liquid crystal material when not comprised in a gel.

Depending on the type of technology used for generating an image in the projection system, polarization of the light might be necessary. For example, Liquid Crystal cells require polarized light.

If this is the case, the polarization of the light needs to be preserved, and a polarization preserving diffuser should be used. An example of a polarization preserving LC element is a nematic anisotropic gel with a homogenous orientation. Such a gel scatters only one of the polarizations effectively.

Other technologies, such as Digital Micromirror Devices (DMD) and grating light valves, do not require polarized light. Consequently, in these cases, the liquid crystal element can be a polarization altering diffuser. A polarization altering diffuser will in addition to angular diversity induce polarization diversity, which may serve to even further reduce speckle.

Examples of a polarization altering LC element are polymer dispersed crystals (PDLC) or an anisotropic gel including one of nematic, ferroelectric or chiral liquid crystal material.

Other aspects, benefits and advantageous features of the present invention will be apparent from the following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more apparent from the accompanying drawings, which are provided by way of non- limiting examples.

Figure 1 shows an overview of a laser projection system in accordance with an embodiment of the present invention.

Figure 2a shows an example of how a time varying voltage applied to a liquid crystal element in accordance with an embodiment of the present invention affects the scattering of light emitted by the laser over time.

Figure 2b shows an alternative of the time varying voltage showed in Figure 2a.

Figure 2c shows yet another alternative of the time varying voltage showed in Figure 2a.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 shows in an exemplifying manner a laser projection system 1 in accordance with an embodiment of the present invention.

The system 1 in the illustrated example comprises a laser 2, from which light is emitted. Alternatively, there can be several lasers 2, for instance to accomplish several different colors of light.

The emitted light passes through illuminating optics 3 and is supplied to a modulator 4, which induces an effect for modulating the intensity of the light beam. The modulator 4 can be a single pixel, a line array of pixels, or a two dimensional array of pixels, and it may induce phase change, diffraction, scattering, and refraction as well as deflection.

The beam next passes through contrast producing optics 5. The choice of suitable contrast optics 5 is depending on the effect produced by the modulator 4. Contrast producing optics 5 can comprise polarisisers, diaphragms, and lenses, such as schlieren optics.

In the illustrated example, the beam next passes through a diffuser 6, i.e. a fast switching liquid crystal element. The diffuser 6 can be placed at an intermediate focal plane in the optical path, and alternatively, elsewhere in the optical path, e.g. in the optics 3, preceding the modulator .Next in the illustrated example, the beam passes through deflection optics 7, e.g. a scanner. When the modulator 4 is not adapted to produce the entire image, but only a fraction of the image, such as one pixel or an array of pixels, deflection optics 7 are used to build the entire image by scanning a beam across a screen 9. In any case, the light is focused onto the screen 9 by a lens 8, which focuses the modulated light onto the screen 9. As indicated in figure 1, the deflection optics 7 can be positioned before the lens 8. Alternatively, the deflection optics 7 can be positioned after the lens 8.

Contol electronics 10 control the laser 2, the modulator 4, the deflector 7 as well as the time varying diffuser 6.

In the illustrated example, the screen 9 is designed for front projection and thus the observer 11 is situated on the same side of the screen 9 as the laser front projection system 1, but alternatively, the screen 9 can be designed for rear projection. The screen can be any surface suitable for displaying the image formed by the modulated light. It can be reflective (front projection) or a transmissive (rear projection) screen, depending on the implementation.

The diffuser 6 can be a polarization altering diffuser, such as for instance, a polymer dispersed crystal (PDLC) or an isotropic gel diffuser 6 including one of nematic, ferroelectric or chiral liquid crystal material. Alternatively, the diffuser 6 can be a polarization preserving diffuser, such as for instance a switchable nematic anisotropic gel with a homogeneous orientation.

In the illustrated example, the diffuser 6 is arranged to scatter the light within a focusing area of the lens 8. Accordingly, the diffuser 6 scatters the light in small angles, so as to restrict the light beam from spreading outside the focusing area of the collecting optics, thus contributing to the intended image to be projected with high brightness and minimal light losses.

The desired beam profile of a parallel beam, after passing through the scattering cell, i.e. the diffuser 6, needs to have full width at half maximum (HWFM) of not more than 45°. HWFM most desirably needs to be in the range 2-30°.The diffuser 6 can be made by polymerization of a mixture reactive molecules and liquid crystals and should it be preferred, the diffuser 6 can also be polymerized in a pattern wise manner to produce a patterned morphology within the cells.

A well-known procedure to produce PDLC is by mixing a reactive monomer with a conventional liquid crystal, to obtain an isotropic mixture. Upon polymerization, phase separation is induced leading to the formation of liquid crystal droplets in the polymer matrix. As an electric field is applied to the PDLC system after polymerization, the PDLC system shows scattering effects.

Anisotropic gels are obtained by producing a mixture of a monomer and a liquid crystal, which is in the liquid crystal state. Upon polymerization, the system remains macroscopically oriented. As opposed to PDLC, gels are transparent after polymerization. Following applying an electric field, gels become translucent and induce light scattering. Nematic gels with a negative dielectric anisotropy in homotropic orientation show polarization-altering (polarization independent) scattering in the same way chiral gels do. Gels with positive dielectric anisotropy in homogeneous orientation show polarization- preserving (linear polarization direction dependent) scattering. Maximum angular scattering induced by such a system can be adjusted by the layer thickness morphology and the birefringence of the liquid crystal.

Applied to the diffuser 6 is a time varying voltage 12, which in the illustrated example is the same across the diffuser surface and which can be realized in different ways. Alternatively, the pixilated switching area cells within the diffuser 6 can be induced by using pixilated electrodes, as well as pixilated ferroelectrics is an option. In such cases, the applied time varying voltage 12 can vary across the surface of the diffuser 6 opposed to being the same across the surface.

The time varying voltage 12 generates an electric field pattern. The voltage 12 applied to the diffuser 6 gives rise to a time varying scattering 21a, 21b, 21c (Figure 2a, 2b, 2c), i.e. angular diversity, of the passing beam.

The diffuser cells need to have switching characteristics, which enables switching of the different switching areas within the cells to induce variation in the deflection angle of the scattered light, leading to suppression of speckle.

As is clear from Figure 2a, the applied voltage 12 is not necessarily continuous, but can be a pulsed signal 22. Nor does it have to have the same frequency. Each time a pulse (negative or positive) is applied to the diffuser 6 it generates an electric field across the surface layer of the diffuser cell, causing scattering 21a to increase. Due to leakage decays as a function of time, the effective voltage 22 across the cell, and thus the scattering 21a, decreases. Each pulse therefore provides an entire period of scattering variation, as indicated in Figure 2a. As the cell reacts the same way regardless of pulse polarity, the frequency of the varying scattering 21a will be twice that of the voltage 22.

Alternatively, as illustrated in an exemplifying manner in Figure 2b, the time varying voltage 12 applied to the diffuser 6 can be amplitude modulated. Applying an amplitude modulated voltage 12 makes the amplitude, and not the frequency, of the applied time varying voltage 12 affect the resulting time varying scattering pattern 21b. To accomplish a scattering pattern 21b which varies over time, a high frequency AC voltage 23 is applied, for which the amplitude varies as a function of time, causing the scattering pattern 21b to vary correspondingly. In this case, the negative or positive voltage 23 applied to the diffuser 6 is never equal to zero, resulting in that decay of the effective voltage across the diffuser cell is prevented.

As illustrated in Figure 2c, it is also possible to apply a continuously varying voltage pattern with a fixed amplitude 24, if the liquid crystal is fast enough to follow the frequency of the applied voltage 24. This will result in scattering 21c. It is likewise possible to vary both the shape and the magnitude of the applied voltage pattern.

As long as the scattering 21a, 21b, 21c does not give rise to intensity fluctuations, the frequency of the time varying voltage 12 is not critical.

However, if the time varying scattering 21a, 21b, 21c creates varying intensity of the image, such variations may be detected by the eye of the observer 11. Therefore, in such cases, the switching of the diffuser 6 is preferably faster than the reaction of the eye of the observer 11, i.e. a frequency greater than around 50 Hz is required.

Additionally, any impact on the average intensity caused by the scattering 21a, 21b, 21c is preferably constant for each displayed image frame. Otherwise, this also could be detected by the eye of the observer 11.

The present invention has been described above by way of example. Several variants of the present invention are, however, conceivable. For instance, additional and alternative components to those mentioned by way of example in Figure 1 should be considered part of the scope of the present invention. Such and other similar obvious alternatives must be considered to be comprised by the present invention as defined by the appended claims.

Claims

CLAIMS:

1. A projection system (1) comprising at least one coherent light source (2), a modulator (4) for modulating light from said light source, and a lens (8) for projecting modulated light from the modulator onto a screen (9), characterized by a fast switching liquid crystal (LC) element (6) positioned in the optical path between said at least one coherent light source and said lens (8), and adapted to scatter light passing said LC element; and a driver for varying the scattering of said LC element, so as to introduce angular diversity and thereby suppress speckle in an image on said screen.

2. The projection system according to claim 1, wherein said liquid crystal element is arranged to scatter light within a focusing area of collecting optics following said LC element.

3. The projection system according to claim 2, wherein said collection optics is said lens (8).

4. The projection system according to claim 2 or 3, wherein said LC element has a beam profile full width half maximum (FWHM) less than 45°, and preferably less than 20°.

5. The projection system according to any one of the preceding claims, wherein said driver is adapted to vary said scattering (21a, 21b, 21c) with a frequency greater than 50 Hz.

6. The projection system according to any one of the preceding claims, wherein said driver is adapted to vary said scattering (21a, 21b, 21c) in such a way that the average intensity of a fully open pixel in each image frame is held essentially constant.

7. The projection system according to any one of the preceding claims, further comprising deflection optics (7) for scanning the modulated light across the screen (9).

8. The projection system according to any one of the preceding claims, wherein said liquid crystal element is pixilated.

9. The projection system according to any one of the preceding claims, wherein said liquid crystal element is a polarization altering diffuser.

10. The projection system according to claim 1, wherein said liquid crystal element is a polymer dispersed crystal (PDLC).

11. The projection system according to claim 1 , wherein said liquid crystal element include one of nematic, ferroelectric or chiral liquid crystal material.

12. The projection system according to claim 1, wherein said liquid crystal element is a switchable anisotropic gel.

13. A method for reducing speckle in an image projected on a screen by a projection system (1) comprising at least one coherent light source (2), a modulator (4) for modulating light from said light source, and a lens (8) for projecting modulated light from the modulator onto a screen (9), characterized by varying scattering of a liquid crystal (LC) element (6) positioned in the optical path between said at least one coherent light source and said lens (8), so as to introduce angular diversity in the projected light.

14. The method according to claim 13, wherein the scattering is varied by applying a voltage to the LC element.

15. The method according to claim 14, said voltage is a series of discrete pulses, such that the scattering increases for a leading flank of each pulse, and decreases for a trailing flank of each pulse.

16. The method according to claim 14, said voltage is amplitude modulated, such that the scattering adjusts to each amplitude level of the voltage.

17. The method according to one of claims 13-16, wherein said scattering is controlled to vary with a frequency greater than 50 Hz.

18. The method according to one of claims 13-17, wherein said scattering is controlled to vary in such a way that the average intensity of a fully open pixel in each image frame is held essentially constant.