CA2366566C - Method and device for reducing speckle formation on a projection screen - Google Patents
Method and device for reducing speckle formation on a projection screen Download PDFInfo
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- CA2366566C CA2366566C CA002366566A CA2366566A CA2366566C CA 2366566 C CA2366566 C CA 2366566C CA 002366566 A CA002366566 A CA 002366566A CA 2366566 A CA2366566 A CA 2366566A CA 2366566 C CA2366566 C CA 2366566C
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
- G02B27/48—Laser speckle optics
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
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Abstract
The invention relates to a method and a device for reducing the speckle that is formed on a projection display when a coherent light source (1) is used.
Before being projected, the light coming from the light source strikes an electrically controllable optical element (4) with a spatially inhomogeneous index of refraction and penetrates said optical element, whereby the index of refraction within the period of projection is temporally altered. This leads to the averaging out of the speckle pattern on the projection screen (6). A multimode light source is preferably used for illumination and/or the light coming from the light source is split into several spatial modes in order to increase the effect. The light from the light source is preferably split by being coupled into a multimode light-conducting fibre. Said optical element is advantageously a liquid crystal element consisting of at least two liquid crystal layers, to which a position-dependent voltage is applied in order to produce a position-dependent index of refraction and whose birefringence is compensated by the appropriate alignment of the layers in relation to each other.
Before being projected, the light coming from the light source strikes an electrically controllable optical element (4) with a spatially inhomogeneous index of refraction and penetrates said optical element, whereby the index of refraction within the period of projection is temporally altered. This leads to the averaging out of the speckle pattern on the projection screen (6). A multimode light source is preferably used for illumination and/or the light coming from the light source is split into several spatial modes in order to increase the effect. The light from the light source is preferably split by being coupled into a multimode light-conducting fibre. Said optical element is advantageously a liquid crystal element consisting of at least two liquid crystal layers, to which a position-dependent voltage is applied in order to produce a position-dependent index of refraction and whose birefringence is compensated by the appropriate alignment of the layers in relation to each other.
Description
~ CA 02366566 2001-10-11 METHOD AND DEVICE FOR REDUCING SPECKLE FORMATION
ON A PROJECTION SCREEN
Field of the Invention The present invention is directed to a method and a device for reducing speckle formation on a projection screen.
Background of the Invention Speckle patterns are irregular, fine-grain light distributions which occur when illuminating white walls, projections screens and other surfaces - referred to in the following as projection screens - using widened, coherent light, in particular laser light. The speckle pattern is formed when a spot of light is imaged on the projection screen, due to the high coherence caused by interference of the light waves scattered at various points on the projection screen.
Therefore, the interference pattern exhibits the stochastic fine structure of the reflecting screen. The average size of a speckle grain depends on the aperture of the coherently illuminated spot on the screen. The larger the light spot is, the finer is the graininess of the speckle pattern. The contrast in the speckle grains is determined by the coherence of the light source. The speckle pattern disappears when the coherence length of the light falls perceptibly below the average roughness of the screen.
To optically reproduce images, laser-based projection methods are being used to an increasing degree. In contrast to image rendition using cathode-ray tubes or liquid-crystal displays, the laser projection technique has the advantage of fundamentally enabling a high-quality image to be attained NY01 410503 v 1 REVISED PAGES
with an unlimited image size. In this context, the laser beam for displaying the image to be rendered is rasterized similarly to an electron beam in a picture tube via a projection screen.
The speckle formation encountered in projection methods using lasers or other coherent light sources is disadvantageous.
Speckles occur, in particular, when the image is built up from individual image points, line-by-line, and, to this end, laser beams are focused on the projection screen. Due to the small image points, the speckle pattern is usually coarse-grained and is perceived by the observer as a disturbing glittering of the individual image points.
Various basic approaches are known for suppressing speckle formation. An overview is given in the article by Toshiaki Iwai and Toshimitsu Asakura, "Speckle Reduction in Coherent Information Processing", Proceedings of the IEEE, vol. 84, no.
5, May 1996, pp. 765-780. The methods described can be broken down into methods for controlling spatial coherence, controlling temporal coherence, each implemented by manipulating the light source, spatial scanning, spatial averaging, and speckle reduction through digital image processing.
It is known, for example, to use a pulsed laser light source having a small pulse length, thereby reducing the coherence length of the laser light and minimizing speckle formation.
However, this only permits the use of laser systems, which are able to be simply modulated, externally or internally.
Furthermore, the spatial coherence of the laser light can be reduced by passing the laser light through a rotating ground glass screen or by scattering it at one or a plurality of optical diffusers. It is also known to couple coherent laser light into a multimode optical fiber and to deform the fiber by subjecting it to rotation or vibration. At the end of the fiber, the light emerges, having been separated into a NY01 410503 v 1 2 REVISED PAGES
multiplicity of modes in the local space, each mode having traversed a different optical path and, therefore, having different phase positions. By vibrating or rotating the fiber, the mode distribution is varied over time. Thus, a temporal and spatial average is generated over the phase pattern being formed, and an incoherent, even if multimode, light source is provided. Here, the disadvantage is that this mechanical approach for thoroughly mixing the modes can adversely affect the stability of the overall arrangement.
To reduce the temporal coherence of the laser light, it is known to vary the wavelength of the laser light or to use a plurality of wavelengths at the same time. For example, to reduce speckles, a method has been proposed which is based on a change in the wavelengths of laser diodes caused by mode jumps. Other lasers as well, which are subject to random fluctuations, come into consideration for this.
An alternative approach for reducing speckle formation is described by German Patent 196 45 976 Cl. It provides for using a projection screen, whose projection depth is greater than the coherence length, so that the reflected or transmitted wave field becomes incoherent. This entails the disadvantage that the image points are diffusely enlarged by the surface structure of the screen, as well as the limitation of always having to use a specially prepared screen for image rendition.
From the WO 96 21 883 A, a projection screen is known, whose surface is formed in irregular fashion, inter alia, for purposes of reducing speckles, such that the Fourier spectrum of the surface exhibits higher frequencies than that of a pixel structure projected onto the screen.
From the DE 195 08 754 Al, a method is known for reducing the interference of a coherent light beam, the light being polarized variably with respect to location, in a direction NY01 410503 v 1 3 REVISED PAGES
perpendicular to the direction of propagation. In this case, the circumstance is utilized that different polarization states of the light are no longer able to completely interfere with one another. The required polarization states can be produced, for example, with the assistance of LCD matrices.
From the DE 107 10 660 A, a device is known for removing screen speckles when working with scanning laser-image projection, the laser beam being split with the assistance of an ultrasound cell in which density waves travel, by the diffraction of the density waves into various orders of diffraction of different frequencies. The beam components are superposed using a lens. In this manner, a moving system of interference patterns is formed on the projection screen, so that the forming speckles overlap one another in the eye of the observer due to the integration process, and become averaged out in time and space.
U.S. Patent 3,941,456 A describes a device for reducing granulation, which occurs when transmitting optical information using a high-grade, coherent light beam. The light beam propagates through an ultrasound cell in which, depending on the excitation, standing or traveling density waves form.
The density waves influence the refractive index locally, so that the light beam propagates through zones having different refractive indices, resulting in a reduction in the granulation.
From the publication "Perceived Speckle Reduction in Projection Display Systems" in IBM Technical Disclosure Bulletin, U.S., IBM Corp. New York, vol. 40, no. 7, July 1, 1997, pp. 9-11, XP 000728388, ISSN 0018-8689, it is known to reduce speckles in that the light beam propagates through a liquid crystal, whose refractive index is influenced by an electrical field.
From U.S. 4,647,158 A, a method and a device are known, which NY01 410503 v 1 4 REVISED PAGES
are used to convert a beam of coherent light using a controllable diffraction grating into a beam of incoherent light.
Technical Objective The object of the present invention is, therefore, to provide a method and a device for reducing speckle formation, which will avoid the disadvantages of the related art.
Summary of the Invention The objective is achieved according to one aspect of the present invention by a device for reducing speckle formation on a projection screen, using a coherent light source, comprising an electrically controllable optical elejnent having a spatially inhomogeneous refractive index, which is variable over time, the optical element being configured between the light source and the projection screen, wherein the electrically controllable optical element includes a liquid crystal element, to which a temporally variable voltage gradient may be applied to control the temporally and spatially dependent refractive index.
In another aspect, there is provided a method for operating such a device wherein the light coming from the light source, before the projection, strikes an electrically controllable optical element having a spatially inhomogeneous refractive index, passing through the same, the refractive index being varied over time within the projection period.
Advantageous further embodiments of the method and of the device are characterized in the dependent claims.
The functional principle of embodiments of the present invention is as follows: The light generated by the coherent light source, in particular by a laser, strikes the optical element before the actual projection device, which directs the light to the projection screen, and, in fact, in the simplest embodiment of the present invention, always at a fixed angle of incidence.
The optical element has the property of deflecting the light in a manner that is varied over time. Because the refractive index of the optical element is spatially inhomogeneous over the irradiated surface, the incident beam is deflected in a direction that differs from the initial beam direction. To achieve the desired effect when the given refractive index profile exhibits maximal differences in refractive indices, the beam may be widened before striking the light-deflecting optical element, and, after that, collimated again. This spatial deflection of the beam is modified by varying the refractive index profile over time. Even at the smallest angle variations of less than one degree in the beam direction, this leads to an averaging out of the speckle patterns on the projection screen. However, in the simplest variant of the present invention, it also leads to a slight widening of the light spot on the screen. To ensure that the eye merely perceives an averaged image, the temporal variation in the refractive index should be carried out repeatedly within the reaction time of the eye, i.e., the optical element should be driven at switching frequencies of about 100 Hz. In the case of a rasterized projection of an image, where the projected light spot dwells for just a certain time on a projection location on the screen, the refractive index may be repeatedly temporally varied within this dwell period.
In one advantageous further embodiment of the present invention, the thus achieved effect of speckle reduction is reinforced by using a multimode light source, and/or by separating the light coming from the light source into a plurality of spatial modes. Thus, the light striking the optical element is composed of a plurality of modes, each having a particular spatial characteristic, such as beam profile and angle of emergence, which are superimposed on one another. Because of the inhomogeneous refractive index, the NY01 410503 v 1 6 REVISED PAGES
individual modes are variably spatially deflected; they are spatially intermixed due to the temporal variation of the refractive index. Therefore, the speckle patterns are averaged out. In the process, the need for mechanically intermixing by using vibrators or rotating elements in the optical path of rays is advantageously eliminated. Essentially the same effect is achieved, however. In this context, the differences in refractive indices are to be selected such that no substantial widening in the projected light spot occurs.
In the case of rasterized imaging of the light on the projection screen, the temporal variation in the refractive index of the optical element for averaging out the speckle patterns is to be selected such that the spatial modes are spatially deflected multiple times within the dwell period of one image spot on the projection screen, at least, however, once between two images.
To split up the light radiated by a single-mode light source, it is preferably coupled into a first multimode optical fiber.
Depending on the in-coupling conditions and, as the case may be, also on the mechanical stressing of the fiber, modes other than the original mode are excited and transmitted, so that, after propagating through the fiber, the light is composed of a superposition of a plurality of spatial modes. The light then strikes the optical element. Depending on the location of incidence, the individual spatial components are deflected in slightly different directions due to the different refractive indices.
Advantageously linked to the optical element is an additional optical fiber, whose light is used for the projection. By coupling the light into this fiber, the light may be fed in a defined manner to the actual projection device. In this manner, one avoids a "blurring" of the light spot on the projection display, while the effect of averaging the speckles is retained.
NY01 410503 v 1 7 REVISED PAGES
Preferably, this fiber that is connected downstream from the optical element is a multimode fiber, which, as the case may be, is added to the first fiber connected upstream from the optical element. Since the deflection directions are varied by the optical element, in this case of the second multimode fiber, different modes are excited and transmitted each time.
Thus, the second fiber renders possible a further intermixing and averaging of the coherence effects.
As an optical element, a liquid crystal element is advantageously used. Liquid crystals are semiliquid solutions or mixtures of large molecules, which orient themselves to one another in the liquid, resulting in a birefringent liquid crystal layer. The birefringence may be influenced by an externally applied electric field. In the process, both a voltage-proportional variation, as well as a non-linear, steplike variation occurs in the case of a threshold voltage.
Because of these electrooptical properties, liquid crystals may be used to control the phase of a light wave passing through them. To convert the present invention to practice, a liquid crystal element is used, for example. A spatially variable voltage, for example a voltage gradient, is applied thereto to produce a spatially inhomogeneous refractive index distribution. When working with elements having voltage-proportional birefringence, the birefringence changes accordingly. A thus produced birefringent gradient acts for a polarization direction as a refractive index gradient, which deflects a light beam of this polarization. When working with the small refractive index gradients that can be generated within the operating range and the small layer thicknesses of known liquid-crystal cells, the spatial deflection is slight, but suffices for mixing modes along the lines of the present invention. As described above, the effect may be reinforced by adding a downstream multimode fiber.
One especially advantageous further embodiment is constituted by a device having an optical element, which remains isotropic NY01 410503 v 1 8 REVISED PAGES
in response to the application of a voltage, i.e., it may allow a spatially varying refractive index to be established, but it does so without being birefringent. The birefringent properties of simple liquid-crystal elements having one single liquid crystal cell (partial element) lead to a change in the polarization when passing through the element. For most applications, however, it is beneficial and desirable to have a polarization-independent manipulability.
Liquid-crystal elements, which remain isotropic in response to the application of a voltage, have been proposed by the non-prepublished German Patent Application 198 52 890.6. They are composed of two or more liquid-crystal layers as partial elements, in particular of helical, smectic, ferroelectric liquid crystals, which are so oriented in relation to one another, that their birefringence is compensated for all applied voltages. For example, two layers are aligned orthogonally to one another, so that the slow axis of the first layer is aligned perpendicularly the fast axis of the second layer, and the fast axis of the first layer is aligned perpendicularly the slow axis of the second layer. An isotropic refraction of the entire layer sequence remains; the polarization of transmitted light is retained.
In accordance with the present invention, the optical elements described in German Patent 198 52 890.6 are further developed in such a way that, instead of a constant voltage, a voltage gradient is produced over the surface of the cells. The spatially dependent voltage is to be selected at the various cells of the optical element such that the polarization of the light is the same independently of the pass-through location upstream and downstream from the optical element. Especially advantageous is the use of an optical element made up of two such liquid crystal layers, the slow axis of the first layer being aligned perpendicularly the fast axis of the second layer, and the fast axis of the first layer being aligned perpendicularly the slow axis of the second layer. A voltage NY01 410503 v 1 9 REVISED PAGES
gradient is applied to both layers so that the voltage at one location is more or less the same, orthogonally to the beam direction for both layers. In general, it suffices to have one voltage gradient in one spatial direction. Advantageous is, however, when the voltage gradient is applied alternately in the x- and y-directions. Alternatively, a rotating field can be applied to the liquid crystal layers.
To average out the speckle patterns, it is necessary to change the spatially dependent refractive index by switching over the applied voltage. This is done repeatedly over the reaction time of the eye. As a rule, laser projection systems produce the image on a point-to-point basis using a raster procedure.
In such a case, the image spot must dwell on the projection screen for a time period tl, which is greater than the time period t2 of the refractive index variation, preferably at least five times t2.
Accordingly, for displays having, for example, 1000 times 1000 image points and 100 individual images per second, the required switching time for the liquid crystal cell would amount to approximately 0.5 GHz. At the present time, such high switching frequencies can, in fact, be reached using electrooptical crystals, not, however, by using liquid crystal cells. To improve the image quality of such highly resolving systems as well, by reducing the speckle formation, projection systems are advantageously employed which not only use one laser, but an entire laser array, for example columns having a plurality of lasers. When such a system is used, a greater number of lines of the image can be simultaneously constructed and projected. In this manner, one reduces the period of time tl and, accordingly, also the requisite switching time t2 when each individual projection laser is provided with an optical element, in particular a liquid crystal element, along the lines of the present invention. If 100 lasers are simultaneously used for projection purposes in the above example, then switching frequencies of about 5 MHz are NY01 410503 v 1 10 REVISED PAGES
required. This is already within the range of present-day liquid crystal development.
In many cases, however, an adequate speckle suppression can even be effected by a switching frequency that is on the order of the image frequency of the projection system.
Brief description of the drawing in which:
Figure 1 shows a laser projection system having a device for reducing speckle formation;
Figures 2, 3 illustrate a switchable liquid crystal element for generating a refractive index gradient.
Figure la depicts a laser projection system having a device for reducing speckle formation through mode mixing. The projection system includes a laser 1 as a light source. In color displays, a plurality of lasers having different wavelengths are used. The laser light is coupled by an optical arrangement 2, shown schematically here as a lens, into a first multimode optical fiber Ml, whose output is mapped onto the input of a second multimode optical fiber M2; compare Figure lb. Situated at the output of the second multimode optical fiber M2 is the actual projection unit 5, which is used to project the laser beam point-by-point onto projection screen 6.
The mode-mixing unit 4 is schematically shown in Figure lb.
The light emerging from first multimode fiber Ml is already constituted of a superposing of a plurality of modes. It strikes an optical element 7 having a spatially inhomogeneous, electrically variable refractive index. The electrical driving of element 7 is schematically indicated by a signal line 8.
Optical element 7 is capable of deflecting incident and transmitted light in a spatially dependent fashion. For this reason, the image of the in-coupled light changes at the output of element 7 and at the input of second fiber M2, respectively. The individual modes from fiber Ml are coupled, NY01 410503 v 1 11 REVISED PAGES
as spatially altered modes, at various locations and at various angles, into second multimode fiber M2 and, consequently, excite in M2 a mode that differs from the original one. This appears at the output of M2 and, therefore, on projection screen 6, likewise at a slightly different location. If the light-deflecting properties of optical element 7 are quickly varied within the reaction time of the eye, the result is that the individual modes are scrambled on projection screen 6. Since each mode has a different phase lag, the speckle patterns are averaged out and become blurred on the projection screen. In this context, the image spot dwells on the projection screen for a time period tl, which is greater than the time period t2 of the refractive index variation, preferably at least five times t2.
An averaging of speckles among various individual images requires only multiple variations in the refractive index within the reaction time of the eye. For this, about 1000 Hz suffice.
Figure 2 illustrates a switchable liquid crystal element 9 having birefringence that may be compensated, for producing a refractive-index gradient. Liquid crystal element 9 is composed of two cells, each having a liquid crystal layer 10, 11, which is disposed between two transparent electrodes 13, 13', and 14, 14', respectively. The molecules orient themselves within the layers, it being possible to influence the electrooptical properties by the voltage applied between the particular electrodes. The orientations of the indicatrices of the two liquid crystal layers 10, 11 are described by vectors I1 and 12. They are oriented in a direction normal to one another and to the beam direction of incident light beam 12. In accordance with the present invention, a voltage gradient is applied to both cells, in a direction normal to the beam direction, in this case in the y-direction. The spatial voltage characteristic is to be selected, in this context, such that voltage V(y) at locus y NY01 410503 v 1 12 REVISED PAGES
is the same for both cells. Consequently, the orthogonal alignment of the indicatrices is retained for each y-value, so that layer packet 10, 11 remains isotropic. By properly selecting the resistance of the electrodes, the current is kept low in the electrodes.
To set the average cell voltage, a voltage VO is applied to both cells at locus y=yO. Applying the voltage gradient deflects light beam 12, which is incident upon the cells at right angles, slightly in the y-direction; as schematically output directions 12' and, subsequent to switching over, by 12", respectively. This deflection is used for mode mixing between two multimode glass fibers, e.g., in accordance with Figure 1, and, thus, for suppressing the formation of speckle patterns in accordance with the present invention.
An improvement in speckle reduction through intensified mode intermixing is attained by using liquid crystal elements having a more complex deflection property. An example of such a liquid crystal element 9' is shown in Figure 3. Its structure that includes two liquid crystal layers 10, 11 essentially corresponds to that of Figure 2. In contrast to the liquid crystal element of Figure 2, here electrodes 15, 15' and 16, 16', respectively, are additionally provided. They may be used to apply a voltage gradient to the particular cell, simultaneously or alternatively to the voltage gradient in the y-direction, perpendicularly thereto in the x-direction. Preferably, the direction is quickly changed. As a result, the direction of the voltage gradient and, thus, the deflection direction of the incident light may be altered for the intermixing of the modes. Here, the equalization condition for compensating for the birefringence of the individual cell is the matching of the voltages in the individual cells at each irradiated locus (x, y).
Industrial Applicability:
NY01 410503 v 1 13 REVISED PAGES
The present invention is advantageously suited for commercial use, to improve the image quality of laser projections systems by suppressing speckle patterns.
NY01 410503 v 1 14 REVISED PAGES
ON A PROJECTION SCREEN
Field of the Invention The present invention is directed to a method and a device for reducing speckle formation on a projection screen.
Background of the Invention Speckle patterns are irregular, fine-grain light distributions which occur when illuminating white walls, projections screens and other surfaces - referred to in the following as projection screens - using widened, coherent light, in particular laser light. The speckle pattern is formed when a spot of light is imaged on the projection screen, due to the high coherence caused by interference of the light waves scattered at various points on the projection screen.
Therefore, the interference pattern exhibits the stochastic fine structure of the reflecting screen. The average size of a speckle grain depends on the aperture of the coherently illuminated spot on the screen. The larger the light spot is, the finer is the graininess of the speckle pattern. The contrast in the speckle grains is determined by the coherence of the light source. The speckle pattern disappears when the coherence length of the light falls perceptibly below the average roughness of the screen.
To optically reproduce images, laser-based projection methods are being used to an increasing degree. In contrast to image rendition using cathode-ray tubes or liquid-crystal displays, the laser projection technique has the advantage of fundamentally enabling a high-quality image to be attained NY01 410503 v 1 REVISED PAGES
with an unlimited image size. In this context, the laser beam for displaying the image to be rendered is rasterized similarly to an electron beam in a picture tube via a projection screen.
The speckle formation encountered in projection methods using lasers or other coherent light sources is disadvantageous.
Speckles occur, in particular, when the image is built up from individual image points, line-by-line, and, to this end, laser beams are focused on the projection screen. Due to the small image points, the speckle pattern is usually coarse-grained and is perceived by the observer as a disturbing glittering of the individual image points.
Various basic approaches are known for suppressing speckle formation. An overview is given in the article by Toshiaki Iwai and Toshimitsu Asakura, "Speckle Reduction in Coherent Information Processing", Proceedings of the IEEE, vol. 84, no.
5, May 1996, pp. 765-780. The methods described can be broken down into methods for controlling spatial coherence, controlling temporal coherence, each implemented by manipulating the light source, spatial scanning, spatial averaging, and speckle reduction through digital image processing.
It is known, for example, to use a pulsed laser light source having a small pulse length, thereby reducing the coherence length of the laser light and minimizing speckle formation.
However, this only permits the use of laser systems, which are able to be simply modulated, externally or internally.
Furthermore, the spatial coherence of the laser light can be reduced by passing the laser light through a rotating ground glass screen or by scattering it at one or a plurality of optical diffusers. It is also known to couple coherent laser light into a multimode optical fiber and to deform the fiber by subjecting it to rotation or vibration. At the end of the fiber, the light emerges, having been separated into a NY01 410503 v 1 2 REVISED PAGES
multiplicity of modes in the local space, each mode having traversed a different optical path and, therefore, having different phase positions. By vibrating or rotating the fiber, the mode distribution is varied over time. Thus, a temporal and spatial average is generated over the phase pattern being formed, and an incoherent, even if multimode, light source is provided. Here, the disadvantage is that this mechanical approach for thoroughly mixing the modes can adversely affect the stability of the overall arrangement.
To reduce the temporal coherence of the laser light, it is known to vary the wavelength of the laser light or to use a plurality of wavelengths at the same time. For example, to reduce speckles, a method has been proposed which is based on a change in the wavelengths of laser diodes caused by mode jumps. Other lasers as well, which are subject to random fluctuations, come into consideration for this.
An alternative approach for reducing speckle formation is described by German Patent 196 45 976 Cl. It provides for using a projection screen, whose projection depth is greater than the coherence length, so that the reflected or transmitted wave field becomes incoherent. This entails the disadvantage that the image points are diffusely enlarged by the surface structure of the screen, as well as the limitation of always having to use a specially prepared screen for image rendition.
From the WO 96 21 883 A, a projection screen is known, whose surface is formed in irregular fashion, inter alia, for purposes of reducing speckles, such that the Fourier spectrum of the surface exhibits higher frequencies than that of a pixel structure projected onto the screen.
From the DE 195 08 754 Al, a method is known for reducing the interference of a coherent light beam, the light being polarized variably with respect to location, in a direction NY01 410503 v 1 3 REVISED PAGES
perpendicular to the direction of propagation. In this case, the circumstance is utilized that different polarization states of the light are no longer able to completely interfere with one another. The required polarization states can be produced, for example, with the assistance of LCD matrices.
From the DE 107 10 660 A, a device is known for removing screen speckles when working with scanning laser-image projection, the laser beam being split with the assistance of an ultrasound cell in which density waves travel, by the diffraction of the density waves into various orders of diffraction of different frequencies. The beam components are superposed using a lens. In this manner, a moving system of interference patterns is formed on the projection screen, so that the forming speckles overlap one another in the eye of the observer due to the integration process, and become averaged out in time and space.
U.S. Patent 3,941,456 A describes a device for reducing granulation, which occurs when transmitting optical information using a high-grade, coherent light beam. The light beam propagates through an ultrasound cell in which, depending on the excitation, standing or traveling density waves form.
The density waves influence the refractive index locally, so that the light beam propagates through zones having different refractive indices, resulting in a reduction in the granulation.
From the publication "Perceived Speckle Reduction in Projection Display Systems" in IBM Technical Disclosure Bulletin, U.S., IBM Corp. New York, vol. 40, no. 7, July 1, 1997, pp. 9-11, XP 000728388, ISSN 0018-8689, it is known to reduce speckles in that the light beam propagates through a liquid crystal, whose refractive index is influenced by an electrical field.
From U.S. 4,647,158 A, a method and a device are known, which NY01 410503 v 1 4 REVISED PAGES
are used to convert a beam of coherent light using a controllable diffraction grating into a beam of incoherent light.
Technical Objective The object of the present invention is, therefore, to provide a method and a device for reducing speckle formation, which will avoid the disadvantages of the related art.
Summary of the Invention The objective is achieved according to one aspect of the present invention by a device for reducing speckle formation on a projection screen, using a coherent light source, comprising an electrically controllable optical elejnent having a spatially inhomogeneous refractive index, which is variable over time, the optical element being configured between the light source and the projection screen, wherein the electrically controllable optical element includes a liquid crystal element, to which a temporally variable voltage gradient may be applied to control the temporally and spatially dependent refractive index.
In another aspect, there is provided a method for operating such a device wherein the light coming from the light source, before the projection, strikes an electrically controllable optical element having a spatially inhomogeneous refractive index, passing through the same, the refractive index being varied over time within the projection period.
Advantageous further embodiments of the method and of the device are characterized in the dependent claims.
The functional principle of embodiments of the present invention is as follows: The light generated by the coherent light source, in particular by a laser, strikes the optical element before the actual projection device, which directs the light to the projection screen, and, in fact, in the simplest embodiment of the present invention, always at a fixed angle of incidence.
The optical element has the property of deflecting the light in a manner that is varied over time. Because the refractive index of the optical element is spatially inhomogeneous over the irradiated surface, the incident beam is deflected in a direction that differs from the initial beam direction. To achieve the desired effect when the given refractive index profile exhibits maximal differences in refractive indices, the beam may be widened before striking the light-deflecting optical element, and, after that, collimated again. This spatial deflection of the beam is modified by varying the refractive index profile over time. Even at the smallest angle variations of less than one degree in the beam direction, this leads to an averaging out of the speckle patterns on the projection screen. However, in the simplest variant of the present invention, it also leads to a slight widening of the light spot on the screen. To ensure that the eye merely perceives an averaged image, the temporal variation in the refractive index should be carried out repeatedly within the reaction time of the eye, i.e., the optical element should be driven at switching frequencies of about 100 Hz. In the case of a rasterized projection of an image, where the projected light spot dwells for just a certain time on a projection location on the screen, the refractive index may be repeatedly temporally varied within this dwell period.
In one advantageous further embodiment of the present invention, the thus achieved effect of speckle reduction is reinforced by using a multimode light source, and/or by separating the light coming from the light source into a plurality of spatial modes. Thus, the light striking the optical element is composed of a plurality of modes, each having a particular spatial characteristic, such as beam profile and angle of emergence, which are superimposed on one another. Because of the inhomogeneous refractive index, the NY01 410503 v 1 6 REVISED PAGES
individual modes are variably spatially deflected; they are spatially intermixed due to the temporal variation of the refractive index. Therefore, the speckle patterns are averaged out. In the process, the need for mechanically intermixing by using vibrators or rotating elements in the optical path of rays is advantageously eliminated. Essentially the same effect is achieved, however. In this context, the differences in refractive indices are to be selected such that no substantial widening in the projected light spot occurs.
In the case of rasterized imaging of the light on the projection screen, the temporal variation in the refractive index of the optical element for averaging out the speckle patterns is to be selected such that the spatial modes are spatially deflected multiple times within the dwell period of one image spot on the projection screen, at least, however, once between two images.
To split up the light radiated by a single-mode light source, it is preferably coupled into a first multimode optical fiber.
Depending on the in-coupling conditions and, as the case may be, also on the mechanical stressing of the fiber, modes other than the original mode are excited and transmitted, so that, after propagating through the fiber, the light is composed of a superposition of a plurality of spatial modes. The light then strikes the optical element. Depending on the location of incidence, the individual spatial components are deflected in slightly different directions due to the different refractive indices.
Advantageously linked to the optical element is an additional optical fiber, whose light is used for the projection. By coupling the light into this fiber, the light may be fed in a defined manner to the actual projection device. In this manner, one avoids a "blurring" of the light spot on the projection display, while the effect of averaging the speckles is retained.
NY01 410503 v 1 7 REVISED PAGES
Preferably, this fiber that is connected downstream from the optical element is a multimode fiber, which, as the case may be, is added to the first fiber connected upstream from the optical element. Since the deflection directions are varied by the optical element, in this case of the second multimode fiber, different modes are excited and transmitted each time.
Thus, the second fiber renders possible a further intermixing and averaging of the coherence effects.
As an optical element, a liquid crystal element is advantageously used. Liquid crystals are semiliquid solutions or mixtures of large molecules, which orient themselves to one another in the liquid, resulting in a birefringent liquid crystal layer. The birefringence may be influenced by an externally applied electric field. In the process, both a voltage-proportional variation, as well as a non-linear, steplike variation occurs in the case of a threshold voltage.
Because of these electrooptical properties, liquid crystals may be used to control the phase of a light wave passing through them. To convert the present invention to practice, a liquid crystal element is used, for example. A spatially variable voltage, for example a voltage gradient, is applied thereto to produce a spatially inhomogeneous refractive index distribution. When working with elements having voltage-proportional birefringence, the birefringence changes accordingly. A thus produced birefringent gradient acts for a polarization direction as a refractive index gradient, which deflects a light beam of this polarization. When working with the small refractive index gradients that can be generated within the operating range and the small layer thicknesses of known liquid-crystal cells, the spatial deflection is slight, but suffices for mixing modes along the lines of the present invention. As described above, the effect may be reinforced by adding a downstream multimode fiber.
One especially advantageous further embodiment is constituted by a device having an optical element, which remains isotropic NY01 410503 v 1 8 REVISED PAGES
in response to the application of a voltage, i.e., it may allow a spatially varying refractive index to be established, but it does so without being birefringent. The birefringent properties of simple liquid-crystal elements having one single liquid crystal cell (partial element) lead to a change in the polarization when passing through the element. For most applications, however, it is beneficial and desirable to have a polarization-independent manipulability.
Liquid-crystal elements, which remain isotropic in response to the application of a voltage, have been proposed by the non-prepublished German Patent Application 198 52 890.6. They are composed of two or more liquid-crystal layers as partial elements, in particular of helical, smectic, ferroelectric liquid crystals, which are so oriented in relation to one another, that their birefringence is compensated for all applied voltages. For example, two layers are aligned orthogonally to one another, so that the slow axis of the first layer is aligned perpendicularly the fast axis of the second layer, and the fast axis of the first layer is aligned perpendicularly the slow axis of the second layer. An isotropic refraction of the entire layer sequence remains; the polarization of transmitted light is retained.
In accordance with the present invention, the optical elements described in German Patent 198 52 890.6 are further developed in such a way that, instead of a constant voltage, a voltage gradient is produced over the surface of the cells. The spatially dependent voltage is to be selected at the various cells of the optical element such that the polarization of the light is the same independently of the pass-through location upstream and downstream from the optical element. Especially advantageous is the use of an optical element made up of two such liquid crystal layers, the slow axis of the first layer being aligned perpendicularly the fast axis of the second layer, and the fast axis of the first layer being aligned perpendicularly the slow axis of the second layer. A voltage NY01 410503 v 1 9 REVISED PAGES
gradient is applied to both layers so that the voltage at one location is more or less the same, orthogonally to the beam direction for both layers. In general, it suffices to have one voltage gradient in one spatial direction. Advantageous is, however, when the voltage gradient is applied alternately in the x- and y-directions. Alternatively, a rotating field can be applied to the liquid crystal layers.
To average out the speckle patterns, it is necessary to change the spatially dependent refractive index by switching over the applied voltage. This is done repeatedly over the reaction time of the eye. As a rule, laser projection systems produce the image on a point-to-point basis using a raster procedure.
In such a case, the image spot must dwell on the projection screen for a time period tl, which is greater than the time period t2 of the refractive index variation, preferably at least five times t2.
Accordingly, for displays having, for example, 1000 times 1000 image points and 100 individual images per second, the required switching time for the liquid crystal cell would amount to approximately 0.5 GHz. At the present time, such high switching frequencies can, in fact, be reached using electrooptical crystals, not, however, by using liquid crystal cells. To improve the image quality of such highly resolving systems as well, by reducing the speckle formation, projection systems are advantageously employed which not only use one laser, but an entire laser array, for example columns having a plurality of lasers. When such a system is used, a greater number of lines of the image can be simultaneously constructed and projected. In this manner, one reduces the period of time tl and, accordingly, also the requisite switching time t2 when each individual projection laser is provided with an optical element, in particular a liquid crystal element, along the lines of the present invention. If 100 lasers are simultaneously used for projection purposes in the above example, then switching frequencies of about 5 MHz are NY01 410503 v 1 10 REVISED PAGES
required. This is already within the range of present-day liquid crystal development.
In many cases, however, an adequate speckle suppression can even be effected by a switching frequency that is on the order of the image frequency of the projection system.
Brief description of the drawing in which:
Figure 1 shows a laser projection system having a device for reducing speckle formation;
Figures 2, 3 illustrate a switchable liquid crystal element for generating a refractive index gradient.
Figure la depicts a laser projection system having a device for reducing speckle formation through mode mixing. The projection system includes a laser 1 as a light source. In color displays, a plurality of lasers having different wavelengths are used. The laser light is coupled by an optical arrangement 2, shown schematically here as a lens, into a first multimode optical fiber Ml, whose output is mapped onto the input of a second multimode optical fiber M2; compare Figure lb. Situated at the output of the second multimode optical fiber M2 is the actual projection unit 5, which is used to project the laser beam point-by-point onto projection screen 6.
The mode-mixing unit 4 is schematically shown in Figure lb.
The light emerging from first multimode fiber Ml is already constituted of a superposing of a plurality of modes. It strikes an optical element 7 having a spatially inhomogeneous, electrically variable refractive index. The electrical driving of element 7 is schematically indicated by a signal line 8.
Optical element 7 is capable of deflecting incident and transmitted light in a spatially dependent fashion. For this reason, the image of the in-coupled light changes at the output of element 7 and at the input of second fiber M2, respectively. The individual modes from fiber Ml are coupled, NY01 410503 v 1 11 REVISED PAGES
as spatially altered modes, at various locations and at various angles, into second multimode fiber M2 and, consequently, excite in M2 a mode that differs from the original one. This appears at the output of M2 and, therefore, on projection screen 6, likewise at a slightly different location. If the light-deflecting properties of optical element 7 are quickly varied within the reaction time of the eye, the result is that the individual modes are scrambled on projection screen 6. Since each mode has a different phase lag, the speckle patterns are averaged out and become blurred on the projection screen. In this context, the image spot dwells on the projection screen for a time period tl, which is greater than the time period t2 of the refractive index variation, preferably at least five times t2.
An averaging of speckles among various individual images requires only multiple variations in the refractive index within the reaction time of the eye. For this, about 1000 Hz suffice.
Figure 2 illustrates a switchable liquid crystal element 9 having birefringence that may be compensated, for producing a refractive-index gradient. Liquid crystal element 9 is composed of two cells, each having a liquid crystal layer 10, 11, which is disposed between two transparent electrodes 13, 13', and 14, 14', respectively. The molecules orient themselves within the layers, it being possible to influence the electrooptical properties by the voltage applied between the particular electrodes. The orientations of the indicatrices of the two liquid crystal layers 10, 11 are described by vectors I1 and 12. They are oriented in a direction normal to one another and to the beam direction of incident light beam 12. In accordance with the present invention, a voltage gradient is applied to both cells, in a direction normal to the beam direction, in this case in the y-direction. The spatial voltage characteristic is to be selected, in this context, such that voltage V(y) at locus y NY01 410503 v 1 12 REVISED PAGES
is the same for both cells. Consequently, the orthogonal alignment of the indicatrices is retained for each y-value, so that layer packet 10, 11 remains isotropic. By properly selecting the resistance of the electrodes, the current is kept low in the electrodes.
To set the average cell voltage, a voltage VO is applied to both cells at locus y=yO. Applying the voltage gradient deflects light beam 12, which is incident upon the cells at right angles, slightly in the y-direction; as schematically output directions 12' and, subsequent to switching over, by 12", respectively. This deflection is used for mode mixing between two multimode glass fibers, e.g., in accordance with Figure 1, and, thus, for suppressing the formation of speckle patterns in accordance with the present invention.
An improvement in speckle reduction through intensified mode intermixing is attained by using liquid crystal elements having a more complex deflection property. An example of such a liquid crystal element 9' is shown in Figure 3. Its structure that includes two liquid crystal layers 10, 11 essentially corresponds to that of Figure 2. In contrast to the liquid crystal element of Figure 2, here electrodes 15, 15' and 16, 16', respectively, are additionally provided. They may be used to apply a voltage gradient to the particular cell, simultaneously or alternatively to the voltage gradient in the y-direction, perpendicularly thereto in the x-direction. Preferably, the direction is quickly changed. As a result, the direction of the voltage gradient and, thus, the deflection direction of the incident light may be altered for the intermixing of the modes. Here, the equalization condition for compensating for the birefringence of the individual cell is the matching of the voltages in the individual cells at each irradiated locus (x, y).
Industrial Applicability:
NY01 410503 v 1 13 REVISED PAGES
The present invention is advantageously suited for commercial use, to improve the image quality of laser projections systems by suppressing speckle patterns.
NY01 410503 v 1 14 REVISED PAGES
Claims (18)
1. A device for reducing speckle formation on a projection screen, using a coherent light source, comprising an electrically controllable optical element having a spatially inhomogeneous refractive index, which is variable over time, the optical element being configured between the light source and the projection screen, wherein the electrically controllable optical element includes a liquid crystal element, to which a temporally variable voltage gradient may be applied to control the temporally and spatially dependent refractive index.
2. The device as recited in Claim 1, wherein the liquid crystal element has at least two liquid crystal layers, disposed one behind the other in the path of rays, each of whose fast and slow optical axes are disposed in parallel to the layer in question, and the slow and fast axes are rotated with respect to one another such that the polarization of the light upstream and downstream from the liquid crystal element is the same.
3. The device as recited in Claim 1, wherein the slow axis of the first layer is aligned perpendicularly the fast axis of the second layer, and the fast axis of the first layer is aligned perpendicularly the slow axis of the second layer.
4. The device as recited in one of the Claims 1 through 3, wherein, to separate the light coming from the light source into a plurality of spatial modes, a first multimode optical fiber is provided, which is connected upstream from the optical element.
5. The device as recited in one of the Claims 1 through 4, wherein a second multimode optical fiber is situated at the output of the electrically controllable optical element.
6. The device as recited in Claim 1, wherein, a first multimode optical fiber is connected upstream of the electrically controllable, light-deflecting element, which variably deflects the individual modes of the first multimode optical fiber, depending on the control voltage.
7. The device as recited in Claim 6, wherein, configured downstream from the electrically controllable, light-deflecting element is a second multimode optical fiber, the individual modes of the first multimode optical fiber being variably coupled into the second multimode optical fiber, depending on the control voltage.
8. The device as recited in Claim 6 or 7, wherein the light-deflecting element is a liquid crystal element, to which a voltage gradient is applied, in which the dependency of the polarization direction of the light is canceled as the result of compensation in at least two partial elements.
9. The device as recited in Claim 8, wherein respective fast and slow axes of the at least two partial elements are aligned perpendicularly to one another.
10. The device as recited in Claim 8 or 9, wherein an electric rotating field is applied to each of the partial elements, so that the voltage gradient and, thus, the deflecting direction assume alternating directions in space.
11. A method for operating a device as recited in Claim 1, wherein the light coming from the light source, before the projection, strikes an electrically controllable optical element having a spatially inhomogeneous refractive index, passing through the same, the refractive index being varied over time within the projection period.
12. The method as recited in Claim 11, wherein a multimode light source is used.
13. The method as recited in Claim 11, wherein before striking the optical element, the light coming from the light source is separated into a plurality of spatial modes, which are superposed on one another.
14. The method as recited in Claim 13, wherein the light is coupled into the first multimode fiber and propagates through the same.
15. The method as recited in one of the Claims 11 through 14, wherein, in the case of rasterized imaging of the light onto the projection screen, the refractive index of the optical element, is varied multiple times within the dwell period of one image spot on the projection screen.
16. The method as recited in Claim 15, wherein the refraction index is a refractive index profile.
17. The method as recited in one of the Claims 11 through 16, wherein, after propagating through the optical element, the light is coupled into an optical fiber.
18. The method as recited in Claim 17, wherein the light is coupled into a second multimode fiber.
Applications Claiming Priority (5)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19916306.5 | 1999-04-12 | ||
| DE19916306 | 1999-04-12 | ||
| DE19924519A DE19924519A1 (en) | 1999-04-12 | 1999-05-28 | Method and device for reducing the formation of speckles on a projection screen |
| DE19924519.3 | 1999-05-28 | ||
| PCT/EP2000/002794 WO2000062114A1 (en) | 1999-04-12 | 2000-03-30 | Method and device for reducing the formation of speckle on a projection screen |
Publications (2)
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| CA2366566A1 CA2366566A1 (en) | 2000-10-19 |
| CA2366566C true CA2366566C (en) | 2009-07-21 |
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| CA002366566A Expired - Fee Related CA2366566C (en) | 1999-04-12 | 2000-03-30 | Method and device for reducing speckle formation on a projection screen |
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| EP (1) | EP1171793B1 (en) |
| JP (1) | JP4455771B2 (en) |
| AT (1) | ATE225947T1 (en) |
| CA (1) | CA2366566C (en) |
| WO (1) | WO2000062114A1 (en) |
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| US6736321B2 (en) | 1995-12-18 | 2004-05-18 | Metrologic Instruments, Inc. | Planar laser illumination and imaging (PLIIM) system employing wavefront control methods for reducing the power of speckle-pattern noise digital images acquired by said system |
| US6629641B2 (en) | 2000-06-07 | 2003-10-07 | Metrologic Instruments, Inc. | Method of and system for producing images of objects using planar laser illumination beams and image detection arrays |
| US7028899B2 (en) | 1999-06-07 | 2006-04-18 | Metrologic Instruments, Inc. | Method of speckle-noise pattern reduction and apparatus therefore based on reducing the temporal-coherence of the planar laser illumination beam before it illuminates the target object by applying temporal phase modulation techniques during the transmission of the plib towards the target |
| US7077319B2 (en) | 2000-11-24 | 2006-07-18 | Metrologic Instruments, Inc. | Imaging engine employing planar light illumination and linear imaging |
| US7140543B2 (en) | 2000-11-24 | 2006-11-28 | Metrologic Instruments, Inc. | Planar light illumination and imaging device with modulated coherent illumination that reduces speckle noise induced by coherent illumination |
| JP3595297B2 (en) * | 2001-11-22 | 2004-12-02 | 株式会社東芝 | Laser light source device |
| JP4546830B2 (en) | 2002-09-30 | 2010-09-22 | アプライド マテリアルズ イスラエル リミテッド | Dark field inspection system |
| JP4748297B2 (en) * | 2004-05-28 | 2011-08-17 | 日亜化学工業株式会社 | Image display device |
| EP1734771A1 (en) * | 2005-06-14 | 2006-12-20 | SONY DEUTSCHLAND GmbH | Illumination optics, illumination unit and image generation unit |
| WO2007007389A1 (en) * | 2005-07-11 | 2007-01-18 | Mitsubishi Denki Kabushiki Kaisha | Speckle removing light source and illuminator |
| JP4920679B2 (en) * | 2006-04-12 | 2012-04-18 | パナソニック株式会社 | Projection display device and speckle reduction element |
| JP5326195B2 (en) * | 2006-07-07 | 2013-10-30 | 凸版印刷株式会社 | Projector and rear projection display device |
| JP5245218B2 (en) * | 2006-07-07 | 2013-07-24 | 凸版印刷株式会社 | Projector, transmissive screen, and rear projection display device |
| JP2008058869A (en) * | 2006-09-04 | 2008-03-13 | Toppan Printing Co Ltd | Projector, rear projection type display and transmission screen |
| WO2008087575A1 (en) * | 2007-01-19 | 2008-07-24 | Koninklijke Philips Electronics N.V. | Speckle reduction in a projection system |
| AU2008332909A1 (en) * | 2007-12-04 | 2009-06-11 | Bae Systems Plc | Improvements in or relating to diffuser screens |
| EP2110703B1 (en) * | 2008-04-15 | 2010-08-18 | JDS Uniphase Corporation | Retarder-based despeckle device and method for laser illumination systems |
| GB2462444A (en) * | 2008-08-06 | 2010-02-10 | Optyka Ltd | Image projection apparatus and method |
| GB2467181B (en) * | 2009-01-27 | 2014-03-05 | Optyka Ltd | Speckle removal for a laser scanning projector |
| JPWO2016208047A1 (en) * | 2015-06-26 | 2017-10-05 | 株式会社日立製作所 | Optical phase control device, optical phase control method, optical information recording / reproducing device, and optical information recording / reproducing method |
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| DE2315658C3 (en) * | 1973-03-29 | 1980-11-20 | Philips Patentverwaltung Gmbh, 2000 Hamburg | Method and device for reducing or eliminating the granulation occurring in laser beam projections |
| JPS5643786A (en) * | 1979-09-18 | 1981-04-22 | Nippon Telegr & Teleph Corp <Ntt> | Semiconductor laser combinating apparatus |
| GB8410973D0 (en) * | 1984-04-30 | 1984-06-06 | Crosfield Electronics Ltd | Modifying coherent radiation |
| WO1996021883A2 (en) * | 1995-01-14 | 1996-07-18 | Optica Nova Onab Ab | Projection screen |
| DE19501525C1 (en) * | 1995-01-19 | 1996-04-04 | Schneider Rundfunkwerke Ag | Interference prevention system for coherent light bundle |
| DE19508754C2 (en) * | 1995-03-10 | 1999-06-02 | Ldt Gmbh & Co | Method and apparatus for reducing interference from a coherent light beam |
| DE19710660C2 (en) * | 1997-03-14 | 2000-11-09 | Daimler Chrysler Ag | Device for removing or reducing image speckles |
| JPH11337888A (en) * | 1998-05-22 | 1999-12-10 | Hitachi Ltd | Speckle pattern dispersion device and laser beam irradiation system |
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- 2000-03-30 CA CA002366566A patent/CA2366566C/en not_active Expired - Fee Related
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- 2000-03-30 JP JP2000611122A patent/JP4455771B2/en not_active Expired - Fee Related
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| JP2002541526A (en) | 2002-12-03 |
| EP1171793A1 (en) | 2002-01-16 |
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| CA2366566A1 (en) | 2000-10-19 |
| EP1171793B1 (en) | 2002-10-09 |
| WO2000062114A1 (en) | 2000-10-19 |
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