Note: Descriptions are shown in the official language in which they were submitted.
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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
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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
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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
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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
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28030-87
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
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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
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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.
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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
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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
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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
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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,
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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
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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:
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The present invention is advantageously suited for commercial
use, to improve the image quality of laser projections systems
by suppressing speckle patterns.
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