|Número de publicación||WO2010015478 A1|
|Tipo de publicación||Solicitud|
|Número de solicitud||PCT/EP2009/058635|
|Fecha de publicación||11 Feb 2010|
|Fecha de presentación||7 Jul 2009|
|Fecha de prioridad||6 Ago 2008|
|Número de publicación||PCT/2009/58635, PCT/EP/2009/058635, PCT/EP/2009/58635, PCT/EP/9/058635, PCT/EP/9/58635, PCT/EP2009/058635, PCT/EP2009/58635, PCT/EP2009058635, PCT/EP200958635, PCT/EP9/058635, PCT/EP9/58635, PCT/EP9058635, PCT/EP958635, WO 2010/015478 A1, WO 2010015478 A1, WO 2010015478A1, WO-A1-2010015478, WO2010/015478A1, WO2010015478 A1, WO2010015478A1|
|Solicitante||Optyka Limited, Descharmes, Nicolas|
|Exportar cita||BiBTeX, EndNote, RefMan|
|Citas de patentes (5), Citada por (1), Clasificaciones (13), Eventos legales (4)|
|Enlaces externos: Patentscope, Espacenet|
IMAGE PROJECTION APPARATUS AND METHOD
This invention relates to an image projection apparatus and method and in particular to speckle reduction in an image projection apparatus, particularly for projected images or cinema applications when the light source is a narrow band and high power laser. Some methods of reducing or removing speckle are known.
Thus US 4,155,630 discloses improved image creation in a coherent light imagery system by directing diffused laser light onto a mirror with a 3-D rocking motion so that reflected rays sweep a 2-D area and focusing the reflected light through a diffuser before collimation. The mirror may be mounted on a piezoelectric translator/tilter. In particular the disclosure is directed to elimination of speckle by random spatial phase modulation. US 4,155,630 also refers to previously known methods of speckle reduction obtained by moving one diffuser with respect to another.
Vόlker, A. C. et al, "Laser speckle imaging with an active noise reduction scheme"; Optics Express 13(24) 28 Nov 2005 #8982 pp 1-6 discloses suppression of statistical noise in laser speckle imaging by illuminating an object surface through a slowly rotating diffuser, laser speckle imaging being a technique for measuring local dynamic properties in scattering media by monitoring temporal fluctuations of light scattered by the media. In the absence of movement the speckle has similar properties in an entire field of view. High contrast areas correspond to slow movement and low contrast areas correspond to regions of high movement. Reference is made to the removal of speckle in the prior art by a motionless and a moving diffuser.
US 2004/0239889 discloses a liquid crystal projector in which a laser diode provides auxiliary red colour. A ferroelectric shaking or vibrating device vibrates the laser diode to make visual noise inconspicuous. The disclosure refers to a shaken or oscillated mirror in the prior art in a laser beam path for virtually removing unwanted effects of the coherency of laser light and to a display device in which an image itself is shaken or oscillated. US 2004/0239889 appears to disclose vibration of a laser diode holder; of an optical system or of a "light path holder" using a ferroelectric vibrating element. Shaking includes oscillating, vibrating and wobbling - the shaking device may reciprocate, oscillate or rotate. Instead of a ferroelectric vibrating element a motor with an eccentric cam may be used. US 3,588,217 discloses an apparatus for displaying a hologram in which a mid-section of a fibre optic device forming part of a coherent light path is moved with respect to ends of the fibre optic device to reduce speckle noise by changing a path length in the fibre optic device. A flexible mid-section of a fibre optic may be tapped with a finger to provide the required motion. Alternatively, a collar mounted on the fibre optic may be moved in a reciprocal, circular or other motion by electromagnets. The disclosure refers to reduction of speckle in the prior art by rotating a thin transparent wedge in the optical path but this disadvantageously also changes an angle of incidence of the light path on the hologram.
US 2003/0039036 discloses a laser projector including a spatial light modulator and a fly's eye integrator with a moving diffuser between a laser light source and the spatial light modulator. The disclosure refers to attempts to reduce speckle in the prior art by broadening a line width of the laser light or to delay beamlets relative to each other (US 5,224,200). Alternatively a screen may be vibrated or dynamically altered (US 5,272,473) or a multi-mode optical fibre vibrated to cause mode scrambling (US 3,588,217). Alternatively, a diffusing element is moved or vibrated for example in an intermediate image plane (US 4,035,068). In a hologram illumination system, a diffuser is rotated in the focus of a beam expander (US 3,490,827). Alternatively, a light valve may be illuminated through a rotating diffuser such as a variable-thickness plate rotated in the illumination of a lightpipe homogenizer (US 5,313,479). US 2003/0039036 itself discloses a diffuser to advantageously reduce source brightness and coherence. The fly's eye integrator amplifies the effect of the diffuser on despeckling. To reduce speckle further the diffuser is given a linear, rotary or random motion in which a frequency of the motion is faster than the flicker frequency - say 40 Hz. The motion may be produced by an electric motor to provide circular or elliptical motion or by a vibrator motor with an imbalanced rotor mounted with the diffuser on springs. Alternatively, a voltage controlled linear actuator may be used or a rotating wheel. Placement of the diffuser in the illumination system ensures that the diffuser does not degrade image quality. A preferred location of the diffuser is immediately before the fly's eye integrator. Alternatively, the diffuser can be placed before the collimating lens or at a focus of a diverging lens. Other possible locations are within or after the fly's eye integrator. Multiple diffusers can be used to magnify the despeckling effect or to decouple despeckling from reduction of brightness of the laser beam, in which at least one of the diffusers is moved. An array of partially reflecting mirrors may be used as a beam expander to impart optical path differences between adjacent beams in conjunction with a fly's eye array to reduce speckle even without motion of the diffuser - although diffuser motion may be used for more complete removal of speckle. A linear spatial light modulator may be used with a diffuser which diffuses light along a length of a light valve array.
It is an object of the present invention at least to ameliorate disadvantages in the prior art. According to a first aspect of the invention there is provided an image projection apparatus arranged to reduce speckle in a projected image, the image projection apparatus comprising a multimode optical fibre, an output end of which is optically coupled to a light scattering member wherein the light scattering member is optically coupled to a light homogenizer. Conveniently, an input of the multimode optical fibre is coupled to an output of a laser light source by a first light coupling member.
Conveniently, the multimode optical fibre is coupled to the light scattering member by a second light coupling member.
Advantageously, an output of the light homogenizer is output to a micro display optically coupled to projection optics means to form the projected image.
Advantageously, the multimode optical fibre is moveable with respect to remaining elements of the image projection apparatus by first motion enabling means arranged to promote mode mixing in the multimode optical fibre.
Advantageously, the first coupling member is moveable with respect to remaining elements of the image projection apparatus by second motion enabling means.
Conveniently, at least one of the first and second motion enabling means comprises vibration means.
Advantageously, the image projection apparatus further comprises a coherent light source. Conveniently, the coherent light source comprises laser light source means. Conveniently, the multimode optical fibre comprises a stepped index multimode optical fibre.
Conveniently, the light scattering member comprises at least one of opal glass scattering means, nanoparticles or sol gel means.
Alternatively, the light scattering member comprises a plurality of diffusing members. Conveniently, the second light coupling means comprises reflective means optimised as a function of properties of the scattered light and the light homogenising means to reflect light scattered from the light scattering member into an input of the light homogenizing means. Conveniently, the second light coupling means comprises ellipsoidal or parabolic mirror means arranged to focus light scattered from the light scattering member into an input of the light homogenizing means.
Conveniently, the image projection apparatus comprises lens means for focusing light from the light scattering member into an input of the light homogenising means. Conveniently, the light homogenising means comprises light tunnel means.
Conveniently, the light tunnel means tapers outwards from an input end to an output end.
Conveniently, the light tunnel tapers with a taper angle of substantially 4 degrees to 8 degrees.
Advantageously, the light scattering member is located within the light tunnel means. Conveniently, an output end of the multimode fibre passes through an aperture in an end mirror in an input end of the light tunnel means to deliver light to the light scattering element.
Conveniently, the light scattering means is located on light transparent means arranged transversely of the light tunnel means. Alternatively, the light homogenising means comprises fly's eye lens means.
According to a second aspect of the invention, there is provided a method of reducing speckle in a projected image produced by an image projection apparatus, the method comprising passing light successively through a multimode optical fibre, a light scattering member and a light homogenizer. Conveniently, the method further comprises passing the light through a first light coupling member coupling an input of the multimode optical fibre to an output of a coherent light source.
Conveniently, the method further comprises passing the light through a second light coupling member coupling the multimode optical fibre to the light scattering member. Conveniently, the method further comprises outputting light from the light homogenizer to a micro display and forming the projected image with projection optics means optically coupled to the micro display.
Advantageously, the method further comprises moving a portion of the multimode optical fibre with respect to remaining elements of the image projection apparatus to promote mode mixing in the multimode optical fibre.
Advantageously, the method further comprises moving the first coupling member with respect to remaining elements of the image projection apparatus.
Advantageously, the method comprises vibrating at least one of the multimode optical fibre and the first coupling means.
Conveniently, the multimode optical fibre comprises a stepped index multimode optical fibre.
Conveniently, the light scattering member comprises at least one of opal glass scattering means, nanoparticles or sol gel means. Conveniently, the light scattering member comprises a plurality of diffusing members.
Advantageously, the method comprises focusing light scattered from the light scattering member with ellipsoidal or parabolic mirror means into an input of the light homogenizing means.
Advantageously, the method comprises reflecting light scattered from the light scattering means, with reflective means optimised as a function of properties of the scattered light and the light homogenising means, into an input of the light homogenizing means.
Conveniently the method comprises focusing light scattered from the light scattering member with ellipsoidal or parabolic mirror means into an input of the light homogenizing means. Conveniently, the light homogenising means comprises light tunnel means.
Conveniently, the light tunnel means tapers outwards from an input end to an output end.
Conveniently, the light tunnel tapers with a taper angle of substantially 4 degrees to 8 degrees.
Advantageously, the light scattering member is located within the light tunnel means. Advantageously, the method comprises delivering light to the light scattering element from an output end of the multimode fibre passing through an aperture in an end mirror in an input end of the light tunnel means.
Conveniently, the light scattering means is located on light transparent means arranged transversely of the light tunnel means.
Alternatively, the light homogenising means comprises fly's eye lens means.
The invention will now be described, by way of example, with reference to the accompanying drawings in which:
Figure 1 is a schematic drawing of an apparatus according to the invention; Figure 2 shows simulation and measurement of light distribution using different shapes of light tunnel homogenizer;
Figure 3 is a graph of maximum output angle of a light tunnel vs tunnel angle; Figures 4a - 4c show possible configurations for collecting scattered light;
Figure 5a - 5b are diagrams showing the advantage of locating the diffuser or scatterer within a light tunnel;
Figure 6 is a plot of angular transmission of Opalika® glass from Schott North America, Inc.
Figure 7 is a drawing of a fibre-based projector according to the invention; Figure 8 is a diagram of an optical system for coupling light from a laser into a fibre core;
Figure 9 is a drawing of allowed modes inside a multimode fibre; and
Figure 10 is a drawing illustrating differences in transmission in a single mode and multimode step and graded index fibres.
In the Figures, like reference numbers denote like parts. Electromagnetic scattering is a known process by which electromagnetic radiation, including light, is scattered by a small spherical volume of different refractive index from the surrounding medium, such as a particle, bubble, droplet, or even a density fluctuation. For Rayleigh's scattering model, the spherical volume must be much smaller in diameter than a wavelength of the scattered wave. In contrast to Rayleigh scattering, the Mie solution to the scattering problem is valid for all possible ratios of diameter to wavelength, although the technique requires numerical summation of infinite sums. Multiple elastic scattering of laser light in a medium containing a large number of nanoparticles destroys both spatial and temporal coherence of the laser light. At the spatial level, the multiplicity of short and long paths and the multiplicity of directions that light can take before exiting a medium induces multiple random interferences and therefore significantly reduces the coherence volume. This random interference could be speckle grain pattern. Therefore, less angle diversification is needed, in comparison to classical diffusers. At the temporal level, phase and time retardation induced by the scattering process decreases the coherence time of the incident beam and therefore reduces contrast of residual perceptual speckle. In other words, fewer photons have a same phase in an Airy disc, and therefore residual subjective speckle contrast is much lower. Scattering by nanoparticles provides a combination of a smaller coherence volume and a smaller coherence time and therefore large delays between wavefront trains. It will be understood that by "nanoparticles" are to be understood particles with sizes ranging from 1 nanometre to 10 microns in size. Therefore, speckle present in an image displayed by a laser light projector has much less observable speckle following such scattering. Vibration of the optical elements modifies a residual speckle pattern and therefore is able to remove perceived residual speckle.
Figure 2 shows results of simulation and measurement of light distribution in a light engine for different shapes of light tunnel homogeniser: a converging light tunnel 21 with circular cross-section in the top row, a diverging light tunnel 22 with circular cross-section in the middle row, and a light tunnel 23 with rectangular cross-section in the bottom row. A cone angle of a light source 20 at the entrance of the light tunnel is 30 degrees in the schematic diagram of Figure 3. In practice this may be any angle from 0 to 90 degrees.
A light tunnel 21, 22 can have a converging or diverging shape as shown in the upper and middle rows of Figure 2 respectively, a converging light tunnel 21 increases a numerical aperture (NA) of a light beam 210 at an exit of the tunnel 21. As shown in the middle row, a diverging tunnel 22 decreases the numerical aperture of the light beam 220 at an exit of the tunnel 22. A tunnel 23 with parallel faces as shown in the lower row does not change the numerical aperture of the emergent beam 230. An optimal orientation of the tunnel allows a significant reduction in the numerical aperture of the light beam. This feature may be used to collect light from a high numerical aperture fibre or from a diffusing or scattering element in order to create a less divergent beam that is easier to focus on a micro display and to get through the projection lens. The graph of Figure 3 shows a maximum output angle 31 as a function of a tunnel angle when the input light illumination is 90 degrees. There is clearly an optimal tunnel angle 312, at approximately 6 degrees, which minimizes the output angle. This value will vary with a length of the tunnel and its cross section. Typically it may be between 4 degrees and 8 degrees.
A light tunnel can be used not only to collect a maximum amount of light produced by a high numerical aperture system and to decrease the numerical aperture of the light beam, but also to reduce or remove speckle, by introducing a diffusing or scattering element before or inside the light tunnel. The scattering or diffusing element as well as the multiple reflections of the light rays inside the tunnel induce a phase delay between the rays and therefore decrease the coherence volume and consequently speckle.
Finally, any vibration of a portion of the optical system with respect to remaining portions of the optical system, for example of the light tunnel, diffusers, coupling lens, rigid fold mirror or fibre, additionally reduces speckle by averaging the residual speckle patterns. Some systems according to the invention including a diffuser or scattering element inside a light pipe or tunnel integrator are presented and discussed below.
In a one embodiment shown in Figure 4a, a scattering element 411 is at a focal point of an ellipsoidal mirror 421. Light from an optical fibre 410 entering axially through a rear of the mirror is scattered by the scattering element 411, collected by the mirror 421 and focused into a light tunnel 431. The scattering element 411 has a mirror at its back side remote from the optical fibre so that light transmitted through the scattering element is reflected back towards the ellipsoidal mirror 421 and thereby focussed into the light tunnel 431.
Another solution, shown in Figure 4b, otherwise similar to the embodiment of Figure 4a, is to collect light transmitted through the scattering element 411 which does not have a mirror at its rear side by a lens 421 located between the back side of the scattering element 411 and the light tunnel 431. The lens 421 and ellipsoidal mirror 421 both focus light into the light tunnel 431. This configuration is suitable for cinema projectors.
Replacement of the ellipsoidal mirror of Figure 4a by a parabolic mirror 441, as shown in Figure 4c, results in a further embodiment which is suitable for light engines for producing collimated light for entry into a fly's eye array light homogeniser (not shown).
Alternatively, referring to Figure 5b, the diffuser or scattering element 511 may be located inside the light pipe or tunnel rather than before an entrance to the light tunnel as shown in Figure 5a. Light from an optical fibre 510 is introduced directly into the light tunnel 531 through a pinhole 512 in an end mirror wall 513 of the light tunnel 531. The scattering element 511 scatters part of the light that is redirected by the mirror 513 towards an opposed end of the tunnel 531. The scattering element 511 is preferably embedded in a glass plate 514 so that the reflected light is redirected through the glass plate towards the tunnel end with minimal losses. The multiple reflections reduce speckle. Any vibration of an angle diversifying active element, such as a deformable membrane or a tip-tilt arrangement for a rigid mirror, additionally reduces speckle.
Optimized incident beam inside the tunnel
Figure 6 shows angular transmission of Opalika® glass available from Schott North America, Inc., 555 Taxter Road, Elmsford, NY 10523.
An optical lens with an NA=O.6 collects light inside a cone with the angle +-37 degrees. All the rest of the light is lost. Therefore, a diverging tunnel provides a significant advantage, since it collects all the transmitted light.
An experimental set up of a first embodiment of a projector 10 according to the invention is illustrated in Figure 1. Laser light at 532 nm is coupled into a first end 121 of a multimode fibre 12, with a core diameter of at least 400 microns. Preferably, but not necessarily, the core diameter is 1 mm. A second end 122 of the multimode fibre 12 opposed to the first end 121 is close to an opal light scatterer 13 placed in front of a first end of a tunnel rod integrator or light pipe 14. It will be understood that instead of opal glass some other form of light scattering material could be used such as a medium comprising nanoparticles or a sol gel. An optimised or free form mirror 421, which may be ellipsoidal, reflects light back-scattered by the opal light scatterer back through the light scatterer and into the light tunnel. Essentially the mirror is designed to reflect light through the aperture and loss vs cost are the parameters that will define the final design. A projection lens 15 proximate a second end of the tunnel rod integrator 14 is a rear projection television (RPTV) lens. A distance from the fibre end 122 to the opal light scatterer 13 is between 0 mm and 5 mm. Figure 7 is a drawing of an optical module 70 of a second embodiment of a fibre-based laser projector according to the invention. The projector in its simplest form consists of an optical arrangement of a laser source 71 coupled by a coupling unit 77 to a multimode optical fibre 76 which is coupled to at least two diffusers or one scatterer 72, which comprise the speckle reduction component, followed by a light homogenizer 73 which contributes to presenting the despeckled light in a uniform distribution of intensity to a micro display 74 and projection optics 75. The laser source 71 may be a free beam laser, a laser diode, an array of laser diodes, or some other very coherent light source. The optical fibre 76 is a multimode fibre, that allows easy coupling of light into its core, and that enables efficient mode scrambling. Typically the multimode fibre may have a core diameter from a few microns to a few millimetres in section or comprise a bundle of fibres. The light homogenizer 73 can be a light tunnel or fly's eye arrays or a flat hat holographic beam shaping element. In the example of a fibre-based projector 70 in Figure 7, key elements to remove speckle are the diffusers or scatterers 72, the optical fibre 76 and the homogeniser 73. The coupling unit 77 can include a lens, a selfoc lens or a graded index (GRIN) lens. The vibrator 78 can be, for example, a piezoelectric element, a magnetic actuator or a motor.
6.1 Fibre technology Figure 8 is a drawing of a light coupling system into an optical fibre core comprising the coupling unit 77.
The characteristics of the focused beam must match the optical fibre parameters for good coupling efficiency, as illustrated in Figure 8. For multimode fibres this is straightforward. General guidelines are: • The focused spot should be comparable to or smaller than the core size.
• The incident half cone angle should not exceed the arcsine of the NA of the fibre (e.g. 11° for 0.2 NA and 17° for 0.3 NA).
Mode scrambling is a technique that distributes the optical power in an optical fibre among all the guided modes. When light is launched into an optical fibre, modes are excited to varying degrees depending on conditions of the launch, such as input cone angle, spot size, fibre core size and axial centration.
Some allowed modes are shown in Figure 9. Any micro deformation of the fibre and or any change in the coupling of light into the fibre core induces a different distribution of light in the propagating modes inside the core or a change in the coupling between the modes, and therefore generates a different phase of the waveform at the fibre end and generates different phase retardations. Figure 10 is a drawing showing differences between a single mode step index fibre 101, a multimode step index fibre 102 and a multimode graded index fibre 103. Step index fibres 102, 103 induce more phase retardation than a graded index fibre 103.
6.2 Speckle reduction with a multimode fibre Dynamic speckle reduction requires a passive and a dynamic component.
The passive component is needed to obtain the smallest speckle grains possible, in order to improve and enable an easier averaging process. Indeed, the fully developed speckle pattern is the result of random interferences generated by random optical paths created by the transmission or reflection on a diffuser or rough surface. If the granularity of the rough surface is high enough, or if a scattering process is involved, the speckle is fully developed, and the grain size is close to the local Airy disc. One diffuser is usually not sufficient to obtain the smallest speckle grain size. Therefore two or more diffusers are implemented, or at least one scatterer. A fully developed speckle is used because it is easier to average small interference structure than large ones. The dynamic component is responsible for the averaging process. A change in the illuminated section of a rough surface or diffuser results in a changed random interference pattern on a screen and/or on a micro display. A similar effect occurs if the incident wavefront on the diffuser, or scatterer, is modified, or if its phase distribution is modified. The motion or the vibration of the fibre or of any other optical element that couples light into the fibre changes the distribution of modes inside the fibre core and therefore modifies the phase distribution of the illumination wavefront that is incident on the diffuser or scatterer. It consequently generates different uncorrelated speckle patterns on the screen. Moreover, if the speckle patterns observed on the screen change over time faster than an integration time of the eye, than, during this integration time of the eye to create one image, the different patterns are added and integrated. The integration by the eye results in the reduction of the standard deviation on the speckle contrast and subsequently in the reduction of the perceptual speckle contrast. According to Poisson's law, standard deviation of the speckle contrast value is consequently reduced by N/2 where N is the number of different speckle patterns during the integration time. This is a fundamental principle of speckle reduction with dynamic averaging.
The light homogenizer 73 increases the angle diversification effect and therefore increases the change in the speckle pattern when the modes inside the fibre are modified. High power lasers 71 are usually specified with a maximum output beam drift. This drift can be caused by, for example, a cooling system or thermal expansions inside the laser cavity. Drift of the exit laser beam can be enough to generate sufficient mode scrambling to reduce speckle.
Vibrations due to the cooling system can also be sufficient to reduce speckle
The optical fibre 76 can be adhered to a dither piezoelectric element 78. A sinusoidal signal applied to the piezoelectric element expands the piezoelectric element and therefore stretches or vibrates the fibre.
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|Patente citante||Fecha de presentación||Fecha de publicación||Solicitante||Título|
|CN103092049A *||16 Ene 2013||8 May 2013||北京工业大学||All-solid digital holography imaging system capable of reducing speckle noise|
|Clasificación internacional||G03B21/00, H04N9/00, G02B27/48|
|Clasificación cooperativa||G03B21/208, G03B21/2033, H04N9/3152, G02B27/48, H04N9/3161|
|Clasificación europea||H04N9/31R5L, H04N9/31R5B, G02B27/48, G03B21/20, H04N9/31V|
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