WO2009133111A1

WO2009133111A1 - Optical system for speckle reduction

Info

Publication number: WO2009133111A1
Application number: PCT/EP2009/055142
Authority: WO
Inventors: Omar Sqalli
Original assignee: Optyka Limited
Priority date: 2008-04-29
Filing date: 2009-04-28
Publication date: 2009-11-05

Abstract

An optical system comprising a coherent light source and optical elements for directing light from the coherent light source to a target. The optical elements including a dynamically controlled moveable optical element arranged to change ray paths of the coherent light and a homogeniser. Drives are provided for moving the optical elements and a controller for controlling the drives. The optical elements are controllable by the controller to cause changes of speckle patterns at the target over a period of time less than the temporal resolution of the eye of an observer and to reduce a speckle contrast ratio.

Description

OPTICAL SYSTEM FOR SPECKLE REDUCTION

Introduction

The present invention relates to the field of illumination systems using spatially and/or temporally coherent light sources. More particularly, the present invention relates to the reduction of speckle and/or other interference patterns observed in images displayed, areas illuminated, and images acquired using coherent light sources.

Speckle and/or other interference patterns arise through constructive and destructive interference when spatially and/or temporally coherent light is scattered by transmission through, or reflection off, a rough surface. We consider the term speckle as synonymous with interference patterns. We understand the term "coherent" to mean spatially and/or temporally coherent. We use the term "coherence volume" to mean a correlated region within a beam of light where speckle can arise.

Speckle is manifest as unwanted bright and dark granular and/or more structured artefacts unrelated to image content that degrade image quality. It is of particular concern in image display systems such as projectors, rear-projection televisions, near- to-eye displays, head-mounted displays, and head-up displays. Speckle is also of concern in image acquisition systems such as raman, confocal, and fluorescence microscopes.

A well-known approach to reducing speckle is to superimpose N uncorrelated speckle patterns to achieve up to a sqrt(N) reduction in speckle contrast ratio (the standard deviation of intensity variation divided by the mean intensity). A speckle pattern uncorrelated to another can be formed when rays in the illumination beam follow different optical paths during transmission through, or reflection off, an optical element with appropriate reflective, refractive, or diffractive characteristics. A common optical element used is a diffuser that diversifies ray angles within the illumination beam. A sequence of uncorrelated speckle patterns can be formed over time by moving the optical element or varying its characteristics appropriately. If the frequency of motion or variation is greater than the observer's temporal frequency of visual perception, or the sensor's temporal frequency of acquisition, then the uncorrelated speckle patterns are superimposed.

US 2007/0223091 describes how an appropriately refracting element is rotated through the illumination beam to diversify ray angles.

US 2007/0251916 describes how a diffusing element in the illumination beam is mechanically vibrated to diversify ray angles.

US 2007/7244032 describes how a laser diode or optics focussing light into an appropriate tunnel integrator element, or directing the light out of the tunnel integrator to the micro display, are actuated in order to diversify ray angles.

US 2006/081381 describes how a rotating matrix of lenses in front of an appropriately tunnel integrator element diversifies ray angles to reduce speckle.

US 1979/4155630 describes how a random vibrating rigid mirror positioned between two diffusing elements diversifies ray angles to reduce speckle.

US 1971/358271 describes how the change in radius of a multimode fibre achieved by bending the fibre diversifies ray angles to reduce speckle.

Problems associated with solutions that depend on rotation through the illumination beam, translation, and/or vibration of an optical element include size, scale, weight, cost, and power consumption. A problem associated with many solutions is their failure to achieve the maximum sqrt(N) reduction in speckle contrast ratio due to some correlation remaining between the N speckle patterns. Another problem is that if there is a temporal and/or other constraint which limits the number of images N that can be superimposed, the maximum sqrt(N) reduction in speckle contrast ratio may be insufficient to achieve the desired image or illumination quality. Statements of Invention

According to the invention, there is provided an optical system comprising: coherent light source means; optical elements for directing light from the coherent light source means to a target, said elements including: a dynamically controlled moveable optical element arranged to change ray paths of the coherent light; homogenising means arranged to homogenize the coherent light; drive means for moving the moveable optical element; and controller means for controlling said drive means; wherein the moveable optical element is controllable by the controller means to causes changes of speckle patterns at the target over a period of time less than the temporal resolution of the eye of an observer and so to reduce a speckle contrast ratio.

Conveniently, the optical elements include diffusing means arranged to diffuse coherent light from the coherent light source means.

In one embodiment, the elements include at least one diffuser for diversifying ray angles and so reducing sizes of coherence volumes.

In a further embodiment, the controlled element is located between two diffusers.

In one embodiment, the controlled element comprises a rigid mirror, and the controller drives the rigid mirror.

In another embodiment, the controlled element comprises a rigid optical surface that is rotated and/or translated.

In a further embodiment, the source means comprises a plurality of light sources. In one embodiment, the source means comprises a plurality of light sources and an optical integrator.

In another embodiment, the target is an image formation device.

Detailed Description of the Invention

The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1 illustrates various elements of a projection display system 100 in accordance with an embodiment of the present invention;

Fig. 2 shows a projected image of a target (illuminated with coherent light) in which coarse and fine speckle patterns are visible, a plot of pixel intensity levels across a line of the image, and a plot of spatial frequencies across the same line of the image;

Fig. 3 shows a projected image of a target (illuminated with coherent light) in which fine speckle patterns are visible, a plot of pixel intensity levels across a line of the image, and a plot of spatial frequencies across the same line of the image;

Fig. 4 shows a projected image of a target (illuminated with coherent light) in which speckle patterns are not easily visible, a plot of pixel intensity levels across a line of the image, and a plot of spatial frequencies across the same line of the image; and

Fig. 5 shows a projected image of a target (illuminated with non-coherent light) in which speckle patterns are not visible (although there are a variety of other problems due to the apparatus that degrade image quality), a plot of pixel intensity levels across a line of the image, and a plot of spatial frequencies across the same line of the image.

This invention provides a system and method for reducing the speckle observed in images displayed, images acquired, or areas illuminated with spatially and/or temporally coherent light sources.

The system comprises at least one optical element which has constant optical properties but its effect in the system is dynamically controlled by moving it. This element is referred to as a "moveable constant optical property element". It is used to change optical paths of rays in the illumination beam with the effect of changing the speckle patterns formed over time. In addition, the system can comprise at least one static optical property element with appropriate reflective, refractive, or diffractive characteristics to reduce the sizes of coherence volumes. The system can operate to form uncorrelated speckle patterns within coherence volumes smaller than the effective size of the image integrating elements, at speeds faster than the effective image integrating period, such that speckle perceived by an observer or acquired by a sensor is reduced or effectively eliminated.

Fig. 1 illustrates a projection display system 100 in accordance with an embodiment of the present invention. A sequence of elements define an optical path between the coherent light source 101 and a surface 114 onto which the image is projected. The coherent light source 101 may be a single line or a red, green, and blue semiconductor laser diodes, such as Novalux Inc. NECSEL diodes, or Philips-Lumileds Inc. Luxeon LEDs (light-emitting diodes,) or a combination of semiconductor laser diodes and light-emitting diodes, respectively. The light source may be a free beam light source or a fibre coupled light source. It is common to use a multimode fibre or a bundle of fibres to reduce the coherence volume of the light.

A source image formation device 112 can be illuminated sequentially to form colour fields of the image by turning on and off any channel of the light sources 101. The source image formation device 111 is a pixellated microdisplay such as a Texas Instruments Inc. DMD (digital micromirror device) display panel, Sony Corp. SXRD LCOS (liquid crystal on silicon) display panel, or Epson Inc. HTPS (high-temperature polysilicon) LCD (liquid crystal display) panel. Additional diffusers are needed in the optical path to increase the contrast when LCD or TFT panels are used.

Optical systems 102 A and 103 A collect light from the light source 101 and direct the light towards a rigid mirror 105 A that reflects the light towards the optical element 106A. Optical systems 104A and 107A comprise static optical elements that act as diffusers to diversify ray angles within the illumination beam between the optical elements 104A and 107A. These could be light-shaping holographic diffusers or diffractive optical elements from Edmunds Optics Inc. The nominal maximum angle of diffusion is such that as little light as possible is lost from the illumination system. The optical elements 104A and 107A could be a stack of diffusers. One of the elements 104A or 107A could be removed to increase the light transmission of the system. The optical element 106A focuses the light into the tunnel integrator 108 A. It is important to keep the diffusing element 107A close to the aperture of the tunnel integrator in order to avoid light losses.

The optical integrator 108 A may incorporate an integrating reflective tunnel such as LightTunnel from Oerlikon Balzers Ltd, or any waveguide that has a rectangular shaped aperture at the exit.

A movable constant optical property element 105 A is a rigid mirror mounted on an actuator 105B, such as a motors, piezo elements, pneumatic actuators, electromagnetic actuators. The combination 105 A and 105B can dynamically diversify reflected ray angles, so change the optical paths of rays, within the illumination beam. The shapes and frequencies of its deformation are determined by the controller of the element 105B. A single actuation frequency, a frequency ramp or a random pattern of frequencies could be applied on the actuator 105B.

The movable constant optical property element 105 A is located between the static diffusers 104 A and 107 A. An optical system 109A and HOA directs the illumination beam onto the source image formation device 112 through the complex prism 111. The projection optical system 113 magnifies the source image from the microdisplay and forms a real image at a screen or other surface 114. The real image is perceived by the eye of the observer or any image capture device.

The effects of the diffusers 104 A and 107 A are such that the coherence volumes within the illumination beam are reduced in size such that they are small with respect to the spatial resolution of the eye of the observer. The resulting fine speckle patterns can be seen in Fig. 3. The effect of the movable constant optical property element 105 A-B changing the optical paths of rays is such that a different spatial distribution of coherence volumes is achieved, and that different speckle patterns within the coherence volumes can arise. When the system operates to change speckle patterns arising over a period of time less than the temporal resolution of the eye of the observer, a sequence of speckle patterns are superimposed and so the speckle contrast ratio is reduced. This can be seen in Fig. 4. Fig. 4 compares favourably to Fig. 5 in which no speckle is visible because non-coherent illumination was used.

Alternative Embodiments

In another embodiment of the invention there are no static diffusing optical elements 104 A and 107 A. This can result in the sizes of the coherence volumes being large with respect to the spatial resolution of the eye. This can result in speckle patterns that are large with respect to the image observed by the eye. These can be seen in Fig. 2. The movable constant optical property element 105 A is used to change the optical paths of rays which results in a changing sequence of different speckle patterns over time. If the distribution of coherence volumes is not also changed significantly, the resulting speckle patterns may not be sufficiently uncorrelated so that speckle contrast ratio is not reduced or slightly reduced.

In another embodiment of the invention there is one static diffusing optical element 104A positioned before the movable constant optical property element 105 A, with no static diffusing optical element 107A positioned after the moveable constant optical property element 105 A. This can result in the sizes of the coherence volumes being smaller with respect to the spatial resolution of the eye. This can result in speckle patterns that are smaller with respect to the image observed by the eye. The moveable constant optical property element 105 A is used to change the optical paths of rays which results in a sequence of different speckle patterns over time. The tunnel integrator 108A should be long enough to decrease the coherence volume and increase the random phase shift between the optical rays. Because the coherence volume are smaller than the eye resolution, the moveable constant optical property element 105 A can change in this case significantly their distribution over time to create a more uncorrelated sequence of speckle patterns to reduce the speckle contrast ratio.

In another embodiment of the invention there is one static diffusing optical element 107A positioned after the moveable constant optical property element 105 A, with no static diffusing optical element 104A positioned before moveable constant optical property element 105 A. This can result in the sizes of the coherence volumes being smaller with respect to the spatial resolution of the eye. This can result in speckle patterns that are smaller with respect to the image observed by the eye. The moveable constant optical property element 105 A is used to change the optical paths of rays which results in a sequence of different speckle patterns over time. This achieves a more significant distribution of coherence volumes over time to create a more uncorrelated sequence of speckle patterns to reduce the speckle contrast ratio.

In another embodiment there may be more than one moveable constant optical property element 105 A positioned on the optical path between the sources of illumination 101 and the microdisplay 112.

In other embodiments the diffusing optical elements 104A and/or 107A can be made of ground glass or other rough surface, or opal or other non-homogeneous medium.

The actuators 102B connected to the element 102A, 103B connected to the element 103 A, 104B connected to the element 104A, 105B connected to the element 105 A, 106B connected to the element 106A, 107B connected to the element 107 A, 108B connected to the element 108 A, 109B connected to the element 109A, HOB connected to the element HOA could be piezo actuators, electromagnetic actuators, inductive actuators, pneumatic actuators. The actuators 102B, 103B, 104B, 105B, 107B, 106B, 108B, 109B, and HOB are simultaneously or sequentially or alternatively used to actuate the elements 102 A, 103 A, 104 A, 105 A, 107A, 106 A, 108 A, 109A, and HOA and subsequently to diversify ray angles in the optical path. The sequence of uncorrelated speckle patterns formed over time faster than the observer's temporal frequency by moving any of the actuators or a group of actuation elements 102B, 103B, 104B, 105B, 107B, 106B, 108B, 109B, and HOB results in the reduction or the total removal of the perceptual speckle by the eye.

The invention is not limited to the embodiments described but may be varied in construction and detail.

Claims

1. An optical system comprising: coherent light source means; optical elements for directing light from the coherent light source means to a target, said elements including: a dynamically controlled moveable optical element arranged to change ray paths of the coherent light; homogenising means arranged to homogenize the coherent light; drive means for moving the optical elements; and controller means for controlling said drive means; wherein the optical elements are controllable by the controller means to cause changes of speckle patterns at the target over a period of time less than the temporal resolution of the eye of an observer and so to reduce a speckle contrast ratio.

2. An optical system as claimed in claim 1, wherein the optical elements further comprise diffusing means arranged to diffuse coherent light from the coherent light source means.

3. An optical system as claimed in claim 2, wherein the diffusing means is arranged to diversify ray angles and so reduce sizes of coherence volumes.

4. An optical system as claimed in claim 3, wherein the dynamically controlled moveable optical element is located between two diffusers.

5. An optical system as claimed in any preceding claim, wherein the dynamically controlled moveable optical element comprises a rigid mirror, and the controller means drives the rigid mirror.

6. An optical system as claimed in any preceding claim, wherein the dynamically controlled moveable optical element comprises a rigid optical surface that is rotated and/or translated.

7. An optical system as claimed in any preceding claim, wherein the coherent light source means comprises a plurality of light sources.

8. An optical system as claimed in any preceding claim, wherein the coherent light source means comprises a plurality of light sources and an optical integrator.

9. An optical system as claimed in any preceding claim, wherein the target is an image formation device.

10. An optical system as claimed in any of the preceding claims wherein the homogenising means is a tunnel integrator.