WO2008152183A1

WO2008152183A1 - Display screen based on particles carried in a fluid stream

Info

Publication number: WO2008152183A1
Application number: PCT/FI2007/050364
Authority: WO
Inventors: Ismo Rakkolainen
Original assignee: Fogscreen Inc.
Priority date: 2007-06-15
Filing date: 2007-06-15
Publication date: 2008-12-18

Abstract

A display device (500) comprises an optically thin projection screen (110) comprising scattering centers carried in gas or liquid, a projector (200) to project a light beam (B0) onto said screen (110) in order to display a pixel (P1) of an image (610), and a position sensor (450) to detect the position of the eye of a viewer (E1), wherein said display device (500) is adapted to maintain a scattering angle (φ 1) smaller than or equal to 30 degrees, wherein said scattering angle (φ 1) is the angle between a viewing direction (DVW) and the direction (DBE) of said light beam (B0), said viewing direction (DVW) being defined by a line from said pixel (P1) to the position of eye of the viewer. Consequently, the image resolution may be maintained when the viewer moves or stands at various positions.

Description

DISPLAY SCREEN BASED ON PARTICLES CARRIED IN A FLUID STREAM

FIELD OF THE INVENTION

The present invention relates to displaying images on a screen formed of scattering centers carried in gas or liquid.

BACKGROUND

Images projected on a walk-through particle screen provide a fancy effect e.g. in exhibitions and shows. The screen may be e.g. a curtain of ultrasonically nebulised water particles carried in an air stream.

US patent 6,819,487 discloses an apparatus to provide a non-solid projection screen, said apparatus comprising a flow device to form a substantially laminar transfer flow, and one or more supplies to supply scattering centers and/or the initial materials needed in the formation thereof to said transfer flow.

However, as a projection surface, the walk-through screen is a light- scattering slab having a finite thickness. It is typically difficult or impossible to reduce the thickness of a walk-through screen. Thus, unlike in the case of solid diffusing back-lit display screens, the option of increasing the image resolution by reducing the thickness of the screen is not available.

SUMMARY

The object of the invention is to provide a device for displaying images on a walk-through screen.

According to a first aspect of the invention, there is provided a display device 500 comprising

- an optically thin projection screen 1 10 comprising scattering centers carried in gas or liquid, - at least one projector 200 to project a light beam BO onto said screen 1 10 in order to display a pixel P1 of an image 610, and

- a position sensor 450 to detect the position of the eye of a viewer E1 , said display device 500 being adapted to maintain a scattering angle φ1 smaller than or equal to 30 degrees, wherein said scattering angle φ1 is the angle between a viewing direction DVW and the direction DBE of said light beam BO, said viewing direction DVW being defined by a line from said pixel P1 to the position of eye of the viewer.

According to a second aspect of the invention, there is provided a method of projecting an image 610 onto an optically thin projection screen 1 10 comprising scattering centers carried in gas or liquid, said method comprising:

- projecting at light beam BO onto said screen 1 10 in order to display a pixel P1 of an image 610,

- sensing the position of the eye of a viewer E1 ,

- adjusting the direction of said light beam BO such that a scattering angle φ1 is kept smaller than or equal to 30 degrees, wherein said scattering angle φ1 is the angle between a viewing direction DVW and the direction DBE of said light beam BO, said viewing direction DVW being defined by a line from said pixel P1 to the position of eye of the viewer.

A sufficient image resolution may be maintained when the viewer moves or stands at various positions with respect to a reference frame. The reference frame may be fixed e.g. to the screen itself or to a room where the display device is located.

In an embodiment, a sharp image may be displayed to the viewer when the viewer moves or stands at various positions with respect to a reference frame, wherein the screen is substantially immobile with respect to said reference frame. However, the scattering centers themselves may move e.g. vertically within the boundaries of said screen, wherein the boundaries of said screen remain immobile.

In an embodiment the optical power of the projector may be reduced. In an embodiment, glare may be minimized or avoided.

In an embodiment, substantially sharp images may be displayed for two or more viewers standing at different positions.

The embodiments of the invention and their benefits will become more apparent to a person skilled in the art through the description and examples given herein below, and also through the appended claims.

BRIEF DESCRIPTION OF THE FIGURES

In the following examples, the embodiments of the invention will be described in more detail with reference to the appended drawings in which

Fig. 1 shows, in a three dimensional view, a display device comprising a walk-through screen

Fig. 2 shows, in a top view, an illuminated volume corresponding to a pixel,

Fig. 3a shows, in a top view, viewing of a pixel when the viewing direction deviates from the direction of the projecting beam by a small angle,

Fig. 3b shows, in a top view, viewing of a pixel when the viewing direction deviates from the direction of the projecting beam by a large angle,

Fig. 4a shows the apparent width of the illuminated volume corresponding to a pixel when the viewing direction deviates from the direction of the projecting beam by a small angle, as seen by the viewer of Fig. 3a,

Fig. 4b shows the apparent width of the illuminated volume corresponding to a pixel when the viewing direction deviates from the direction of the projecting beam by a large angle, as seen by the viewer of Fig. 3b,

Fig. 5a shows, in a top view, moving of a projector in order to reduce the apparent width of the illuminated volume corresponding to a pixel,

Fig. 5b shows, in a top view, moving of the projector and the screen in order to reduce the apparent width of the illuminated volume corresponding to a pixel,

Fig. 5c shows, in a top view, providing a light beam by a second projector instead of a first projector in order to reduce the apparent width of the illuminated volume corresponding to a pixel,

Fig. 5d shows, in a top view, reflecting a light beam by a second mirror portion instead of a first mirror portion in order to reduce the apparent width of the illuminated volume corresponding to a pixel,

Fig. 5e shows, in a top view, projecting of a light beam through the screen and reflecting the light beam back into the screen in order to illuminate a volume corresponding to a pixel,

Fig. 6a is a polar plot showing angular distribution intensity of Mie- scattered light from 10 μm water particles carried in air,

Fig. 6b shows the angular distribution of intensity of Mie-scattered light from 1 μm and 10 μm water particles carried in air,

Fig. 7 shows, by way of example, the angular distribution of intensity of light scattered from a screen based on ultrasonically nebulised water droplets.

Fig. 8a shows, in a top view, a light beam transmitted through the screen and impinging on the eye of a viewer, Fig. 8b shows, in a top view, positioning of an eye such that a bright image may be perceived and glare may be avoided or reduced,

Fig. 9a shows, in a top view, formation of a glare spot on the screen,

Fig. 9b shows the glare spot as seen by the viewer in the situation of Fig. 9a,

Fig. 9c shows brightness distribution along the line LIN 1 of Fig. 9b,

Fig. 10a shows, in a top view, darkening of a portion of the projected image in order to eliminate the glare spot,

Fig. 10b shows a projected image and a darkened portion as seen by the viewer in the situation of Fig. 10a,

Fig. 10c shows brightness distribution along the line LIN1 of Fig. 10b,

Fig. 1 1 a shows, in a top view, positioning of the light beams such that the darkened portion is outside the projected image,

Fig. 1 1 b shows, the projected image, as seen by the viewer in the situation of Fig. 1 1 a,

Fig. 1 1 c shows brightness distribution along the line LIN1 of Fig. 1 1 b,

Fig. 1 1 d shows a first portion and a second portion of a projected image,

Fig. 1 1 e shows a projected image comprising a sharp portion and a less sharp portion, Fig. 12a shows, in a top view, projecting of an image by two projectors such that the glare spot may be avoided,

Fig. 12b shows the projected image, as seen by the viewer in the situation of Fig. 12a,

Fig. 12c shows brightness distribution along the line LIN1 of Fig. 12b,

Fig. 13a shows, in a top view, gradual darkening of the projected light beams in order to reduce the brightness of the glare spot,

Fig. 13b shows, a projected image and a darkened portion as seen by the viewer in the situation of Fig. 13a,

Fig. 13c shows, by way of example, brightness distribution along the line LIN1 of Fig. 13b,

Fig. 14 shows, in a three dimensional view, simultaneous projecting of different images to different viewers, and

Fig. 15 shows a walk-through display unit.

DETAILED DESCRIPTION

Referring to Fig. 1 , a display device 500 may comprise a back-lit substantially planar screen 1 10, a projector 200 to project an image 610 onto the screen, a position sensor 450 to detect the position of the eye E1 of a viewer, a positioning unit 300 to adjust the direction and/or the position of a light beam BO emitted from the projector 200, and a control unit 400 to control the positioning unit 400 according to the detected position of the eye E1 .

The projector 200 emits a light beam BO to illuminate a pixel P1. In addition, the projector may emit further light beams to illuminate further pixels. The pixel Pi together with the further pixels constitute the projected image 610.

The control unit 400 may also control the projector 200, e.g. in order to control the brightness of the displayed image 610 or the brightness of a portion of said image 610.

The screen 1 10 may be substantially in a plane defined by directions SX and SY. The direction SY is perpendicular to the direction SX. The direction SZ is perpendicular to the directions SX and SY.

The positioning unit 300 may move the projector 200 in the direction SX and/or in the direction SY with respect to a fixed reference REF1 . In addition, the positioning unit 300 may change the direction of the projector 200, i.e. to rotate the projector around at least one axis. The projector 200 may may rotated e.g. in vertical direction and/or horizontal direction. The positioning unit 300 may be implemented e.g. by using guideways and servomotors.

The eye E1 of the viewer may move with respect to the fixed reference REF1 and with respect to the boundaries of the screen 1 10, e.g. by walking to different positions. The control unit 450 and the positioning unit 300 are adapted to move the projector with respect to the reference REF1 according to the moving position of the eye E1 of the viewer.

The fixed reference REF1 may be e.g. a corner of a room, as shown in Fig. 1 , or a predetermined location on the floor of said room.

The position of the body of the viewer or the position of the head of the viewer may be used as an estimate for the position of the eye E1. However, the position sensor 450 may also be so accurate that it actually detects the position of the pupil of the eye E1 .

The position sensor 450 may be a single camera capable of measuring distance to an object. The position sensor 450 may also be a combination of two or more cameras adapted to detect the position by stereoscopic algorithms. The viewer may also stand on a pressure- sensitive film which acts as the position sensor 450. The position sensor 450 may be based e.g. on an ultrasonic sonar. The position of the viewer may be determined by measuring time-of-flight of one or more laser pulses reflected from the viewer. Potentiometers mechanically linked to the viewer may be used as position sensors 450. Hall-effect sensors moved in a magnetic field may be used as position sensors when the Hall-effect sensor or a magnet is attached to the viewer. The sensing of the position may be based on inertial sensing. The acceleration of the viewer may be measured in one or more directions, and the position of the viewer may be calculated based on said measured acceleration and on his initial position. The position sensor 450 or a part of said sensor 450 may be carried by the viewer, and the position information may be transmitted e.g. optically or by radio frequency communication to the control unit 400. In addition to the position sensor 450, a gaze direction sensor may be coupled to the control unit 400 in order to determine which part of an image the viewer is looking at.

The screen 1 10 comprises light-scattering elements carried in fluid, e.g. in gas or liquid. The scattering elements are distributed in a volume. The screen 1 10 may be a walk-through screen, which means that the viewer or at least his hand may penetrate through the screen. The screen may be formed by providing e.g. ultrasonically or pneumatically nebulised water droplets into an air stream. The scattering centers may also be e.g. oil particles, smoke particles or ammonium chloride particles. The gas may also be e.g. nitrogen or argon.

The screen 1 10 may be e.g. a projection screen disclosed in US 6,819,487, herein incorporated by reference. The screen 1 10 may also be formed by a stream of bubbles rising in water.

The screen 1 10 has a finite thickness t1. The scattering centers are distributed along said thickness t1. The thickness may be e.g. in the range of 10 to 100 mm. The screen 1 10 is typically optically thin in the visible wavelength region 400 to 760 nm, i.e. less than 50% of the optical power of the light beam BO is scattered by the scattering centers when the beam is transmitted through the screen 1 10.

The scattering centers of the screen 1 10 may also be arranged to scatter less than 30% of the optical power of the light beam BO. A very high density of scattering centers and/or a thick screen 1 10 may be arranged to scatter less than 70% of the optical power of the light beam BO.

The control unit 400 may also be adapted to stretch the displayed image e.g. in the lateral direction SX if the viewer moves to the side of the screen 1 10.

As a safety feature, the position sensor 450 may be adapted to stop the movement of the projector if the viewer E1 approaches too close to the moving projector.

Referring to Fig. 2, an individual pixel P1 of the displayed image 610 is actually an illuminated volume V1 within the screen 1 10, due to the finite thickness t1. Depending on focusing and the divergence of the light beam BO, the volume V1 has the form of a hour-glass, truncated cone, or a cylinder. The exact position of said pixel P1 may be interpreted to be at the narrowest part of the volume V1.

Referring to Fig. 3a, a scattering angle φ1 is an angle between the direction DBE of the light beam BO and a viewing direction DVW. The viewing direction DVW is defined by a line from the pixel P1 to the eye E1. The apparent width w1 of the pixel P1 is small when the scattering angle φ1 is small.

Referring to Fig. 3b, the apparent width w1 of the pixel increases with increasing scattering angle φ1 , when compared with the situation of Fig. 3a. Fig. 4a shows a view perceived by the viewer in the situation of Fig. 3a. Fig. 4b shows a view perceived by the viewer in the situation of Fig. 3b.

The resolution of the image 610 displayed for the viewer may be improved if the apparent width of the pixels P1 can be reduced. The apparent width W1 of the pixel P1 may be reduced e.g. by limiting the scattering angle φ1.

Referring to Fig. 5a, the projector 200 may be moved with respect to the fixed reference REF1 to keep the scattering angle φ1 smaller than a predetermined value when the viewer is moving.

Referring to Fig. 5b, the screen 1 10 and the projector may also be together moved with respect to the fixed reference REF1 to keep the scattering angle φ1 smaller than a predetermined value when the viewer is moving with respect to the reference REF1 . Thus, the available width of the screen 1 10 may be maintained constant if the screen rotates.

Also the screen 1 10 may be turned with respect to the reference REF1 in order to preserve the full width of the displayed image. However, the apparatus for generating the screen may be large, heavy and even dangerous to rotate. It is typically enough to move only the projector in order to maintain the desired resolution.

Referring to Fig. 5c, the device 500 may comprise two or more projectors 200a, 200b adapted to keep the scattering angle φ1 smaller than a predetermined value. The light beam BO illuminating the pixel P1 may be provided by a second projector 200b instead of a first projector 200a in order to limit the scattering angle φ1. Thus, the light beams provided by the different projectors 200a, 200b may be adapted to provide substantially the same image on the screen while the ratio of a first optical power provided by a first projector 200a to a second optical power provided by a second projector 200b may be different corresponding to different pixel locations in the displayed image, said first optical power being the optical power of a light beam provided by the first projector 200a to illuminating a pixel and said second optical power being the optical power of a light beam provided by the second projector 200b to illuminating the same pixel. In other words, beams provided by the different projectors may be blended together seamlessly so that they have spatially varying intensity ratios.

The device 500 may comprise a surrounding bank of several projectors 200a, 200b.

Referring to Fig. 5d, the display device 500 may comprise one or more mirror portions 250a, 250b to re-direct a light beam BO provided by a projector 200. The projector 200 may be e.g. rotated to direct the beam to a second mirror portion 250b instead of a first mirror portion 250a, to keep the scattering angle φ1 smaller than a predetermined value.

Referring to Fig. 5e, as the screen 1 10 is optically thin, the light beam BO may be transmitted through the screen before being reflected by a mirror portion 250 back into the screen to illuminate the pixel P1. The backward scattering efficiency is much weaker than the forward scattering efficiency. Although the image formed by the backward scattered light may be unsharp or and/or displaced with respect to the screen, it may be that the dim image formed by backward scattered light does not significantly disturb viewing of the bright image formed by the forward-scattered light.

Fig. 6a is a polar plot showing angular distribution of the intensity of scattered light for 10 μm water droplets carried in air. The intensity values are shown in logarithmic scale (in arbitrary units). It may be noticed that increasing of the scattering angle φ1 from zero to approximately 3° reduces the intensity approximately by a factor of 100. A further increase from 3° to 30° reduces the intensity approximately by a factor of 10. Yet, a further increase from 30° to 60° reduces the intensity approximately by a factor of 10.

The same behaviour may be noticed also from the curve of Fig. 6b, which shows the angular distribution of the intensity of scattered light for 10 μm water droplets carried in air. Fig. 6b shows also the angular distribution of the intensity of scattered light for 1 μm water droplets carried in air. The wavelength for light in case of Figs 6a and 6b is 650 nm and the light has perpendicular polarization. The 1 μm and the 10 μm curve are associated with the same droplet density (i.e. same number of droplets per cubic meter). It may be noticed that the scattering efficiency is over 1000 times higher for a 10 μm droplet than for a 1 μm droplet. However, if the plots would be redrawn for the same mass of water per volume unit (e.g. grams / cubic meter), the difference between the scattering efficiency would be much smaller.

It may be noticed from Figs. 6a and 6b that in case 10 μm particles, the brightness of the displayed image may be increased by a factor of 10 if the scattering angle is kept below 30° instead of allowing angles near 60°. In case 10 μm particles, the brightness of the displayed image may be increased by a factor of 100 if the scattering angle is kept below 2° instead of allowing angles near 30°. In case 1 μm particles, the brightness of the displayed image may be increased by a factor of 10 if the scattering angle is kept below 12° instead of allowing angles near 60°. In case 1 μm particles, the brightness of the displayed image may be increased by a factor of 10 if the scattering angle is kept below 3° instead of allowing angles near 12°.

The angle β denotes an angle where the scattered intensity has decreased by a factor of 10 from its maximum value. To the first approximation, the angle β depends on the average size of the scattering centers. Fig. 6b shows the angle β for 10 μm droplets.

Referring to Fig. 7, the screen 1 10 may comprise e.g. ultrasonically or pneumatically nebulised water particles, whose mass median diameter (MMD) is typically in the range of 1 to 10 μm. The droplet size distribution is typically polydisperse in a real application. Fig. 7 shows, by way of example, a measured angular distribution of the intensity light scattered from a screen 1 10 based on ultrasonically nebulised water droplets. The ordinate scale is in arbitrary units (a.u.). The unit has not been matched with the arbitrary unit shown in Figs. 6a and 6b. It may be noticed that the angle β is substantially equal to 1 1 degrees. The fraction of unscattered light may be e.g. 72% for a screen 1 10 based on ultrasonically nebulised water droplets.

Referring to Fig. 8a, the brightness of the displayed image 610, i.e. the intensity of light scattered by the scattering centers of the screen 1 10 is at maximum when the scattering angle is near zero. However, as the screen is optically thin, a part of the original light beam BO provided by the projector 200 impinges on the eye E1 of the viewer. The resulting glare may be rather annoying or even dangerous if the beam BO is provided by a powerful projector 200.

The scattering centers may also be fluorescing or phosphorescing particles and the projector 200 may be adapted to provide UV (ultraviolet) light in order to induce fluorescence. In that case glare may not be a problem in terms of a visual perception, but it may be a more serious physical problem due to UV-light induced damage to tissues and fluids of the eye E1.

Referring to Fig. 8b, the scattering angle φ1 may be kept greater than the half-divergence α of the light beam BO in order to avoid glare. In other words, a high-intensity glare spot appearing in the image may be avoided by moving the projector 200 and/or by using several projectors 200 providing light from different directions.

However, it may be that avoiding glare by moving only one projector 200 is not possible for all pixels of an image 610. Consequently, as a further measure, the intensity of a portion of the displayed image 610 has to be substantially reduced. In other words, the intensity of the light beams which illuminate the pixels of said portion has to be reduced. This may mean that the image comprises a darkened portion.

If the device 500 comprises further projectors, they may be used to illuminate said portion from further directions in order to illuminate the whole image 610.

While keeping the scattering angle φ1 greater than the half-divergence α of the light beam BO, the scattering angle φ1 may also be kept smaller than 30 degrees in order to maintain the resolution of the image 610.

While keeping the scattering angle φ1 greater than the half-divergence α of the light beam BO, the scattering angle φ1 may also be kept smaller than a predetermined limit, e.g. smaller than the angle β in order to keep the maximum brightness of the displayed image 610 greater than a predetermined limit. Consequently, a projector which provides less optical power may be used, which saves energy and costs.

Referring to Fig. 9a, an observer may be positioned with respect to the projected image 610 and the projector 200 such that the transmitted portion of a light beam BO impinges on the eye E1 of the observer, wherein said beam BO illuminates a pixel P1 of said image 610. Consequently, the observer sees a glare spot at a point 620 on the screen 1 10. The point 620 is located at the intersection of the screen 1 10 and a line LHS, said line LHS being drawn from the output aperture of the projector to the eye E1 of the observer. A further projected light beam Ba provides a pixel on the left side of the image 610 and a light beam Bb provides a pixel on the right side of the image 610.

Fig. 9b shows the image 610 and a glare spot 630, as seen by the observer of Fig. 9a. The projected image 610 may be e.g. a star pattern.

Fig. 9c shows brightness intensity distribution along a line LIN1 shown in Fig. 9b. x denotes spatial coordinate in the direction SX and IN denotes intensity. The abscissa values are in arbitrary units. The ordinate values are shown in arbitrary units in logarithmic scale.

The aim may be, for example, to provide a substantially uniform brightness for the projected image. However, the glare spot 630 may appear e.g. one or two orders of magnitude brighter than the image

610. The glare spot may be harmfully bright to the eye E1 of the observer and the glare spot may make it difficult to see details of the image 610.

Referring to Fig 10a, one or more light beams projected by a projector 200 may be darkened in order to eliminate the glare spot. In particular, light beams whose direction deviates from the line LHS by an angle smaller than or equal to δ may be darkened. The angle δ may have a fixed value, e.g. 2 degrees. The angle δ may also be selected to be e.g. greater than or equal to the angle β (Figs 6b, 7 and 8b). Beams Bc and Bd indicate boundaries of the darkened solid angle. Although the path of the beam BO has also been drawn, the intensity of the beam BO is substantially equal to zero in case of Fig. 10a.

Fig. 10b shows the image as seen by the observer in the situation of Fig. 10a. The glare spot is now eliminated and replaced by a darkened portion 630.

Fig. 10c shows brightness distribution along a line LIN1 shown in Fig. 10b. The abscissa and the ordinate values are as in Fig. 9. The brightness of the darkened portion may have substantially equal to zero.

Referring to Fig. 1 1 a, the projector 200 may be moved and/or light beams may be provided by another projector such that the glare spot is moved outside the image 610. The position of the projector 200 may be moved with respect to the screen 1 10. The position of the projector 200 may be different than e.g. in Fig. 9a or 10a, while displaying the same image 610. In particular, the brightness of the pixel P1 may now be modulated with respect to its neighbouring pixels, i.e. spatial modulation becomes possible in the vicinity of the pixel P1. Spatial modulation in the vicinity of the pixel P1 was not possible in the case of Figs. 10a and 1 1 a because the brightness of the pixel P1 and of the neighbouring pixels was substantially set to zero in order to eliminate the glare spot.

The glare spot may still be darkened. In particular, light beams whose direction deviates from the line LHS by a limit angle smaller than or equal to δ may be darkened. The limit angle δ may have a fixed value, e.g. 2 degrees.

Moving of the darkened portion 630 outside the projected image 610 may mean that the scattering angle φ1 for some pixels P2 becomes greater than 30 degrees. Consequently said pixels P2 may appear blurred to the viewer E1. Nevertheless, at least the pixel P1 and a group of pixels in the vicinity of the pixel P1 appear sharp to the viewer E1.

Fig. 1 1 b shows the image as seen by the observer in the situation of Fig. 1 1 a. The darkened portion 630 appears now to be shifted outside the displayed image 610, and does not disturb the viewing of the image 610.

Fig. 1 1 c shows brightness distribution along a line LIN1 shown in Fig. 1 1 b. The abscissa and the ordinate values are as in Fig. 9. The whole image may have substantially uniform brightness.

Referring to Fig. 1 1 d, a projected image 610 may comprise a first portion 61 1 and a second portion 612. The first portion 61 1 may comprise features 613 of primary importance such as text, numerical values, or features of a human face. The second portion may comprise features 614, 615 of secondary importance, e.g. decorative features such as star patterns or flower patterns. The control unit 400 may now be adapted to adjust the position and the direction of one or more projectors 200 according to the detected position of the viewer E1 such that the features of the first portion 61 1 appear substantially sharp to the viewer E1 , wherein the features 614, 615 of the second portion 612 are allowed to be less sharp than the features 613. The expression "sharp" may mean herein that the scattering angle φ1 is smaller than or equal to 30 degrees for pixels of the first portion 61 1. The scattering angle φ1 may be allowed to be greater than 30 degrees for pixels residing in the second portion 612. Features 615 which are farther away from the intersection point 620 appear less sharp than features 614 which are closer to the intersection point 620. Moving of projector 200 so that the darkened portion 630 would be moved completely out of area occupied by the image 610 might mean that it might be impossible to display sharp features 613 in the first portion 61 1. Consequently, it may be acceptable that the projector 200 is moved only to such an extent that the darkened portion 630 resides in the second portion 612 It may be acceptable that some of the secondary features 614 are obscured by the darkened portion 630, wherein it is ensured that the first portion is not obscured by the darkened portion 630.

In an embodiment, it may be even acceptable that the darkened portion 630 resides partially or completely in the first portion 61 1.

Referring to Fig. 12a, further light beams Be and Bf may be projected by a second projector 200b, in order to fill a darkened portion left by a first projector 200a. The brightness of the pixel P1 may be modulated by controlling the second projector 200b.

Fig. 12b shows the image as seen by the observer in the situation of Fig. 12a. A complete image 610 may be seen, without the glare spot.

Fig. 12c shows brightness distribution along a line LIN1 shown in Fig.

12b. The abscissa and the ordinate values are as in Fig. 9. The whole image may have substantially uniform brightness.

Referring to Fig. 13a, one or more light beams projected by a projector

200 may be completely or partially darkened in order to eliminate the glare spot or in order to reduce the brightness of said glare spot. Light beams whose direction deviates from the line LHS by an angle smaller than or equal to δ may be darkened. The angle δ may have a fixed value, e.g. 5 degrees. The angle δ may also be selected to be e.g. greater than or equal to the angle β (Figs 6b, 7 and 8b). Beams Bc and

Bd indicate boundaries of the darkened solid angle.

Fig. 13b shows the image as seen by the observer in the situation of Fig. 13a. The darkened portion 630 may have a gradual brightness distribution. Fig. 13c shows brightness distribution along a line LIN1 shown in Fig. 13b. The abscissa and the ordinate values are as in Fig. 9. The darkened portion 630 may have a gradual brightness distribution.

It is emphasized that the brightness at the center of the darkened portion does not need to reach zero value. For example, the apparent brightness of the darkened portion 630, as seen by the observer E1 , may be adjusted be substantially equal to the average brightness of the displayed image. This is possible because the position of the observer E1 is known with respect to the screen 1 10.

The display device 500 converts a digital image into a real image 610. The brightness of each pixel of the image 610 may be adjusted by the control unit 400 so that the projected image 610, when seen by the observer E1 , corresponds to the digital image as closely as possible.

The brightness of the darkened portion 630 maybe adjusted to be substantially equal to the brightness of the other parts of the real image 610, wherein said image 610 is observed by the viewer E1 and wherein the digital image has substantially uniform brightness.

The positions of the observer E1 and the projector 200 are known with respect to the screen 1 10, and it may be determined e.g. by the control unit 400 which of the provided light beams coincides with the line LHS. Light beams whose direction deviates from the line LHS by an angle smaller than or equal to e.g. 5 degrees may be digitally or electronically dimmed in order to reduce the brightness of a glare spot caused by light having a scattering angle φ1 less than 5 degrees. The dimming may comprise multiplying pixel values of the digital image by coefficients smaller than one. The dimming coefficients may be arranged and/or used as a two-dimensional array of coefficients.

The intensity of the glare spot may also be reduced by e.g. positioning a movable blocking element (not shown) between the imaging optics of the projector 200 and an image-generating array in said projector. The blocking element may be e.g. a slab which is moved with respect to the projecting optics according to the movements of the viewer. The blocking element may be moved by a mechanical actuator controlled by the control unit 400.

The blocking element may also be positioned between the projector 200 and the viewer. In that case the distance between the blocking element and the projector may be e.g. in the range of 30% - 70% of the distance between the projecting optics of the projector and the screen 1 10. The blocking element may be moved according to the movements of the viewer.

Referring to Figs. 10a to 13c, and in particular to Figs 1 1 d and 1 1 e, the display device 500 may be adapted to display the pixels of a selected portion 61 1 of the image 610 as sharp pixels in addition to reducing the brightness of the glare spot. In other words, the scattering angle φ1 may be kept smaller than or equal to 30 degrees for each pixel of a group residing in said selected portion 61 1 , in addition to reducing the brightness of those pixels for which the scattering angle φ1 is smaller than or equal to 5 degrees. The limit angle δ may also have another value instead of said 5 degrees, e.g. 2 degrees, 3 degrees, 10 degrees, or the limit angle δ may be equal to the angle β, and depending on the angular scattering properties of the screen 1 10, depending on the divergence of the beam BO.

The darkened portion 630 may also have a form which deviates from the circular form. The darkened portion 630 may have an elliptical form. In particular, the horizontal width of the darkened portion 630 (in the direction SX) may be greater than the height of said portion (in the direction SY) in order to simultaneously eliminate the glare spot for both eyes of the viewer.

Referring back to Figs 6a, 6b and 7, if an angle between the viewing directions of two or more persons is greater than e.g. 30 degrees, and if the scattering angle φ1 for each viewer is kept substantially small, e.g. smaller than 15 degrees or smaller than the angle β, then substantially sharp separate images 610 may be simultaneously displayed to both persons by using two or more projectors 200. Said images 610 may be different images or the same image. The side of the screen corresponds to 180 degrees of viewing angles. Thus even 4 to 6 (=180730°) persons may view different images at one side of the screen 1 10.

However, the different images may still appear to be substantially superposed when the angular separation between the viewing directions of adjacent viewers is near 30 degrees, i.e. the image quality may be only moderate.

As the back-scattering efficiency is typically very low (Figs. 6a and 6b) then both sides of the screen 1 10 may also be used for displaying different images 610.

Referring to Fig. 14, the display device 500 may be adapted to show two or more different images 610a, 610b, 610c to two or more different observers E1 , E2, E3. A first projector 200a may project a first image 610a seen by an observer E1. A second projector 200b may project a second image 610b seen by an observer E2. A third projector 200c may project a third image seen by an observer E3. Although the images 610a, 610b, 610c may be partially or completely overlapping, the first image 610a appears much brighter to the first observer E1 than the second image 610b. Thus, the first observer may see effectively only the first image 610a.

However, depending on the angular scattering efficiency of the screen 1 10, the second image 610b may still be substantially visible to the first observer E1 when the angle γ1 between the viewing directions of said viewers is in the range of 30 to 55 degrees. In order to ensure a greater difference between the brightness of the first image 610a shown to the first observer E1 and the second image 610b, the angle γ1 between the viewing directions of said two observers E1 , E2 may be greater than 55 degrees. In that case different images may be displayed for 2 to 3 viewers positioned at one side of the screen and/or for 4 to 6 viewers positioned on both sides of the screen. γ2 denotes an angle between the viewing directions of the second observer E2 and the third observer E3. Referring to Fig. 15, a screen unit 100 for generating the screen 1 10 may comprise a flow conditioning unit 120 to provide a substantially laminar air flow and a particle unit 140 to add or form scattering centers SC1 into the laminar air flow. The screen unit 100 may comprise one or more fans 160a, 160b to provide an air flow to the flow conditioning unit.

Flow conditioning unit 120 may be e.g. e.g. a honeycomb tube flow conditioner. The height hi of the stable screen 1 10 depends on the thickness t2 of the laminar air stream provided by the flow conditioning unit 120. The height hi of the screen 1 10 may be increased by providing a thicker air stream.

The particle unit 140 may be implemented e.g. by using a perforated duct which distributes water droplets generated by an ultrasonic nebuliser.

The screen unit 100 may further comprise a suction unit (not shown) to collect the air stream and the scattering centers on the other side of the screen 1 10. The orientation of the air flow is typically vertical from up to down. However, any orientation may be used.

A screen unit 100 may also be implemented without the flow conditioning unit 120 and even without the fans 160a, 160b. However, in that case the screen 1 10 may be substantially less stable and it has fluctuating vortexes.

Referring back to Fig. 1 , the position sensor 450 and/or a further sensor may be further adapted to detect the gaze direction of the eye E1 of the viewer, i.e. to analyze at which point of the image 610 the viewer is actually looking. Although the viewing direction was defined to be a line from the point P1 to the eye E1 , the viewer may actually look at another point different from P1.

It may be that a sharp image can not be provided over the whole area of the image 610 or the screen 1 10. Thus, the control unit 450 may be adapted to maximize the sharpness of the image 610 at the specific point which the viewer is looking at.

The control unit 450 may also change the projected image according to the position of the viewer and/or the gaze direction.

The sensor 450 or a further sensor may be adapted to sense the position of the hand or finger of the viewer in order to implement an interactive touch-sensitive screen. In other words, the viewer may use his hand or an object as a pointer to indicate a point and/or to make a selection among alternatives displayed on the screen. The viewer may move or change the displayed image 610 by moving his hand.

The displayed image may be stretched, moved or modified in the horizontal direction SX, in the vertical direction SY according to the horizontal and/or vertical position of the viewer.

The viewer may wear e.g. shutter or polarizing goggles coupled to the device 500, and the device 500 may be adapted to display stereoscopic (3D) images to the viewer.

Images of three-dimensional objects may also be displayed to the viewer without using goggles. The same image may be shown to both eyes of the viewer wherein the image is changed according to the position of the viewer such that the image corresponds to different aspects of an object, or to different aspects of several objects. For example, the viewer may move his head to a first position to look how the first side of a projected object looks like, and to a second position to look a second side of said object. The object may be e.g. a cube which appears to float in the air. For example, the viewer may walk to a second position in order to see what is behind a projected wall.

The display device 500 may also be adapted to eliminate only the glare spot 630, wherein the scattering angle φ1 is not actively kept below 30 degrees for a predetermined pixel P1 . However, in that case the projected image may occasionally appear substantially blurred to a moving viewer. For the person skilled in the art, it will be clear that modifications and variations of the devices according to the present invention are perceivable. The figures are schematic, apart from Figs 6a, 6b and 7. The particular embodiments described above with reference to the accompanying drawings and table are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Table 1 : Numerical values for the measured angular intensity distribution shown in Fig. 7

Claims

1. A display device (500) comprising

- an optically thin projection screen (1 10) comprising scattering centers carried in gas or liquid,

- at least one projector (200) to project a light beam (BO) onto said screen (1 10) in order to display a pixel (P1 ) of an image (610), and

- a position sensor (450) to detect the position of the eye of a viewer (E1 ), said display device (500) being adapted to maintain a scattering angle (φ1 ) smaller than or equal to 30 degrees, wherein said scattering angle (φ1 ) is the angle between a viewing direction (DVW) and the direction (DBE) of said light beam (BO), said viewing direction (DVW) being defined by a line from said pixel (P1 ) to the position of eye of the viewer.

2. The display device (500) according to claim 1 comprising a positioning unit (300) to move at least one projector (200) in order to maintain said scattering angle (φ1 ) smaller than or equal to 30 degrees.

3. The display device (500) according to claim 1 or 2 comprising a control unit 400, a first projector (200a), and a second projector (200b), said control unit (400) being adapted to allocate the projecting of said light beam (BO) from said first projector (200a) to said second projector (200b) in order to maintain said scattering angle (φ1 ) smaller than or equal to 30 degrees.

4. The display device (500) according to claim 2 or 3 wherein said control unit (400) is adapted to keep the scattering angle (φ1 ) greater than or equal to 5 degrees for said pixel (P1 ).

5. The display device (500) according to any of the preceding claims 1 to 4, wherein said display device (500) is adapted to project a plurality of light beams (BO, Ba, Bb, Bc, Bd) in order to display said image (610), said display device (500) being adapted to determine which one of the provided light beams coincides with a line (LHS) drawn from a first projector (200a) to said eye of the viewer (E1 ), said display device (500) being adapted to digitally or electronically dim light at least one light beam whose direction deviates from the said line (LHS) by an angle smaller than or equal to 5 degrees.

6. The display device (500) according to any of the preceding claims 1 to 5, wherein a projector (200) is adapted to project a plurality of light beams (BO, Ba, Bb, Bc, Bd) in order to display a predetermined portion of an image (610), said display device (500) being adapted to shift the position of said projector (200) from a first position to a second position based on the position of said viewer (E1 ), wherein in said first position the direction of at least one of said light beams deviates less than 5 degrees from a line (LHS) drawn from said first position to the eye of the viewer (E1 ), and in said second position the directions of said light beams deviate more than 5 degrees from said line (LHS)

7. The display device (500) according to any of the preceding claims 1 to 6, wherein a first projector (200a) is adapted to display a first image (610a) to a first viewer (E1 ) and a second projector (200b) is adapted to display a second image (610b) to a second viewer (E2), an angle (γ1 ) between the viewing directions of said viewers (E1 , E2) being greater than or equal to 55 degrees.

8. The device (500) according to any of the claims 1 to 7 wherein said screen (1 10) is formed of water droplets carried in air.

9. A method of projecting an image (610) onto an optically thin projection screen (1 10) comprising scattering centers carried in gas or liquid, said method comprising: - projecting at light beam (BO) onto said screen (1 10) in order to display a pixel (P1 ) of an image (610),

- sensing the position of the eye of a viewer (E1 ),

- adjusting the direction of said light beam (BO) such that a scattering angle (φ1 ) is kept smaller than or equal to 30 degrees, wherein said scattering angle (φ1 ) is the angle between a viewing direction (DVW) and the direction (DBE) of said light beam (BO), said viewing direction (DVW) being defined by a line from said pixel (P1 ) to the position of eye of the viewer.

10. The method according to claim 9 comprising - projecting a plurality of light beams (BO, Ba, Bb, Bc, Bd) by a projector (200) in order to display said image (610),

- determining which one of the provided light beams substantially coincides with a line (LHS) drawn from said projector (200) to said eye of the viewer (E1 ), and - dimming at least one light beam whose direction deviates from the said line (LHS) by an angle smaller than or equal to 5 degrees.

1 1. The method according to claim 9 or 10 comprising:

- displaying a first image (610a) to a first viewer (E1 ), and - displaying a second image (61 Ob) to a second viewer (E2), wherein an angle (γ1 ) between the viewing directions of said viewers (E1 , E2) is greater than or equal to 55 degrees.