WO2004086123A1 - Three-dimensional display - Google Patents

Three-dimensional display Download PDF

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Publication number
WO2004086123A1
WO2004086123A1 PCT/IB2004/050284 IB2004050284W WO2004086123A1 WO 2004086123 A1 WO2004086123 A1 WO 2004086123A1 IB 2004050284 W IB2004050284 W IB 2004050284W WO 2004086123 A1 WO2004086123 A1 WO 2004086123A1
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WO
WIPO (PCT)
Prior art keywords
display
light
screen
display device
display screen
Prior art date
Application number
PCT/IB2004/050284
Other languages
French (fr)
Inventor
Coen T. H. F. Liedenbaum
Ralph Kurt
Original Assignee
Koninklijke Philips Electronics N.V.
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Application filed by Koninklijke Philips Electronics N.V. filed Critical Koninklijke Philips Electronics N.V.
Publication of WO2004086123A1 publication Critical patent/WO2004086123A1/en

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Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04NPICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N13/00Stereoscopic video systems; Multi-view video systems; Details thereof
    • H04N13/30Image reproducers
    • H04N13/388Volumetric displays, i.e. systems where the image is built up from picture elements distributed through a volume
    • H04N13/395Volumetric displays, i.e. systems where the image is built up from picture elements distributed through a volume with depth sampling, i.e. the volume being constructed from a stack or sequence of 2D image planes
    • GPHYSICS
    • G02OPTICS
    • G02BOPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B30/00Optical systems or apparatus for producing three-dimensional [3D] effects, e.g. stereoscopic images
    • G02B30/50Optical systems or apparatus for producing three-dimensional [3D] effects, e.g. stereoscopic images the image being built up from image elements distributed over a 3D volume, e.g. voxels
    • G02B30/52Optical systems or apparatus for producing three-dimensional [3D] effects, e.g. stereoscopic images the image being built up from image elements distributed over a 3D volume, e.g. voxels the 3D volume being constructed from a stack or sequence of 2D planes, e.g. depth sampling systems

Definitions

  • the invention relates to a display device for providing an image with depth perception.
  • Such devices are sometimes also called 3D display devices or 3D display systems.
  • a three-dimensional representation of graphics and video is one of the holy grails in the display field.
  • Several methods have been devised which give the viewer the impression that he is watching a 3 -dimensional image.
  • a 3D perception can be created in several manners.
  • a three-dimensional impression can be created by using stereo pairs (two different images directed at the two eyes of the viewer).
  • stereo pairs two different images directed at the two eyes of the viewer.
  • the images may be time-multiplexed on a 2D display, but this requires the viewers to wear glasses with e.g. LCD shutters.
  • the stereo images When the stereo images are displayed at the same time, the images can be directed to the appropriate eye by using a head-mounted display, or by using polarized glasses (the images are then produced with orthogonally polarized light).
  • the glasses worn by the observer effectively route the views to each eye. Shutters or polarizer's in the glasses are synchronized to the frame rate to control the routing.
  • a disadvantage of such a system is that the two images produce only a limited "look-around" capability.
  • glasses have to be worn to produce any effect. This is unpleasant for those observers who are not familiar with wearing glasses and a potential problem for those already wearing glasses, because the extra pair of glasses does not always fit.
  • the two stereo images can also be split at the display screen by means of a splitting screen such as a lenticular screen or a parallax barrier.
  • a splitting screen such as a lenticular screen or a parallax barrier.
  • the principle is shown in e.g. Figures 3 and 4 of United States Patent US 6,275,254.
  • these displays with splitting screens do not require special glasses to view the 3D image, they conventionally work only for one viewer at a fixed position in space, making them unsuitable for normal every-day use, such as, for instance, in a living room, where more than one viewer may be present and the position of the viewer vis-a-vis the image screen may vary considerably.
  • the display device according to the invention is characterized in that the display device comprises
  • an addressable attenuator close to the at least one front display screen for attenuating light emanating from the rear display screen at positions corresponding to addressed pixels in the front display screen.
  • images are formed one behind the other.
  • a 3D image is formed by forming image infonnation with different depth content on the rear and front display screens.
  • the words "rear” and "front” are used to indicate that the one is behind the other, or the other is in front of the one, i.e. this terminology refers to their relative position, and should not be inteipreted as to signify any restriction of the position of said screens vis-a-vis other elements of the device.
  • image screens may be provided to increase the number of image layers.
  • the front display screen is substantially transparent to the light emanating from the rear display screen so that a rear image shines through a front display screen. Formation of a front image on a front display screen does not substantially influence formation of a rear image on a rear display screen.
  • the device comprises an addressable attenuator close to the front display screen.
  • the addressable attenuator At positions corresponding to addressed pixels in a front display screen, the addressable attenuator attenuates the light coming from the at least one rear display screen.
  • the addressable attenuator positioned close to the front display screen, partially or totally blocks the light from the rear image for the addressed pixels (i.e. those for which light is formed) of the front display screen.
  • the rear image is thus attenuated and does not mix or only partially mixes with the front image.
  • the light from the rear image is not attenuated and thus passes through the front display screen.
  • the combination of the at least one rear display screen, the at least one front display screen and the attenuator close to the at least one front display screen makes a foreground figure formed on a front screen stand out in respect of the background image on a rear screen, as it would in real life.
  • the attenuator is a stopping screen, i.e. the attenuator becomes opaque at positions corresponding to addressed pixels for the front display screen, i.e. it totally blocks light coming from behind, and thus only the foreground figure, i.e. the image on the front screen, is visible at said positions.
  • the attenuator has such an attenuation action that the attenuator can be made partially transparent to selected pixels in the at least one front display screen.
  • the image comprises partially or totally transparent image parts (such as e.g. a ghost, stained glass, fog or partially transparent textiles)
  • the information data on the front image does not only comprise data on the luminance and color but also data on the transparency of the pixel in such preferred embodiments. Via a control, these transparency data are used to control the amount of light that passes the attenuator for the relevant pixels. A more lifelike imaging of (semi)transparent objects is thereby possible.
  • the attenuator is preferably positioned between the at least one rear display screen and the at least one front display screen, preferably attached to the at least one front screen.
  • the attenuator does not attenuate the light from the at least one front screen, but only attenuates the light from the at least one rear screen.
  • the attenuator is positioned in front of the front screen.
  • the emitted light from the at least one front screen as well as from the at least one rear screen goes through the attenuator. Distinction between the rear and front images and thereby selective passing of light from the at least one rear display screen is possible when the polarization of the rear and front images differs and the attenuator comprises polarization-dependent light-addressable transmission elements and means for selecting the polarization of transmitted light. Selection is also possible when the rear and front images are emitted at different time slots.
  • the display device comprises means (such as a circuit or hardware and/or software) to form the images on the at least one rear and the at least one front display screen inte ⁇ nittently, i.e. when an image on the at least one rear display screen is formed, no image is formed on the at least one front display screen, and vice versa.
  • Intermittent formation of the images in combination with the fact that the front screen is transmissive to a rear image involves no or only little or moderate light absorption of a rear image by a front screen. The front screen is off when the rear screen is on, and vice versa.
  • the possibility of crosstalk between the rear and the front image is reduced. Furthermore, if the rear and front screens are arranged in such a way that the front screen is always substantially transmissive to light coming from behind, the front display screen can be addressed when the rear screen is emitting, and vice versa.
  • the inventive device and method of displaying 3D images require no additional devices such as e.g. special goggles to be used by the viewer.
  • the depth perception is due to the fact that the images are displayed on a number of screens, rather than on the viewer's position. Thus, the problem of a very narrow viewing zone is also eliminated. Depth is perceived no matter what the position of the viewer is vis-avis the device.
  • the attenuator provides a natural look to the displayed images, by making foreground figures standing out from the background in a natural manner.
  • the addressing of the addressable stopping screen is coupled to the information on the at least one front display screen, but attenuates the light of an image on the at least one rear display screen.
  • the device according to the invention may have only one rear and only one front display screen, but in more sophisticated embodiments, the display device has more than two display screens arranged one behind the other.
  • the device comprises a first rear display screen and a final front display screen, and in addition one or more display screens in between the first rear display screen and the final front display screen.
  • Each of the one or more display screens in between the first rear display screen and the final front display screens acts as a front display screen for each display screen positioned behind said display screen, and as a rear display screen for each display screen positioned in front of said display screen.
  • the one or more display screens in between the final rear and front display screen are each provided with an attenuator to attenuate light from behind, and the display screens themselves are each transparent to light coming from behind. At those positions where the display screen with which it is associated emits light, the attenuator attenuates the light coming from behind.
  • the provision of more than two display screens, each (with the exception of the first rear screen) with an attenuator close to, and preferably attached to the relevant screen enables a multi-layered image to be made, enhancing the 3-D impression.
  • an attenuator is attached to each display screen, but for the first rear display screen.
  • each attenuator is positioned behind (seen from the viewer's position) the associated display screen.
  • photoluminescent materials i.e. materials that emit visible light when light is incident on them
  • electroluminescent materials i.e. materials that emit light when a potential difference is applied to them.
  • a preferred embodiment of the display device according to the invention is characterized in that at least one of the display screens has areas comprising nanocrystals selected from the group of nanorubes and nano wires which show photoluminescence, and the display device is provided with a light source and selection means for selectively and addressably exciting the areas.
  • the nanocrystals show photoluminescence, i.e. when impinged by light (usually UV light), the nanocrystals emits visible light.
  • a light source and means for selectively and addressably exciting the areas an image may be formed on the display screen with the nanocrystals.
  • any photoluminescent material may be used.
  • the use of photoluminescent nanocrystals has the advantage that nanocrystals are very stable emitters. Ageing of materials, especially under UV light and in a normal atmosphere is a major problem. Most of the photoluminescent materials show considerable ageing effects. Nanocrystals are very stable emitters.
  • the ageing effect may form a considerable problem.
  • ageing problems affect the image as a whole, resulting in, for instance, somewhat less bright colors, or off-colors.
  • slight discolorations are often not readily perceivable by a viewer, for lack of comparison.
  • red is perceived as red, even if it is e.g. slightly brownish.
  • a layered image is shown. Any discoloration due to ageing may result in differences in color between one display screen and another if this discoloration is different in different display screens.
  • a slight off-color due to an ageing effect which is hardly perceivable in standard display devices, may become perceivable in a device according to the invention, because differences in ageing cause the "same" color (i.e. the signals are tuned in such a way that the same color should have been displayed, but for the ageing effect) to be displayed differently on different image screens.
  • the display screens are provided with areas with photoluminescent materials, the photoluminescent materials (e.g.
  • At least two of the display screens comprise areas having nanotubes or nano wires. These areas are arranged in such a way that the orientation of the plane of polarization of luminescent light (for the display screen furthest remote from the viewer, in this paragraph further denoted as "rear” screen for simplicity), and absorbed light (for the screen closest to the viewer, in this paragraph further denoted as "front” screen for simplicity) differ respectively.
  • the invention makes use of the strong anisotropic optical properties (absorption and emission show polarization dependence) obtainable by nanocrystals.
  • the alignment of the nanotubes or nanowires (and therefore the axis of anisotropy) in a rear screen differ with respect to the alignment of the nanotubes or nanowires in a front screen, different polarization planes for emission (by the nanocrystals in the rear screen) and absorption (by the nanocrystals in the front screen) are provided.
  • the light emitted by the rear screen has a polarization which is different from the plane of polarization for absorption of the front screen and thus substantially passes through said front screen without exciting it.
  • the intensity of the rear image is therefore not decreased or only moderately decreased on transmission through the front screen, and the front screen is not excited, or only excited to a small extent by light passing through it.
  • This provides a simple, yet good independence of the image on the rear and the front screen.
  • the best results are obtained by making the alignment of the nanocrystals substantially perpendicular to each other.
  • the front screen has such an absorption spectrum that the light emitted by the rear screen is substantially not absorbed by, and substantially does not excite the nanocrystals in the front screen. This reduces absorption, increases the intensity of the image and reduces crosstalk between images.
  • the display device comprises the at least two screens of a photoluminescent material, preferably nanocrystals, having different planes of polarization of absorption, and a spatial modulator and a means for polarizing the light in a plane of polarization corresponding to the plane of polarization for absorption of the at least two screens are arranged between the light source (e.g. blue, UV, or DUV) and the screen.
  • the light source e.g. blue, UV, or DUV
  • Nanocrystals show a prominent polarization dependence of the absorption.
  • the spatial modulator it is possible to form an x-y image. Depth-addressing of the at least two screens is done by switching the polarization of the (blue, UV, DUV) light incident on the at least two screens. For a given polarization state of the polarizer, light with a particular polarization is passed through the polarizer. Due to the polarization dependence of the absorption of the nanocrystals, the polarized light is substantially absorbed in one of the screens but is not absorbed, or at least absorbed to a much smaller amount, in the other display screen. The x-y image, fo ⁇ ned by the spatial modulator, is then substantially only visible on one of the display screens and not on the other.
  • the spatial modulator provides the x-y image (i.e. the image to be displayed, whereas the switchable polarizer provides the z-(depth) addressing
  • both of the at least two display screens comprise photoluminescent nanocrystals.
  • Nanocrystals form a class of preferred materials, because they have a prominent polarization dependence of absorption and also because the emitted light has a similar polarization dependence, and thus the emitted light from one screen is not absorbable, or only absorbable to a small extent by the screen which is more to the front.
  • at least one of the display screens comprises electroluminescent materials, preferably nanocrystals, and electrodes to electrically excite the electroluminescent materials.
  • Electroluminescent nanocrystals are stable emitters, presenting similar advantages as discussed in relation to photoluminescent nanocrystals.
  • the means for selectively activating the relevant screen is formed by the electrodes and a circuit to selectively activate the areas, i.e. selectively apply voltages across areas to induce electroluminescence.
  • the types of circuits and electrodes usable for selectively addressing areas may be any type, e.g. those used in active and passive matrix displays such as LCDs and plasma display devices.
  • the nanocrystals in different display screens are preferably arranged in such a way that the plane of polarization of the light emitted by the screens differs.
  • a combination is possible of one or more display screens comprising electroluminescent materials, preferably nanocrystals, and one or more display screens having photoluminescent materials, preferably nanocrystals.
  • the display device comprises only two display screens, but in other embodiments, the device may comprise more screens.
  • the number of screens is preferably four or less, preferably three or less, most preferably two.
  • Nanocrystals have a relatively strong polarization dependence for absorption and emission, allowing a number of screens to be used. However, the more screens are used, the stronger the polarization dependence has to be to prevent non-selected screens from being excited or to prevent unwanted absorption of light. Such excitation of non-selected screens will cause crosstalk between screens, diminishing the depth perception. Unwanted absorption of light diminishes the intensity of light.
  • the polarization directions are preferably orthogonal to each other, a good separation of images can be achieved without (or at least with little) crosstalk.
  • the polarization planes of the nanocrystals are preferably substantially orthogonal to each other.
  • the display screens with nanocrystals may be monochromatic, but they are preferably color screens, i.e. they produce a color image.
  • the display screens comprise several types of nanocrystals emitting in individual colors (RGB).
  • the nanocrystals emit light in a relatively small light band (i.e. they have a color).
  • the screens comprise broadband emitting nanocrystals and additional color filters.
  • Figures 1A and IB schematically illustrate the basic concept of a device according to the invention.
  • Figure 2 schematically illustrates an embodiment of a device according to the invention.
  • Figure 3 shows a principle of z-addressing and image formation on different image planes as usable for embodiments having photoluminescent display screens.
  • Figure 4 shows a detail of a device according to the invention having narrow band nanocrystals on one of the image plates 1, 2 to obtain a color image.
  • Figure 5 illustrates the anisotropic behavior of absorption of light by nanocrystals.
  • Figure 6 illustrates the anisotropic behavior of emission of light by nanocrystals as an example.
  • Figure 7 illustrates the emission spectrum of InP nanowires as shown for bulk InP (showing a peak around 900 nm), for nanowires (diameter 5 nm, showing a peak around 770 nm) and InP tubes (showing a peak around 580 nm).
  • Figure 8 schematically illustrates an embodiment of the device according to the invention, using electroluminescent materials.
  • Figure 9 illustrates schematically a method of producing a screen based on InP wires.
  • Figure 10 illustrates a transparent LED based on a compound semiconductor nanotube.
  • Figure 11 shows an example of a device in which use is made of the polarization dependence of the emitted light to be able to increase the number of plates.
  • the Figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the Figures.
  • the invention aims to provide a novel device and a method to realize a 3D- display.
  • FIG. 1 A shows a device comprising a rear screen 1, which produces a rear image, in this example in times slots tl.
  • the device further comprises a front screen 2, which produces an image, in this example in time slots t2, i.e. when the rear screen is not emitting light.
  • the front screen is transmissive to the light coming from the rear screen 1. The light from the rear screen thus passes through the front screen. This gives an image with depth perception.
  • the device comprises a switchable attenuator 2' in between the rear screen 1 and the front screen 2. This attenuator 2' stops at those positions in the front screen 2 where pixels are activated, while the light comes from the rear screen 1.
  • the areas where the light is stopped is dependent on the information displayed on front screen 2 (t2). If the device comprises more than two screens, each screen is provided with at least one attenuator, stopping or partly stopping the light coming from any screen situated behind the relevant screen.
  • the attenuator 2' prevents this.
  • the attenuator 2' is preferably positioned close to, and preferably attached to the front screen 2. The closer the attenuator 2' is positioned in relation to the front screen 2, the better the result.
  • the attenuator may be positioned in between the screen 1 and screen 2, as in this example, with the further advantage that light from the second screen does not impinge on the rear screen.
  • the attenuator may be positioned in front of the screen2, provided that the light coming from screen 1 is distinguishable from the light coming from screen 1.
  • Such a distinction may be a difference in polarization, in which the attenuator 2 'is arranged to attenuate light with the polarization of screen 1, or a difference in time slots in which light is emitted by screens 1 and 2, in which the attenuator is arranged and switched in such a way that it attenuates when light is emitted by screen 1.
  • the insight on which the provision of the attenuator 2' is based is that, although the display device would work without the provision of the stopping screen, it will be a normal case in a 3d display that information (images) is (are) displayed in the foreground and in the background at the same time.
  • Our every-day experience is that, in a 3- dimensional world, we would like to be confirmed by that display.
  • foreground information for instance, a subject or a person
  • the background is therefore only visible around the foreground figure, not through the foreground figure.
  • the stopping screen provides a more "real-life" image in which the foreground figure stands out from the background.
  • the stopping action of the control for the attenuator 2' is such that the attenuator 2' can be made at least partially transparent to selected pixels in the front screen 2. If the image comprises partially or totally transparent image parts (such as e.g.
  • the information data on the front image therefore does not only comprise data on the luminance and color but also data on the transparency of the pixel.
  • these transparency data are used to control the amount of light that passes through the stopping screen 2' for the relevant pixels. A more lifelike imaging is thereby possible. Partial attenuation can be done e.g. by choosing the polarization angle to be such that at least some of the light from screen 1 passes, or by time selection, i.e.
  • one or more of the display screens comprises photoluminescent materials and/or electroluminescent materials. If photoluminescent materials are used, the device comprises a light source, e.g. a blue, UV or DUV lamp. In preferred embodiments, one or more of the display screens comprise nanocrystals.
  • Nanocrystals i.e. nanotubes and nanowires are small bodies having a more or less cylindrical or prismatic shape. Whenever hereinafter reference is made to their orientation, this relates to the orientation of their central cylindrical or prismatical axis. They provide a strong degree of stability.
  • a composite, layered image is provided. A lack of stability produces ageing. Ageing produces discolorations or greying in the image. Since a layered image is produced in a device according to the invention, discolorations are more likely to strike the eye than those in single layer images. The stability of nanocrystals provides a high quality image in this respect.
  • Nanowires sometimes also called filaments or whiskers, have been described for a variety of materials, inter alia, nanowires of indium phosphide (InP) (X. Duan et al,
  • C carbon
  • Si silicon
  • Carbon nanotubes are particularly well studied. They are single and/or multi- layered cylindrical carbon structures of basically graphitic (sp2-) configured carbon. The existence of both metallic and semiconducting nanotubes has been confirmed experimentally. It has recently been found that single- walled 4 A carbon nanotubes aligned in channels of an AlP0 4 -5 single crystal exhibit optical anisotropy in that the carbon nanotubes are nearly transparent in the wavelength range from 1.5 ⁇ m to 200 nm, when the light electric field is polarized perpendicularly to the central axis, and that strong absorption is observed in the spectral range from 600 nm to at least 200 nm, when the light electric field is polarized parallel to the central axis (Li Z M et al., Phys. Rev. Lett. 87 (2001), 127401-1 - 127401-4).
  • nanotubes or nanowires other than those consisting of carbon Similar properties have been found for nanotubes or nanowires other than those consisting of carbon. In embodiments in which photoluminescent materials are provided in the device, nanocrystals are preferred.
  • Photoluminescent nanocrystals conveniently combine the following features: They absorb light, said absorption properties being effective in a broad range of wavelengths, said absorption also being a function of the orientation of the nanocrystals relative to a plane of polarization of said light, and the orientation of nanocrystals can be directed and/or stabilized mechanically and/or by an electric field.
  • the direction of the nanocrystals can be arranged in such a way that they show absorption for light polarized in a particular plane, while being transmissive to differently polarized light.
  • the nanotubes exhibit a very large anisotropy in optical absorption, e.g. depending on the relative orientation of the incident radiation and the physical orientation of the tubes, one can realize a difference in absorption coefficient of approximately 10 ⁇ 6 in the UV region.
  • well-aligned nanotubes absorb light with a polarization parallel to the tube axis, while the nanotubes are transparent to light with a perpendicular polarization.
  • nanocrystals have been shown to have luminescence properties.
  • single- alled carbon nanotubes formed in micro-channels of zeolite crystals emit light in the visible range upon excitation (N. Nagasawa et al, Journal of Luminescence 97 (2002), 161-167).
  • Such properties are also known from other types of nanocrystals and nanowires (J.-M. Bonard et al., Phys. Rev. Lett. vol. 81, no. 7, 1441 (1998); M. H. Huang et al, Science vol. 292 (2001), 1897; K. Yamamoto et al., J. Phys. D: Appl. Phys. 31 (1998), 34-36; X. Duan et al., Nature 409 (2001), 66; J.
  • the emitted (luminescent) light is polarized depending on the orientation of the light emitting nanocrystal. It is reported that the plane of polarization of the emitted light is often the same as that of the absorbed light (N. Nagasawa et al., supra).
  • the nanocrystals Upon excitation, the nanocrystals exhibit emission in the visible range, which has as an extra feature that the emission is polarized depending on the orientation of the nanocrystals. Only nanocrystals in the absorbing mode are excited and can therefore emit light. It is reported that the emitted light has the same polarization as the absorbed light.
  • the excitation can be realized by means of light, e.g.
  • a species of nanocrystals is any set of nanotubes and/or nanowires whose emission spectra comprise light of the same wavelength upon excitation.
  • the device in accordance with a preferred embodiment of the invention has two display screens provided with nanocrystals.
  • the crystals in different screens have different absorption vs polarization characteristics.
  • the device and method of such a preferred embodiment of the device according to the invention entails addressing of image layers in the display by a difference in polarization, which yields the 3D-effect without having to resort to additional means.
  • An embodiment of a device in accordance with the preferred embodiment of the invention mentioned in the previous paragraph is schematically shown in Figure 2, which shows a schematic build-up of an example of the display according to the invention in which photoluminescent nanocrystals are used.
  • a 3D display 9 has a plurality (in this example two) of image formation planes 1 (rear), 2 (front) upon which an image is generated by illumination through an optical system.
  • the optical system comprises an unstructured large-area light source 3 (backlight) (e.g. blue, UV or DVD) that can controlled in intensity by a controller 7.
  • the output of this light source is passed through a modulator device 4.
  • the modulator device 4 is characterized in such a way that it consists of a plurality of pixels that can each be individually addressed and changed in light transmission characteristics. This modulator device could be realized by using a modified TFT LCD plate including driver ICs.
  • a light transmission pattern can be formed across the area of the device 4 in correspondence with the desired 2D (x-y) graphical representation as decoded from the image-processor 8, which in its turn receives the data from the outside world (network, cable, antenna, computer, CD-ROM).
  • the illumination pattern thus formed is led through another modulator 5 which controls the polarization state of each complete x-y pattern.
  • the 3 rd dimension is introduced (z-decoding) as explained below.
  • An example of such a modulator could be an unpatterned Liquid Crystal cell.
  • this scheme will work when at least one of the at least one rear and the at least one front, spatially separated display screen both comprise areas comprising photoluminescent material which differ in the plane of polarization for absorption and when a means for selectively passing polarized light is provided in between the light source and at least one of the at least one rear and the at least one front, spatially separated display screen.
  • the modulator 5 By controlling the polarization state of the light passing said means, here the modulator 5, the polarization of the exciting light is controlled, and thereby the z-decoding is controlled.
  • the invention is not restricted to such embodiments.
  • Z-addressing using incident light may also be performed by using two photoluminescent materials having different excitation wavelengths ranges, and performing z-addressing by letting light of a different wavelength fall on the luminescent materials.
  • z-addressing using polarization switching is a preferred embodiment, because it allows swift switching and provides a relatively simple set-up.
  • the pattern is led through an optical lens system 6 which takes care of the proper magnification, projection and collimation on said image planes 1 and 2.
  • the lens system can also be used to focus the x-y patterns that have been z- decoded by different polarization onto the corresponding image planes 1 or 2 by using birefringence.
  • the attenuator 2' action is controlled by control 7.
  • the stopping screen may e.g. be an array of LCD Cells, corresponding to the array of pixels for the front screen, which can be switched between a transparent and a substantially opaque state.
  • the information for modulator 4 for the front screen is used to control which LCD cells of the stopping screen are opaque and which are transparent.
  • any piece of hardware such as a controller
  • any circuit or sub-circuit designed for performing a controlling action as well as any piece of software (computer program or sub-program or set of computer programs, or program code(s)) designed or programmed to perform a controlling action according to the invention) as well as any combination of pieces of hardware and software acting as such, alone or in combination, without being restricted to the given examples of embodiments.
  • the image planes 1 and 2 are essentially made of an optically transparent substrate (glass/plastic) upon which a layer is formed of the photoluminescent nanocrystals.
  • the nanotubes are aligned in a preferred direction as illustrated in Fig. 3 and the principal axes (or alignment axes of the image planes) are oriented in such a way that they are perpendicular to each other (e.g. in the y-direction on plane 1 and in the x-direction on plane 2).
  • Figure 3 shows a possible means of z-addressing and image formation on either image plane 1 or 2 by separating the two time steps t_ and t 2 , respectively.
  • t_ (described in the left part of Fig.
  • the "background” or “rear” image information is an x-y light intensity pattern with a polarization in the y-direction addressing image plane 1 due to the orientation of its nanotubes in the y-direction.
  • the excited pixels in plane 1 will emit light (either white or of the corresponding color as described below), which is also polarized in the y-direction.
  • a color image reproduction of the "background” or “rear” image is produced at the depth of plane 1.
  • a "foreground” or "front” x-y image is polarized in the x-direction (perpendicular polarization state) resulting in a corresponding color "foreground” or "front” image on plane 2.
  • the proposed 3D-TV consists of both described components, i.e. one image plane 2, which is sensitive to x-polarized light, and one image plane 1 which is sensitive to y- polarized light. It is useful to mention that both these parts in this preferred embodiment are orthogonal functions, i.e. they do not influence each other. Image plane 1 is transparent to x- polarized light, and vice versa. Fast switching of the polarization states at ti and t 2 is achieved by modulator 5.
  • the attenuator 2' is positioned between the image planes 1 and 2, preferably close to image plane 2.
  • an x-y attenuating image i.e. areas opaque to light from plane 1, or at least partially blocking the light
  • tl an x-y attenuating image (i.e. areas opaque to light from plane 1, or at least partially blocking the light) is produced, corresponding to the image produced on plane 2 during time t2.
  • photoluminescent materials apart from the preferred nanocrystals, other photoluminescent materials may be used.
  • photoluminescent materials are described in e.g. Science, vol. 29, pages 835 to 837, by Weder et al, in which polarized photoluminescent (PL) layers based on a mixture of organic molecules are described, which PL layers are characterized by either anisotropic absorption or anisotropic emission or both.
  • PL photoluminescent
  • Such materials also allow production of display screens having photoluminescent materials, and they also belong to the preferred type of materials having anisotropic absorption and/or emission characteristics.
  • mixtures of organic materials are relatively unstable, especially in view of the fact that UV light is used to excite them.
  • the nanocrystals can be aligned in a preferential direction, for instance, by applying an electric AC or DC field of the order of 0.2 V/ ⁇ m [ see e.g. Kunitoshi Yamamoto, Seiji Akita and Yoshikazu Nakayama, J. Phys. D: Appl. Phys. 31 (1998) 34-36 and X. Duan, Y. Huang, Y. Cui, J. Wang, CM. Lieber, "InP nanowires as building blocks for nanoscale electronic and optoelectronic devices” Nature 409 (2001) 66.]
  • the nanocrystals are aligned to obtain large-area surfaces with controlled anisotropic characteristics (aligned nanocrystals). Since the nanocrystals exhibit a large anisotropy in absorption, these surfaces are selectively addressable (by choosing the polarization of the incoming light) and finally the emission of light in the visible region to generate a luminescent image on the image planes.
  • the display device is a color display device.
  • the spectrum emitted by backlight for instance, UV
  • the spectrum emitted by backlight is such that it is able to excite the nanocrystals to emit visible light, i.e. falling within their absorption band and having enough energy.
  • backlight for instance, UV
  • RGB color generation three cases (realization options) are possibly based on either narrow-band A) or broad-band B) or white light C) visible light-emitting nanotubes/nanowires.
  • the light emission characteristics can be influenced by controlling various parameters: type of nanotube (material, single or multiwalled), diameter, doping, chirality.
  • FIG. 4 shows a detail of a device according to the invention having narrow-band nanocrystals on one of the image plates 1, 2 to obtain a color image.
  • nanotubes characterized by e.g. red light emission
  • a stripe- like pattern layer 10 in Fig. 4
  • the nanotubes as obtained from an external synthesis process are originally dissolved in a proper liquid/viscous solvent. By applying an electric field, the nanotubes will align with respect to the electric field lines. In this state, the solvent is fixed (e.g.
  • the next patterned layer 11 comprising e.g. green light- emitting nanotubes is deposited by following the same procedure.
  • the last layer 12 is produced, consisting of aligned blue emitting nanotubes.
  • Image plane 2 is produced in the same way as image plane 1 but with the nanotubes aligned at a different angle, preferably substantially perpendicular to those in plane 1.
  • nanotubes or nanowires directly on pixel sidewalls of the image plates.
  • electric field- assisted catalytic CVD growth can be used.
  • Some typical, but not restrictive sizes of nanocrystal areas are indicated in Fig. 4. When narrow-band nanocrystals are used, the use of color filters is not necessary, dependent on the narrowness of the emission band.
  • the nanocrystals generate light within a broad band.
  • a pixelated color filter array is preferably placed on or near each image plane to filter out the desired color component (R,G or B).
  • These filters could be placed between the plate with nanocrystals and the light source, but preferably between the plate with nanocrystals and the viewer.
  • the color filters should preferably have their full characteristics for a particular polarization and be transparent to, and optically neutral-colored for other polarization states. This feature can be obtained by using liquid crystal-based (cholesteric) colorfilters in combination with ⁇ /4 plates.
  • Another method of creating the desired color filter characteristics is to define double bandpass filters. One bandpass is located at the desired region in the optical spectrum (e.g.
  • an optical filter should be placed at the exit face of the display 9 to remove the spectrum of the excitation source (usually a normal low-pass filter in the UV).
  • nanocrystals are mentioned as preferred materials, but the same possibilities exist for other photoluminescent materials.
  • Carbon nanotubes are known to exhibit anisotropic behavior. Recently, the polarized absorption spectra of single-wall nanotubes (SWNT) with an almost mono- dispersed size and perfectly aligned in the channels of zeolite AFI single crystals were measured. With a 0.4 nm diameter, these nanotubes are the smallest CNT known.
  • Figure 5 shows polarized optical absorption spectra of SWNT. In a large wavelength range, the absorption of light with E
  • FIG. 6 shows the photoluminescence of the 0.4 nm SWNTs arrayed in the channels of zeolite crystal, excited using the 488-nm line of an Ar-ion laser, at a varied excitation polarization angle ⁇ .
  • the insets show the experimental setup (bottom), and the polarization dependence of the light-emission intensity on the polarization angle (top).
  • Figures 7 shows by way of example that the emission spectrum of nanowires can be adjusted by controlling the diameter.
  • Figure 7 shows the emission spectrum of InP nanowires for bulk InP (showing a peak around 900 nm), for nanowires (diameter 5 nm, showing a peak around 770 nm) and InP tubes (showing a peak around 580 nm). This effect is called confinement. It can also be used for all other nanostructures to precisely tune the emission color.
  • the device comprises photoluminescent emitters such as nanocrystals.
  • nanocrystals may also exhibit electroluminescence, i.e. when a voltage difference is applied across nanocrystals, they emit light.
  • Fig. 8 schematically illustrates an embodiment of the device according to the invention, using electroluminescent materials.
  • the device has a rear (1) and a front screen (2), comprising electrodes 81 and 82, in between which electroluminescent materials 83, preferably nanocrystals are provided.
  • electroluminescent materials 83 preferably nanocrystals are provided.
  • nanocrystals oriented between the electrodes are present.
  • Figure 9 illustrates schematically a method of producing a screen based on InP wires.
  • a porous alumina (A1 2 0_) matrix is formed by anodizing an aluminum film on a substrate 91. Pores with a diameter in the range of 5-50 nm are formed with a density of 10 10 pores.cm 2 . Gold (Au) particles are electrochemically deposited at the bottom of the pores. By using the VLS growth mechanism, nanowires or nanotubes are grown inside the pores. By switching the target during the synthesis, a p-n junction within a wire or a tube is formed. After growth, the top surface is polished and a transparent electrode (ITO) is deposited. Light 92 is induced by applying a voltage V across the p-n-junction and exits the transparent ITO layer.
  • ITO transparent electrode
  • Figure 10 illustrates a transparent LED based on a compound semiconductor nanotube 101.
  • An n-p junction is established in the tube.
  • the tube is contacted with an ohmic contact to the n-type part, the electron-injector electrode (e), and an ohmic contact to the p- type part, the hole-injector electrode (h). Injection of electrons and holes induces light at the p-n junction.
  • the LED is made on a Si0 2 substrate so that light may pass through the substrate.
  • one of the screens such as the rear screen, may be of the photoluminescent type, i.e. having photoluminescent materials, whereas the other (or another) screen, e.g. the front screen or one or more screens in between, may be of a transparent electroluminescent type.
  • Figure 11 shows an example of a device in which use is made of the polarization dependence of the emitted light so as to be able to increase the number of plates.
  • this device there are three screens 1, 1' and 2, each with nanocrystals whose plane of polarization differs 45 degrees with the others.
  • the modulator 5 sequentially transmits light with the proper polarization (at time slots tl, t2, t3) and is coupled to a modulator 5' transmitting light of the same polarization at the same time slots.
  • a display device provides an image with depth perception.
  • the device comprises two or more displays screens (1, 2), and means (4, 5) for addressing pixels on said display screens.
  • the front display screen is substantially transparent to light coming from display screens behind said screen and comprises an addressable attenuator (2') for attenuating light emanating from behind at positions corresponding to addressed pixels in the front display screen.
  • the attenuator may be established in several manners.
  • all image information is provided by the modulated backlight and corresponding polarization i.e. depth addressing.
  • the blocking layer has to be electrically addressed pixel-wise, i.e. active control is required.
  • CNTs could be used again (there are also species which only show absorption but no luminescence in the visible range), but a switchable plate has to be realized, which could be done electrophoretically.
  • anisotrope materials such as liquid crystal materials could be used (use of TFT LCD).
  • nanocrystals are described in the embodiments. Although they form preferred embodiments, the invention in its broadest scope is not restricted to the use of nanocrystals.
  • materials namely fluorescent organic dyes which, when prepared in a particular way (i.e. under mechanical stretching), obtain similar un-isotropic optical properties as nanocrystals, namely polarized absorption + luminescence.
  • fluorescent organic dyes which, when prepared in a particular way (i.e. under mechanical stretching), obtain similar un-isotropic optical properties as nanocrystals, namely polarized absorption + luminescence.
  • LEEC light- emitting electrochemical cells
  • PolyLED polymer light-emitting diode
  • smOLED small molecule organic LED
  • the invention is also useful for applications in which (nearly) static images are shown, for instance, in medical 3-D imaging or for showing static structures, such as, for instance, in architecture, for 3-D imaging of complex molecules or mathematical 3-D objects.
  • a combination of different types of display screens is possible, for instance, a set-up in which a light source and two photoluminescent screens with crossed orientations are used, as schematically shown in Figures 2 and 3, followed by one or more electroluminescent screens.
  • One further improvement may be formed by a filter positioned in front of the device to filter the light coming from the light source.
  • the UV or DUV light source is usually a bright source and many materials may age when hit by UV light, so that stopping the UV light after it has passed through the last photoluminescent screen may be useful.
  • the phrase "positioned in front of the device" may mean: after the last photoluminescent screen.
  • a UV filter could advantageously be positioned between the photoluminescent and electroluminescent screens.
  • a further improvement may be formed by an absorption compensation means in the control means.
  • screens are transparent to light coming from behind, total absorption may differ from screen to screen.
  • means for compensating such absorption are preferably provided, which would mean in practical terms that the light coming from the rear screen is somewhat brighter than from a screen which is more to the front, so as to give substantially the same intensity for the viewer. This can be done by controlling the spatial modulator. A slightly different way of doing so would be fonned in a scheme in which the rear screen is excited during a time slot tl, and a front screen is excited during time slots t2 (see Figures 2 and 3). By making time slots tl slightly longer than time slots t2, a compensation may be accomplished in accordance with the difference in absorption. Further variations will be apparent to a person skilled in the art.

Abstract

A display device provides an image with depth perception. The device comprises two or more displays screens (1, 2) and means (4, 5) for addressing pixels on said display screens. The front display screen is substantially transparent to light coming from display screens behind said screen and comprises an addressable attenuator (2') for attenuating light emanating from behind at positions corresponding to addressed pixels in the front display screen.

Description

Three-dimensional display
FIELD OF THE INVENTION
The invention relates to a display device for providing an image with depth perception. Such devices are sometimes also called 3D display devices or 3D display systems.
BACKGROUND OF THE INVENTION
A three-dimensional representation of graphics and video is one of the holy grails in the display field. Several methods have been devised which give the viewer the impression that he is watching a 3 -dimensional image. A 3D perception can be created in several manners. A three-dimensional impression can be created by using stereo pairs (two different images directed at the two eyes of the viewer). There are several ways to produce stereo images. The images may be time-multiplexed on a 2D display, but this requires the viewers to wear glasses with e.g. LCD shutters. When the stereo images are displayed at the same time, the images can be directed to the appropriate eye by using a head-mounted display, or by using polarized glasses (the images are then produced with orthogonally polarized light). The glasses worn by the observer effectively route the views to each eye. Shutters or polarizer's in the glasses are synchronized to the frame rate to control the routing. A disadvantage of such a system is that the two images produce only a limited "look-around" capability. Furthermore, glasses have to be worn to produce any effect. This is unpleasant for those observers who are not familiar with wearing glasses and a potential problem for those already wearing glasses, because the extra pair of glasses does not always fit.
Instead of near the viewer's eyes, the two stereo images can also be split at the display screen by means of a splitting screen such as a lenticular screen or a parallax barrier. The principle is shown in e.g. Figures 3 and 4 of United States Patent US 6,275,254. Although these displays with splitting screens do not require special glasses to view the 3D image, they conventionally work only for one viewer at a fixed position in space, making them unsuitable for normal every-day use, such as, for instance, in a living room, where more than one viewer may be present and the position of the viewer vis-a-vis the image screen may vary considerably. OBJECT AND SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an alternative to the known device. To this end, the display device according to the invention is characterized in that the display device comprises
- at least one rear and at least one front, spatially separated, display screen, and means for addressing pixels on said display screens for providing a rear image on the rear display screen and a front image on the front display screen, wherein the front display screen is substantially transparent to light emanating from the rear display screen;
- an addressable attenuator close to the at least one front display screen for attenuating light emanating from the rear display screen at positions corresponding to addressed pixels in the front display screen.
By providing a rear and a front, spatially separated, display screen, i.e. one (the rear screen) behind the other (the front screen) as seen by a viewer, with a distance between the display screens, images are formed one behind the other. A 3D image is formed by forming image infonnation with different depth content on the rear and front display screens. The words "rear" and "front" are used to indicate that the one is behind the other, or the other is in front of the one, i.e. this terminology refers to their relative position, and should not be inteipreted as to signify any restriction of the position of said screens vis-a-vis other elements of the device. In particular, as will be further evident, image screens may be provided to increase the number of image layers.
The front display screen is substantially transparent to the light emanating from the rear display screen so that a rear image shines through a front display screen. Formation of a front image on a front display screen does not substantially influence formation of a rear image on a rear display screen. The substantially independent formation of the images, positioned one behind the other as seen by a viewer, provides a 3D effect. Although this setup does provide a 3D perception, it would also mean that a rear image does not just "shine through" the front screen but also through a front image displayed on the front screen, giving the composite image a ghostly appearance.
Besides the at least one rear and the at least one front display screen, the device according to the invention comprises an addressable attenuator close to the front display screen. At positions corresponding to addressed pixels in a front display screen, the addressable attenuator attenuates the light coming from the at least one rear display screen. Thus the addressable attenuator, positioned close to the front display screen, partially or totally blocks the light from the rear image for the addressed pixels (i.e. those for which light is formed) of the front display screen. At the pixels addressed for the front image (i.e. those pixels that are active, i.e. emit light, the rear image is thus attenuated and does not mix or only partially mixes with the front image. For the other, non-light emitting, pixels of the front image, the light from the rear image is not attenuated and thus passes through the front display screen. The combination of the at least one rear display screen, the at least one front display screen and the attenuator close to the at least one front display screen makes a foreground figure formed on a front screen stand out in respect of the background image on a rear screen, as it would in real life. In simple embodiments, the attenuator is a stopping screen, i.e. the attenuator becomes opaque at positions corresponding to addressed pixels for the front display screen, i.e. it totally blocks light coming from behind, and thus only the foreground figure, i.e. the image on the front screen, is visible at said positions. In preferred embodiments, the attenuator has such an attenuation action that the attenuator can be made partially transparent to selected pixels in the at least one front display screen. If the image comprises partially or totally transparent image parts (such as e.g. a ghost, stained glass, fog or partially transparent textiles), it is advantageous to have the possibility to select whether or not, and to which extent and for which pixels, the light from the rear image is attenuated by the attenuator. To this end, the information data on the front image does not only comprise data on the luminance and color but also data on the transparency of the pixel in such preferred embodiments. Via a control, these transparency data are used to control the amount of light that passes the attenuator for the relevant pixels. A more lifelike imaging of (semi)transparent objects is thereby possible.
The attenuator is preferably positioned between the at least one rear display screen and the at least one front display screen, preferably attached to the at least one front screen.
In this preferred embodiment, the attenuator does not attenuate the light from the at least one front screen, but only attenuates the light from the at least one rear screen. However, embodiments in which the attenuator is positioned in front of the front screen are also possible. In such embodiments, the emitted light from the at least one front screen as well as from the at least one rear screen goes through the attenuator. Distinction between the rear and front images and thereby selective passing of light from the at least one rear display screen is possible when the polarization of the rear and front images differs and the attenuator comprises polarization-dependent light-addressable transmission elements and means for selecting the polarization of transmitted light. Selection is also possible when the rear and front images are emitted at different time slots. By synchronizing the stopping action with the time slots corresponding to the displayed front and rear images, each of the rear and front images can be passed selectively. Within the framework of an embodiment of the invention, the display device comprises means (such as a circuit or hardware and/or software) to form the images on the at least one rear and the at least one front display screen inteπnittently, i.e. when an image on the at least one rear display screen is formed, no image is formed on the at least one front display screen, and vice versa. Intermittent formation of the images in combination with the fact that the front screen is transmissive to a rear image involves no or only little or moderate light absorption of a rear image by a front screen. The front screen is off when the rear screen is on, and vice versa. Furthermore, the possibility of crosstalk between the rear and the front image is reduced. Furthermore, if the rear and front screens are arranged in such a way that the front screen is always substantially transmissive to light coming from behind, the front display screen can be addressed when the rear screen is emitting, and vice versa.
Basically, in all embodiments, the inventive device and method of displaying 3D images require no additional devices such as e.g. special goggles to be used by the viewer. The depth perception is due to the fact that the images are displayed on a number of screens, rather than on the viewer's position. Thus, the problem of a very narrow viewing zone is also eliminated. Depth is perceived no matter what the position of the viewer is vis-avis the device. The attenuator provides a natural look to the displayed images, by making foreground figures standing out from the background in a natural manner.
It is to be noted that the addressing of the addressable stopping screen is coupled to the information on the at least one front display screen, but attenuates the light of an image on the at least one rear display screen.
In its simplest embodiment , the device according to the invention may have only one rear and only one front display screen, but in more sophisticated embodiments, the display device has more than two display screens arranged one behind the other. In such embodiments, the device comprises a first rear display screen and a final front display screen, and in addition one or more display screens in between the first rear display screen and the final front display screen. Each of the one or more display screens in between the first rear display screen and the final front display screens acts as a front display screen for each display screen positioned behind said display screen, and as a rear display screen for each display screen positioned in front of said display screen. The one or more display screens in between the final rear and front display screen are each provided with an attenuator to attenuate light from behind, and the display screens themselves are each transparent to light coming from behind. At those positions where the display screen with which it is associated emits light, the attenuator attenuates the light coming from behind. The provision of more than two display screens, each (with the exception of the first rear screen) with an attenuator close to, and preferably attached to the relevant screen enables a multi-layered image to be made, enhancing the 3-D impression.
Preferably, an attenuator is attached to each display screen, but for the first rear display screen. The closer the attenuator is to the display screen it is associated with, the better the effect. Preferably, but not exclusively, each attenuator is positioned behind (seen from the viewer's position) the associated display screen.
Two types of materials are most useful for the display screens, namely photoluminescent materials, i.e. materials that emit visible light when light is incident on them, and electroluminescent materials, i.e. materials that emit light when a potential difference is applied to them.
A preferred embodiment of the display device according to the invention is characterized in that at least one of the display screens has areas comprising nanocrystals selected from the group of nanorubes and nano wires which show photoluminescence, and the display device is provided with a light source and selection means for selectively and addressably exciting the areas.
In these embodiments, the nanocrystals show photoluminescence, i.e. when impinged by light (usually UV light), the nanocrystals emits visible light. In combination with a light source and means for selectively and addressably exciting the areas, an image may be formed on the display screen with the nanocrystals. Within the larger framework of the invention, any photoluminescent material may be used. However, the use of photoluminescent nanocrystals has the advantage that nanocrystals are very stable emitters. Ageing of materials, especially under UV light and in a normal atmosphere is a major problem. Most of the photoluminescent materials show considerable ageing effects. Nanocrystals are very stable emitters. Since a multilayered image comprising an overlay of several images is made in a device according to the invention, the ageing effect may form a considerable problem. Normally, ageing problems affect the image as a whole, resulting in, for instance, somewhat less bright colors, or off-colors. However, such slight discolorations are often not readily perceivable by a viewer, for lack of comparison. As long as a viewer does not have a way of perceiving off-color, red is perceived as red, even if it is e.g. slightly brownish. In a device according to the invention, a layered image is shown. Any discoloration due to ageing may result in differences in color between one display screen and another if this discoloration is different in different display screens. A slight off-color due to an ageing effect, which is hardly perceivable in standard display devices, may become perceivable in a device according to the invention, because differences in ageing cause the "same" color (i.e. the signals are tuned in such a way that the same color should have been displayed, but for the ageing effect) to be displayed differently on different image screens. In a preferred embodiment, at least two of the display screens are provided with areas with photoluminescent materials, the photoluminescent materials (e.g. preferably nanocrystals) in the at least two display screens being arranged in such a way that the orientation of the plane of polarization of emitted light of one of said at least two display screens differs from the plane of orientation of absorption of another of said at least two display screens, the latter being positioned more towards the viewer. In this preferred embodiment of the invention, at least two of the display screens comprise areas having nanotubes or nano wires. These areas are arranged in such a way that the orientation of the plane of polarization of luminescent light (for the display screen furthest remote from the viewer, in this paragraph further denoted as "rear" screen for simplicity), and absorbed light (for the screen closest to the viewer, in this paragraph further denoted as "front" screen for simplicity) differ respectively. The invention makes use of the strong anisotropic optical properties (absorption and emission show polarization dependence) obtainable by nanocrystals. By making the alignment of the nanotubes or nanowires (and therefore the axis of anisotropy) in a rear screen differ with respect to the alignment of the nanotubes or nanowires in a front screen, different polarization planes for emission (by the nanocrystals in the rear screen) and absorption (by the nanocrystals in the front screen) are provided. The light emitted by the rear screen has a polarization which is different from the plane of polarization for absorption of the front screen and thus substantially passes through said front screen without exciting it. The intensity of the rear image is therefore not decreased or only moderately decreased on transmission through the front screen, and the front screen is not excited, or only excited to a small extent by light passing through it. This provides a simple, yet good independence of the image on the rear and the front screen. In this respect, the best results are obtained by making the alignment of the nanocrystals substantially perpendicular to each other. Preferably, the front screen has such an absorption spectrum that the light emitted by the rear screen is substantially not absorbed by, and substantially does not excite the nanocrystals in the front screen. This reduces absorption, increases the intensity of the image and reduces crosstalk between images.
In preferred embodiments, the display device according to the invention comprises the at least two screens of a photoluminescent material, preferably nanocrystals, having different planes of polarization of absorption, and a spatial modulator and a means for polarizing the light in a plane of polarization corresponding to the plane of polarization for absorption of the at least two screens are arranged between the light source (e.g. blue, UV, or DUV) and the screen.
Some photoluminescent materials show a polarization dependence of polarization. Nanocrystals, in particular, show a prominent polarization dependence of the absorption.
Using the spatial modulator, it is possible to form an x-y image. Depth-addressing of the at least two screens is done by switching the polarization of the (blue, UV, DUV) light incident on the at least two screens. For a given polarization state of the polarizer, light with a particular polarization is passed through the polarizer. Due to the polarization dependence of the absorption of the nanocrystals, the polarized light is substantially absorbed in one of the screens but is not absorbed, or at least absorbed to a much smaller amount, in the other display screen. The x-y image, foπned by the spatial modulator, is then substantially only visible on one of the display screens and not on the other. Switching the polarization and concurrently switching the x-y infonnation on the spatial modulator provides an image on the other of the at least two screens. The spatial modulator provides the x-y image (i.e. the image to be displayed, whereas the switchable polarizer provides the z-(depth) addressing
Switching the polarity of the incident light and concurrently the pixel information, and thus the related display information, will reproduce said display information exclusively on that image screen which is in the absorption mode while the other is in the transmission mode. By switching between the polarization modes and synchronously providing the related display information, a viewer will experience the build-up of a 3D- image on several planes, i.e. in space. In this preferred embodiment, both of the at least two display screens comprise photoluminescent nanocrystals. The great advantage of this embodiment is that the light source, the spatial modulator and also the polarizer are elements that are common to both screens. This embodiment is not restricted to devices having two display screens. More than two display screens having photoluminescent materials with a polarization dependence in the absorption may be present in the device. Nanocrystals form a class of preferred materials, because they have a prominent polarization dependence of absorption and also because the emitted light has a similar polarization dependence, and thus the emitted light from one screen is not absorbable, or only absorbable to a small extent by the screen which is more to the front. In other embodiments, at least one of the display screens comprises electroluminescent materials, preferably nanocrystals, and electrodes to electrically excite the electroluminescent materials.
Various nanocrystals show electroluminescence, i.e. they emit visible light when a potential difference is applied across the nanocrystals. Electroluminescent nanocrystals are stable emitters, presenting similar advantages as discussed in relation to photoluminescent nanocrystals.
The means for selectively activating the relevant screen is formed by the electrodes and a circuit to selectively activate the areas, i.e. selectively apply voltages across areas to induce electroluminescence. The types of circuits and electrodes usable for selectively addressing areas may be any type, e.g. those used in active and passive matrix displays such as LCDs and plasma display devices.
When two or more display screens having electrolumiscent nanocrystals are present in the device, the nanocrystals in different display screens are preferably arranged in such a way that the plane of polarization of the light emitted by the screens differs. Within the concept of the invention, a combination is possible of one or more display screens comprising electroluminescent materials, preferably nanocrystals, and one or more display screens having photoluminescent materials, preferably nanocrystals.
In preferred embodiments, the display device comprises only two display screens, but in other embodiments, the device may comprise more screens. When using nanocrystals, the number of screens is preferably four or less, preferably three or less, most preferably two. Nanocrystals have a relatively strong polarization dependence for absorption and emission, allowing a number of screens to be used. However, the more screens are used, the stronger the polarization dependence has to be to prevent non-selected screens from being excited or to prevent unwanted absorption of light. Such excitation of non-selected screens will cause crosstalk between screens, diminishing the depth perception. Unwanted absorption of light diminishes the intensity of light. With three or preferably two screens, in which case the polarization directions are preferably orthogonal to each other, a good separation of images can be achieved without (or at least with little) crosstalk. When two screens are used, the polarization planes of the nanocrystals (in absorption and/or emission) are preferably substantially orthogonal to each other.
The display screens with nanocrystals may be monochromatic, but they are preferably color screens, i.e. they produce a color image. In a preferred embodiment, the display screens comprise several types of nanocrystals emitting in individual colors (RGB). In such embodiments, the nanocrystals emit light in a relatively small light band (i.e. they have a color).
In a different embodiment, the screens comprise broadband emitting nanocrystals and additional color filters. These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: Figures 1A and IB schematically illustrate the basic concept of a device according to the invention.
Figure 2 schematically illustrates an embodiment of a device according to the invention.
Figure 3 shows a principle of z-addressing and image formation on different image planes as usable for embodiments having photoluminescent display screens.
Figure 4 shows a detail of a device according to the invention having narrow band nanocrystals on one of the image plates 1, 2 to obtain a color image.
Figure 5 illustrates the anisotropic behavior of absorption of light by nanocrystals. Figure 6 illustrates the anisotropic behavior of emission of light by nanocrystals as an example.
Figure 7 illustrates the emission spectrum of InP nanowires as shown for bulk InP (showing a peak around 900 nm), for nanowires (diameter 5 nm, showing a peak around 770 nm) and InP tubes (showing a peak around 580 nm). Figure 8 schematically illustrates an embodiment of the device according to the invention, using electroluminescent materials.
Figure 9 illustrates schematically a method of producing a screen based on InP wires. Figure 10 illustrates a transparent LED based on a compound semiconductor nanotube.
Figure 11 shows an example of a device in which use is made of the polarization dependence of the emitted light to be able to increase the number of plates. The Figures are not drawn to scale. Generally, identical components are denoted by the same reference numerals in the Figures.
DESCRIPTION OF EMBODIMENTS
The invention aims to provide a novel device and a method to realize a 3D- display.
Figures 1A and IB illustrates the principle of the device in accordance with an example of an embodiment of the invention. Fig. 1 A shows a device comprising a rear screen 1, which produces a rear image, in this example in times slots tl. The device further comprises a front screen 2, which produces an image, in this example in time slots t2, i.e. when the rear screen is not emitting light. The front screen is transmissive to the light coming from the rear screen 1. The light from the rear screen thus passes through the front screen. This gives an image with depth perception. The device comprises a switchable attenuator 2' in between the rear screen 1 and the front screen 2. This attenuator 2' stops at those positions in the front screen 2 where pixels are activated, while the light comes from the rear screen 1. This is the basic concept of the invention, namely a number of emissive screens with various brightness on which different images may be formed, wherein the light of screen 1 passes through screen 2, to give a depth perception, in combination with an attenuator to attenuate the light coming from the screen 1. In this example of the embodiment, this is done at times tl. The areas where the light is stopped is dependent on the information displayed on front screen 2 (t2). If the device comprises more than two screens, each screen is provided with at least one attenuator, stopping or partly stopping the light coming from any screen situated behind the relevant screen. Without the attenuator 2', light from the rear screen would shine through the image displayed on the front screen, giving an impression as schematically indicated in Figure IB in which the girl's bicycle seems to be cut into pieces. Noticeable discolorations could occur in color displays. The attenuator 2' prevents this. The attenuator 2' is preferably positioned close to, and preferably attached to the front screen 2. The closer the attenuator 2' is positioned in relation to the front screen 2, the better the result. The attenuator may be positioned in between the screen 1 and screen 2, as in this example, with the further advantage that light from the second screen does not impinge on the rear screen. Such backwardly directly light could reduce the contrast of the overall image, especially if the foreground image were bright and the background image dim. However, although this a preferred embodiment, the attenuator may be positioned in front of the screen2, provided that the light coming from screen 1 is distinguishable from the light coming from screen 1. Such a distinction may be a difference in polarization, in which the attenuator 2 'is arranged to attenuate light with the polarization of screen 1, or a difference in time slots in which light is emitted by screens 1 and 2, in which the attenuator is arranged and switched in such a way that it attenuates when light is emitted by screen 1.
The insight on which the provision of the attenuator 2' is based is that, although the display device would work without the provision of the stopping screen, it will be a normal case in a 3d display that information (images) is (are) displayed in the foreground and in the background at the same time. Our every-day experience is that, in a 3- dimensional world, we would like to be confirmed by that display. This means that foreground information (for instance, a subject or a person) usually covers only a part of the viewed scene, which part is furthermore not transparent. The background is therefore only visible around the foreground figure, not through the foreground figure.
Color mixing effects will also occur in color devices . Dependent on the distance between the image screens and the viewing distance, this effect will also depend on content and on the viewing angle, i.e. the position of the viewer vis-a-vis the display device. As explained, the provision of the stopping screen according to the invention provides a more "real-life" image in which the foreground figure stands out from the background. In preferred embodiments, the stopping action of the control for the attenuator 2' is such that the attenuator 2' can be made at least partially transparent to selected pixels in the front screen 2. If the image comprises partially or totally transparent image parts (such as e.g. a ghost, stained glass, fog or partially transparent textiles), it is advantageous to have the possibility to select whether or not, to which extent and for which pixels the light from the rear image is blocked by the attenuator 2'. In such preferred embodiments, the information data on the front image therefore does not only comprise data on the luminance and color but also data on the transparency of the pixel. Via the control , these transparency data are used to control the amount of light that passes through the stopping screen 2' for the relevant pixels. A more lifelike imaging is thereby possible. Partial attenuation can be done e.g. by choosing the polarization angle to be such that at least some of the light from screen 1 passes, or by time selection, i.e. by stopping light during a portion of time slots tl, and passing it during the rest, or any combination thereof. In embodiments, one or more of the display screens comprises photoluminescent materials and/or electroluminescent materials. If photoluminescent materials are used, the device comprises a light source, e.g. a blue, UV or DUV lamp. In preferred embodiments, one or more of the display screens comprise nanocrystals.
Nanocrystals, i.e. nanotubes and nanowires are small bodies having a more or less cylindrical or prismatic shape. Whenever hereinafter reference is made to their orientation, this relates to the orientation of their central cylindrical or prismatical axis. They provide a strong degree of stability. In a device according to the invention, a composite, layered image is provided. A lack of stability produces ageing. Ageing produces discolorations or greying in the image. Since a layered image is produced in a device according to the invention, discolorations are more likely to strike the eye than those in single layer images. The stability of nanocrystals provides a high quality image in this respect.
Nanowires, sometimes also called filaments or whiskers, have been described for a variety of materials, inter alia, nanowires of indium phosphide (InP) (X. Duan et al,
Nature 409 (2001), 66; J. Wang et al, Science 293 (2001), 1455-1457), zinc oxide (ZnO) (M. Huang et al.s Science 292 (2001), 1897-1899), gallium arsenide (GaAs) and gallium phosphide (GaP) (K. Haraguchi et al., Appl. Phys. Lett. 60 (1992), 745; X. Duan et al, Nature 409 (2001), 66), silicon carbide (SiC) (S. Motojima et al., J. Crystal Growth 158 (1996), 78-83), boron nitride (BN) (W. Han et al., Applied Physics Letters 73, 21 (1998) 3085), nickel dichloride (NiCl2) (Y. Rosenfeld Hacohen et al., Nature 395 (1998) 336), molybdenum disulfide (M0S2) (M. Remskar et al, Surface reviews and Letters, vol. 5 no. 1 (1998) 423) and tungsten disulfide (WS2) (R. Tenne et al, Nature 360 (1992) 444). Two materials are currently known to form nanotubes: carbon (C) (Iijima, S., Nature 354 (1991), 56-58; Ebbesen T W and Ajayan P M, Nature 358 (1992), 220) and silicon (Si) (B. Li et al., Physical Review B 59, 3, (1999) 1645).
Carbon nanotubes are particularly well studied. They are single and/or multi- layered cylindrical carbon structures of basically graphitic (sp2-) configured carbon. The existence of both metallic and semiconducting nanotubes has been confirmed experimentally. It has recently been found that single- walled 4 A carbon nanotubes aligned in channels of an AlP04-5 single crystal exhibit optical anisotropy in that the carbon nanotubes are nearly transparent in the wavelength range from 1.5 μm to 200 nm, when the light electric field is polarized perpendicularly to the central axis, and that strong absorption is observed in the spectral range from 600 nm to at least 200 nm, when the light electric field is polarized parallel to the central axis (Li Z M et al., Phys. Rev. Lett. 87 (2001), 127401-1 - 127401-4).
Similar properties have been found for nanotubes or nanowires other than those consisting of carbon. In embodiments in which photoluminescent materials are provided in the device, nanocrystals are preferred.
Photoluminescent nanocrystals conveniently combine the following features: They absorb light, said absorption properties being effective in a broad range of wavelengths, said absorption also being a function of the orientation of the nanocrystals relative to a plane of polarization of said light, and the orientation of nanocrystals can be directed and/or stabilized mechanically and/or by an electric field.
Thus, the direction of the nanocrystals can be arranged in such a way that they show absorption for light polarized in a particular plane, while being transmissive to differently polarized light. The nanotubes exhibit a very large anisotropy in optical absorption, e.g. depending on the relative orientation of the incident radiation and the physical orientation of the tubes, one can realize a difference in absorption coefficient of approximately 10Λ6 in the UV region. I.e. well-aligned nanotubes absorb light with a polarization parallel to the tube axis, while the nanotubes are transparent to light with a perpendicular polarization. Furthermore, nanocrystals have been shown to have luminescence properties.
For example, single- alled carbon nanotubes formed in micro-channels of zeolite crystals emit light in the visible range upon excitation (N. Nagasawa et al, Journal of Luminescence 97 (2002), 161-167). Such properties are also known from other types of nanocrystals and nanowires (J.-M. Bonard et al., Phys. Rev. Lett. vol. 81, no. 7, 1441 (1998); M. H. Huang et al, Science vol. 292 (2001), 1897; K. Yamamoto et al., J. Phys. D: Appl. Phys. 31 (1998), 34-36; X. Duan et al., Nature 409 (2001), 66; J. Wang et al, Science vol. 293 (2001), 1455). The emitted (luminescent) light is polarized depending on the orientation of the light emitting nanocrystal. It is reported that the plane of polarization of the emitted light is often the same as that of the absorbed light (N. Nagasawa et al., supra). Upon excitation, the nanocrystals exhibit emission in the visible range, which has as an extra feature that the emission is polarized depending on the orientation of the nanocrystals. Only nanocrystals in the absorbing mode are excited and can therefore emit light. It is reported that the emitted light has the same polarization as the absorbed light. The excitation can be realized by means of light, e.g. UV light, in which case the device comprises a light source, but also by means of electroluminescence, because nanocrystals have been found to exhibit also electroluminescence, wherein the emitted light has a strong polarization dependence, as is the case for photoluminescence. In the case of electroluminescent nanocrystals, electrodes are provided to apply voltage differences across the nanocrystal areas. Within the scope of this invention, a species of nanocrystals is any set of nanotubes and/or nanowires whose emission spectra comprise light of the same wavelength upon excitation.
The device in accordance with a preferred embodiment of the invention has two display screens provided with nanocrystals.
The crystals in different screens have different absorption vs polarization characteristics. The device and method of such a preferred embodiment of the device according to the invention entails addressing of image layers in the display by a difference in polarization, which yields the 3D-effect without having to resort to additional means. An embodiment of a device in accordance with the preferred embodiment of the invention mentioned in the previous paragraph is schematically shown in Figure 2, which shows a schematic build-up of an example of the display according to the invention in which photoluminescent nanocrystals are used. A 3D display 9 has a plurality (in this example two) of image formation planes 1 (rear), 2 (front) upon which an image is generated by illumination through an optical system. Associated with the front image formation plane 2 is an attenuator 2'. The optical system comprises an unstructured large-area light source 3 (backlight) (e.g. blue, UV or DVD) that can controlled in intensity by a controller 7. The output of this light source is passed through a modulator device 4. The modulator device 4 is characterized in such a way that it consists of a plurality of pixels that can each be individually addressed and changed in light transmission characteristics. This modulator device could be realized by using a modified TFT LCD plate including driver ICs. By proper driving of the whole set of pixels through an electronic controller 7, a light transmission pattern can be formed across the area of the device 4 in correspondence with the desired 2D (x-y) graphical representation as decoded from the image-processor 8, which in its turn receives the data from the outside world (network, cable, antenna, computer, CD-ROM).
The illumination pattern thus formed is led through another modulator 5 which controls the polarization state of each complete x-y pattern. By doing so, the 3 rd dimension is introduced (z-decoding) as explained below. An example of such a modulator could be an unpatterned Liquid Crystal cell. In general, this scheme will work when at least one of the at least one rear and the at least one front, spatially separated display screen both comprise areas comprising photoluminescent material which differ in the plane of polarization for absorption and when a means for selectively passing polarized light is provided in between the light source and at least one of the at least one rear and the at least one front, spatially separated display screen. By controlling the polarization state of the light passing said means, here the modulator 5, the polarization of the exciting light is controlled, and thereby the z-decoding is controlled. The invention is not restricted to such embodiments. Z-addressing using incident light may also be performed by using two photoluminescent materials having different excitation wavelengths ranges, and performing z-addressing by letting light of a different wavelength fall on the luminescent materials. However, z-addressing using polarization switching is a preferred embodiment, because it allows swift switching and provides a relatively simple set-up.
Finally, the pattern is led through an optical lens system 6 which takes care of the proper magnification, projection and collimation on said image planes 1 and 2. In an advanced set-up, the lens system can also be used to focus the x-y patterns that have been z- decoded by different polarization onto the corresponding image planes 1 or 2 by using birefringence. The attenuator 2' action is controlled by control 7. The stopping screen may e.g. be an array of LCD Cells, corresponding to the array of pixels for the front screen, which can be switched between a transparent and a substantially opaque state. The information for modulator 4 for the front screen is used to control which LCD cells of the stopping screen are opaque and which are transparent. In this example, for simplicity of the drawing, all control actions are drawn by a single control 7. It will be clear to a person skilled in the art that, although a common control circuit as shown in Figure 2 may be advantageous, each control of a part of the device (light source, spatial modulator, polarization modulator, stopping screen, even lens system) may be provided separately from one or more of the other controls within the concept of the invention. The words "Control", "control means" or control circuit" are to be interpreted broadly and to comprise e.g. any piece of hardware (such as a controller), any circuit or sub-circuit designed for performing a controlling action as well as any piece of software (computer program or sub-program or set of computer programs, or program code(s)) designed or programmed to perform a controlling action according to the invention) as well as any combination of pieces of hardware and software acting as such, alone or in combination, without being restricted to the given examples of embodiments.
In this embodiment, the image planes 1 and 2 are essentially made of an optically transparent substrate (glass/plastic) upon which a layer is formed of the photoluminescent nanocrystals. The nanotubes are aligned in a preferred direction as illustrated in Fig. 3 and the principal axes (or alignment axes of the image planes) are oriented in such a way that they are perpendicular to each other (e.g. in the y-direction on plane 1 and in the x-direction on plane 2). Figure 3 shows a possible means of z-addressing and image formation on either image plane 1 or 2 by separating the two time steps t_ and t2, respectively. At t_ (described in the left part of Fig. 3), the "background" or "rear" image information is an x-y light intensity pattern with a polarization in the y-direction addressing image plane 1 due to the orientation of its nanotubes in the y-direction. The excited pixels in plane 1 will emit light (either white or of the corresponding color as described below), which is also polarized in the y-direction. A color image reproduction of the "background" or "rear" image is produced at the depth of plane 1.
At the time step t2 (illustrated in the right part of Fig. 3), a "foreground" or "front" x-y image is polarized in the x-direction (perpendicular polarization state) resulting in a corresponding color "foreground" or "front" image on plane 2.
The proposed 3D-TV consists of both described components, i.e. one image plane 2, which is sensitive to x-polarized light, and one image plane 1 which is sensitive to y- polarized light. It is useful to mention that both these parts in this preferred embodiment are orthogonal functions, i.e. they do not influence each other. Image plane 1 is transparent to x- polarized light, and vice versa. Fast switching of the polarization states at ti and t2 is achieved by modulator 5.
By controlling the sequence of the pattern formation in modulator 4, controlling the polarization state with 5 and optionally controlling the backlight intensity of 5, one can obtain image formation in planes 1 and 2. Since they are placed apart by a certain distance, the viewer will experience the build-up of an image in space (or depth), provided that the change-over between the formation of images on the image planes 1 and 2 is effected quickly enough within the retention time of the human eye.
The attenuator 2' is positioned between the image planes 1 and 2, preferably close to image plane 2. During time step tl, an x-y attenuating image (i.e. areas opaque to light from plane 1, or at least partially blocking the light) is produced, corresponding to the image produced on plane 2 during time t2.
This will produce an image as schematically shown in Figure 1A. Without the attenuator 2', an image as schematically shown in Fig. IB is produced. As described above in the preferred embodiment, image screens comprising nanocrystals are used.
It is to be noted that, apart from the preferred nanocrystals, other photoluminescent materials may be used. Examples of such photoluminescent materials are described in e.g. Science, vol. 29, pages 835 to 837, by Weder et al, in which polarized photoluminescent (PL) layers based on a mixture of organic molecules are described, which PL layers are characterized by either anisotropic absorption or anisotropic emission or both. Such materials also allow production of display screens having photoluminescent materials, and they also belong to the preferred type of materials having anisotropic absorption and/or emission characteristics. Compared to nanocrystals, however, mixtures of organic materials are relatively unstable, especially in view of the fact that UV light is used to excite them. Returning to the preferred embodiment comprising nanocrystals, there are various ways of aligning the nanotubes or nanowires.
The nanocrystals can be aligned in a preferential direction, for instance, by applying an electric AC or DC field of the order of 0.2 V/μm [ see e.g. Kunitoshi Yamamoto, Seiji Akita and Yoshikazu Nakayama, J. Phys. D: Appl. Phys. 31 (1998) 34-36 and X. Duan, Y. Huang, Y. Cui, J. Wang, CM. Lieber, "InP nanowires as building blocks for nanoscale electronic and optoelectronic devices" Nature 409 (2001) 66.]
Furthermore, directed/aligned growth of nanotubes is achieved by catalytic CVD methods, while simultaneously applying an electric field [ see e.g. Y. Zhang, A.
Chang..., "Electric field directed growth of aligned single walled carbon nanotubes" Applied Physics Letters, vol. 79, no. 19, 3155 (2001)]
In the device of the invention in accordance with a preferred embodiment, the nanocrystals are aligned to obtain large-area surfaces with controlled anisotropic characteristics (aligned nanocrystals). Since the nanocrystals exhibit a large anisotropy in absorption, these surfaces are selectively addressable (by choosing the polarization of the incoming light) and finally the emission of light in the visible region to generate a luminescent image on the image planes.
For numerous cases, such as medical imaging systems, a working 3D monochrome display would already be a significant improvement.
In preferred embodiments, the display device is a color display device. For all display devices, the spectrum emitted by backlight (for instance, UV) is such that it is able to excite the nanocrystals to emit visible light, i.e. falling within their absorption band and having enough energy. For RGB color generation, three cases (realization options) are possibly based on either narrow-band A) or broad-band B) or white light C) visible light-emitting nanotubes/nanowires.
In principle, one can distinguish different types of existing light-emitting nanotubes/nanowires characterized by narrow-band (A), broader band (B) and broad-band (white) (C) visible light emission. It is known that the light emission characteristics can be influenced by controlling various parameters: type of nanotube (material, single or multiwalled), diameter, doping, chirality.
A) Narrow-band emitting nanocrystals. Figure 4 shows a detail of a device according to the invention having narrow-band nanocrystals on one of the image plates 1, 2 to obtain a color image. As a first step, nanotubes, characterized by e.g. red light emission, are deposited pixelwise in a stripe- like pattern (layer 10 in Fig. 4) on a transparent glass substrate, for instance, by means of e.g. microcontact printing or any other suitable method. The nanotubes as obtained from an external synthesis process are originally dissolved in a proper liquid/viscous solvent. By applying an electric field, the nanotubes will align with respect to the electric field lines. In this state, the solvent is fixed (e.g. by polymerization or UV hardening). After this step, the next patterned layer 11 comprising e.g. green light- emitting nanotubes is deposited by following the same procedure. Subsequently, the last layer 12 is produced, consisting of aligned blue emitting nanotubes. Image plane 2 is produced in the same way as image plane 1 but with the nanotubes aligned at a different angle, preferably substantially perpendicular to those in plane 1.
Besides using pre-synthesized nanotubes, it is also possible to grow nanotubes or nanowires directly on pixel sidewalls of the image plates. For this method, electric field- assisted catalytic CVD growth can be used. Some typical, but not restrictive sizes of nanocrystal areas are indicated in Fig. 4. When narrow-band nanocrystals are used, the use of color filters is not necessary, dependent on the narrowness of the emission band.
B) Nanocrystals with a rather broad emission spectrum.
In such devices, the nanocrystals generate light within a broad band.
In order to generate R,G and B, a pixelated color filter array is preferably placed on or near each image plane to filter out the desired color component (R,G or B). These filters could be placed between the plate with nanocrystals and the light source, but preferably between the plate with nanocrystals and the viewer. The color filters should preferably have their full characteristics for a particular polarization and be transparent to, and optically neutral-colored for other polarization states. This feature can be obtained by using liquid crystal-based (cholesteric) colorfilters in combination with λ/4 plates. Another method of creating the desired color filter characteristics is to define double bandpass filters. One bandpass is located at the desired region in the optical spectrum (e.g. R, G or B region) and the other bandpass coincides with the spectrum of the backlight. To avoid the exit of non-absorbed light from the backlight, an optical filter should be placed at the exit face of the display 9 to remove the spectrum of the excitation source (usually a normal low-pass filter in the UV).
C) Broad-band nanotubes combined with usual color filter. In such devices, the nanocrystals generate white light. In order to generate R,G and B, a pixelated color filter array should be placed on or near each image plane which will filter out the desired color component (R,G or B). For a description of preferred characteristics of such filters, reference is made to point B above.
The difference between possibility B (broad-band emission by the nanocrystals) and C (white light emission by the nanocrystals) is that, in C, all of the screen can be covered with the same nanocrystals emitting white light (i.e. no structure in the screen in so far as nanocrystals are concerned), and color is achieved by the function of the filters alone, thus requiring high quality filters, whereas in B, the nanocrystals do emit colored light, but with a rather broad band which is fine-tuned by light filters, which may be of a relatively lower quality. The advantage of embodiment B over C is that the construction is simplified, but the disadvantage is that more of the light emitted by the white light-emitting nanocrystals is blocked by the color filters (thus a reduction of efficiency).
In the above examples A, B and C, nanocrystals are mentioned as preferred materials, but the same possibilities exist for other photoluminescent materials.
By way of illustrating the anisotropic behavior of absorption of light by nanocrystals, an example is given and illustrated in Figure 5.
Carbon nanotubes (CNT) are known to exhibit anisotropic behavior. Recently, the polarized absorption spectra of single-wall nanotubes (SWNT) with an almost mono- dispersed size and perfectly aligned in the channels of zeolite AFI single crystals were measured. With a 0.4 nm diameter, these nanotubes are the smallest CNT known. Figure 5 shows polarized optical absorption spectra of SWNT. In a large wavelength range, the absorption of light with E || c (θ = 0°) is remarkably high, whereas the material is nearly transparent to light polarized perpendicularly to the tube axis (θ = 90°). The slight increase observed at higher photon energies falls within the reported measurements from the epoxy used to hold the sample. The inset in Fig. 5 clearly demonstrates the anisotropy of the material. As a consequence, light with the right polarization is absorbed, whereas light with a polarization perpendicular to the right polarization is not absorbed.
By way of illustrating the anisotropic behavior of emission of light by nanocrystals, an example is given and illustrated in Figure 6. In 1998, field emission-induced luminescence during electron field emission on single-wall and multi-wall carbon nanotubes was observed and reported by Bonard et al. in Phys Rev Lett, vol. 81, no. 7, 1441 (1998). The luminescent spectra measured at that time are reproduced in Fig. 6. This Figure shows the photoluminescence of the 0.4 nm SWNTs arrayed in the channels of zeolite crystal, excited using the 488-nm line of an Ar-ion laser, at a varied excitation polarization angle θ. The insets show the experimental setup (bottom), and the polarization dependence of the light-emission intensity on the polarization angle (top).
Figures 7 shows by way of example that the emission spectrum of nanowires can be adjusted by controlling the diameter. Figure 7 shows the emission spectrum of InP nanowires for bulk InP (showing a peak around 900 nm), for nanowires (diameter 5 nm, showing a peak around 770 nm) and InP tubes (showing a peak around 580 nm). This effect is called confinement. It can also be used for all other nanostructures to precisely tune the emission color.
In the examples hitherto shown, the device comprises photoluminescent emitters such as nanocrystals. As stated above, nanocrystals may also exhibit electroluminescence, i.e. when a voltage difference is applied across nanocrystals, they emit light.
Fig. 8 schematically illustrates an embodiment of the device according to the invention, using electroluminescent materials. The device has a rear (1) and a front screen (2), comprising electrodes 81 and 82, in between which electroluminescent materials 83, preferably nanocrystals are provided. In this example, nanocrystals oriented between the electrodes are present. By applying appropriate voltages across the nanocrystals, light can be induced. Due to the anisotropy of the polarization of the light emitted by the nanocrystals, light emitted by screen 1 passes through screen 2.
Figure 9 illustrates schematically a method of producing a screen based on InP wires.
A porous alumina (A120_) matrix is formed by anodizing an aluminum film on a substrate 91. Pores with a diameter in the range of 5-50 nm are formed with a density of 1010 pores.cm2. Gold (Au) particles are electrochemically deposited at the bottom of the pores. By using the VLS growth mechanism, nanowires or nanotubes are grown inside the pores. By switching the target during the synthesis, a p-n junction within a wire or a tube is formed. After growth, the top surface is polished and a transparent electrode (ITO) is deposited. Light 92 is induced by applying a voltage V across the p-n-junction and exits the transparent ITO layer. Figure 10 illustrates a transparent LED based on a compound semiconductor nanotube 101. An n-p junction is established in the tube. The tube is contacted with an ohmic contact to the n-type part, the electron-injector electrode (e), and an ohmic contact to the p- type part, the hole-injector electrode (h). Injection of electrons and holes induces light at the p-n junction. The LED is made on a Si02 substrate so that light may pass through the substrate.
A number of different ways of making color displays have been discussed (possibilities A (narrow band), B (broad band + filters) and C (white light plus filters), while the same possibilities exist for electroluminescent materials.
Within the scope of the invention, one of the screens, such as the rear screen, may be of the photoluminescent type, i.e. having photoluminescent materials, whereas the other (or another) screen, e.g. the front screen or one or more screens in between, may be of a transparent electroluminescent type.
Figure 11 shows an example of a device in which use is made of the polarization dependence of the emitted light so as to be able to increase the number of plates. In this device, there are three screens 1, 1' and 2, each with nanocrystals whose plane of polarization differs 45 degrees with the others. The modulator 5 sequentially transmits light with the proper polarization (at time slots tl, t2, t3) and is coupled to a modulator 5' transmitting light of the same polarization at the same time slots.
In summary, the invention may be described as follows. A display device provides an image with depth perception. The device comprises two or more displays screens (1, 2), and means (4, 5) for addressing pixels on said display screens. The front display screen is substantially transparent to light coming from display screens behind said screen and comprises an addressable attenuator (2') for attenuating light emanating from behind at positions corresponding to addressed pixels in the front display screen.
It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinbefore. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features, even if not explicitly recited in the claims. Reference numerals in the claims do not limit their protective scope. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements other than those stated in the claims. Use of the article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The present invention has been described in terms of specific embodiments, which are illustrative of the invention and should not to be construed as limiting. The invention may be implemented in hardware, firmware or software, or in a combination of them. Other embodiments are within the scope of the following claims.
The attenuator may be established in several manners. In some embodiments, all image information is provided by the modulated backlight and corresponding polarization i.e. depth addressing. The blocking layer has to be electrically addressed pixel-wise, i.e. active control is required. As an example of such a layer, CNTs could be used again (there are also species which only show absorption but no luminescence in the visible range), but a switchable plate has to be realized, which could be done electrophoretically. However, also other anisotrope materials such as liquid crystal materials could be used (use of TFT LCD). There are three requirements: (not necessarily), active control (fast switching between the absorbing and the transparent state), no influence on light with a different polarization state. The last requirement is not essential if the switching is fast enough for active absorption to only occur during the corresponding time cycle of that sub-image. This principle could also be used to address layers 1 and 2. In addition to the configuration of a 3D display described above, where the image infonnation is decoded in modulators 4 and 5 for position and depth information, respectively, a second configuration using addressable matrices at 1,2 might be realized. Each pixel of these matrices can be addressed individually and by pixelwise orienting the nanotubes by means of an electric field. In this configuration, the nanotubes can operate in an active mode (light emitted as described above) or in an inverse mode (absorption of light, characteristic 1). The backlight 3 still produces non-polarized light, while 4 and 5 are not required.
As specific photoactive materials for obtaining a display device according to the invention, nanocrystals are described in the embodiments. Although they form preferred embodiments, the invention in its broadest scope is not restricted to the use of nanocrystals. There is another group of materials, namely fluorescent organic dyes which, when prepared in a particular way (i.e. under mechanical stretching), obtain similar un-isotropic optical properties as nanocrystals, namely polarized absorption + luminescence. For a description of such materials, reference is made to e.g. Science, vol. 29, 6 February 1998, pages 835-837, "Incorporation of photoluminescent Polarizers into Liquid Crystal Displays", by Weder et al.
Yet further different types of transparent emitters exist, such as LEEC (light- emitting electrochemical cells), some PolyLED (polymer light-emitting diode) or smOLED (small molecule organic LED)-materials. Such materials may be used in embodiments of the invention. It is to be noted that some of these materials such as LEEC have a relatively long switching time, which would make them less suitable for applications in which fast switching times are relevant such as the display of movies. However, the invention is also useful for applications in which (nearly) static images are shown, for instance, in medical 3-D imaging or for showing static structures, such as, for instance, in architecture, for 3-D imaging of complex molecules or mathematical 3-D objects.
A combination of different types of display screens is possible, for instance, a set-up in which a light source and two photoluminescent screens with crossed orientations are used, as schematically shown in Figures 2 and 3, followed by one or more electroluminescent screens.
One further improvement may be formed by a filter positioned in front of the device to filter the light coming from the light source. The UV or DUV light source is usually a bright source and many materials may age when hit by UV light, so that stopping the UV light after it has passed through the last photoluminescent screen may be useful. The phrase "positioned in front of the device" may mean: after the last photoluminescent screen. In devices in which e.g. a number of photoluminescent screens are used, followed by one or more electroluminescent screens, such a UV filter could advantageously be positioned between the photoluminescent and electroluminescent screens.
A further improvement may be formed by an absorption compensation means in the control means. Although screens are transparent to light coming from behind, total absorption may differ from screen to screen. Thus, means for compensating such absorption are preferably provided, which would mean in practical terms that the light coming from the rear screen is somewhat brighter than from a screen which is more to the front, so as to give substantially the same intensity for the viewer. This can be done by controlling the spatial modulator. A slightly different way of doing so would be fonned in a scheme in which the rear screen is excited during a time slot tl, and a front screen is excited during time slots t2 (see Figures 2 and 3). By making time slots tl slightly longer than time slots t2, a compensation may be accomplished in accordance with the difference in absorption. Further variations will be apparent to a person skilled in the art.

Claims

CLAIMS:
1. A display device for providing an image with depth perception, characterized in that the display device comprises
- at least one rear (1) and at least one front (2), spatially separated, display screen, and means (4, 5) for addressing pixels on said display screens for providing a rear image on the rear display screen and a front image on the front display screen, wherein the front display screen is substantially transparent to light emanating from the rear display screen
- an addressable attenuator (2') close to or attached to the at least one front display screen for attenuating light emanating from the rear display screen at positions corresponding to addressed pixels in the front display screen.
2. A display device as claimed in claim 1, characterized in that the attenuator (2s) is positioned between the at least one rear (1) display screen and the at least one front (2) display screen.
3. A display device as claimed in claim 1, characterized in that the attenuator is arranged in such a way that, in operation, the attenuator has such an attenuation action that the attenuator (2') partially attenuates for selected pixels in the at least one front display screen.
4. A display device as claimed in claim 1, characterized in that the display device comprises means for forming the images on the at least one rear and the at least one front display screen intermittently (tl, t2).
5. A display device as claimed in claim 1, characterized in that the display device comprises a first rear (1) and a final (2) front display screen, and in addition one or more display screens (1') between the first rear and final front display screen, and in that the one or more display screens (1') between the first rear and final front display screen are transparent to light coming from behind and are provided with an attenuator to attenuate light from behind.
6. A display device as claimed in claim 1 or 2, characterized in that the display device comprises only a single rear (1) and a single front (2) display screen.
7. A display device as claimed in claim 1, characterized in that at least one of the at least one rear and at least one front, spatially separated display screen has areas comprising photoluminescent material and a light source (3) to excite said areas, and the means for addressing pixels (4) on said display comprise a spatial modulator (4) in the light path between source (3) and the photoluminescent material.
8. A display device as claimed in claim 7, characterized in that at least one of the at least one rear and the at least one front, spatially separated display screen has areas comprising photoluminescent material, and in that the plane of polarization for absorption for the display screens differs, and in that a means for selectively passing polarized light is arranged between the light source and at least one of the at least one rear and the at least one front, spatially separated display screen.
9. A display device as claimed in claim 7 or 8, characterized in that the at least two display screens both comprise photoluminescent material, and in that the photoluminescent materials in the at least two display screens are ananged in such a way that the orientation of the plane of polarization of emitted light of one of said at least two display screens differs from the plane of orientation of absorption of another of said at least two display screens, the latter being positioned more towards the viewer.
10. A display device as claimed in claim 7, 8 or 9, characterized in that at least one of the display screens has areas comprising nanocrystals selected from the group of nanotubes and nanowires which show photoluminescence.
11. A display device as claimed in claim 7, characterized in that it comprises a filter positioned in front of the photoluminescent screen(s) to filter the light coming from the light source.
12. A display device as claimed in claim 1, characterized in that at least one of the at least one rear and at least one front, spatially separated display screen has areas (83) comprising electroluminescent material and electrodes (81, 82) to apply voltages differences across the material.
13. A display device as claimed in claim 12, characterized in that the electroluminescent material comprises electroluminescent nanocrystals.
14. A display device as claimed in claim 7, 8, 9 or 12, characterized in that at least one of the screens has areas comprising materials selected from the group of PolyLED (PLED), OLED, smOLED, or LEEC or a combination.
15. A display device as claimed in claim 13 or 14, characterized in that the at least two display screens both comprise electroluminescent material and electrodes, and in that the electroluminescent materials in the at least two display screens are arranged in such a way that the orientation of the plane of polarization of emitted light of one of said at least two display screens differs from the plane of orientation of emission of another of said at least two display screens.
16. A display device as claimed in claim 7 or 13, characterized in that at least one screen comprises electroluminescent or photoluminescent material for emission of white light, and color filters.
17. A display device as claimed in claim 7 or 13, characterized in that at least one screen has three types of color areas comprising electroluminescent or photoluminescent materials of three different colors.
18. A display device as claimed in claim 17, characterized in that the display device comprises color filters associated with the color areas.
19. A display device as claimed in claim 1, characterized in that the device comprises a regulator to regulate the light intensity of each screen so as to correct for absorption losses or perception enors.
PCT/IB2004/050284 2003-03-24 2004-03-18 Three-dimensional display WO2004086123A1 (en)

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WO2005055331A1 (en) * 2003-12-02 2005-06-16 Koninklijke Philips Electronics N.V. Pixel arrangement for an emissive device
GB2425673A (en) * 2005-04-25 2006-11-01 Boeing Co 3D display using quantum dots
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US7742673B2 (en) 2007-09-28 2010-06-22 General Electric Company Thermal mangement article having thermal wave guide
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EP2535761A2 (en) * 2010-03-04 2012-12-19 Tovis Co. Ltd. Multi-layer image display device
EP2535761A4 (en) * 2010-03-04 2014-07-23 Tovis Co Ltd Multi-layer image display device
US9661316B2 (en) 2010-03-04 2017-05-23 Tovis Co., Ltd. Multi-layer image display device
CN101917641A (en) * 2010-09-08 2010-12-15 北京利亚德电子科技有限公司 LED stereoscopic display, displaying method and signal receiver
TWI450391B (en) * 2010-12-23 2014-08-21 Advanced Optoelectronic Tech Led stereoscopic display
DE102012014645A1 (en) * 2012-07-24 2014-01-30 Blexton Management Ltd. Multilayer image display device for use in e.g. notebook, has polarization filter assigned to liquid crystal layer, where light from light source is guided through retardation element before light reaches observer

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