WO2012164496A1

WO2012164496A1 - Autostereoscopic display device

Info

Publication number: WO2012164496A1
Application number: PCT/IB2012/052695
Authority: WO
Inventors: Philip Steven Newton; Mark Thomas Johnson; Bart Kroon; Marcellinus Petrus Carolus Michael Krijn; Adrianus Sempel; Siebe Tjerk De Zwart
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2011-05-30
Filing date: 2012-05-30
Publication date: 2012-12-06

Abstract

An autostereoscopic display device uses a display arrangement such as an emissive display arrangement or a reflective display arrangement and an autostereoscopic lens arrangement. A light controlling layer is provided between the display arrangement and the lens arrangement, with a refractive index lower than the surrounding materials. This reduces waveguiding in the lens arrangement by increasing the total internal reflection for shallow angles (near the plane of the display) before the light enters the lens arrangement.

Description

Autostereoscopic display device

FIELD OF THE INVENTION

This invention relates to an autostereoscopic display device of the type that comprises a display panel having an array of display pixels for producing a display and an imaging arrangement for directing different views to different spatial positions.

BACKGROUND OF THE INVENTION

A first example of an imaging arrangement for use in this type of display is a barrier, for example with slits that are sized and positioned in relation to the underlying pixels of the display. In a two-view design, the viewer is able to perceive a 3D image if his/her head is at a fixed position. The barrier is positioned in front of the display panel and is designed so that light from the odd and even pixel columns is directed towards the left and right eye of the viewer, respectively.

A drawback of this type of two-view display design is that the viewer has to be at a fixed position, and can only move approximately 3 cm to the left or right. In a more preferred embodiment there are not two sub-pixel columns beneath each slit, but several. In this way, the viewer is allowed to move to the left and right and perceive a stereo image in his/her eyes all the time.

The barrier arrangement is simple to produce but is not light efficient. A preferred alternative is therefore to use a lens arrangement as the imaging arrangement. For example, an array of elongate lenticular elements can be provided extending parallel to one another and overlying the display pixel array, and the display pixels are observed through these lenticular elements.

The lenticular elements are provided as a sheet of elements, each of which comprises an elongate semi-cylindrical lens element. The lenticular elements extend in the column direction of the display panel, with each lenticular element overlying a respective group of two or more adjacent columns of display pixels.

In an arrangement in which, for example, each lenticule is associated with two columns of display pixels, the display pixels in each column provide a vertical slice of a respective two dimensional sub-image. The lenticular sheet directs these two slices and corresponding slices from the display pixel columns associated with the other lenticules, to the left and right eyes of a user positioned in front of the sheet, so that the user observes a single stereoscopic image. The sheet of lenticular elements thus provides a light output directing function.

In other arrangements, each lenticule is associated with a group of four or more adjacent display pixels in the row direction. Corresponding columns of display pixels in each group are arranged appropriately to provide a vertical slice from a respective two dimensional sub-image. As a user's head is moved from left to right, a series of successive, different, stereoscopic views are perceived creating, for example, a look-around impression.

Known autostereoscopic displays use liquid crystal displays to generate the image.

There is increasing interest in the use of emissive displays, such as

electroluminescent displays, for example organic light emitting diode (OLED) displays, as these do not need polarizers, and potentially they should be able to offer increased efficiency since the pixels are turned off when not used to display an image, compared to LCD panels which use a continuously illuminated backlight.

There is also increasing interest in the use of reflective displays, such as electrophoretic displays and electrowetting displays.

This invention is based on the use, within an autostereoscopic display system, of a display arrangement that is emissive or reflective.

Emissive displays such as OLED displays and reflective displays such as electrophoretic displays differ significantly from LCD displays in how the light is emitted from the pixel. OLED pixels are emitters that emit light over a wide range of directions, and electrophoretic pixels are reflectors that reflect light over a wide range of directions. In the context of the present invention, such emitters and reflectors are also called diffuse emitters and diffuse reflectors, respectively. For a conventional (2D) display, OLED displays have a clear advantage over LCD displays that require a backlight and which, without taking special measures, emit light only in a narrow beam. However, the diffuse emission of the OLED material also poses a challenge as a lot of light is recycled inside the organic layers and is not emitted giving rise to a low efficiency. To improve, this various solutions have been sought to improve the out-coupling of the light out of the OLED.

However this improvement for 2D displays is in fact a problem for 3D autostereoscopic OLED displays. The solutions for increasing the light output cannot be used in autostereoscopic lenticular displays, as the light intended to be emitted from one lenticular lens may be reflected in the glass to a neighbouring lens. This reduces contrast and increases crosstalk.

Reflective displays such as electrophoretic and electrowetting displays may give rise to similar drawbacks as discussed above for emissive displays in the form of OLED displays.

Thus, there is a conflict between the desire for using emissive and reflective displays and the desire for low crosstalk within a 3D autostereoscopic display.

SUMMARY OF THE INVENTION

According to the invention, there is provided an autostereoscopic display device comprising:

a display arrangement comprising a stack of layers;

an autostereoscopic lens arrangement comprising a plurality of lenses over the - display arrangement, wherein a plurality of pixels is provided beneath each lens; and

- a light controlling layer between the display arrangement and the

autostereoscopic lens arrangement, wherein the light controlling layer has a refractive index lower than the material of the layers on each side of the light controlling layer.

In an embodiment of the invention, the display arrangement is an emissive display, such as an electroluminescent display, for example an OLED display. In a further embodiment of the invention, the display arrangement is a reflective display, such as an electrophoretic display or an electrowetting display.

The function of the light controlling layer is to increase the amount of total internal reflection compared to an arrangement without the layer, such that light of shallow angles, which can contribute to waveguiding in the lens material, is prevented from entering the lens material.

When the display arrangement is an electroluminescent display arrangement having a top emitting structure, the stack of layers can comprise a passivation layer, a cathode layer, a light emitting layer and an anode layer, wherein the passivation layer is adjacent the light controlling layer. The light controlling layer is then contacted on one side by the lens arrangement and on the other side by the outermost layer of the stack of layers of the electroluminescent display arrangement.

The light controlling layer preferably has a refractive index at least 0.1 less than the refractive index of the material of the lens arrangement, to provide the desired improvement. This refractive index difference can be more than 0.2 or even more than 0.3. For other emissive or reflective display arrangements, the light controlling layer can be contacted on one side by the stack of layers of the display arrangement and on the other side by a device substrate.

The light controlling layer then preferably has a refractive index at least 0.1 less than the refractive index of the material of the substrate, to provide the desired improvement. This refractive index difference can be more than 0.2 or even more than 0.3.

In both cases, the light controlling layer preferably also has a refractive index at least 0.1 less than the outer layer of the display arrangement, to provide a surface at which total internal reflection takes place. Again, this refractive index difference can be more than 0.2 or even more than 0.3.

The lens arrangement preferably comprises glass, although it may instead comprise a transparent polymer or other transparent material. The light controlling layer can comprise air, and an array of spacers is then provided. It may instead comprise a solid layer with the desired (low) refractive index, in which case spacers are not needed.

The autostereoscopic lens arrangement can comprise a plurality of lenticular lenses extending in a pixel column direction or inclined at an acute angle to the pixel column direction, wherein each lens covers a plurality of pixel columns.

If the display arrangement is an electroluminescent display arrangement, the stack of layers of the display preferably comprises a cathode layer, a light emitting layer and an anode layer, wherein the cathode layer is adjacent the light controlling layer, and comprises an ITO layer. This defines a top emitting display with a transparent top cathode. The invention can however also be applied to other types of emissive displays and to reflective displays.

The invention also provides a method of displaying autostereoscopic images, comprising:

generating a pixellated image using a display arrangement comprising a stack of layers;

passing the light of the pixellated image through a light controlling layer to an autostereoscopic lens arrangement,

wherein the light controlling layer has a refractive index lower than the refractive index of the materials on opposite sides of the light controlling layer. BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic perspective view of a known autostereoscopic display device;

Fig. 2 shows how a lenticular array provides different views to different spatial locations;

Fig. 3 schematically shows the structure of a single pixel of an OLED display, and in the form of a backward emitting structure;

Fig. 4 shows how the light paths are affected when applying a lenticular lens to a top emitting structure;

Fig. 5 shows a first example of pixel structure in accordance with the invention; and

Fig. 6 shows a second example of pixel structure in accordance with the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an autostereoscopic display device using a display arrangement and an autostereoscopic lens arrangement. A light controlling layer is provided between the display arrangement and the lens arrangement, with a refractive index lower than the surrounding material layers. This reduces waveguiding in the lens arrangement by increasing the total internal reflection for shallow angles (near the plane of the display) before the light enters the lens arrangement.

Hereinbelow, embodiments of the present invention will be described on the basis of an electroluminescent display, which is an example of an emissive display. The skilled person will understand that the invention can be applied in lenticular lens based autostereoscopic display arrangements comprising any kind of emissive display, and also in lenticular lens based autostereoscopic display arrangements comprising any kind of reflective display, as in all these display types light will be directed (via emission or via reflection) from a pixel to the lenticular lenses over a wide range of directions.

The basic operation of a known 3D autostereoscopic display will first be described. Fig. 1 is a schematic perspective view of a known direct view autostereoscopic display device 1 using an LCD panel to generate the images. The known device 1 comprises a liquid crystal display panel 3 of the active matrix type that acts as a spatial light modulator to produce the display.

The display panel 3 has an orthogonal array of display pixels 5 arranged in rows and columns. For the sake of clarity, only a small number of display pixels 5 are shown in the Figure. In practice, the display panel 3 might comprise about one thousand rows and several thousand columns of display pixels 5.

The structure of the liquid crystal display panel 3 as commonly used in autostereoscopic displays is entirely conventional. In particular, the panel 3 comprises a pair of spaced transparent glass substrates, between which an aligned twisted nematic or other liquid crystal material is provided. The substrates carry patterns of transparent indium tin oxide (ΓΓΟ) electrodes on their facing surfaces. Polarising layers are also provided on the outer surfaces of the substrates.

Each display pixel 5 comprises opposing electrodes on the substrates, with the intervening liquid crystal material therebetween. The shape and layout of the display pixels 5 are determined by the shape and layout of the electrodes. The display pixels 5 are regularly spaced from one another by gaps.

Each display pixel 5 is associated with a switching element, such as a thin film transistor (TFT) or thin film diode (TFD). The display pixels are operated to produce the display by providing addressing signals to the switching elements, and suitable addressing schemes will be known to those skilled in the art.

The display panel 3 is illuminated by a light source 7 comprising, in this case, a planar backlight extending over the area of the display pixel array. Light from the light source 7 is directed through the display panel 3, with the individual display pixels 5 being driven to modulate the light and produce the display.

The display device 1 also comprises a lenticular sheet 9, arranged over the display side of the display panel 3, which performs a view forming function. The lenticular sheet 9 comprises a row of lenticular elements 11 extending parallel to one another, of which only one is shown with exaggerated dimensions for the sake of clarity.

The lenticular elements 11 are in the form of convex cylindrical lenses, and they act as a light output directing means to provide different images, or views, from the display panel 3 to the eyes of a user positioned in front of the display device 1. The device has a controller 13 which controls the backlight and the display panel.

The autostereoscopic display device 1 shown in Fig. 1 is capable of providing several different perspective views in different directions. In particular, each lenticular element 11 overlies a small group of display pixels 5 in each row. The lenticular element 11 projects each display pixel 5 of a group in a different direction, so as to form the several different views. As the user's head moves from left to right, his/her eyes will receive different ones of the several views, in turn.

In the case of an LCD panel, a light polarising means must also be used in conjunction with the above described array, since the liquid crystal material is birefringent, with the refractive index switching only applying to light of a particular polarisation. The light polarising means may be provided as part of the display panel or the imaging arrangement of the device.

Fig. 2 shows the principle of operation of a lenticular type imaging arrangement as described above and shows the backlight 20, display device 24 such as an LCD and the lenticular array 28. Fig. 2 shows how the lenticular arrangement 28 directs different pixel outputs to three different spatial locations 22', 22", 22"'. These locations are all in a so-called viewing cone, in which all views are different. The views are repeated in other viewing cones, which are generated by pixel light passing through adjacent lenses. The spatial locations 23', 23", 23"' are in the next viewing cone.

The use of an OLED display avoids the need for a separate backlight and polarizers. OLED promises to be the display technology of the future. However, a problem currently with OLED display is the light extraction out of the device. Without taking any measures the light extraction out of the OLED can be as low as 20 %.

Fig. 3 schematically shows the structure of a single pixel of an OLED display, and in the form of a backward emitting structure (i.e. through the substrate).

The display comprises a glass substrate 30, a transparent anode 32, a light emissive layer 34 and a mirrored cathode 36.

The lines represent the path light can take when emitted from a point 38 in the organic layer. As the light is emitted from the source it can travel in all directions. When the light reaches the transition from one layer to another layer the difference between the refractive index of each of the layers determines whether the light can escape one layer and get into the next. The refractive index is determined by the speed of light in the material and is given by Snell's law:

sin 0>2 -{.¾ nt v = velocity (m/s) n = refractive index (unities s)

In the example of Fig. 3, the refractive index of the organic material forming the light emissive layer 34 is high (n = 1.8) while the refractive index of glass is 1.45.

When the angle of incidence of light that travels from a material with a high refractive index to a material with a low refractive index is large enough, the light cannot leave the material. The angle of incidence the critical angle and is given by a = arcsin(n2/nl) for the organic material into glass. This gives 54 degrees.

Thus, it is clear that a lot of the light generated in the organic layer never leaves the layer but stays inside the material, where it is re-absorbed and drives another photon emission or turns into heat.

The same happens for the light that does leave the organic layer and enters the glass substrate. A lot of light cannot leave the glass at the glass to air interface.

Several solutions have been proposed both for ensuring the coupling of light out of the organic layers into the glass and for coupling the light out of the glass into the air.

The article by D. S. Mehta et. Al, "Light out-coupling strategies in organic light emitting devices" Proc. of ASID'06, 8-12 Oct, New Delhi gives an overview of the various solutions.

Whilst OLED devices are typically bottom emitting, and emit light through the glass substrate, another approach is to make the OLED stack top emitting such that the light emits through a transparent cathode and a thin encapsulating layer and not through the glass substrate. In general, different approaches to increasing the light extraction work better (or only) with either top or bottom emitting OLED structures.

The invention is described below based mainly on the use of a top-emitting OLED display. However the basic principle behind this invention can also be used with a bottom emitting OLED display, and all embodiments are applicable to both top and bottom emitting OLED structures. Whilst the known solutions help to improve the light extraction efficiency up to 80% for lighting applications and for 2D displays, they do not provide a good solution for autostereoscopic displays. A problem occurs when fitting a lenticular lens on the OLED display for creating an autostereoscopic TV. Even with a top emitting OLED, light will still be injected into a relatively thick glass layer causing the problems highlighted above, and a substantial amount of light will remain in waveguide mode in the glass. In principle, using a lenticular lens improves the light extraction from the glass into air as compared to a bottom emitting OLED but for a 3D display this has the side effect of reducing contrast and increasing crosstalk. This is a particular issue for 3D displays. For 2D displays, in many cases adjacent pixels will display the same colour (i.e. white or coloured areas of a screen, lines of single colour etc.) so that if any light escapes from a neighbouring pixel, this will simply add to the desired colour. However, in a 3D display, adjacent pixels do not in general have any relationship to each other, as they belong to different views and will generally be of different colour content. Thus, if any light escapes from a neighbouring pixel, this will seriously affect the quality of the image.

Furthermore, a substantial amount of light will still stay in waveguide mode in the glass. Part of this will be re-absorbed.

Fig. 4 shows how the light paths are affected when applying a lenticular lens to a top emitting structure. The top emitting structure comprises a glass substrate 40, mirrored anode 42, light emissive layer defining pixels 44 and a transparent cathode 46. A sealing and passivation layer 48 is between the cathode 46 and the glass lenticular array 49.

As illustrated in Fig. 4, light is generated in the organic layer and some light enters the glass of the lenticular arrangement 49. Some of the light will stay in waveguide mode in the glass by virtue of the internal reflections 50 and enter the optical path of a neighbouring view (or pixel/subpixel). Here it may be reflected back and leave through the lens (as shown for light ray 52) or it may be re-absorbed in the pixel.

If the light does leave the lens of the neighbouring view it will create crosstalk.

The invention provides a solution in which a layer is introduced between the lenticular and the OLED with a low refractive index.

Fig. 5 shows an example of the invention, in which the layer comprises air. As shown, the air layer 50 is between the top cathode 46 and the lenticular lens array 49.

Rays of light with a large angle of incidence are reflected off the air interface back into the light emitting material layer. The result is that less light remains in waveguide mode inside the glass (or other material) of the lenticular sheet, therefore it reduces the crosstalk between neighbouring views.

The layer 50 can be any material with a low refractive index. In this context, a "low" refractive index is one which is lower than the materials of the layers on opposite sides, so that a critical angle is defined and there is increased total internal reflection.

For example, the passivation layer 48 can be glass or polycarbonate with a refractive index in the range 1.3-1.6. The lower refractive index of the air of 1.0 means that the critical angle is in the range 38 to 50 degrees, so that all light steeper than the critical angle will be internally reflected at the air interface and thereby prevented from entering the glass of the lenticular array.

There will be no internal reflection at the interface between the air layer and the glass of the lenticular array, since there is an increase in refractive index.

The refractive index of the passivation layer 48 can be lower than that of the top layer 46 of the OLED stack.

The lower the critical angle, the greater the reduction in light that can waveguide within the material of the lens. A lower critical angle is obtained by having a greater difference in refractive index.

With direct contact between the electroluminescent display stack and the glass of the lenticular array (as in a conventional arrangement), there is already a boundary at which total internal reflection occurs. To increase the range of angles for which internal reflection arises and thereby reduce waveguiding, the additional material layer should have a refractive index lower than the material of the lenticular array. The additional material layer also needs to have a lower refractive index than the contacted layer of the electroluminescent display stack, so that a total internal reflection boundary is created.

Thus, the additional material has a lower refractive index than the passivation layer (or the cathode if no passivation layer is needed) to create a total internal reflection surface, and lower than the refractive index of the lenticular to create an improvement to the suppression of waveguiding compared to the case with no additional material layer.

If the lenticular array is glass, it will typically have a refractive index of 1.45, and the additional material layer should preferably have a refractive index at of 1.35 or less.

More generally, the refractive index of the additional layer should be at least 0.1 lower than the refractive index of the lenticular material (which may be a polymer instead of glass) and at least 0.1 lower than the refractive index of the layer of the layer of the display panel which is in contact with the additional material layer (on the opposite side of the additional material layer to the lenticular array).

The refractive index can more preferably be more than 0.2, 0.3 or 0.4 below the refractive index of the layers on both opposite sides of the additional material layer.

The thickness of the additional material layer 50 is not critical, but thinner is better. A thickness similar to the thickness of the OLED layers is suitable (100s of nanometers), although increasing the thickness above the wavelength of the light may be preferable, for example to the range 1 to 50 μιη.

The material with a low refractive index may be inserted during production when applying the lenticular sheet onto the OLED display. If the material consists of air, spacers can be used to maintain a homogeneous distance between the lenticular sheet and the OLED display. These spacers are shown schematically in Fig. 5 as 52. They may be at each lens boundary as shown or they may be less frequently provided. The spacers can be absorbing, and they can also extend into the passivation layer 48 to block light waveguiding within the layer 48.

Instead of using air, another material with a low refractive index approaching the refractive index of 1.0 of air can be used, such as an aerogel. More generally, any material can be used that has a refractive index that is at least lower than that of the surrounding layers, which may be graded films of Si02 and Ti02, nanorods of Si02, Teflon etc.

The solution of the invention is somewhat counterintuitive as usually great effort is taken to ensure a very tight lamination of the lenticular sheet onto the display. For OLED displays the invention involves introducing an air gap or other material spacing that reduces the light extraction from the OLED into the lenticular to reduce crosstalk.

The example above is for a top emitting display. The invention can also be applied to a bottom emitting display.

Fig. 6 shows a bottom emitting display. The substrate 40 is between the lens arrangement 49 and the display stack. The display stack comprises a transparent anode 42, the light emitting layer 44 and a mirrored cathode 46. In this case, the anode is transparent. A passivation layer 48 is on the top of the stack (with respect to the substrate 40) over the cathode 46. The anode 42 makes direct contact with the additional light controlling layer 50, which in this example is shown as a solid.

The anode can for example be ITO, with a refractive index of around 1.7. The additional layer serves the same function of increasing the amount of internal reflection than would arise if the electroluminescent display stack (the anode in particular) is in direct contact with the substrate 40.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. An autostereoscopic display device comprising:

a display arrangement comprising a stack of layers;

an autostereoscopic lens arrangement (49) comprising a plurality of lenses over the display arrangement, wherein a plurality of pixels is provided beneath each lens; and a light controlling layer (50) between the display arrangement and the autostereoscopic lens arrangement (49), wherein the light controlling layer has a refractive index lower than the material of the layers on each side of the light controlling layer (50).

2. A device as claimed in claim 1, wherein the display arrangement is an emissive display arrangement.

3. A device as claimed in claim 2, wherein the emissive display arrangement is an electroluminescent display arrangement

4. A device as claimed in claim 1, wherein the display arrangement is a reflective display arrangement.

5. A device as claimed in any one of claims 1 to 4, wherein the light controlling layer (50) is contacted on one side by the lens arrangement (49) and on the other side by the stack of layers of the display arrangement.

6. A device as claimed in claim 3, wherein the stack of layers comprises a passivation layer (48), a cathode layer (46), a light emitting layer (44) and an anode layer (42), wherein the passivation layer (48) is adjacent the light controlling layer.

7. A device as claimed in any one of claims 1 to 6, wherein the light controlling layer (50) has a refractive index at least 0.1 less than the refractive index of the material of the lens arrangement (49).

8. A device as claimed in any one of claims 1 to 6, wherein the light controlling layer (50) has a refractive index at least 0.2 less than the refractive index of the material of the lens arrangement.

9. A device as claimed in claim 1, wherein the light controlling layer (50) is contacted on one side by the stack of layers of the display arrangement and on the other side by a device substrate (40).

10. A device as claimed in any preceding claim, wherein the light controlling layer (50) has a refractive index at least 0.1 less than the adjacent layer of the display arrangement stack.

11. A device as claimed in claim 11, wherein the light controlling layer (50) has a refractive index at least 0.2 less than the adjacent layer of the display arrangement stack.

12. A device as claimed in any preceding claim, wherein the material of the lens arrangement (49) comprises glass.

13. A device as claimed in any preceding claim, wherein the light controlling layer (50) comprises air, wherein an array of spacers (52) is provided for defining the air spacing.

14. A device as claimed in any preceding claim, wherein the autostereoscopic lens arrangement comprises a plurality of lenticular lenses extending in a pixel column direction or inclined at an acute angle to the pixel column direction, wherein each lens covers a plurality of pixel columns.

15. A method of displaying autostereoscopic images, comprising:

generating a pixellated image using a display arrangement comprising a stack of layers, and

- passing the light of the pixellated image through a light controlling layer (50) to an autostereoscopic lens arrangement,

wherein the light controlling layer (50) has a refractive index lower than the refractive index of the materials on opposite sides of the light controlling layer.