WO2012038856A1

WO2012038856A1 - Multi-view display device

Info

Publication number: WO2012038856A1
Application number: PCT/IB2011/053994
Authority: WO
Inventors: Marcellinus Petrus Carolus Michael Krijn
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2010-09-21
Filing date: 2011-09-13
Publication date: 2012-03-29
Also published as: TW201218179A

Abstract

A display device has a backlight arrangement which has a first backlight device for generating illumination in the form of a set of spaced lines. This is used to illuminate the display panel to project different images to different spatial locations and thereby provide multi-view output. A second backlight device is for generating illumination in the form of a continuous area of illumination covering the backlight area. This is for a single-view mode.

Description

Multi-view display device

FIELD OF THE INVENTION

This invention relates to a multi-view 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

Multi-view display devices can be for providing different views to different locations (for example a driver and his front seat passenger) or for generating an

autostereoscopic display output. This detailed description below is based on an

autostereoscopic display, but the concepts described apply equally to multi-view 2D displays.

A first example of an imaging arrangement for use in this type of display is a parallax barrier, for example with slits that are sized and positioned in relation to the underlying pixels of the display. In the case of an autostereoscopic display, 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. Hence, the viewer perceives different views of an image the left and right eye, creating the 3D experience. Alternatively, the barrier is such that the views are separated to such an extent that they end up at positions allowing different viewers to see one of the views to create a dual-view 2D display.

A drawback of this type of two-view 3D 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 eyes all the time.

The parallax 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.

The above described device provides an effective three dimensional display. However, it will be appreciated that, in order to provide stereoscopic views, there is a necessary sacrifice in the horizontal resolution of the device. This sacrifice in resolution is unacceptable for certain applications, such as the display of small text characters for viewing from short distances. For this reason, it has been proposed to provide a display device that is switchable between a two-dimensional mode and a three-dimensional (stereoscopic) mode.

One way to implement this is to provide an electrically switchable lenticular array. In the two-dimensional mode, the lenticular elements of the switchable device operate in a "pass through" mode, i.e. they act in the same way as would a planar sheet of optically transparent material. The resulting display has a high resolution, equal to the native resolution of the display panel, which is suitable for the display of small text characters from short viewing distances. The two-dimensional display mode cannot, of course, provide a stereoscopic image.

The loss of resolution is still a problem in the 3D mode. A major dilemma is caused by the fact that on the one hand a large number of views per angle is needed for a good 3D impression and on the other hand a small number of views is needed for a sufficiently high resolution (i.e. number of pixels) per view. A high number of views means the image resolution per view is reduced because the total number of available pixels of the display panel has to be distributed among the views. In the case of an n-view 3D display with vertical lenticular lenses, the perceived resolution of each view along the horizontal direction will be reduced by a factor of n relative to the 2D case. In the vertical direction the resolution will remain the same. The use of a barrier or lenticular that is slanted can reduce this disparity between resolution in the horizontal and vertical direction. In that case, the resolution loss can be distributed evenly between the horizontal and vertical directions.

There are a number of problems associated with the use of switchable lenticulars.

The material parameters of the LC layer and replica have to be chosen very carefully in order not to get a residual lens action in the 2D mode. There still may be a residual lens action when viewing the display at an angle. In other words, the 2D mode is compromised. The shape of the replica has to be very accurate and an LC-based switchable lenticular is relatively expensive.

There is therefore a need for an alternative method for realizing a single-view- multi-view switchable display not having the drawbacks outlined above.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an alternative single-multiview display device. The invention is defined by the independent claims. The dependent claims provide advantageous embodiments.

According to the invention, there is provided a multi-view display device as claimed in claim 1.

The device of the invention has a backlight arrangement which has a first backlight device for generating illumination in the form of a set of spaced lines. This is used to illuminate the display panel to project different images to different spatial locations and thereby provide multi-view output. A second backlight device is provided for generating illumination in the form of a continuous area of illumination covering the backlight output area. This is for a single-view mode.

This approach locates the measures to provide single- view-multi- view switching behind the display panel as part of the backlight design instead of in front of the display panel. When the device is operated in a single- view mode, both backlight devices are illuminated whereas in a multi-view mode only the first backlight device is illuminated.

Each backlight device can comprise an edge lit illumination panel. This means a mature technology can be used for the two backlight arrangements.

The set of spaced lines of the first backlight arrangement can be defined by painted areas on a surface of the panel or by surface contours on a surface of the panel.

The invention can be used for an autostereoscopic display device or for a multi-view display.

The invention also provides a method of operating a display device as claimed in claim 10.

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 shows a display device of the invention; and

Fig. 4 is used to show more clearly the mult i- view generation.

DETAILED DESCRIPTION

The known device 1 of Fig. 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 may also be of the passive type e.g. in case of smaller area displays.

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 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 (ITO) electrodes on their facing surfaces. Polarizing 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.

Fig. 1 also shows schematically a controller 13 for driving the display device.

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.

The skilled person will appreciate that a light polarizing means must 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 polarization. The light polarizing 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 different spatial locations.

The invention is based on the use of a striped backlight to illuminate different pixels in different directions, in order to generate multiple spatially separated views. In order to provide switchability between 2D (i.e. single-view) and 3D (i.e. multi-view) modes, a second backlight is provided.

Fig. 3 shows an example of a display of the invention.

In Fig. 3, a 2D-3D switchable display is shown based on a regular LCD panel 30 such as e.g. that described for the known device of Fig. 1 in combination with two backlights 32, 34.

Both backlights are based on a light-guide. Light of several LEDs 36 is coupled into each light guide from one or more edges. Light travels within this light-guide by total-internal-reflection (TIR) which is virtually loss-less. Light is coupled out of the light guide by means of an out-coupling structure. The out-coupling structure can consist of dots or lines of paint. Alternatively, it can consist of surface indentations having a certain shape or roughness.

For the first edge-lit light-guide 32, enabling a 3D mode, the out-coupling structure consists of a pattern of thin parallel lines 33 spaced apart at some distance (pu_ne in Fig. 3). The result will be a pattern of line sources. These line sources in combination with the pixels of the LCD panel enable rendering 3D: the location of each pixel relative to the location of an underlying line source determines a view direction. Each pixel corresponds to several similar views (equivalent to the repeated view phenomenon arising in lenticular arrangements). There are cones of views with unique views within each cone. The angular size of each cone of views is (by approximation): Q_r = n^- c d

Here, n is the effective index of refraction of the material in between the pixels and the line sources whereas d is the physical distance between pixels and line sources.

Fig. 4 shows more clearly how the multiple views are formed, for 3 views, VI , V2, V3. In this example, the pitch between the light lines is an integer multiple of the (sub-) pixel pitch, so that each light line generates the same set of angles to the pixels above. In this way, the same set of illumination directions is generated from each light line. The dotted lines show the view repetitions which arise in the next viewing cone. However, the pitch does not necessarily have to be an integer times the (sub-) pixel pitch; a non- integer number is also allowed. This is similar to the case where there is a lenticular in front of a display panel having a pitch that is a non- integer times the sub-pixel pitch. This helps to reduce the moire effect caused by the periodic nature of the pixel array with black matrix and the lenticular array.

The overall width of the field of view will be limited by the total internal reflection within the light guide - in that beyond a given angle, the light from the line light source will not escape from the light guide, and will continue a total internal reflection path within the light guide, until it is again scattered by the light source. The rays that leave the light-guide will leave it up to angles of 90 degrees. When travelling through the display panel these rays are refracted towards a direction normal to the panel and do not fulfill the condition for TIR. Thus, all rays that leave the light guide will be able to travel through the panel.

In case the line sources are parallel to the columns of pixels, the number of unique views equals

where p_pixei is the sub-pixel size. In case the line sources are slanted with respect to the columns of pixels, this number is larger (e.g. in case of slant 1/6, the number is N_view=2 pn„_e/p pixel).

The width of the line sources should be small (wn„_e in Fig. 3). In practice, it should not exceed the sub-pixel pitch p_pixei. If the line source width is too large, the individual 3D views will become broadened to a level that is unacceptable.

The second edge-lit light-guide 34 enables a 2D mode, and resembles the first light-guide 32, the difference being that it provides a uniform illumination of the LCD panel. The out-coupling structure on this light-guide is such that a uniform illumination is obtained. The first light-guide 32 is substantially transparent for the light produced by the second light- guide.

For the first light-guide 32, light emitted by the line sources should be emitted towards the LCD panel only: light that is emitted into the wrong direction can scatter from the out-coupling pattern of the second light-guide and result into a loss of contrast in each 3D view.

One way to achieve this is to use white paint for the out-coupling structure that is thick enough not to let light through or is coated with an absorbing or reflective layer.

For the first light-guide 32, the width of the light-emitting lines may vary across the light-guide in order to ensure that each line produces the same amount of light. In case the width of all lines is equal, lines furthest away from the LEDs will produce less light than others since at that location there is less light left inside the light-guide.

For the first light-guide 32, although preferred, the out-coupling structure does not necessarily have to consist of straight lines. The lines may be staggered. The structure may also be a two-dimensional pattern of line segments.

A diffuser may be located in between the two light-guides 32,34 to provide a more uniform illumination in the 2D mode.

In the 2D mode, the LEDs of the first light guide 32 used for the 3D mode should be driven such that the intensity produced by the light-emitting lines of the first light- guide 32 equals that of the light emitted by the second (2D mode) light-guide 34. This ensures that the light-emitting lines of the first light guide 32 remain invisible in the 2D mode.

The second light-guide 34 can be any common backlight technology delivering a uniform illumination, including technologies that do not require a light-guide.

The invention thus provides a switchable multi-view display having a regular backlight for a single-view (e.g. 2D) mode and between this backlight and an LCD panel there is an array of closely spaced narrow line sources. Preferably, the array of line sources is produced by an edge-lit light-guide having line-shaped out-coupling structures. These produce light directed towards the LCD panel and not in the opposite direction, towards the regular backlight.

In the 3D mode, the backlight responsible for the 2D mode is switched off, whereas in the 2D mode, the line sources responsible for the 3D mode are switched such as to emit a light level that masks their presence.

The switching between modes can be implemented by the controller (13 in Fig. 1) and under the control of a computer program. The mode switching can be manually performed by the user, or it can be implemented automatically based on detection of the format of the image content being displayed.

One potential issue is the diffraction of light by small-sized pixels, which could result in broadened views.

Due to diffraction, the angular extent of a light ray (a collection of photons travelling in the same direction) will be broadened by an amount:

λ

AQ_D = c with c~l-2.

P pixel

Here, λ is the wavelength of the light and c is some constant of order unity. The angular width of an individual view on the other hand is:

P pixel

Δθ V j

a

where n is the effective index of refraction of the material in between the pixels and line sources and d is the physical distance between the pixels and line sources (see Fig. 3).

It is required that the view broadening caused by diffraction does not exceed the width of a view, in other words Δ(¾< Αθγ. Thus, it is required that:

2 d

P · pixel > θ - n

Together with a viewing cone size being: θ = n Pline

^C d

With piine being the pitch of the line-sources and the number of views being

(for the example of no slant), the result is:

N.,„„

P pixel > c λ -

As an example if c=2, =450 nm (worst case: blue light), N_view=l 5, and θσ=20°. In that it is required that that p_pixei > 39 μιη. This is satisfied by current pixel dimensions. Thus, diffraction will not play a serious role in practice.

The example above uses a liquid crystal display. However, the advantages of the invention can be obtained when other illuminated display technologies are employed. The invention applies to any display arrangement which uses an illumination source and a shutter- type pixellated display.

The invention has been described in connection with autostereoscopic displays. The invention can also be applied to multi-view displays in which different images are provided to different spatial locations; but these generally are different monoscopic (2D) images for different viewers at different positions although they may also be different stereoscopic images for the different viewers or a combination. Such displays are often referred to as dual view displays or split screen displays.

As mentioned above, the light out-coupling structure can be a surface coating such as paint or else a surface patterning or shaping, such as triangular grooves (to change the reflection angle and thereby permit escape from the light guide) or surface roughening for example by sandblasting. Alternatively, stripes of light emitting material may be used, such as phosphor (yellow) or organic luminescent material. In this case, blue light could be coupled into the light guide, and white light could be generated and released from the light guide.

The preferred implementation has both light guides on for the single- view (2D) mode. However, this is not essential, particularly if the line sources of the first backlight are small enough that the shadow they create of the second backlight is not perceptible by the viewer. If the shadow is perceptible, the first backlight is turned on to regenerate a uniform illumination field.

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. A single processor or other unit may fulfill the functions of several items recited in the claims. 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. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.

Claims

CLAIMS:

1. A multi-view display device, comprising:

a backlight arrangement (32,34); and

a display panel (30) for modulating the light output from the backlight arrangement (32,34) and comprising a plurality of pixels;

wherein the backlight arrangement comprises:

a first backlight device (32) for generating illumination in the form of a set of spaced lines for illuminating the display panel (30) to project different images to different spatial locations; and

a second backlight device (34) for generating illumination in the form of a continuous area of illumination covering the backlight arrangement area,

wherein the first backlight device (32) is between the display panel (30) and the second backlight device (34), and outside the area of the spaced lines is transparent to the light of the second backlight device.

2. A device as claimed in claim 1, wherein the device is operable in a single- view mode, in which both backlight devices (32,34) are illuminated and a multi-view mode in which only the first backlight device (32) is illuminated.

3. A device as claimed in claim 2, comprising a display controller (13) for controlling the mode of operation.

4. A device as claimed in any preceding claim, wherein the first backlight device (32) comprises an edge lit illumination panel.

5. A device as claimed in claim 4, wherein the set of spaced lines are defined by painted areas on a surface of the illumination panel.

6. A device as claimed in claim 4, wherein the set of spaced lines are defined by surface contours on a surface of the illumination panel.

7. A device as claimed in any preceding claim, comprising an autostereoscopic display device.

8. A device as claimed in claim 7, for generating at least three individual 2D views.

9. A device as claimed in any one of claims 1 to 6, wherein the device is dual view display device capable of providing at least two different 2D and/or 3D images to two different viewers positioned at different positions in front of the display.

10. A device as claimed in any preceding claim, wherein the display panel (30) comprises a liquid crystal display.

11. A method of operating a display device for providing both a single- view output and a multi-view display output, comprising:

in a first, multi-view mode, controlling a first backlight device (32) of a backlight arrangement to generate illumination in the form of a set of spaced lines for illuminating the display panel (30) to project different images to different spatial locations; and

in a second, single- view mode, controlling a second backlight device (34) of the backlight arrangement, behind the first backlight device (32) with respect to a display panel (30), to generate illumination in the form of a continuous area of illumination covering the backlight area,

wherein in each mode, the light output from the backlight arrangement (32,34) is modulated by the display panel.

12. A method as claimed in claim 11, wherein in the second, single-view mode, the first backlight device (32) of the backlight arrangement is controlled to generate illumination in the form of a set of spaced lines.

13. A method as claimed in claim 11 or 12, comprising generating an

autostereoscopic display image in the first mode.

14. A method as claimed in claim 13, for generating at least three individual 2D views.

15. A computer program comprising computer program code means for implementing the method of any one of claims 8 to 14 when said program is run on a computer.