Patent Grant
8205999
This application is the National Stage of international Application No. PCT/IB06/051824 filed on Jun. 8, 2006, which claims the priority to European Patent Application Ser. No. 05105218.1 filed on Jun. 14, 2005.
The present invention relates to multi view displays, i.e. displays arranged to simultaneously provide different image information depending on from which direction the display is regarded.
Such multi view displays are useful e.g. in automotive applications, where they can be mounted on the midconsole of a car and display different images for the driver and for the passenger.
A special case (dual view) is a display which has a low emission in the direction of the normal of the display, while providing a normal image at oblique angles (e.g. +/−30 degrees) to the left and right.
Such a dual view display can be constructed in several ways. A well known way is by introducing a barrier layer on top of a conventional display (e.g. an LCD). The barrier layer consists of a horizontal array of slits aligned in the vertical direction. These apertures allow light from one display pixel to be visible under viewing angles to the left of the display normal, and light from its neighbor pixel to be visible only under right oblique angles. In this way two independent images can be displayed: one to the left, and one to the right. An example of such a display is given GB 2403367.
However, in directions close to the display normal, both views will be visible, a phenomenon known as “cross talk”. An observer regarding the display from this angle will see the image of view1 and view2 merged into one image. This is undesirable, and can in automotive applications be a dangerous situation, as it can distract the driver.
To solve this problem, either the backlight needs to be modified so that it emits only into desired directions, or the display itself, so that it prevents transmission of light into undesired directions. Two possible solutions have been proposed:
a double color filter acting both as color filter and as barrier layer. This solves the problem but requires the use of new optical components that need to be aligned with existing components and is therefore expected to be expensive.
a specially designed LC mode that has the property of absorbing the light in the forward direction. By combining such an LCD with the above described barrier layer, one has solved the cross talk problem. However, this solution requires the use of a specially designed LCD.
It is an object of a first aspect of the present invention to overcome or at least reduce these problems, and to provide a dual view display with little or no cross talk.
This and other objects are achieved by a multi view display comprising a light emitting element including an optical cavity formed by a first and a second reflecting layer, at least said second reflecting layer being semi-transparent, and a light emitting layer arranged between said reflecting layers, wherein said optical cavity is designed so that light emitted in at least two preferred viewing directions has a higher intensity than light emitted in other directions. As one example, light emitted along a normal to the display has lower intensity than light emitted at a viewing angle with respect to said normal.
For the purpose of the present invention, the term intensity is used as the light intensity at one wavelength in case of (near) monochromatic emission, or as the integrated light intensity over relevant wavelengths involved in case of non-monochromatic light.
The invention is based on resonance phenomena in the optical cavity, making it possible to realize a light emitting element that intrinsically emits more light in the preferential viewing direction than in other directions. This emission profile makes the light emitting element very useful in a multi view display, reducing the problems of cross talk.
The desired emission profile is realized by tuning the optical properties of the optical cavity. The principal effect of the optical cavity on the light emission is to redistribute the photon density of states such that only certain wavelengths, which correspond to allowed cavity modes, are emitted in a given direction.
According to a preferred embodiment, the light emitting layer comprises an electroluminescent material. For example, the light emitting element can be an organic light emitting diode (OLED), based on e.g. small molecule organics (smOLED) or polymer organics (pLED).
A ratio between light intensity emitted along the normal and light intensity emitted in a given plane on both sides of the normal and at a predefined viewing angle with respect to said normal, can preferably exceed 2, and even more preferably exceed 5. A ratio around 13 has been shown to provide satisfactory results.
Preferably, emitted light intensity as a function of viewing angle in said plane has a maximum essentially in said viewing angle, and a minimum essentially in the normal direction. Such a design will increase the mentioned ratio.
The optical cavity preferably includes at least one layer having an index of refraction with a real part lower than 1.5, preferably lower than 1.45, and most preferably lower than 1.4. Such a low refraction index has been found to present advantageous conditions for designing the cavity.
The second, semi-transparent reflective layer preferably has a reflective coefficient larger than 0.5, preferably larger than 0.7 and most preferably larger than 0.8. It can be shown that such values for the reflective coefficients provide advantageous conditions for designing the cavity. A larger reflective coefficient results in an emission profile with a more accentuated peak, thus increasing the mentioned ratio. The first reflective layer is preferably a perfect reflector, i.e. reflective coefficient equal to one.
The light emitting material is preferably adapted to emit light within a narrow wavelength range, typically less than 100 nm and preferably less than 50 nm. In a special case, the light emitting material is adapted to emit monochromatic light. This makes design according to the above criteria easier to implement. However, the invention is by no means limited to monochromatic light, and the required light intensity distribution can be accomplished also with a spectral distribution of light. In this case, the above equations must be considered in the wavelength domain, and compared to the emitted spectrum.
The light emitting element can be adapted to act as a backlight, and the display can then further comprise a light modulating panel (e.g. an LCD) for modulating light emitted from said backlight. Alternatively, the light emitting element is constructed with a patterned pixel structure to form an addressable display panel controllable by a display driver.
It is an object of a second aspect of the present invention to improve the perpendicular viewing angle range of a multi view display, e.g. the vertical viewing angle range in a multi view display with horizontally differentiated views.
This aspect of the invention relates to a multi view display comprising means for generating multi view images, and an asymmetric diffuser arranged between the image generating means and a viewer, which asymmetric diffuser is adapted to scatter light essentially only in one direction. Such a diffuser has the effect to extend the emission profile in one direction, making it more elongated. By orienting the asymmetric diffuser correctly, this provides an improved angular viewing angle in a direction perpendicular to the angular spread of the multiple views of the display. If, for example, the display is arranged to have different viewing zones in the horizontal direction, the asymmetric diffuser can be oriented to extend the emission profile in the vertical direction, thus allowing greater variations in the vertical position of the viewer.
The second aspect of the invention can advantageously be combined with the first aspect, i.e. the means for generating multi view images can comprise an optical cavity. As light emitted from the optical cavity typically is circular symmetric, the asymmetric diffuser thus significantly improves the angular viewing range.
This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing a currently preferred embodiment of the invention.
a illustrates the desired viewing angle ranges for a dual view display.
b shows a diagram of emitted light intensity as a function of viewing angle corresponding to the performance in
The following description will be based on a dual view display, as an example of a multi view display. A dual view display can be used for automotive applications and can display different images in different directions. An example of an application, shown in
According to the illustrated embodiment of the present invention, the display is based on a light emitting element including an optical cavity, designed to have an emission profile suitable to realize the desired viewing zones. As an example, the optical cavity can be based on OLED technology. The light emitting element can comprise a multitude of individually addressable pixels, to be controlled by a display driver thereby forming the entire display. Alternatively, the light emitting element can act as a backlight in combination with suitable modulation technique (e.g. LCD).
In this architecture, the interference characteristics of the optical cavity are defined by a number of properties, the most important being the optical distance d between the reflecting surfaces 11, 12, the optical distance d1 between the location of the light generation in layer 14 and the reflecting surface 11 on the non-emitting side of the cavity, and the reflective coefficients r1 and r2 of the two reflecting surfaces.
The optical distances d and d1 are a function of the corresponding physical distances and the refractive indexes of the materials in the space 13. In the simple case where the light emitting material is the only material in the space 13, the optical distances are simply the physical distance multiplied by the refractive index of this material. When several layers of different materials are present, the optical distance becomes a more complex function.
Regarding the distance d1, it is important to note that light typically is generated only in a small fraction of the thickness of the light emitting layer. For example, in case of a pLED, the light emitting layer often has a thickness of about 100 nm, while light is only generated in the “recombination zone”.
The reflecting properties of the surfaces 11, 12 can be obtained by using metals or semitransparent conducting films possibly in combination with dielectric overcoats. Often, as shown below in
As an example,
Similarly, the optical cavity of a sub-pixel can be optimized for red and blue emission. In case a display comprises a white-light emitting organic material in combination with a color filter, the sub-pixels can also be optimized to emit light according to the desired angular emission profile.
It should be noted that light emission from an OLED is typically circle symmetric. As a consequence, the angular emission profile is obtained for all azimuthal angles, i.e. at a given distance from the origin of light, as illustrated in
In the following, some design rules and conditions for achieving the desired cavity modes will be described, with reference to
Two types of resonances occur in such an optical cavity: wide-angle resonance and multiple-beam resonance. Here, we will use the term resonance to indicate all constructive interference effects. The resonance will depend on the optical distance d between the reflecting surfaces and the position of the light emitting layer, i.e. the optical distance d1 between the location of light generation in the emission layer and the reflecting surface 11. In the expressions below, it is assumed that that there is only one medium present between the reflecting surfaces 11, 12, making the optical distance equal to the physical distance multiplied by the real part of the refractive index ne of this material.
Wide-angle resonance is related to interference of light emitted directly in the direction of the viewer (towards the semi-transparent electrode, 12), and light reflected from the opposite reflecting surface (dashed lines 25 and 26 in
where θ1 is the phase shift resulting from the reflection in surface 11, ne is the (real part of the) refractive index of the medium, and N is an integer. The internal propagation angle θ′ is given by Snell's law
sin θ=ne sin θ′. (2)
The emission enhancement is minimal when
Multiple beam resonance is the result of interference between light resulting from wide-angle interference that is reflected back and forth many times (dashed lines 27 and 28 in
where M is again an integer, and θ1 is the phase shift resulting from reflection at the semi-transparent surface 12.
Because typically −π<θ1, θ2<0, the lowest value for N and M is zero. As mentioned above, if other materials are present between the reflecting surfaces, in addition to the light emitting medium, Eqs. (1), (3) and (4) must be replaced by more complex equivalents, but the resonance phenomena are very similar, and the resonances for a certain λ and θ occur at similar values of the optical thicknesses.
An example of a desired intensity distribution as a function of viewing angle was shown in
Monochromatic Emission
In general, it is not possible to have a wide-angle resonance maximum exactly at θ=θview and a minimum exactly at θ=0. It is possible, however, to have a resonance maximum exactly at θ=θview and a minimum close to θ=0, or to have a resonance minimum exactly at θ=0 and a maximum close to θ=θview. In both cases the optical distance d1 must be near
where θ′view is related to θview by Snell's law as in Eq. (2). This follows from subtracting Eq. (1) with θ′=θ′view from Eq. (3) with θ′=0, and neglecting the small change in φ1 as a function of θ.
In order to have a resonance maximum exactly at θ=θview, an optical distance d1 must be selected that satisfies Eq. (1) with θ′=θ′view (N=1, 2, 3, . . . ) and is as close to {tilde over (d)}1 as possible. In order to have a resonance minimum exactly at θ=0, an optical distance d1 must instead be selected that satisfies Eq. (3) with θ′=0, again as close to {tilde over (d)}1 as possible.
The multiple-beam resonance maximum should coincide with the wide-angle maximum. Thus the optical distance d must be chosen so that Eq. (4) with θ′=θ′view is satisfied. Obviously, M≧N, and it has been found that it is advantageous to select M=N.
In practice it may be necessary to allow for certain tolerances, and it has been found that the numerical values of the mentioned parameters (except the integers) can be allowed to deviate from values exactly satisfying the given relationships by around 10%, but preferably not more than around 5%.
If the device is optimized as above, there is always a second maximum in the intensity distribution (see
Emission with Spectral Distribution
In this case, the problem should be considered in the wavelength domain.
Let λmax(θ) denote the wavelength corresponding to a certain resonance maximum (wide-angle with given d1 and N or multiple-beam with given d and M). If the cavity contains only the light emitting medium and the dependence of θ1 and θ2 on θ are neglected, it follows from Eqs. (1) and (4) that
λmax(θ)=λmax(θ)cos θ′. (6)
Thus the resonance maxima shift by
Δλmax=λmax(θ)(1−cos θ′view) (7)
to lower wavelengths if the viewing angle increases to θview.
i: the wavelength distance between the resonance maxima is approximately the same for wide-angle and multiple-beam interference, i.e. if N=M
ii: this distance is at least approximately the width of the spectrum Δspec. Assuming that the phase shifts θ1 and θ2 can be approximated as −π (the value for an ideal mirror) it leads to the requirement
where 600 nm is used as the characteristic value of λmax(0) for the relevant resonance.
Let λ0 be the wavelength so that the overlap between the spectrum and a typical resonance peak is maximal if the resonance maximum is at λ0. It should be clear that, in an optimal device, the optical thickness d must be chosen so that λmax(θview) for the relevant resonance is near λ0. We refer to λ0 as the dominant wavelength in this context.
The ratio of the emitted intensities at θ=θview and θ=0 is further optimized if the shift Δλmax is at least the high-wavelength half-width Δspec+ of the spectrum (the wavelength distance between the maximum and high-wavelength edge of spectrum). For a cavity filled primarily with LEP material, Δλmax will typically be much smaller than Δspec+. It can be increased by including a low-n layer in the cavity. Low-n here means that the material is transparent in the visible spectrum and has a refractive index n1 with Re n1<1.4 for at least a part of the visible spectrum. The low-n layer is added to the layers in the cavity that are needed for operation of the device and typically have refractive index n>1.4. The increased shift is the result of the relatively small propagation angle θ′ in the high-n material, as defined by Eq. (3) (with ne replaced by n1). It is favorable to give the required layers their minimum thickness, so that the low-n layer is as dominant for the average θ′ as possible. The upper limit for the shift is given by
750 nm (1−cos θview), (9)
where 750 nm has been used as limit for λmax(0). It follows from assuming n=1 in the entire cavity.
The optical thickness d corresponding to M-values satisfying Eq. (8) may be too small to allow inclusion of enough low-n material to significantly increase Δλmax. For a wide spectrum (as the example in
Because the FWHM of a multiple-beam resonance depends on the product |r1 r2|, it can be tuned with the reflection coefficient at the semi-transparent surface (remember that |r1| should always be large). A larger |r2| gives a narrower resonance and thus a better intensity ratio. A drawback is a lower maximum intensity because the maximum overlap between spectrum and resonance is reduced.
The resonance maxima occur at slightly different wavelengths for the two polarizations TE and TM. A complete optimization must therefore be a compromise that takes both contributions into account.
The intensity enhancement shown in
When the resonances shift with viewing angle, different parts of the spectrum are emphasized. This means that the reduction of the intensity is accompanied by a change in the color of the emitted light. This can be suppressed by color filters, which are known per se.
The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, it should be noted that there are many different OLED architectures that comprise an optical cavity and, therefore, can be tuned to emit light according to the desired angular emission profile. Moreover, both small-molecule and polymer based organics can be used for light generation. In order to further improve the ratio between light along the normal and along an oblique angle, a barrier can also be introduced to block all undesired light, for instance along the normal of the display. Examples of such a barrier are known per se.
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|---|---|---|---|---|
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| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2006/134519 | 12/21/2006 | WO | A |
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