Optical: systems and elements – Holographic system or element – With optical waveguide
Reexamination Certificate
2002-09-23
2004-06-15
Dunn, Drew A. (Department: 2872)
Optical: systems and elements
Holographic system or element
With optical waveguide
C359S015000, C362S035000
Reexamination Certificate
active
06750996
ABSTRACT:
The invention relates to a waveguide having an entry side face for coupling light into the waveguide and an exit surface for emission of waveguided light from the waveguide. The invention further relates to an edge-lit illumination arrangement comprising such a waveguide and a display device comprising such a waveguide or edge-lit illumination arrangement.
Waveguides of the type mentioned in the opening paragraph can be suitably used for supplying light from a light source arranged near an entry side face of the waveguide to a surface such as a display surface of, in particular, a non-emissive display device, such as a liquid crystal display (LCD), arranged near an exit surface of the waveguide. Typically, the exit surface extends parallel to the direction of waveguiding and is much larger in area than the entry side face. The light source being arranged on a side of the waveguide, a compact, in particular thin, lighting arrangement is obtained and display device comprising such can be obtained. Such an illumination arrangement is also referred to in the art as an edge-lit illumination arrangement.
In transmissive displays the light provided by the waveguide competes with ambient light to present the information displayed to the viewer and in this case the waveguide is typically used in a back light illumination arrangement in which case the display is arranged between the waveguide and a viewer. In a reflective display the waveguide may be used in a back or front light illumination arrangement. Since a reflective display makes use of ambient light to make the displayed information visible to the viewer, reflective displays are in particular preferred in low power applications such as hand held, palm top and other portable applications. A reflective display may be provided with an edge-lit illumination arrangement which need only be used if ambient lighting conditions are such to render the information presented by the display poorly visible using ambient light alone. Ambient light is understood to mean light provided by an external light source either natural or artificial.
In order to direct the light from the light source from an entry side face to an exit surface the waveguide comprises light outcoupling means. An example of such outcoupling means is a relief (micro)structured exit surface such as disclosed in patent application WO 99/22268. A disadvantage of such a relief structure is that the dimension of the relief structure elements is such that the relief structure as such is visible to a viewer, in particular when ambient light alone is sufficient to use the display. Using a volume hologram as the outcoupling does not have this disadvantage because the spatial modulation in refractive index recorded in the volume hologram has characteristic dimensions which cannot be resolved by the unaided eye.
A waveguide for use in a front illumination device having such a volume hologram is disclosed in U.S. Pat. No. 6,048,071. The front illumination device disclosed therein is mounted on the front surface of a reflective-type LCD. In order to provide polarized light to the LCD a polarizer is provided between the front illumination device and the LCD which absorbs one polarization of the ambient light or light from the front light. A similar front light arrangement is disclosed in JP-A-11-232919 and its English-language abstract.
It is an object of the invention, inter alia, to alleviate these drawbacks. In particular it is an object to provide a waveguide which can be suitably used in an edge-lit arrangement in particular one of the front light type and which, if used in such an edge-lit arrangement, provides an edge-lit arrangement which has a high brightness and/or makes highly efficient use of the light coupled at the entry side. Also, usage of the waveguide in accordance with the invention may result in an LCD of a simpler design by reducing the number of components thereof.
This object is achieved in accordance with the invention by a waveguide comprising an entry side face for coupling light into the waveguide, an exit surface for emission of light from the waveguide, and polarization-selective light outcoupling means for coupling out waveguided light through the exit surface polarization selectively, the polarization-selective outcoupling means comprising a volume hologram configured to selectively diffract waveguided light towards the exit surface.
The waveguide in accordance with the invention has polarization-selective outcoupling means which enables the waveguide to emit polarized light from its exit surface. If used in a back light illumination arrangement, this has the advantage that the polarizer for polarizing the light emitted by the waveguide can be dispensed with or can be made much thinner thus reducing the number of components and achieving further integration. Since polarizers contribute significantly to the total thickness of an LCD panel a significant reduction in thickness and weight is achieved. If used in a front light arrangement, wherein a polarizer is arranged between a viewer and the display cell, the polarized light emitting waveguide may be arranged between the polarizer and the display cell, in which case the polarizer helps to reduce the reflections coming off the front light when the display is lit by ambient light. Moreover, since the polarization-selective outcoupling means may be realized using non-absorptive means, ie diffractive, refractive, reflective and/or transmissive means which selectively couples out light of a first polarization while keeping the light of a second polarization captured in the waveguide, the waveguide in accordance with the invention is capable of making an edge-lit arrangement comprising such waveguide more efficient and/or more bright, if the waveguide comprises or is combined with means to at least partially convert the light having the second polarization into light having the first polarization and then offer the converted light again to polarization-selective outcoupling means. To some extent such recycling of waveguided light is inherent in any waveguide in accordance with the invention as imperfections, such as stress deformations which are generally inevitably always introduced to some extent during manufacture of the waveguide, cause the waveguide light to become depolarized as it traverses down the waveguide thus converting at least partially light of a second polarization to light of the first which converted is then offered to the polarization-selective outcoupling means further down the waveguide. Alternatively or additionally, other recycling means may be provided to increase the brightness and/or efficiency and/or polarization contrast of the light outcoupling of the waveguide.
The polarization-selective outcoupling means of the waveguide in accordance with the invention comprises a volume hologram configured to selectively diffract waveguided light towards the exit surface of the waveguide. The use of a hologram to couple out light from the waveguide has the advantage that, in contrast to micro relief patterned waveguides, it does not disturb the propagation of light incident on the volume hologram in transmission, an aspect which is of particular importance in front light applications. Furthermore, holograms allow good control over the angular distribution of the light coupled out in terms the principal direction(s) of outcoupling and/or the magnitude of the range of outcoupled angles. For example, the light coupled out can be made highly collimated or strongly diverging, and normal or near-normal outcoupling with respect to the exit surface may be achieved. Also, waveguided light can be coupled out selectively to a, typically one, side of the volume hologram (hereinafter also referred to as the front side) an aspect which is of particular relevance in front light arrangements as it allows selective direction of light to the display panel and away from the viewer.
Although any hologram configured to selectively diffract waveguided light towards the exit surface can be used in the polarization-sele
Bastiaansen Cees
Broer Dirk Jan
Cornelissen Hugo Johan
Jagt Hendrik Johannes Boudewijn
Boutsikaris Leo
Dunn Drew A.
Koninklijke Philips Electronics , N.V.
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