Computer graphics processing and selective visual display system – Plural physical display element control system – Display elements arranged in matrix
Reexamination Certificate
1998-10-27
2001-12-11
Hjerpe, Richard (Department: 2774)
Computer graphics processing and selective visual display system
Plural physical display element control system
Display elements arranged in matrix
C345S102000
Reexamination Certificate
active
06329968
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a display device comprising a display panel with a first substrate provided with electrodes at the location of pixels, a second light-transmitting substrate and liquid-crystalline material between the two substrates, an illumination system which is situated on the side of the second substrate facing away from the liquid-crystalline material and which includes a light guide of an optically transparent material having an exit surface facing the display panel and a number of end faces, an entrance face for light being situated opposite at least one of said end faces, whereby the light can be coupled in at that end face of the light guide.
Such reflective display devices are used, for example, in portable equipment such as laptop computers, mobile telephones, personal organizers etc. With a view to saving energy, it is desirable that the light source can be turned off when there is sufficient ambient light.
A display device of the above-mentioned type is described in “Compact Front Lighting for Reflective Displays”, SID 96 Applications Digest, pp. 43-46. In said document a description is given of a light guide to which an optical compensator is added to preclude image distortion due to a groove structure (microprisms) on the viewer's side of the light guide. Image distortion can be attributed to the fact that the groove structure has different slopes, which results in multiple images. The groove structure is necessary to deflect light rays in the direction of the display panel. In said publication, multiple images are precluded by providing an optical compensator having a complementary groove structure.
In order to function properly, the air gap between both component parts (the light guide and the optical compensator) must have a small accurately defined thickness (obtained, for example, by means of spacers) throughout the plane. This requires a very accurate mounting step, while each local variation in thickness of the air gap gives rise to variation in optical behavior and image distortion.
SUMMARY OF THE INVENTION
It is an object of the invention to provide, inter alia, a solution to the above problem.
To achieve this, a display device in accordance with the invention is characterized in that a polarizer is situated between the light guide and the display panel, and the light guide comprises polarizing means for polarizing the incoming light.
The polarizer may be integrated in the display device.
The function of the polarizing means is, on the one hand, similar to that of the groove structure in the known device, i.e. deflecting light rays originating from the light source in the direction of the display panel, while, on the other hand, only polarized light is passed at the exit surface. The polarized light is subsequently passed by the polarizer (in this case the directions of polarization of the polarizing means and the polarizer are presumed to be identical). For this purpose, the polarizing means are preferably situated at the location of an exit surface of the light guide.
Reflected light which is passed by the polarizer is also passed, in the direction of the viewer, by the polarizing means and the other part of the light guide. As the polarizing means are integrated in the light guide, the above-mentioned mounting step can be dispensed with (and variation in optical behavior no longer occurs).
Light beams incident from the viewer's side, which are incident at an angle (with the normal) which is smaller than the critical angle at the light guide-air interface, are coupled out irrespective of their direction of polarization. The polarizer passes light of the proper polarization, which light is subsequently reflected by the reflective electrodes of the display panel and passed again by the polarizer and the light guide.
As a result of polarization, an unpolarized beam originating from the light source is separated into two mutually perpendicularly polarized beam components. Such a polarization separation is obtained, for example, by making the unpolarized beam impinge on an interface between a region with isotropic material of refractive index n and a region with anisotropic material of refractive indices n
o
and n
e
, where one of the two indices n
o
or n
e
is equal or substantially equal to n
p
. If an unpolarized beam is incident on such an interface, the beam component which does not experience a difference in refractive index at the transition between isotropic and anisotropic material, is passed undeflected, whereas the other beam component is deflected or reflected.
If n
p
is equal or substantially equal to n
o
, the ordinary beam is passed undeflected by an interface between isotropic material and anisotropic material, and if n
p
is equal or substantially equal to n
e
, such an interface passes the extraordinary beam undeflected.
Deflected beam components incident on the light guide-air interface at an angle smaller than the critical angle are coupled out again and the polarizer again passes light of the proper polarization, which is subsequently reflected by the reflective electrodes of the display panel.
Preferably, the direction of polarization of the polarizing means is the same as that of the polarizer. In that case, in display panels based, for example, on (twisted) nematic liquid-crystal effects and ferro-electric effects no light is lost. If the directions of polarization of the polarizing means and the polarizer are different, light loss can be precluded by using a phase plate.
The above-mentioned polarisation-separation can be achieved in various ways.
A first embodiment of the invention is characterized in that the light guide is provided with a layer, for example a coating, of a material which differs from that of the other part of the light guide, and in that the polarizing means are formed by a profiled interface between the layer and the other part of the light guide, the material of the other part of the light guide or of the layer being optically anisotropic, while the other material is optically isotropic.
A second embodiment is characterized in that the light guide is provided with a number of recesses comprising a material which differs from that of the other part of the light guide, and in that the polarizing means are formed by interfaces between the recesses and the other part of the light guide, the material of the other part of the light guide or the material in the recesses being optically anisotropic, while the other material is optically isotropic.
Such a light guide is known per se from WO 97/08582 (PHN 15.415).
In said document, light rays incident on the light guide-air interface at a surface situated opposite the exit surface of the light guide at an angle smaller than the critical angle are prevented from being coupled out by furnishing said surface with a reflector. However, such a reflector cannot be used in an illumination system for reflective panels.
To preclude such coupling-out of light rays, a further preferred embodiment of a display device in accordance with the invention is characterized in that coupling-in means are situated between a light source and the light guide, which means limit the angle between a coupled-in light ray and a plane of the light guide parallel to the exit face to maximally 15 degrees.
Preferably, for the optically isotropic part of the light guide, n
p
is equal or substantially equal to n
o
.
For polymers applicable in such a light guide it generally applies that n
e
is relatively high, and the above-mentioned choice can be realized more readily than n
p
being equal or substantially equal to n
e
. For the optically anisotropic material, preferably, an anisotropic polymer gel or an anisotropic polymeric network is selected.
These materials can very suitably be used as a birefringent material for applying the coating or filling the recesses, requiring only small quantities.
REFERENCES:
patent: 5477239 (1995-12-01), Bush et al.
patent: 5608550 (1997-03-01), Epstein et al.
patent: 5659410 (1997-08-01), Koike et al.
patent: 5719649
Broer Dirk J.
Cornelissen Hugo J.
Dinh Duc Q.
Hjerpe Richard
U.S. Philips Corporation
LandOfFree
Display device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Display device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Display device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2562697