Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
2001-04-30
2002-11-19
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S015000, C349S062000, C349S063000, C349S064000, C349S095000, C349S104000
Reexamination Certificate
active
06483613
ABSTRACT:
TECHNICAL FIELD
The present application relates to a reflective display device such as a reflective liquid crystal display device (LCD). It particularly relates to a reflective display device having a light source to provide illumination, for example in poor light condition. It also relates to a light source, particularly but not necessarily for use with a display device.
BACKGROUND ART
Reflective LCDs are known in the art. For example, the “super-mobile” high reflectivity LCD produced by Sharp Kabushiki Kaisha essentially comprises a polarizer, waveplates, and a liquid crystal layer disposed in front of a reflective layer. The reflective layer has a metallized non-symmetric surface relief structure, which substantially preserves the polarization of light incident on the reflective layer so as to maximize the contrast of the display. The reflective layer is a “high-gain” reflective diffuser using a non-Lambertian surface relief structure.
Such a reflective LCD can use ambient light, so that the power consumption of the LCD is reduced compared to an LCD that comprises a light source provided behind the liquid crystal layer. This is important for portable display equipment such as a personal digital assistants (PDA), camcorders, portable computing equipment, and digital cameras.
Such a conventional reflective LCD does have a number of disadvantages. Clearly, the display will work poorly in dimly lit environments. Moreover, the color balance of the display will be influenced by the color spectrum of the ambient light. Thus, in sunlight, the display device may work well, but in fluorescent or particularly tungsten lighting, the color balance will be degraded. If the LCD is provided with color filters, these may also affect the color balance of the display.
A further disadvantage is that a high-gain reflective layer will work best when the light incident on the LCD is highly collimated overhead lighting incident on the screen of the LCD at a defined angle. Thus, the display position may have to be fixed by the user of the LCD to obtain the best performance, and this is undesirable. It can thus be seen that it would be advantageous to provide a reflective LCD with an auxiliary light source, to increase the quality of the displayed image and also to allow the LCD to operate in conditions of low ambient light.
As described above, a conventional reflective LCD has a reflective layer that is a high-gain reflective diffuser. If a light source is placed close to the surface of the LCD, a number of effects will be produced. First, the gain of the reflective layer will mean that the observer will see illumination structure across the surface of the LCD. This illumination structure will depend on the profile of illumination by the light source, and on the gain of the reflective layer.
Secondly, some light from the light source will be specularly reflected by the front surface of the LCD and from layers within the LCD. If the light source and a user are positioned such that light from the light source is specularly reflected towards the user, this will reduce the contrast of the display and will also be uncomfortable for the user. In order to prevent the user from seeing the specular reflection of the light source, the light source may have to be displaced laterally, i.e., offset, from the display surface. Displacing the light source in this way does, however, introduces a further problem. The LCD may well provide the highest display contrast for light within well-defined input and output cones. If the light source is displaced as described above, the input light may well no longer lie within the preferred input cone to the LCD and this will reduce the imaging efficiency.
FIGS. 1A and 1B
show a prior art reflective LCD. Specifically,
FIG. 1A
is a cross-sectional view of a prior art reflection LCD device with a holographic element for brightness enhancement in front illumination, and
FIG. 1B
illustrates the incident and reflective light for the device of FIG.
1
A.
The illustrated device is based on a glass substrate
1
on which are disposed, in sequence, a reflective layer (internal mirror)
2
, a liquid crystal layer
3
, color filters
4
, an upper glass substrate
5
, a polarizer
6
and a transmission hologram
7
. Electrodes to drive the liquid crystal layer
3
are provided on the glass substrates
1
and
5
.
The transmission hologram
7
is provided to enhance the brightness of the display. The LCD is illuminated by a distant light source, positioned at 34″ offset from the normal axis of the display as shown in FIG.
1
B. Light that is specularly reflected from the front surface of the display, or from internal elements of the LCD, is reflected at an angle of 34″ to the normal to the display.
The transmission hologram
7
deflects light that is incident upon it. Thus, some of the light that passes through the liquid crystal layer
3
and is reflected by the reflective layer
2
is reflected back towards an observer much closer to the axis of the display than the specularly reflected light. As shown in
FIG. 1B
, the light from the display is reflected back at an angle of 14″ to the normal axis, whereas the specularly reflected light (“glare”) is reflected at 34″ to the normal axis. The brightness of the LCD is thus increased, since the display light is angularly separated from the glare.
The prior art display shown in
FIG. 1A
requires to be illuminated with a distant light source that appears collimated at the liquid crystal panel, and it would not function efficiently if the light source were close to the LCD.
U.S. Pat No. 5,663,816 discloses a transmissive LCD in which a reflective holographic directional diffuser is provided at the back side of the LCD. The holographic diffuser is transparent for illumination by a back light, but directionally reflects light when the LCD is illuminated by an external overhead source. The reflection is conical about the axis perpendicular to the reflection side of the LCD.
FIG. 2
shows a further prior art reflective LCD with a holographic light control film. This prior art LCD is disclosed in T. Hotta et al., SPIE Proc., Vol.329, Practical Holography XII, pages 190-195, 1998.
The LCD of
FIG. 2
comprises a liquid crystal layer
15
which is disposed between two glass substrates
13
,
17
provided with electrode layers
14
,
16
for applying voltages to the liquid crystal layer
15
. A first polarizer
12
is provided behind the lower glass plate (substrate)
13
, and a second polarizer
18
is provided above the upper glass plate (substrate)
17
. A volume reflection hologram (holographic light control film)
11
is provided behind the lower polarizer
12
, to concentrate reflected light of a-single color into a specific viewing area (the prior art LCD shown in
FIG. 2
is a monochromatic LCD).
In the prior art LCD shown In
FIG. 2
, the holographic film
11
is separated from the liquid crystal layer
15
by the lower glass substrate
13
, and this will have a relatively large thickness. The spatial separation of the holographic film
11
and the liquid crystal layer
15
will introduce parallax into the display, and this will reduce the resolution of the LCD.
The prior art device described in U.S. Pat. No. 5,663,816 will also suffer from parallax problems.
U.S. Pat. No. 5,659,508 discloses an LCD viewable under ambient light comprising a liquid crystal panel, and a holographic reflective diffuser positioned behind the liquid crystal layer. The diffuser is made as a transmission hologram, and a light-reflective layer is deposited on the rear of the holographic diffuser. Such a holographic element is cheaper to manufacture as a broad band reflector than the other prior art holographic reflectors described above. However, the prior art device described in U.S. Pat. No. 5,659,508 again requires illumination with collimated light, and is not suitable for use with a closely positioned light source or with a high resolution color panel.
FIGS. 3A and 3B
illustrate further prior art reflective LCDs. These are provided
Khazova Marina Vladimirovna
Woodgate Graham John
Renner , Otto, Boisselle & Sklar, LLP
Sharp Kabushiki Kaisha
LandOfFree
Reflective display device and a light source for a display... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Reflective display device and a light source for a display..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Reflective display device and a light source for a display... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2936744