Illumination – Revolving
Reexamination Certificate
2001-12-21
2003-11-11
O'Shea, Sandra (Department: 2875)
Illumination
Revolving
C362S337000, C362S330000
Reexamination Certificate
active
06644824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light unit using a point light source, and a liquid crystal display using the light unit as a front light.
2. Description of the Related Art
Reflection type liquid-crystal displays are notably mounted in portable electronics such as cellular telephones, personal digital assistants (PDA), and notebook personal computers because they allow a user to clearly view images in a light place than transmitted type liquid-crystal displays. The reflection type liquid-crystal display has a surface light source device as a light unit to provide against insufficient ambient light if it is used outdoors at night.
When an attempt is made not to see moiré stripes by setting a desired positional relationship between the arrangement of recesses and projections forming a prism surface of a light guide panel for use in a light unit and the direction in which pixel electrodes reflecting light are arranged, a problem occurs if a point light source is used to provide light incident on the light guide panel. The causes of this problem will be sequentially described below.
As shown in
FIG. 18
, a light unit
102
in a reflection type liquid-crystal display
101
is arranged on the front surface side of the display which is opposite an observer relative to a reflection type liquid-crystal panel
22
based on, for example, a TFT (Thin Film Transistor) method, in order to irradiate the liquid-crystal panel
22
with illumination light, while transmitting reflected light from the liquid-crystal panel
22
, through the liquid-crystal panel so that the observer can view the light. This is called a “front light”. Line light sources using fluorescent lamps or the like have been used for the front light.
As shown in
FIGS. 18 and 20
, the front light
102
has a fluorescent lamp
104
shaped like a thin tube, a reflector
105
that covers three peripheral sides of the fluorescent lamp
104
in the longitudinal direction thereof to reflect light from the fluorescent lamp
104
to emit band-shaped light through an open side of the reflector, and a light guide panel
7
comprising a light transmitting material and shaped like, for example, a plate and which receives direct light from the fluorescent lamp
104
or reflected light from the reflector
105
on a facet
4
and converts the received incident light into planar light to reflect this light to the liquid-crystal panel
22
, while transmitting and guiding the reflected light from the liquid-crystal panel
22
to the observer.
The light guide panel
7
has a prism surface
6
on one side thereof which has a stripe-like pattern of recesses and projections forming a plurality of linear parallel ridge lines, and a flat light applied surface
5
on the back surface thereof. Further, as shown in
FIG. 19
, which is an enlarged view of a portion E of
FIG. 18
, which is a side view of the liquid-crystal display
101
, the prism surface
6
has, for example, steep slopes
12
and gentle slopes
13
which are alternatively formed. The light guide panel
7
receives band-shaped light from the fluorescent lamp
104
through a facet
4
side, converts the incident light into planar light at the prism surface
6
, and then reflect it to the liquid-crystal panel
22
. The light impinging on the liquid-crystal panel
22
is reflected by the liquid-crystal panel
22
, transmitted through the light guide panel
7
again, and then emitted.
On the other hand, the liquid-crystal panel
22
has pixels driven by, for example, the TFT method. As shown in
FIGS. 21 and 22
, the liquid-crystal panel
22
has a TFT substrate
24
having a large number of TFTs
111
and pixel electrodes
112
formed in a matrix, an opposite substrate
25
fixed opposite the TFT substrate
24
via a clearance of several-pm size and having a colored layer (color filter)
115
formed thereon, a liquid crystal layer
26
sealed in the clearance, and a polarizing plate
27
and a quarter wavelength plate
28
both disposed outside the opposite substrate
25
. The pixel electrode
112
is also used as a reflection member.
Moiré stripes, interference stripes resulting from overlapping of groups of parallel lines arranged at a predetermined pitch, may be viewed depending on how the ridge lines on the prism surface
6
overlap the pixel arrangement of the liquid-crystal panel
22
, thereby degrading display quality. The reason why these moiré stripes are viewed will be described below. The stripe-shaped recesses and projections of the prism serve to concentrate light in one direction, thereby creating small differences in brightness along the ridge lines of the recesses and projections. On the other hand, if the pixel electrodes of the liquid-crystal display are used to reflect light, some of the clearances between adjacent pixels fail to reflect light. Since the liquid panel has the pixels arranged in a matrix and composed of the TFTs
111
and the pixel electrodes
112
, as shown in
FIG. 21
, those areas which do not reflect light appear like stripes. If the ridge lines of the prism cross the arrangement direction of the pixels at a small angle, the above described brighter and darker areas are likely to interfere with each other when crossing each other, thereby causing moiré stripes as interference stripes to be viewed. These moiré stripes are most noticeable when the ridge lines
110
of the prism surface
6
are slightly offset from the pixel arrangement direction in the horizontal or vertical direction of the display.
Moiré stripes are not seen by paralleling the ridge lines of the prism and the pixel arrangement direction or setting these directions to cross each other at a large angle. However, the prism is provided in the light guide panel, the pixels are provided in the liquid-crystal panel, and the light guide panel and the liquid-crystal panel are separate components constituting the liquid-crystal display. Accordingly, the ridge lines of the prism and the pixel arrangement direction are likely to cross each other at a small angle in spite of an attempt to parallelize these directions. Thus, instead of parallelization, the ridge lines and the pixel arrangement direction may be set to cross each other at a large value.
Thus, it has been contemplated that the ridge lines may be formed at about 23° from the direction in which the pixels are arranged in a matrix. Such a technique is described in, for example, “Technological Trend of Front Lights” by Akira TANAKA (Monthly Display, June 1999, p. 48 to 53) and Japanese Patent Laid-Open No. 2000-155315. The direction in which the pixels are arranged in a matrix is generally parallel with the longitudinal direction of the fluorescent lamp. Accordingly, as shown in
FIG. 20
, the ridge lines
110
are formed at an angle &thgr; of about 23° from the longitudinal direction of the fluorescent lamp
104
and at a predetermined pitch p0.
However, if the fluorescent lamp is used as a light source, it has high power consumption and requires an inverter that generates a high voltage for lighting. Accordingly, the fluorescent lamp is an obstacle to the reduction of the size and weight of the display. To meet the demand for the reduction of the power consumption, size, and weight the display, a technique has been proposed which converts light from a point light source composed of, for example, white LEDS (light emitting diodes) into false linear light using a light guide, as described in Japanese Patent Laid-Open No. 2000-11723 or the like. Japanese Patent Laid-Open No. 10-260405, which relates to a back light, also describes the technique of converting light from LEDs as a point light source into false linear light.
A front light using such a point light source will be described with reference to, for example, Japanese Patent Laid-Open No. 2000-1723, mentioned above. As shown in
FIGS. 23 and 24
, a front light
201
has a point light source
2
a
composed of, for example, white LEDs, a light guide
3
that converts light emitted from the point light source into band-shaped l
Choate Hall & Stewart
NEC LCD Technologies Ltd.
O'Shea Sandra
Sawhney Hargobind S
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