Optics: measuring and testing – By polarized light examination
Reexamination Certificate
2001-01-29
2004-06-29
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
By polarized light examination
C356S630000
Reexamination Certificate
active
06757062
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and a device for measuring a thickness of a birefringent body having a birefringent index An that is uniform in the thickness direction, such as an alignment-treated liquid crystal layer included in a liquid crystal display element.
2. Description of the Background Art
Liquid crystal display elements are utilized as displays in significantly wide-ranging areas, since they have small power consumption and small size, and are lightweight. In recent years, color liquid crystal display elements of a reflection-type have, among others, come into practical use, and the demand therefor has rapidly expanded.
A reflection-type color liquid crystal display, a conventional reflection-type liquid crystal display and a conventional transmission-type liquid crystal display are herein described for their respective characteristics. A conventional transmission-type liquid crystal display
111
includes, as shown in
FIG. 20
, a liquid crystal cell
100
constituted by a pair of transparent substrates
12
with outer sides interposed by polarizing plates
14
or elliptical polarizing plates. A voltage applied to a liquid crystal layer
11
changes an alignment of liquid crystal molecules and controls an polarization state of light transmitting through the liquid crystal layer. The light transmits through polarizing plate
14
of an exit side, with the controlled polarization state, so that a difference in brightness is generated in accordance with the polarization state, resulting in a desired display. Further, a conventional transmission-type liquid crystal display
112
also uses, as shown in
FIG. 21
, liquid crystal cell
100
which is basically the same as that of the transmission-type, and a reflector
15
is further attached to an outer side of polarizing plate
14
of a backside, for displaying.
However, along with finer picture elements attained in recent years, a conventional method in which reflector
15
is attached to the outside for displaying on conventional reflection-type liquid crystal display
112
have caused a problem in that an image is doubled due to a parallax generated by the thickness of substrate
12
and polarizing plate
14
between reflector
15
and liquid crystal layer
11
. Particularly when it comes to a color display, the doubled image causes color mixture, which significantly lowers a display quality. Further, when the color display is performed using a micro color filter, light entered at an angle passes through color filters of different colors at an incoming side and an outgoing side, lowering color saturation and thereby greatly degrading color reproduction property.
To solve this problem, a configuration is proposed, as shown in
FIG. 22
, in which reflector
15
is disposed within liquid crystal cell
101
. Because this configuration can suppress the doubled image due to the parallax and degrading of the color reproduction, reflection-type color liquid crystal display
113
mainly uses such a configuration in which a reflector is disposed within liquid crystal cell
101
. In reflection-type color liquid crystal display
113
, light is reflected by reflector
15
within liquid crystal cell
101
, which is different from conventional reflection-type liquid crystal display
112
in which light transmitted through liquid crystal cell
100
is reflected after it completely exits from the liquid crystal cell.
Further, in reflection-type liquid crystal displays
112
and
113
, light passes through the same liquid crystal layer
11
twice, so that the thickness of liquid crystal layer
11
has greater effects on the display quality, compared to that of a transmission-type liquid crystal display. This is because a magnitude of a change provided by a member having birefringence to the polarization state of the light entered thereto is proportional to the birefringent index (also referred to as “refractive index anisotropy”) and the thickness of the member.
Thus, measurement of the thickness of liquid crystal layer
11
, i.e. a so-called “cell gap,” for measuring the thickness and uniformity has more significant meaning than the case with the conventional transmission-type liquid crystal display, in order to maintain the display quality of the reflection-type color liquid crystal display.
A conventional technology for measuring a thickness of a liquid crystal layer (hereinafter referred to as “cell gap”) is a measurement method disclosed in Japanese Patent Laid-Open No. 4-307312 (hereinafter referred to as “conventional method 1”). In the conventional method 1, light enters into a liquid crystal cell via a polarizer arranged in a direction which is +45° rotated from an allignment direction of an entrance light side of the liquid crystal cell. Further, exit light transmitted through the liquid crystal cell exits via a polarizer arranged in a direction which is +45° rotated from an alignment direction of an exit light side of the liquid crystal cell, to measure an intensity of the exit light. An operation is performed from a value of a wavelength at which the measured exit light intensity assumes a maximal value or a minimal value, to find a value of a cell gap. In the conventional method 1, the operation is performed using
&Dgr;
n·d=&lgr;
o
·(
m
o
2
−&THgr;
2
/&pgr;
2
)
1/2
as a condition of the maximal intensity, and
&Dgr;
n·d=&lgr;
o
((
m
o
−½)
2
−&THgr;
2
/&pgr;
2
)
1/2
as a condition of the minimal intensity. The respective variables represent the values below.
&Dgr;n: birefringent index (&Dgr;n=extra ordinary index n
e
−ordinary index n
o
)
&lgr;
o
: wavelength to be maximum or minimum
m
o
: degree
&THgr;: twist angle of the liquid crystal layer
An example of a conventional technology for measuring the cell gap by utilizing reflected light is a measurement method disclosed in Japanese Patent Laid-Open No. 10-232113 (hereinafter referred to as “conventional method 2”). In the conventional method 2, mercury lamp light is projected on a substrate to fluorescence-excite alignment films for liquid crystal alignment formed at inner sides of upper and lower substrates, and images are formed for the radiance on a CCD (Charge-Coupled Device) by a lens to measure the cell gap by calculating a distance between the alignment films of the upper and lower substrates from the distance between the images.
The conditional equations by which the exit light intensity is maximum or minimum, used in the conventional method 1, are the equations used only for the transmission-type liquid crystal cell. That is, it is assumed that the entered light passes through the liquid crystal layer only once during which the light is polarized. In the reflection-type liquid crystal cell, however, the light passed through the liquid crystal layer and was polarized is reflected at the reflector, and again passes through the liquid crystal layer for further polarization before exiting. Therefore, the operation for the conventional method 1 cannot be applied, as it is, to the reflection-type liquid crystal cell.
Further, there may be a reflection-type color liquid crystal display, in which a portion of the reflector not used for display, for example, a portion corresponding to a gap between electrodes, is removed by etching or the like. In such a case, the measurement of the cell gap appears to be possible by the conventional method 1, by utilizing transmitted light leaking from the portion where a reflection layer is removed. However, etching of the reflector generates a step in the liquid crystal layer between an etched portion and a nonetched portion, resulting in a possible value difference between the etched portion and a picture element portion used for display. Thus, there is a need, after all, for measuring the cell gap of the picture element portion. Thus, it is required to use the reflection light rather than the transmission light to measure the cell gap.
The conventional method 2 in which the cell gap is measured by using the r
Conlin David G.
Edwards & Angell LLP
Manus Peter J.
Sharp Kabushiki Kaisha
Smith Zandra V.
LandOfFree
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