Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-10-28
2004-12-07
Pert, Evan (Department: 2829)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S161000
Reexamination Certificate
active
06828816
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a method and apparatus of measuring and adjusting a response speed of a liquid crystal display device for automatically establishing an adjusted driver voltage according to modulating data.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) device controls light transmittance of individual liquid crystal cells according to a video signal, thereby displaying an image. An active matrix LCD device includes a switching device for the individual liquid crystal cells, thereby displaying dynamic images (i.e., moving pictures). Thin film transistors (TFT's) are commonly used in the active matrix LCD as the switching devices. However, LCD devices are disadvantageous since they have relatively slow response characteristics due to inherent electro-mechanical properties of the liquid crystal material, such as viscosity and elasticity. Equation (1) demonstrates the dependency of the response characteristics upon the electro-mechanical properties of the liquid crystal material during a period of increasing applied voltage:
&tgr;
r∝
&ggr;d
2
/&Dgr;&egr;|V
2
a
−V
2
F
| (1)
wherein &tgr;
r
is a rise time when a voltage is applied to liquid crystal molecules of the liquid crystal material, V
a
is the voltage applied to the liquid crystal molecules, V
F
is a Freederick transition voltage when the liquid crystal molecules begin an inclined motion, “d” is a cell gap of the individual liquid crystal cells, and &ggr; is a rotational viscosity of the liquid crystal molecules.
In addition, Equation (2) demonstrates the dependency of the response characteristics upon the electro-mechanical properties of the liquid crystal material during a period of decreasing applied voltage:
&tgr;
f∝
&ggr;d
2
/K (2)
wherein &tgr;
f
represents a fall time when the liquid crystal molecules return to an initial position by an elastic restoring force after the voltage applied to the liquid crystal molescules is removed, and K is an elastic constant of the liquid crystal molecules.
FIG. 1
is a schematic diagram of a response speed measuring device employing an oscilloscope according to the related art. In
FIG. 1
, a response speed measuring device employing an oscilloscope including a pattern generator
11
for applying a two-level pulse to a liquid crystal display panel
12
, a photo detector
13
for detecting light intensity of a sample image pattern displayed on the liquid crystal display panel
12
, and an oscilloscope
14
connected to the photo detector
13
.
The pattern generator
11
generates a specific frequency of two-level pulses and applies it to the liquid crystal display panel
12
. The liquid crystal display panel
12
has a liquid crystal material injected between two glass substrates, and data and gate lines are orthogonally positioned on the lower glass substrate. A thin film transistor (TFT) is provided at each intersection between the data and gate lines, wherein the TFT responds to a scanning pulse to supply data to the data lines of a liquid crystal cell. The liquid crystal display panel
12
displays a sample image depending upon a two-level pulse input from the pattern generator
11
.
The photo detector
13
performs a photo-electric conversion of light input from a sample image displayed on the liquid crystal display panel
12
, wherein a current output from the photo detector
13
is proportional to an intensity of the light. The oscilloscope
14
converts a current signal output from the photo detector
13
into a voltage signal and displays the converted signal on a display screen, thereby detecting a response characteristic of the liquid crystal display panel
12
.
FIG. 2
is a response speed measuring device employing an electro-optic characteristic device according to the related art. In
FIG. 2
, a response speed measuring device employing an electro-optic characteristic device including a pattern generator
21
for applying a two-level pulse to a liquid crystal display panel
22
, a photo detector
23
for detecting light intensity of a sample pattern image displayed on the liquid crystal display panel
22
, and a photo-multiplier tube (PMT)
24
connected between the photo detector
23
and the pattern generator
21
.
The pattern generator
21
generates a specific frequency of two-level pulses and applies it to the liquid crystal display panel
22
. The pattern generator
21
includes a monitor for displaying a signal output from the PMT
24
and a driving circuit for the monitor, thereby displaying the signal output from the PMT
24
on the screen of the monitor. Here, the liquid crystal display panel
22
is substantially identical to the liquid crystal display panel
12
shown in FIG.
1
.
The photo detector
23
generates a photo-electric conversion of light input from a sample image displayed on the liquid crystal display panel
22
, wherein a current output from the photo detector
23
is proportional to an intensity of the light. The PMT
24
converts an analog current signal input from the photo detector
23
into a digital voltage signal suitable for the pattern generator
21
, thereby applying the digital voltage signal to the pattern generator
21
.
FIG. 3
is a waveform diagram of a response characteristic of a two-level pulse response speed measuring device according to the related art. In
FIG. 3
, a liquid crystal response speed characteristic device applies a two-level pulse to the liquid crystal display panels (
12
and
22
in
FIGS. 1 and 2
, respectively) to measure a liquid crystal response characteristic (LCRT) of the liquid crystal display panels
12
or
22
according to the pulse signal.
In
FIG. 3
, the liquid crystal response characteristic (LCRT) is changed from a low-level into a high-level at a rise time &tgr;
r
defined by Equation (1) and is changed from a high-level into a low-level at a fall time &tgr;
f
defined by Equation (2). Accordingly, the rise time &tgr;
r
is measured by an interval ranging from 10% charging time until 90% charging time within a time interval when the LCRT is changed from a low-level into a high-level. The fall time &tgr;
f
is measured by an interval ranging from 10% discharging time until 90% discharging interval within a time interval when the liquid crystal response characteristic is discharged from a high-level into a low-level.
FIG. 4
is a response characteristic diagram of a dynamic image of a liquid crystal display device according to the related art. A twisted nematic (TN) mode liquid crystal has a different response speed due to a electro-mechanical characteristic of liquid crystal material positioned within the cell gap. In general, the response speed has a rise time of 20 to 80 ms and a fall time of 20 to 30 ms. Accordingly, since the liquid crystal material has a response speed longer than one frame interval of a moving image (i.e., 16.67 ms in the case of an NTSC system), a voltage charged in the liquid crystal cell progresses into the next frame prior to arriving at a desired characteristic, as shown in FIG.
4
. Thus, a blurring phenomenon causes blurring successive images on the display panel.
In
FIG. 4
, the LCD device cannot generate a desired color and brightness since, upon implementation of a moving image, a display brightness BL fails to achieve a target brightness that corresponds to a change of a data VD from one level into another level due to its slow response speed. Accordingly, in the LCD device, the blurring phenomenon appears from the moving images, and display quality deteriorates due to a reduced contrast ratio. To overcome this, modulation of source data is performed using a pre-determined modulating data in accordance with look-up tables (i.e., high-speed driving strategy), as demonstrated by U.S. Pat. No. 5,495,265 or PCT International Publication No. WO99/05567, which are hereby incorporated by reference.
The high-speed driving strategy increases the quantity &verba
LG.Philips LCD Co. , Ltd.
Morgan & Lewis & Bockius, LLP
Pert Evan
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