Tech Briefs Magazine - September 2021 - PIT-17

TECHNOLOGY LEADERS Optics
Image Sticking - Cause, Test and Solutions
L
CD (liquid crystal display) is the
most widely used display technology.
They are used for automotive,
appliance, telecommunication,
home appliance, industrial, consumer
electronic, military etc. But LCD displays
have some drawbacks, such as slow
response, narrow viewing angle, lower
contrast etc. One annoying phenomenon
often complained about by users is
image sticking.
What is Image Sticking
If a fixed image remains on a display for
a long period of time, the faint outline of
that image will persist on the screen for
some time before it finally disappears.
Normally, it happens to LCD and plasma
screens, but for the purpose of our discussion,
we will focus on TFT LCD displays.
Image sticking is also referred to as
" image persistence " , " image retention " ,
" ghosting " or " burn-in image " .
How Are LCDs Constructed?
To understand better the concept of
image sticking, it is necessary to understand
how LCDs are constructed and
how they work.
An LCD screen includes a thin layer of
liquid crystal material sandwiched
between two electrodes on glass substrates,
with two polarizers on each side. A
polarizer is an optical filter that lets light
waves of a specific polarization pass
through while blocking light waves of
other polarizations. The electrodes need
to be transparent so the most popular
material is ITO (indium tin oxide). Since
an LCD can't emit light itself, normally a
backlight is placed behind an LCD screen
in order to be seen in a dark environment.
The light sources used for a backlight
can be LED (light emitting diode) or
CCFL (cold cathode fluorescent lamps).
The LED backlight is the most popular.
Of course, if you want a color display, a
layer of RGB color filter can be made into
an LCD cell. A touch panel can also be
added in front of an LCD display.
How LCDs Work
The first LCD panel technology in
mass production is called TN (twisted
nematic). The principle behind the
LCDs is that when an electrical field is
not applied to the liquid crystal molecules,
the molecules twist 90 degrees in
the LCD cell. When the light either from
ambient light or from the backlight passes
through the first polarizer, the light is
polarized and twisted with the liquid
crystal molecular layer. When it reaches
the second polarizer, it gets blocked.
The viewer sees the display as black.
When an electric field is applied to the
liquid crystal molecules, they become
untwisted. When the polarized light
reaches the layer of liquid crystal molecules,
the light passes straight through
without being twisted. When it reaches
the second polarizer, it will also pass
through, meaning the viewer sees the
display as bright. Because LCD technology
uses electric fields instead of electric
current (electron passes through), it has
low power consumption.
What Causes Image Sticking?
The cause of LCD image sticking is due
to an accumulation of ionic impurities
inside the liquid crystal materials. When
slight DC voltage occurs, the charged
impurities will move the electrodes and
build up a reversed voltage field. When
the power is removed, the reversed voltage
will kick in to make the LCD molecules
twisted different from the other
part of the LCD, which shows up as the
image sticking. The longer the time, the
more impurities will migrate, the larger
the reversed voltage will be, and the imaging
sticking will appear worse.
Figure 1. Image Sticking
Photonics & Imaging Technology, September 2021
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Tech Briefs Magazine - September 2021

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