1584 Chapter 44
44.8.2 Liquid Crystal Displays
Liquid crystal displays have become ubiquitous. As the
foundation for modern computer and cell phone dis-
plays, LCD technology is used for large flat panel dis-
plays as well as three chip LCD projectors. No matter
the application, LCD technology and how it works is
similar in the way it fundamentally operates.
44.8.2.1 LCD Characteristics
- 3 to 4 inch thick displays (wall or base mount).
- 60 to 400 pounds.
- Panel sizes ranging from under 8 to 108 inch panels.
- 4:3, 16:9, and 16:10 aspect ratio panels.
- No radiation or high-voltage emissions.
- Low power consumption.
- High resolution, up to 4 × HDTV.
- Ideal for computer display and digital signage.
44.8.2.2 LCD Operates in the Following Manner
There’s far more to building an LCD than simply creat-
ing a sheet of liquid crystals. The combination of four
facts makes LCDs possible:
- Light can be polarized.
- Liquid crystal can transmit polarized light or change
the plane of polarization. - The structure of liquid crystals can be changed by
electric field. - There are transparent substances that can conduct
electricity.
To create an LCD, you take two pieces of glass with
polarizing films applied.
A polyimide film is applied to the liquid crystal side
of the glass and then mechanically rubbed to produce
microgrooves.
The two glass plates are assembled together with a
carefully controlled gap dimension.
When LC material is introduced to this cell, the
layers adjacent to the polyimide will align with the
microgroove directions resulting in a helical structure of
LC molecules between the two glass plates, Fig. 44-1.
Liquid crystal displays come in two basic configura-
tions: flat panel displays and projection displays. Both
variations utilize the same basic LCD principle, but
differ in the way that they are illuminated.
In the flat panel, or desktop display, the illumination
comes from bright cold cathode fluorescent lights
behind the display.
In projection LCD displays, the illumination comes
from a bright lamp reflecting off of the LCD and onto
the screen.
LCD monitors make use of thin film transistors
(TFT). TFTs are small switch transistors and capacitors
that sit on a glass substrate in the LCD structure, Fig.
44-2.Each pixel is controlled by one up to four of these
TFTs. To ignite a particular pixel, power is applied to
the correct column and row (just like passive matrix).
Any pixels on the same row and column that are not
targeted simply pass the current on. The transistor at the
target pixel stops the current. The capacitor takes the
current and stores it. It is then able to hold that charge
until the next screen refresh.
Also, by adjusting the amount of voltage to each
pixel, you can control the amount that the crystals will
untwist, thereby allowing varying degrees of color.
For an LCD monitor to produce color, each pixel on
the screen has to have three subpixels, each being a
primary color (red, blue, and green). In this aspect, color
LCDs work the same way as the color CRT. By taking
each of the three colors, each having 256 possible
shades, and blending it all together, the color active
matrix LCD has a possible palette of 16.8 million colors.
Each subpixel has a transistor/capacitor and with this
design process, one can see that there are millions of
transistors necessary to formulate a full TFT screen.
In an LCD monitor, the light source is behind the
panel and illuminates the display from behind. Typically
the lighting is a florescent type but the most recent
development in illuminations is via side emitting LED
display, which improve uniformity, durability, bright-
ness, and the life of the backlight.
LCD projectors utilize three LCD panels or chips as
the imaging devices but unlike LCD monitors, they
differ in the way color and illumination are derived. ByFigure 44-2. TFT-LCD technology.