Meaning of FTN, SFTN, DSTFN?
Paul DuBois
In researching the capabilities of various palmtops, I've run across
the terms FTN, FSTN, and DFSTN in reference to LCD displays. What
do these terms mean?
--
Paul DuBois
dub...@primate.wisc.edu
Home page: http://www.primate.wisc.edu/homepage/dubois
Software: http://www.primate.wisc.edu/software
Michael Schuster
Paul DuBois <dub...@night.primate.wisc.edu> wrote:
>
>In researching the capabilities of various palmtops, I've run across
>the terms FTN, FSTN, and DFSTN in reference to LCD displays. What
>do these terms mean?
excerpted from:
Liquid Crystal Display (FAQ) Frequently Asked Question
Fundamental Liquid Crystal Display Technology:
a multi-part introduction for the basic understanding of
Liquid Crystal Displays
Version 1.00 August 18, 1993 By Scott M. Bruck
[snip]
1.23 Liquid Crystal Display Names and classes
Before discussing the different types of LC displays the topic
of Birefringence must be explained. When a light ray strikes
a crystal ( or crystal-like material), it will be split into
two separate light beams; with one beam perpendicular (offset
by 90 degrees) from the other. Since the beams travel
different paths, they reach the viewer's eyes at slightly
different times. This is an essential point, it may cause the
color or polarity of the display to change when viewed at
angles where the viewer may see both rays.
For active matrix displays, in order to maximize contrast
and gray scale reproducibility, Twisted Nematic (TN) is
utilized. This material is twisted 90 degrees from the top to
bottom glass panels. STN or Super Twisted Nematic is
chemically distinct from TN and the twist angle is usually
greater than 200 degrees. Furthermore, due to the large twist
angle, the actual alignment of the polarizers for STN LCDs are
not perpendicular, but adjusted to find the best direction
(rotation) for optimum display characteristics. The STN
material is rotated in a way so the change from transmission
to dispersion is very abrupt and therefore can respond quickly
to small changes in voltage. Figure 5 illustrates the
response characteristics of a TN curve and Figure 6 shows the
response characteristics of a STN curve which will further
clarify these points.
100% | Figure 5
| Typical Response of a Normal White TN Display
T |
R |************
A | *
N | *
S |<-- Zone I-> *
M | *
I | *
T | *
T | *
A | *
N | <----Zone II--- * --------->
C | *
E | * <--Zone III-->
| *
| *
| *************
0% |________________________________________________________>
Vt (Threshold Voltage)
Applied Voltage
1.24 The TN Liquid Crystal Response Curve
The most prominent feature of the TN response curve is the
central linear region between the two flat areas (Zone II).
Zone I describes the white color of the display when no
electric field is applied. In other words, the display will
transmit virtually all the introduced light. On the other
hand, in Zone III, the display will diffuse light and appear
dark. The middle region can display gray scale or an image
somewhere between White and Black. The key point here is that
you must be able to very carefully control the voltage applied
to the LC cell and maintain it for one duty cycle (before that
pixel is addressed again) in order to produce accurate colors.
For this reason, this type of LC material is primarily used
for active matrix LCDs.
COMMONLY ASKED QUESTION NOTE: because the LC material is
partially twisted in the gray scale area, when looking at a
display at an off angle the colors tend to shift and sometimes
invert due to birefringence.
COMPUTER APPLICATION NOTE: The TN response curve does not have
to be utilized for gray scale, in order to make a simpler
display, improve viewing angle, and use cheaper IC drivers;
the Apple Powerbook 170's TFTs (thin film transistors) drive
the TN response curve directly into region 3. This gives all
the speed/contrast advantages of a TFT display and cheaper
manufacturing cost, but provides no gray scale.
1.25 The STN Liquid Crystal Response Curve
The Key to understanding the STN curve is simply that due to
the addressing method applied, only a small amount of voltage
is available to change the LC material from transmittance to a
dispersion state. For this reason, the shape of the curve has
nearly a 90 degree shift between Zone I and Zone II regions;
in other words, it goes ballistic and nearly straight up !
This property allows the LC material to shift from white to
black at its threshold voltage (VT) without being concerned
with partial transmission (gray scale). Furthermore, the 90
Degree curve shape means that gray scale is not available from
the LC material itself and the driving circuits must provide
the necessary fixes for levels of gray.
STN displays inherently have a yellow on blue appearance
(anyone remember the old Zenith Laptops ?). Because many
individuals found the yellow and blue appearance undesirable,
a number of techniques were developed to convert the STN image
to a black on white scheme. DSTN, developed by Sharp
Corporation, was the first commercial black and white
conversion of the STN display and refers to Double Super
Twisted Nematic. DSTN displays are actually two distinct STN
filled glass cells glued together. The first is a LCD display
as described previously, the second is a glass cell without
electrodes or polarizers filled with LC material for use as a
compensator which increases contrast and gives the black on
white appearance. The drawbacks are a heavier module, a more
expensive manufacturing process, and a more powerful
backlighting system.
FCSTN is Film Compensated STN and is now the most commonly
used STN display technology on the market. FSTN, monochrome
STN, and Polymer film STN are all standard STN displays with a
polymer film applied to the glass as a compensation layer
instead of the second cell as in the case of the DSTN. This
simpler and more importantly cost effective method provides
the preferred black on white image for this display
technology. However, once again, this design lowers the
transmittance of light and requires a more powerful back
lighting system.
[snip]
--
Mike Schuster | schu...@panix.com | 70346...@CompuServe.COM
------------------- | schu...@shell.portal.com | GEnie: MSCHUSTER
Jason Savage
>In researching the capabilities of various palmtops, I've run across
>the terms FTN, FSTN, and DFSTN in reference to LCD displays. What
>do these terms mean?
>--
>Paul DuBois
>dub...@primate.wisc.edu
>Home page: http://www.primate.wisc.edu/homepage/dubois
> Software: http://www.primate.wisc.edu/software
Paul,
Here is the definitions that I have always used for those acronyms:
FTN and FSTN: Flat Screen Twisted Nematic
DFSTN: Dual-Scan Flat Screen Twisted Nematic.
Anybody else care to comment on this?
Jason Savage, CNA
Sav...@MBnet.MB.CA