essay is not intended to explain the exact way a LCD panel works,
I assume people reading this are familiar with the basics of LCD
operation. If you don't here is a good introduction to the
operation and construction of LCD panels:
of TFT LCD
But since the text talks about
black, white and gray pixels a reminder that the entire screen is
made up of three b/w pixels with a color filter for each pixels.
Therefore, a gray-gray transition could be also thought of as one
color or shade to another.
LCD Pixel Response Time.
One of the most important
specifications for a LCD panel is the pixel response time. In the
early days of LCD panels used for video and computer monitor
applications, sported a pretty slow responding pixel response. To
Illustrate the difference here is an example:
To understand what is going on it is
necessary to look at what is happening to the LCD panel as
Notice the Falling Transition
describes the time required for the pixel to go from full
luminescence to minimum luminescence and the Rising Transition
describes the time required for the pixel to go from minimum
luminescence to full luminescence. The combination of these two
times is the pixel response time for a white-black-white (w-b-w)
transition. That is one method of reporting pixel response times
used by some manufacturers.
But that does not tell the entire
story. It turns out when the pixel is excited by the full input
voltage to turn the pixel dark, it also will respond the fastest
because of the driving force. Conversely, when the pixel voltage
is reduced to the minimum it also will respond the fastest. But
when an intermediate transition occurs, say one level of gray to
another level of gray (such as encountered with video) the driving
force is much less and the pixel will respond slower. It not
uncommon for this gray to gray response to take up to 120 ms for a
LCD that is rated at a 25 ms w-b-w response time. Using a
conservative ratio of 3:1 you could extrapolate an 8 ms LCD to 24 ms.
PC World article:
VESA has a technical definition for rise-and-fall,
but the gray-to-gray spec remains undefined. Industry experts agree
that in LCDs, transitions between the fine distinctions of
gray-to-gray are 3 to 4 times slower than those of rise-and-fall.
"The voltage required to turn something from black to white or
white to black is greater, so it's faster. Controlling the voltage
from gray to gray makes it slower," says Samsung's Nichols.
ViewSonic representative uses a car analogy to explain the same
point. "Fully off is a black image, fully on is a white image.
It's similar to a gas pedal on a car--you're either off or on. But
this spec doesn't cover all the speeds in between off and full
throttle. Gray-to-gray is typically slower because it's in smaller
differences that aren't normally overdriven," says Erik Willey,
senior product manager for LCD at ViewSonic.
Newer technologies have fortunately reduced not only the
b-w-b response time, but have also reduced this ratio as well, but
the difference between w-b-w and gray-gray response times still
exist, so it is important to know what response time the
manufacturer is reporting and if they just say “response
time” you can bet it is probably the w-b-w time since it
will make the LCD look the best.
So How Fast Is Necessary?
Here we need to review what is
required to form a picture on a LCD screen. With a 60 Hz refresh
rate that would fully paint a new picture 60 times per second, the
maximum response time for each pixel can be easily calculated.
Take the popular 1366x768 resolution (I'm not going to dwell on SD
resolutions here) there are a total of 1,049,088 pixels.
Refreshed in 1/60 sec will require .0167 seconds or 16.7 ms. If
you step up to the 1920x1080 resolution, there are a total of
2,073,600 and will take the same 16.7 ms. The reason both require
the same time is the pixels are all virtually updated at the same
time as opposed to a CRT pixel that is scanned one after another.
So the minimum desired response time
is going to be 16.7 ms or 20 ms if using PAL 50 Hz video. Now
bear in mind these are gray-gray numbers.
Black Frame Insertion (BFI)
One of the benefits of the faster
response time is the ability to incorporate what is called black
frame insertion. By inserting a black frame in between the normal
video frames, not only does the overall response of the pixel
improve because the gray to black and back to gray will occur
faster than gray-gray transitions in these fast LCDs, but the
inserted black frames between the picture frames allows for
improved black levels approaching those available with a CRT. So
right there you can see the value of a 8 ms LCD since the frames
will be doubled requiring a 120 Hz screen update. On some slower
responding LCDs this black frame is inserted by turning off the
backlight rather than just the LCD pixels.
In Plane Switching (IPS)
This technology first introduced by
Hitachi in 1995 was developed to increase the viewing angle of LCD
panels. It features a LCD cell that has both electrodes on the
same plane or substrate. The downside of IPS is a slower response
time, but that has somewhat been overcome by the newer technology
fast LCDs. For more details on this technology see:
Some manufacturers will advertise
the new Super IPS as improving response time, but it is not due to
the IPS technology, but it rather due to the faster LCD panels.
LCD that combine the 8 ms pixel response times with IPS provide a
fast display with a large viewing angle, typically 170 degrees, or
more, both horizontally and vertically.
World: LCD Specs: Not So Swift