Man-wai Chang said:
I meant, is it continuous, or is it flashing like CCFL?
I can't answer the question, but I can make a few suggestions.
My guess is, it is "flashing".
First of all, LEDs give a relatively consistent color output, if
operated at a constant current. If you attempt to control their
intensity, by changing the current flowing in them, the color
shifts. And good color control is necessary to get a nice white
balance from the backlight.
The next alternative, is called PWM or pulse width modulation.
A constant current is delivered to the LED, for a variable period
of time. The modulation will be at a frequency above the ability
of humans to perceive. For the LED and control circuit, it
probably isn't that critical what the frequency is - in other
words, it is easy for them to make it high enough so you
won't see a flicker.
You might ask, why would intensity adjustment be necessary at all ?
Two reasons.
1) Modern monitors have dynamic contrast specifications. When you
view a movie, the backlight is turned down on demand, frame by frame,
to give the best contrast ratio. This allows 3000:1 contrast from
LCD panels that have only 500:1 contrast from the panel itself.
It allows the quotation of an exaggerated contrast spec.
2) LEDs age with time. A LED will lose 50% of intensity within the
first two years of service. To compensate for the loss of
intensity, a photodetector in the LCD monitor can detect the shift, and
increase the pulse width period to compensate. The compensation
can be applied during the entire life of the monitor. The backlight
level may also be adjusted, according to ambient lighting
conditions in the room.
The old LCD monitors, with their CCFL illumination sources
do much the same thing. CCFLs are modulated at two frequencies.
1) The inverter runs at 25KHz or so. It converts 12VDC to 700-1000 VAC
to start the CCFL running. The CCFL tube runs at a high voltage.
The reason the CCFL inverter runs at 25KHz, is so the noise from
the inverter operation, will be above the range of human hearing.
2) For intensity control, the inverter is run in bursts, at about 200Hz.
The exact frequency is selected, so you won't perceive a flicker.
(It's that old PWM thing again
) This provides a wider range
of light intensity settings, than simply varying the 12VDC fed
into the inverter. And it also allows the CCFL to run with a
consistent coloring (at least, until the tube wears out and
the light turns brownish).
So in some ways, they're equivalent technologies, with similar capabilities.
The ability to provide dynamic contrast, the ability to compensate for
loss of light output with age. And relative freedom to select operating
frequencies which won't be perceived by humans (either visually or
aurally).
The LED provides an advantage on color gamut. It's unclear, whether
in all cases, the LED monitor is really saving a lot of power. I
haven't seen measurement numbers to back up such a statement. And
the LED might last longer, because it isn't dependent on a dirt-cheap
inverter for its operation. That potentially makes it a more reliable
technology. The reliability, really depends on the manufacturing,
and how many corners they cut while making the product.
(Extended Gamut)
http://www.xbitlabs.com/images/monitors/samsung-sm-xl24-xl30/p2s.png
Paul
