Is LED backlit (in LED monitors) flicker-free?

Discussion in 'PC Hardware' started by Man-wai Chang, Sep 13, 2010.

  1. I meant, is it continuous, or is it flashing like CCFL?
     
    Man-wai Chang, Sep 13, 2010
    #1
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  2. Man-wai Chang

    John Doe Guest

    I mean... Why does Hong Kong and its people think that UseNet is
    for selling their namebrand knockoff products?

    How many aliases does the original poster have and use for
    spamming UseNet?

    Only The Shadow knows...
    --
     
    John Doe, Sep 13, 2010
    #2
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  3. Man-wai Chang

    VanguardLH Guest

    Here's the question.
    Oh no, I forgot what was the question after having to wade through a
    signature that comprises 78% of the lines in the post consisting of
    random fluff quotes, proselytizing linux, and pointing at a particular
    gov't agency in a worldwide publicized post.
     
    VanguardLH, Sep 13, 2010
    #3
  4. Man-wai Chang

    Paul Guest

    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
     
    Paul, Sep 13, 2010
    #4
  5. Man-wai Chang

    John Doe Guest

    That depends on the context. If that were entirely true, they
    would be rich.
    I have been messing with spam for a long time here on UseNet.
    No telling what that stuff is supposed to mean.
    --
     
    John Doe, Sep 14, 2010
    #5
  6. LEDs age with time. A LED will lose 50% of intensity
    Does this mean all LED monitors, though cheaper to make, require
    replacement every few years?
     
    Man-wai Chang, Sep 16, 2010
    #6
  7. Man-wai Chang

    Paul Guest

    The LED continues to operate, so it is not a problem. There
    is a graph here, to show the relative loss in intensity of
    various light sources, for comparison. Every generation of
    LEDs, has different properties in this regard.

    http://www1.eere.energy.gov/buildings/ssl/depreciation.html

    I just looked at a recently introduced high power LED, and
    it lists "70% lumen maintenance (L70) at 50,000 hours of operation",
    and 50000 hours is a decent lifetime.

    Paul
     
    Paul, Sep 16, 2010
    #7
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