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More info?)
"Al Dykes" <adykes@panix.com> wrote in message
news:cmauh7$4tm$1@panix5.panix.com...
>
> A high-end CRT gets fuzzy and dim anfter a couple of years.
Well, OK. Actually, what you are seeing here is the aging of the
cathode and phosphor screen, which combine to reduce the
brightness and change the operating characteristics of the
tube. (I.e., it doesn't HAVE to get "fuzzy," but the charactertistics
of the electron gun change such that the focus voltages provided
by the initial adjustment are no longer correct.) It's rarely worth
the trouble, though, to go through a complete readjustment of
the monitor given the main impact of all this aging, which is
to make the cathode a less efficient producer of electrons, and
the phosphor less efficient at converting them into light. This
typically means a lifetime for the CRT of something on the order
of 15-20,000 hours of operation. IF you left the thing running
all the time, that would be a couple of years (one year is
about 8800 hours), but then if you only use the thing 8 hours
a day, five days a week, this might be a useful life of 10 years
or so.
> How does a good LCD change with age and what is the limiting factor
> for life ?
The LCD itself - i.e., the inner workings of the panel, the action
of the liquid-crystal molecules themselves - doesn't really have
much in the way of aging mechanisms that degrade its performance,
assuming that there's no significant net DC voltage applied across
the material for extended periods of time (and this is true for
any drive scheme you're going to come across these days). So
it's tempting to say that the life of the LC itself, assuming that the
panel doesn't get mechanically stressed (cracked, etc.), or operated
under extreme temperatures is practically unlimited. However, what
you DO run into within the panel itself is the eventual failure of one
or more drivers (which will generally take out an entire row or
column of pixels), or (much less likely, but there's way more of them)
the failure of a transistor within the TFT "active matrix" array on the
glass (which knocks out a subpixel). Driver failures are generally
considered to fail the panel, whereas with subpixel losses, it's a
question of how many you can tolerate before you consider the
panel as "failed." In any event, once the "infant mortality" sorts of
fails are past (those failures that occur very early in the life of a
given component, generally due to something wrong in the
construction of that component that wasn't caught when it was
made), the cumulative MTBF of the panel is very likely at least
in the 50-100,000 hours range, meaning that it's very likely to
give you at least a 10 year useful life and possibly quite a bit
more. This again assumes no unusual stresses, either mechanical,
thermal, or electrical.
More likely than a panel failure by far is failure of one or more of
the backlight tubes, and/or the power supplies (you've likely got
at least two to deal with in an LCD monitor, namely the main
supply and the inverter which provides high voltage to drive the
backlights). Backlights of the CCFL type (cold-cathode
fluorescent, which most notebook and monitor panels will be
using) also DO age, similar to the way a CRT ages, meaning
that their light output goes down with time - so you may reach
a point where everything is working, but you find the display
unacceptably dim.
The bottom line, with all of these factors considered, is that
you currently see LCD monitors generally quoting an MTBF in
the low-to-mid tens-of-thousands of hours range, with the
limiting factors being the backlight tubes themselves, followed
by the inverter, power supply, and finally either panel or the
interface board. As with any electronics, mechanical and
thermal stresses outside the normal operating conditions will
shorten the life considerably.
> Does turning an LCD on and off frequently shorten the life. I have in
> mind a laptop that has power saving set to trun the screen off after
> 15 minutes of activity.
As with any electronics, turning the unit off for power-savings
reasons involves a tradeoff. You're trading straight aging
factors for the degradation that results from the on/off
transients. It's really hard to answer this question generally, as
a lot depends on just how often the unit is being power-cycled
vs. how long it would be left on an idle (i.e., burning time on the
"aging" clock, but not really doing anything useful). Turning off
after 15 minutes of inactivity isn't necessarily a bad thing, as long
as it's not being immediately turned back on each and every
time that happens, all day long. That's then the worst possible
case, as you get all of the aging AND the on/off transients.
Bob M.
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