MacFixIt reminds
us that one of the major aggravations with CRT computer displays
was "burn-in", an
unwelcome phenomenon whereby after a length of time left on displaying
the same image or pattern on the screen, a shadow of that image would
persist even after the image had changed.
A severe case of CRT burn-in - 25 years of Pac Man.
What Is Burn-in?
Burn-in is popularly perceived to be a thing of the past with modern
LCD displays, but MacFixIt says you still might experience what it
calls "image persistence" - a problem that's similar in appearance to
classic CRT burn-in, where a constant voltage will cause the crystal
voltage-response curve to change, resulting in them letting more or
less backlight through for a given voltage when compared with
surrounding pixels.
Technically it's not burn-in, but the effect is similar, and it can
happen after just a few hours, according to MacFixIt. In addition, you
may also experience stuck and dead pixels with LCD displays where one
or more of the red, green, or blue colors in a pixel loses the ability
to change intensity, resulting in a persistent color regardless of the
signal being given to the pixel, or the pixel just craps out altogether
leaving a dark spot.
An Excellent Track Record
I've seen CRT burn-in, but I've never experienced it on any of my
own machines. In 13 years of using laptops almost exclusively and
owning some 10 Mac 'Books - variously purchased new, refurbished, and
used - I've never had a stuck or dead pixel.
The former I attribute to being assiduous about putting my computers
to sleep when I'll be away from them for more than a few minutes, and
the only times I ever leave them on overnight is when I'm downloading a
large item over my dialup Internet connection. (Incidentally, I've also
never experienced a hard drive failure in any of my Macs, dating back
to my first one in 1992, and I've had very few hardware issues of any
sort. I'm convinced that the bogie about it being hard on components to
frequently subject them to sleep/wake-up or power down/startup cycles
is highly exaggerated.)
As for dead or stuck pixels, I expect I've had very good luck, but I
suspect that sleeping the computer when practical also helps prolong
LCD screen service life. I've read here and there that having up to 2
to 4 dead pixels on a high resolution TFT display is considered within
the range of "normal," so I guess my various Mac laptops have been
better than normal.
That's a Lot of Pixels!
When you consider that even a modest 1024 x 768 TFT display like the
one in the Pismo I'm typing on
requires 786,432 pixels and each pixel on an LCD monitor requires three
"subpixel" transistors - one for each color (red, green, and blue) -
which works out to 2,359,296 little, bitty transistors in one 1024 x
768 screen. The higher resolution Apple Cinema HD display is made up of
2.3 million pixels with 6.9 million red, green, and blue subpixels.
With those many millions of subpixels, I guess its remarkable that they
can come as close to pixel-perfection as they usually do in a
mass-produced item, and this provides some insight as to why the
display is one of the most expensive components in a laptop
computer.
Apple has addressed the dead pixel issue in a Tech Info Library
article entitled About LCD
Display Pixel Anomalies, noting:
"...occasionally, a transistor does not work
perfectly, which may result in the affected subpixel being turned on
(bright) or turned off (dark). With the millions of subpixels on a
display, it is quite possible to have a low number of faulty
transistors on an LCD. Therefore, a certain number of subpixel
anomalies is considered acceptable. Rejecting all but perfect LCD
panels would significantly increase the retail price for products using
LCD displays. These factors apply to all manufacturers using LCD
technology - not just Apple products."
Apple advises that if you think your Apple product's display
contains an excessively high number of pixel anomalies, you contact
Apple tech support or your local Apple authorized reseller to arrange
an evaluation.
Be aware that Apple will not replace a screen with just one or a
small number of dead pixels. I've heard it suggested that five is the
magic number, but I suspect that there is no arbitrary number threshold
and that the location of the dead pixels, and whether they are
scattered or clustered, would influence the decision to replace or
not.
A Result of Trauma
Dead pixels can also be a result of trauma, like the time a young
friend closed the lid of my son's WallStreet PowerBook,
which had hitherto had a flawless display, after a long game-playing
session on a hot summer afternoon and left it running. It got very hot
after four hours of poorly ventilated running - too hot to touch, after
which there was a dead pixel that tended to come and go
intermittently.
Remedial nostrums include lightly tapping the screen surface in way
of the dead pixels or gently massaging the
surface of the LCD screen in the location of a dead pixel with a
fingertip (at your own risk), which sometimes will stimulate the pixel
back to life, but the best workaround may be to cultivate ignoring them
if there are only one or two.
Prevention
MacFixIt suggests a number of fixes and workarounds for addressing
ailing LCDs but emphasizes sensibly that prevention is better than
cure, noting that the first line of defense for preventing image
persistence could be to use a screen saver that exercises the pixels.
Mac OS X has an attractive and entertaining selection of screen
saver programs that you can access and enable using Desktops &
Screen Savers in System Preferences.
Even better is to frequently turn off your display, with an
important caveat being that while screen dimming helps guard against
burn-in with the old CRT displays, dimming or darkening an LCD
backlight won't help preserve your pixels.
However, the sleep function in OS X works really well, and
wake-up waits are minimal with today's fast Intel processors. You can
configure the computer to automatically put itself to sleep after
idling as little as one minute in the Energy Saver system preferences
panel.
It works for me.
