[Beowulf] cold cathode fluorescent backlighting

Robert G. Brown rgb at phy.duke.edu
Thu Jun 28 13:56:16 PDT 2007

On Thu, 28 Jun 2007, duane at duaneberry.net wrote:

>  Even if a beta source inside a monitor/TV was continuously emitting the
> physical  construction would provide more than enough shielding. For
> power and regulatory considerations I doubt the source is on
> continuously.
>  IMHO moving from CRT to LCD is a good move in general.

Actually, TVs and monitors at one point in time were notorious sources
of soft x-rays from where the electrons hit the glass.  Those of us who
are old enough remember that we we were told not to sit too close
because it could damage our eyes.  Those of us who sat too close anyway
have a higher risk of cataracts and skin cancers.

However, one reason that sets are so damn heavy now, and why they
discourage them from being put into landfills, is that long ago they
mandated lead in the glass in sufficient quantity to block the x-rays.
CRT glass is a whopping 70% lead by mass (less by volume -- it is much
more dense than glass).  It is a landfill hazard as studies have shown
that lead can leach from the glass.

Also, the beta source (generally a hot wire) IS kept hot at all times
the CRT is "on", which can be a lot of the time, especially if you use a
#%Q!* screensaver instead of blank screen for idle mode on a
workstation.  I have measured (with a kill-a-watt) color CRTs drawing
roughly 100W or even a bit over, compared to LCDs drawing around 30W or
even a bit less.

On the biohazard side, the tubes inside an LCD contain mercury vapor,
just like all of those compact fluorescent bulbs.  Sooner or nearly all
that mercury will ALSO make it into the environment.  There is less
mercury per display (in terms of mass) than lead; mercury is more toxic
than lead as heavy metals go.  So it is literally a matter of choosing
your poison (and don't forget the arsenic in semiconductors while you're
at it).

I don't know about the relative toxicity of solid state e.g. LED
designs.  I'm guessing that it would be the least, and would probably
consume the least energy as well.

So yes, I think that LCDs are, on average, far better for the planet and
your pocketbook than CRTs (remember, an 80W power differential can add
up to $100's in power savings over the lifetime of a monitor), but not
perfect.  LEDs, if/when they ever appear (Cree, are you listening?)
would almost certainly be better than either in all ways.


>  Wikipedia is a good source of keywords for use in further research. I
> would not consider Wikipedia an authoritative source of information on
> it's own.

Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu

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