Monitors radiation question.

[miller@FS1.cam.nist.gov (Bruce R. Miller)]

[Disclaimer, I am a physicist, but I've never looked into this specific
issue in detail, so -- generalizations]

    Date: Fri, 21 Jun 91 10:47-0000
    From: p2@porter.asl.dialnet.symbolics.com (Peter Paine)

    Does anyone have any feelings (Pan what are your measurement
    techniques?)  whether a monitor which is dimmed (L-machines/Lemo
    brightness turned down, G-machines Function-c-Refresh, MacIvory Radius
    screen saver stars, etc.) should emit less?

    Sitting at the focus of 4 screens that get used intermittantly during
    the day, I had hoped that dimming the screens would diminish emissions
    to insignificant levels. ie. radiation is caused by the tube beta rays
    striking the phosphore and producing weak X ray? which is absorbed by
    the lead glass?

Here's a common confusion: "Radiation"!  Electromagnetic Radiation
covers a broad (well, infinite actually) spectrum including `heat',
light, microwaves, xrays, gamma rays and the New Improved ALF
(Absurdly Low Frequencies) ie. 60Hz.  And in the popular press, it
is broadened to include radioactive substances, but never mind.

Indeed the electrons which are accelerated and then stopped
(decelerated) at the phosphor will tend to give off high frequency
radiation, maybe even X-rays (10^19 Hz), but this apparently has tended
to be negligible.  And of course, when the apparent clusters of health
problems associated with CRT's first were seen, the engineers
measured the radiation (X-rays !) and said "there practically isn't
any". [ie. they neglected to consider `other' kinds of radiation]
This type of radiation (however large or small it is) should change in
intensity relative to the screen intensity -- dimming the screen reduces
the electron beam current. 
Whether or not this kind of radiation is negligible/safe or unsafe, it is
not at 50/60 hz.

    Or is the only effect a consequence of the 50/60 Hz mains induction
    field - such as is found in homes situated under power line cables?

Close.  The electronics associated with painting the electron beam
across the screen are operating at 50/60 Hz (screen refresh rate), also
at some slightly higher frequencies (horizontal sweep rate). Here there
are relatively high voltages and currents involved, and relatively
strong magnetic fields -- this is what moves the electron beam up/down
side-to-side.   [But, the ~20Kv high voltage used to accelerate the
electron beam forward does not oscillate as I recall].  Color systems
involve higher accelleration voltages and so stronger sweep fields.

It is these fields that are the ones people are concerned about, and
which Paul measured.  They dont change with the intensity and in all
designs I'm aware of, they're always on if the monitor is on.

[BTW: High voltage power lines, or even home power lines, or even even
electric blankets carry 50/60Hz and produce magnetic fields at these
frequencies] 

One problem is that, whatever it does or does not do, this stuff is, in
fact, Radiation!  But its not the same as eating Chernobyl tomatoes or
sticking your head in the microwave.

So far I'm not aware that anybody has come up with a mechanism by which
these weak, low frequency fields could affect people.  But measuring the
effects of low level radiation, even from Chernobyl tomatoes, is
notoriously difficult.  We know what happened to the people in
Hiroshima.  The effects are not necesarily linear though, there may or may
not be thresholds below which there is no effect: If after 1 year
exposed to 10 units a person gets sick, what happens when exposed to 1
unit? 1/10th as sick? Or after 10 years at 1 unit? Or ...
The studies have to deal with small effects over long periods of time,
_any_ flaw in the experiment can mess it up.  Maybe some of the people
were drinking diet soda?

So, in summary, as far as I am aware, there is not yet any concensus on
whether there is an effect at all, much less what level is `safe'.

    If the emissions are not related to the screen illumination intensity,
    ie. dimming completely, viewing as white text on black background, and
    at lowest intensity possibly - then should I be turning the monitor
    power off, except when in active use?

Killing the monitor versus killing the monitee.
I dunno, its up to you.

bruce
miller@cam.nist.gov