I suppose most of us have taken shots of
the TV pictures, and usually with success. There has been plenty of
advice regarding the shutter speeds and the lens apertures etc. But
there is another aspect, which as far as I know, has not been taken into
account. It is possible for you to cause serious damage to your TV tube.
OK. It hasn’t happened to you, and for
about fifteen years of taking TV pictures it hasn’t happened to me
until last month.
After seeing some of George Sparkes’
excellent computer, slides from the TV screen, at Lincoln, I wanted to
make some title slides on the same principle. So I started to do so.
What have I done wrong? Well, I thought it would be a good idea to
measure the brightness of various parts of the screen such as the
highlights and darkest part. of the TV image when you have got it right
it should be useful for reference to other~, displays of different total
brightness. I thought.
So I got out my old Weston Exposure
meter No. 5 to do some close up measurements, and that is where I went
WRONG. The Weston is a moving coil meter, containing a powerful magnet.
When taking readings close to the TV screen I noticed that the Weston
was distorting the raster of the picture and in addition, the colours
were distorted.
If you don’t know the TV lingo, the
“raster” is the complete set of lines and colour dots that make up
the picture area.
When I took the Weston away, I found
that the raster came back to normal but from where the stray magnetic
field had been, there was a darkish cloudy patch in the middle of the
screen, very noticeable when the display is a ‘clear’ screen. I
switched the TV off and on several times: no improvement. Cleaned the
tube with a damp cloth to see if it was static: no improvement. Turned
up the TV brightness and contrast controls from max to min. several
times: the dark patch still remained.
In desperation, I left the TV switched
off for a couple of days. No real improvement, but by wishful thinking,
I imagined that the dark patch could be ‘slightly less,’ perhaps. of
course., I am looking at a blank raster, Not a graphic.
Now this is serious. I wondered if, by
some way, the three-colour ‘fluorescent matrix inside the tube’ had
been damaged, or could the shadow-mask inside the TV tube be permanently
‘magnetised’. The shadow-mask is a sheet ‘of metal, full of little
holes so that the electrons hit the correct fluorescent colour spots on
the matrix. ‘I had no idea of what the mask is made.
In for a penny, in for a pound, (in this
case, lots of pounds) So I guessed that the magnetised mask was the most
likely cause. I have a reel-to-reel tape bulk eraser, which can
demagnetise hefty magnets, such as a magnetised screwdriver or even an
Eclipse pocket magnet. You place the screwdriver on top of the eraser
and slowly withdraw the screwdriver from the eraser until you have no
magnetic “pull” left. Do switch off the eraser when the screwdriver
or anything else made of iron is near the eraser. Otherwise they could
be strongly magnetised.
So I approached the screen with the
eraser and gave the raster a real “thrashing” and then slowly took
the eraser away from the screen, until there was no sign of that
‘cloud’. I had to take the eraser about ten feet away before I
switched it off, to be sure that there was no residual magnetism left in
the shadow-mask.
Well. All is now OK. In fact the screen
seems cleaner than ever before, but it is well not to bring any magnets
near a colour TV screen. I suppose that if you wish to take spot
readings from the screen the best way would be to use something like an
SF1 photometer which can take spot reading on the silver lining of a
cloud, let alone a reading on a small part of the TV picture, six feet
away or, if you have no access to an SEI or simulator, use the camera
exposure readings and bracket them.
‘Also from Area 9 comes some comments
from Member David Harris on reference to Bob’s article on page 10 of
the last National Newsletter (No.36).