Monitors have surprising no moving parts. The only mechanical objects in a monitor are the controls
and power switch. The internal workings of a monitor is all electrical.
The Cathode Ray Tube
Called CRT for short, it is a fancy name for a vacuum tube. Cathode ray tubes are used mostly in
televisions and computer monitors. Every television and monitor that has a lot of depth is made from a
CRT, although the newer flat screen monitors and televisions are made of a different
technology called LCD, or Liquid Crystal Display. This vacuum tube is special in that it has an electron
gun of one end, and a phosphor screen at the other end. The electron gun uses a high voltage of around
10kV to 50kV. This high amount of energy is used to project a steam of electrons, the very
small particles with are electricity. This is know as a cathode ray. These electrons are
projected at the phosphor screen which absorbs their energy, creating a spark of light. The phosphors
glow when struck by electrons, and the electron stream can vary in intensity. If
the electrons have too much energy, the phosphor's energy which it absorbed may leak over into the
adjacent phosphors, creating a blurred image. If the electrons are too weak, the phosphors do not
glow enough, so a dull image is created. Phosphors also come in a different levels of "persistence".
Persistence is the amount of time the phosphor can hold its energy. If the phosphor is being hit
by electrons frequently, for example with a high refresh rate monitor, short persistance
phosphors are used because the phosphors need to switch from bright to off very quickly for a good
picture. In a low refresh rate CRT, like a television, long persistance phosphors are used to prevent the
phoshors to create a flickering effect because they are losing energy before they are recharged again.
The phosphors will eventually wear out, and not be able to absorb enough energy to create a bright
enough picture. This is why some older monitors and televisions seem to have a dull, dark picture.
Making an Image
The cathode ray of electrons can be controlled and directed by a magnetic field. Because electrons are
charged particles, they can be deflected by a magnetic field. CRT's have a powerful magnet between
the electron gun and phosphor screen called a deflection coil. A defection coil can also be referred to
as deflection yoke, or just plain yoke. The magnetic strength of the deflection coil can be varied so
that the electron beam can be swept horizontally from side to side on the phosphor screen. This
sweeping motion is called horizontal sync. CRT's horizontal sync can take place anywhere
from 15.75kHz in televisions and between 85-90kHz in monitors. The deflection coil is not only able to
sweep the ray horizontally, but also vertically as well. This vertical sweep is called vertical
sweep or refresh rate. The beam starts at the top right corner is drawn horizontally on the
phosphor screen, and after each line is finished, the beam is moved down a little and another line is
drawn.
When the beam gets to the bottom left corner of the screen, it starts again from the top right to draw
another frame on the screen.
Interlaced Monitors
Early monitors and all televisions are interlaced. This means that instead of drawing every
horizontal line in sequence, the picture is divided into 2 frames. The first frame draws only the odd
lines, know as the odd field. After that frame is finished all of the even lines are drawn to
complete the picture, known as the even field. This was done because the early CRT's
couldn't draw the complete screen fast enough, so the top half of the phosphor screen would be already
dark by the time the bottom was being drawn. Interlacing fixed this, because the CRT could draw half
lines of the screen quick enough. Newer monitors do not interlace because they are plenty quick
enough to get the whole image on screen.
Colour
So how can one beam of electrons create colour? It can't. Instead, 3 separate beams are used. Each
beam is responsible for one of the primary colours. Light is a little different than the primary colours
that are taught in school. When dealing with light; red, green and blue can be used to make any colour.
This is different than the three other material or pigment primary colours; magenta, yellow, and cyan.
Light is additive, to make white, all of the colours have to be used. The more light, the brighter it is.
Pigment colour is subtractive, to make white, none of the colours have to be used. The more pigment
used, the darker it gets.
