CRT MONITOR cathode-ray tube

CRT

• (Cathode Ray Tube) A vacuum tube used as a display screen in a computer monitor or TV. The viewing end of the tube is coated with phosphors, which emit light when struck by electrons.

• In the past, CRT was a popular term for the entire computer display terminal. Today, "monitor" is the correct term as computer displays have shifted from CRTs to flat LCD panels .

• Likewise, TV sets are widely available in LCD and plasma flat panel technologies. However, the CRT television is not over and done with.

• "Direct view" CRT sets come in wide screen, HDTV models that are more affordable and offer quality equal to or better than LCD and plasma

CRT Basics

horizontaldeflectionplate

phosphor-coatedscreen

heatingfilament

controlgrid

acceleratingsystem

verticaldeflectionplate

electron beam

focusingsystem

cathode

How CRT works

• CRTs receive their picture through an analogue cable, and that signal is

decoded by the display controller, which handles the internal components

of the monitor - think of it as the mini-CPU for the monitor.

• CRTs have a distinctive funnel shape. At the very back of a monitor is an

electron gun. The electron gun fires electrons towards the front through a

vacuum which exists in the tube of the monitor.

• The gun can also be referred to as a cathode - hence the electrons fired

toward are called Cathode Rays.

• These rays correspond to to the red, green and blue channels of

the display and video card.

• At the neck of the funnel-shaped monitor is an anode, which is

magnetised according to instructions from the display controller.

• As electrons pass the anode, they are shunted or pulled in one

direction or the other depending on how magnetic the anode is at

that time.

• This moves the electrons towards the correct part of the screen.

How CRT works

How CRT works

• The electrons pass through a mesh, and this mesh defines the

individual pixels and resolution on the screen. Electrons that pass

through the mesh then hit the phosphor coating which is on the

inside of the glass screen.

• When the particles hit the phosphor, they immediately light up -

causing the light to shine through the front of the monitor, thus

making up the picture on the screen.

• There are three differently colored phosphors for each pixel (known

as phosphor triads), and depending on which phosphor the electron

hits, that's which color the pixel will light up.

How CRT works

Difference in Components

• Different monitors differ in quality, and this is often dependent on the

technology and components used internally.

• Some CRT monitors use a single electron gun at the rear of the monitor

to produce the electrons that will become the red, green and blue

electron rays.

• However, higher quality monitors have an individual gun for each,

which can increase picture quality.

• The metal used for the mesh at the front of the monitor will also affect

quality. Electrons also produce ions because of imperfections in the

vacuum, and these electrons are destructive to image quality if they hit the

phosphor.

• Consequently, meshes are made of relatively thick metal to prevent

phosphor damage.

• However, in better quality monitors, a thinner, yet tougher metal alloy is

used for the mesh. Because it's thinner, it means more light can get

through, making for increased brightness and higher contrast.

Components

Aperture Grille v Shadow Mask

• Each CRT has a sheet of metal at the front of the monitor which (partly) defines the

pixels on the screen. Shadow mask is an older technology, and is literally a piece of

metal with millions of holes in it which allow the different cathode rays through to hit

the phosphour.

• Because a shadow mask covers the whole back of the screen, protecting the phosphor

from stray ions, it also limits the strength of the rays, reducing the brightness of the

monitor.

• Aperture grille is a newer technology which defines the gaps through which electrons

pass using a mesh of wires rather than a sheet with holes in. Whereas a shadow mask

is made of circular holes, the grill is made of vertical slots. Because it is by its nature

thinner, it allows for brighter displays.

• However, the grill is fragile and prone to being knocked around. The grill

is therefore strapped to the monitor using stiff horizontal wires - this is

what causes the distinctive pair of lines across high-end aperture grille

monitors.

• Invar mask is a variant of shadow mask, and uses a thinner, stronger metal

to form the mask, allowing for better image quality whilst remaining

cheaper than aperture grill to produce.

Diagram

Dot pitch and resolution

• Each pixel on the CRT screen is defined by lighting up combinations of the

red, blue and green phosphors that make up the pixel. With a varying strength

of electron gun operating on each phosphor, different colors are produced -

with red, blue and green all fired on maximum strength, that means bright

white is produced.

• Dot pitch is measured on most monitors as the distance, diagonally, between

two phosphors of the same color. However, some manufacturers quote dot

pitch on monitors as the horizontal distance between phosphors, which can

make them appear better specified, on paper, than perhaps they are.

Diagram

Refresh CRT (Vector CRT)

Displayprocessor

CRT

alphanumeric keyboard

stylus

data tablet

interface tohost computer

……MOVE 10 15LINE 100 25CHAR LU CYLINE……JMP

refresh buffer memory

(display commands)

(interaction data)

Advantages of CRT• The cathode rayed tube can easily increase the monitor’s brightness by

reflecting the light.

• They produce more colours

• The Cathode Ray Tube monitors have lower price rate than the LCD

display or Plasma display.

• The quality of the image displayed on a Cathode Ray Tube is superior to

the LCD and Plasma monitors.

• The contrast features of the cathode ray tube monitor are considered

highly excellent.

Disadvantages of CRT

• They have a big back and take up space on desk.

• The electromagnetic fields emitted by CRT monitors constitute a health hazard

to the functioning of living cells.

• CRTs emit a small amount of X-ray band radiation which can result in a health

hazard.

• Constant refreshing of CRT monitors can result in headache.

• CRTs operate at very high voltage which can overheat system or result in an

implosion

• Within a CRT a strong vacuum exists in it and can also result in a implosion

• They are heavy to pick up and carry around