Computer Graphics |Cathode Ray Oscilloscope| Cathode ray tube (video display technology)

Computer Graphics has become a common element in today’s modern world. Be it in user interfaces, data visualization, motion pictures, etc, computer graphics play an important role. The primary output device in a graphics system is a video monitor. Although many technologies exist, the operation of most video monitors is based on the standard Cathode Ray Tube (CRT) design.   A cathode-ray tube is a central technology used in a device called a Cathode Ray Oscilloscope which is the scientific name of the type of television your parents or grandparents used to watch.

Cathode Ray Tubes (CRT) –
A cathode ray tube (CRT) is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface. It modulates, accelerates, and deflects electron beam(s) onto the screen to create the images. Most desktop computer displays make use of CRT for image-displaying purposes. 

Cathode Ray Oscilloscope Circuitry

1. Vertical Amplifier
2. Horizontal amplifier
3. Time base Circuit
4. Power Supplies
5. Cathode Ray Tube
   

Construction of a CRT –

1. The primary components are the heated metal cathode and a control grid.
2. The heat is supplied to the cathode (by passing a current through the filament). This way the electrons get heated up and ejected out of the cathode filament.
3. This stream of negatively charged electrons is accelerated towards the phosphor screen by supplying a high positive voltage.
4. This acceleration is generally produced by means of an accelerating anode.
5. The next component is the Focusing System, which is used to force the electron beam to converge to a small spot on the screen.
6. If there will not be any focusing system, the electrons will be scattered because of their own repulsions and hence we won’t get a sharp image of the object.
7. This focus can be either by means of electrostatic fields or magnetic fields.

Types Of Deflection:

1. Electrostatic Deflection: The electron beam (cathode rays) passes through a highly positively charged metal cylinder that forms an electrostatic lens. This electrostatic lens focuses the cathode rays to the center of the screen in the same way as an optical lens focuses the beam of light. Two pairs of parallel plates are mounted inside the CRT tube. The electrostatic deflection sensitivity of a Cathode Ray Tube is the amount of deflection produced in the electron beam when a voltage of 1V is applied between the plates.
2. Magnetic Deflection: Here, two pairs of coils are used. One pair is mounted on the top and bottom of the CRT tube, and the other pair is on the two opposite sides. The magnetic field produced by both these pairs is such that a force is generated on the electron beam in a direction perpendicular to the direction of the magnetic field and the direction of flow of the beam. One pair is mounted horizontally and the other vertically.

Now as this highly energetic beam strikes the surface of the screen, these electrons are stopped and their kinetic energy is absorbed by the phosphor screen (atoms). Some energy is wasted in heat also, but the majority of the kinetic energy gets transferred to the phosphor atoms. As these atoms receive this huge amount of energy, they get excited to a higher energy level. 

After a short time, these atoms start returning to their original energy level. The original level is at a lower energy level than the excited one, hence the atoms release some energy while coming down. This extra energy is dissipated in the form of a small quantum of light. Thus the de-excitation results in a bright-colored spot on the screen. The frequency (color) of the spot depends on the difference between the two energy levels (excited level and ground-state level). 

Different kinds of phosphors are used in a CRT. The difference is based on the time for how long the phosphor continues to emit light after the CRT beam has been removed. This property is referred to as Persistence. Basically, persistence means how much time is taken by the emitted light to reduce to one-tenth of its original intensity. Now, phosphors with lower persistence require higher refresh rates to maintain a picture on the screen without any flicker. 

A CRT can be used to display a picture in two ways:

1. The electron beam can be directed to draw a line on the screen. The line is drawn repeatedly at high speed which gives the illusion of a permanent image. This method is similar to etching. This type of display is known as vector display and is popular in computer graphics and computer-aided design (CAD)

2. The second type of technique is the raster technique. This works by scanning the electron beam over the screen in the regular pattern of scan line to paint out a picture. Whenever a scan line is identified across the screen by the beam, the beam is modulated. The modulation of the beam is proportional to the brightness of the point in the picture. Once the scan line is drawn, the beam is turned off before moving to the starting point of the next scan line. The resulting pattern is called a raster and is commonly known as a bitmap.