Lecture-02: Cathode Ray Tube (CGA)

                                                                Cathode Ray Tube

Hardware and Software of Computer Graphics:

 Hardware:

*Input Devices-

1. Touch Panels

2. Light Pens

3. Graphics Tablets

4. Mouse

5. Joysticks

6. Keyboard

7. Image Scanner

*Output Devices-

1. Cathode Ray Tube (CRT)

 2. Vector Scan Display (Random Scan Display)

3. Raster Scan Display

4. Colored Monitors

5. LCD, TFT Screens

Software:

1. Photoshop

2. Maya 3D

3. CAD

4. CorelDRAW

Cathode Ray Tube (CRT):

CRT stands for Cathode Ray Tube. CRT is a technology used in traditional computer monitors and televisions. The image on CRT display is created by firing electrons from the back of the tube of phosphorus located towards the front of the screen.

Once the electron heats the phosphorus, they light up, and they are projected on a screen. The color you view on the screen is produced by a blend of red, blue and green light.

                                             Figure: Cathode Ray Tube

Cathode Ray Tube (CRT) – Main Components and Their Functions

A Cathode Ray Tube (CRT) is a vacuum tube that produces images by directing a beam of electrons onto a phosphorescent screen. It was widely used in televisions, computer monitors, and oscilloscopes.

 

1. Electron Gun Assembly

This is the part that produces and controls the electron beam.

a) Heater (Filament)

• Heats the cathode.
• Causes thermionic emission (electrons are released from the cathode surface).

b) Cathode

• Negatively charged electrode.
• Emits electrons when heated.

c) Control Grid

• Controls the number of electrons flowing.
• Adjusts brightness (intensity) of the image.
• More negative voltage → fewer electrons → dim image.

d) Accelerating Anode

• Positively charged.
• Accelerates electrons toward the screen.
• Increases beam speed.

e) Focusing Anode

• Focuses electrons into a narrow beam.
• Acts like a lens to make a sharp image.

➡ These parts together are called the Electron Gun.

 

2. Deflection System

Controls the position of the electron beam on the screen.

a) Vertical Deflection Plates (or Coils)

• Move the beam up and down.

b) Horizontal Deflection Plates (or Coils)

• Move the beam left and right.

By controlling these, the beam scans the screen to form images.

 

3. Fluorescent (Phosphor) Screen

• Coated with phosphorus material.
• When electron beam hits it, light is produced.
• Different phosphors produce different colors.

 

4. Glass Envelope (Vacuum Tube)

• A sealed glass tube.
• Air is removed (vacuum) so electrons can travel freely without collision.

 

5. Aquadag Coating

• Conductive graphite coating inside the tube.
• Collects secondary electrons.
• Maintains proper electrical field inside the CRT.

 

Working Procedure of a CRT Monitor

A CRT (Cathode Ray Tube) monitor works by generating and controlling an electron beam that strikes a phosphor-coated screen to produce images. The working process can be explained step by step:

 

Step 1: Electron Emission

• When the monitor is turned on, the heater (filament) heats the cathode.
• Due to thermionic emission, electrons are released from the cathode.

 

Step 2: Beam Control and Acceleration

• The control grid regulates the number of electrons passing through (controls brightness).
• The accelerating anode increases the speed of electrons.
• The focusing anode narrows the electrons into a sharp beam.

This complete arrangement is called the electron gun.

 

Step 3: Beam Deflection

• The electron beam passes through the deflection system.
• Horizontal deflection coils move the beam left and right.
• Vertical deflection coils move the beam up and down.

The beam scans the screen line by line from top to bottom. This process is called raster scanning.

 

Step 4: Image Formation

• The front screen is coated with phosphor material.
• When the electron beam strikes the phosphor, it emits light.
• By controlling beam intensity and position, different brightness levels are produced.
• Continuous scanning at high speed (refresh rate) makes the image appear stable to the human eye.

Step 5: Continuous Refreshing

• The screen must be refreshed many times per second (e.g., 60–85 Hz).
• This prevents flickering and keeps the image visible.

 

Working in Color CRT Monitor (Extra Point)

• Uses three electron guns (Red, Green, Blue).
• The screen contains RGB phosphor dots.
• By combining different intensities of RGB beams, millions of colors are produced.

 

Advantages of CRT Monitors

• Superior Color Accuracy & Contrast: CRTs offer deep black levels, rich colors, and better contrast ratios than many older LCDs.
• Fast Response Time: Practically instant, eliminating motion blur or ghosting, which is ideal for fast-paced, retro gaming.
• Flexible Resolution: Unlike LCDs, CRTs can display multiple resolutions natively, ensuring, for example, that looks crisp without needing to fill a screen to a fixed resolution.
• No Dead Pixels: Because they use electron beams to paint the screen, they do not suffer from the "dead pixel" issues of LCDs.

Disadvantages of CRT Monitors

• Bulky and Heavy: Large, deep, and heavy, requiring significant desk space.
• High Power Consumption & Heat: They use substantially more electricity than LCD/LED monitors and produce considerable heat.
• Image Degradation & Flickering: They can suffer from screen flickering (causing headaches) and their brightness/color accuracy degrades over time.
• Low Resolution & Sharpness: Generally lower maximum resolutions compared to modern monitors and often have softer, less sharp text rendering.

 

Chapter Questions:

1) Write down working procedure of CRT monitor.

2) Explain different components of CRT monitor with appropriate figure.

3) Advantage and disadvantage of CRT monitor.