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Refresh CRT and Interlacing in Computer Graphics

  • Last Updated : 02 Jun, 2021

Refesh CRT :

  • As we all know, phosphor atoms have phosphorescence properties that cause them to shine. However, the strength of the luminous phosphor dots decreases with time.
  • The electron beam must sweep the whole area of the screen and then repaint it numerous times per second in order to maintain a steady image. Refreshing the screen is the term for this procedure.
  • If the electron beam takes too much time to refresh, it looks like the flickering of the image.
  • To eliminate flickering, the screen image must be regenerated quickly enough that the eye is oblivious to the fact that it is being refreshed.

Refresh RATE :

  • Refresh rate can be defined as the number of times the screen is refreshed per second.
  • It’s SI unit or unit of measurement is Hertz(Hz).
  • A display appears stable if the refresh rate is high enough, but if the refresh rate is too low, the display will flicker, causing eye strain and headaches.

Factors in determining refresh rates :

  • The horizontal scanning frequency and the number of horizontal lines displayed on the monitor are directly proportional to the refresh rate. The horizontal scanning frequency is the number of times in one second that the electron beam sweeps one line before returning to the start of the next. Kilohertz is the unit of measurement for horizontal scanning frequency (kHz).
  • 100,000 lines are scanned per second on a monitor with a horizontal scanning frequency of 100kHz. The number of horizontal lines on the screen is determined by the resolution of the monitor.

Importance of refresh rates :

  • While buying a monitor, one of the features that customers look into is the display refresh rate, which provides users a good experience . A high refresh rate is important in providing a clear picture and avoiding eye fatigue.
  • When electron beams impact the screen in a zig-zag pattern, a picture appears on the screen. The number of times a screen is repainted in one second is called a refresh rate, and it is measured in Hertz (Hz). A display with a refresh rate of 95 Hz, for example, is repainted 95 times per second.
  • A monitor has to be “flicker-free”, meaning that the image is redrawn quickly enough so that the user cannot detect flicker. Today, a refresh rate of 75 Hz or above is considered to be flicker-free.


  • Cathode is heated to produce electrons as a cloud .
  • When the electron is injected through the electron gun , it passes through the control grid .
  • A control grid controls the intensity of electrons in the vacuum tube . If the control grid has a high negative potential , then it allows only a few electrons to pass through it.
  • The brightness of a display is adjusted by altering the voltage on the control grid because the amount of light emitted by the phosphor coating is dependent on the number of electrons impacting the screen. On video displays, a control knob can be used to adjust the brightness of the entire screen.
  • Deflection systems  An electromagnetic field or an electrostatic field can be used to deflect a stream of electrons in some linear manner .When the deflecting plate is at zero potential , the beam produces a spot at the centre.
  • Fluorescence light is emitted while the phosphor is being struck by electrons .
  • Phosphorescence light is emitted once the electron beam is removed .

Difference between Refresh CRT  Interlacing Technique :

  • Interlacing is also a technique for stable images, just like Refresh CRT .
  • In this technique, instead of refreshing the screen every time , the electron beams sweep alternate lines in each pass .
  • Interlacing is faster than Refresh CRT as the refresh rate gets doubled .
  • The odd number of lines are refreshed in the first pass . In the second pass, even numbered lines are drawn .

Interlacing technique

Horizontal retrace and vertical retrace :

  • The process of moving the beam from right to left after completion of Row is known as horizontal retrace .after sweeping across each row (scan line) , the electron beam returns to the left of the screen to start displaying the next scan line or row. This is called horizontal retrace.
  • When the beam reaches the bottom of the screen, it’s taken OFF and rapidly retraced back to the top left to start again and the process of moving back to the top is known as vertical retrace. 

     Interlace vs. Progressive Scan :

  • Interlaced screens display odd lines first : 1-3-5-7 , etc. ; then even lines : 2-4-6-8 ,etc., non-interlaced or progressive scan screens display lines consecutively : 1-2-3-4-5-6,  etc.
  • The old tube televisions (CRTs) were all interlaced. At their greatest resolution, CRT computer monitors were interlaced, and at lower resolutions, they were progressive. Some digital TV standards, such as the high-definition 1080i format, are still interlaced today, and 1080i content is widely shown by television networks. Both interlace and progressive scan technologies are supported by HDTV TVs ( see HDTV and DTV ). DE interlace, vertical scan frequency, 4K 3D TV, and interlaced GIF are all examples of interlaced GIF.
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