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What Is a Scan Line?

By G. Wiesen
Updated May 16, 2024
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A scan line typically refers to one vertical resolution line, which goes across a screen horizontally, on a cathode ray tube (CRT) computer monitor or standard definition television (SDTV) screen. Monitors and television screens that used a CRT tube have a screen that consists of rows arranged horizontally across the screen, which were counted vertically from top to bottom and so were referred to as the vertical resolution. Images displayed on this type of screen were created by an “electron gun” causing images to appear on each pixel of the screen one line at a time. Each line that was displayed was referred to as a scan line, though the process was typically too fast for human perception.

The basic way in which a CRT computer monitor functions is through a tube that includes numerous pixels that are arrayed in a series of horizontal lines across the screen. These lines are counted to determine the vertical resolution of such a screen; an SDTV usually has about 525 with 480 visible lines. Each one of these lines is referred to as a scan line. The image on a screen is created by the pixels of each line being illuminated properly through electrons that move from a filament, called the “electron gun,” at the back of the monitor or TV set toward the screen.

A scan line refers to each line of pixels that are illuminated from top to bottom, and usually from left to right. This process is so fast that the human eye and brain do not typically recognize the series of scans as individual processes, since each complete scan of the screen on a progressive scan television or monitor usually occurs between 30 and 60 times per second. There are two different ways in which each scan line on a screen can be illuminated or scanned, referred to as interlace and progressive scan.

Interlace scanning occurs when every other scan line on a screen is displayed together, alternately. This is typically done with the odd numbered lines being displayed, and then the even numbered lines. The human brain and eye does not pick up on this since the usual 30 to 60 frames per second are split into two so that two sets of frames occur 30 to 60 times per second. Persistence of vision effectively combines the two sets of scanned images into a complete screen. In contrast to this, progressive scan displays scan every line of an image each time the screen is scanned.

Interlace screens are designated with a small “i” while progressive screens are indicated with a small “p.”The number that typically accompanies this letter indicates the scan line count for a particular monitor or screen. An SDTV usually has 480 lines and would be labeled as either 480i or 480p. While a high definition television (HDTV) does not utilize a scan line system in the same way a CRT screen does, the number of rows of pixels are still used to indicate the resolution of the screen, such as 720i or 1080p.

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