scan line

A scan line is a single horizontal pass made by an electron beam (in analog displays) or logical drawing sequence (in digital systems) as it traces across the screen from left to right. On a television or CRT monitor, the beam starts at the top left, sweeps horizontally to paint one line of pixels or picture elements, then returns rapidly to the left edge at a slightly lower vertical position to repeat the process. This happens line by line, top to bottom, many times per second, building up the complete image through the phenomenon of persistence of vision.

The number of scan lines directly determines the vertical resolution of the display. Standard NTSC television used 525 scan lines (of which 480 were visible after accounting for blanking intervals), while PAL systems used 625 lines (575 visible). Computer monitors have varied widely: early VGA displays used 480 lines, while modern high-definition displays use 1080 lines or more. The horizontal frequency (how fast the beam moves left to right) and vertical refresh rate (how many complete passes per second) are interdependent; together they define the display's bandwidth requirements and flicker characteristics.

Each scan line is not continuous image data. The beam must travel back to the left edge without painting (called the horizontal blanking interval or front porch), which typically consumes 20 to 30 percent of the line period. In NTSC, each line takes roughly 63.5 microseconds total. During the vertical blanking interval at the bottom of the frame, typically 21 scan lines are reserved for the beam to return to the top and for synchronization signals; this region was historically used to transmit closed captioning data and vertical interval timecode.

Scan line visibility becomes a practical concern in low-resolution displays, especially those viewed close up, where the individual lines can be perceived as a flicker or pattern. This is why CRT monitors refresh at 60 Hz or higher (in North America and Asia) or 50 Hz (in Europe and elsewhere), to keep flicker below the threshold of human perception. Interlaced scanning, which traces odd-numbered lines in one field and even-numbered lines in the next, was a bandwidth-saving technique that halved the line rate requirement but could introduce visible combing artifacts during motion.

The term remains relevant in modern flat-panel displays and digital projectors, even though they do not use electron beams. Their display controllers still organize pixel output as scan lines, and the vertical and horizontal synchronization signals that once controlled a physical beam are now timing pulses that drive row and column decoders. Understanding scan line structure is essential for video signal integrity, timing constraint analysis, and diagnosing image defects such as rolling lines or vertical distortion.