A raster-scan display forms or updates an image in an ordered sequence of horizontal rows, called scan lines, moving from top to bottom. In a traditional CRT, an electron beam traces each line and varies its intensity to light the screen’s phosphor. “Raster” can also describe an image’s rows and columns of pixels; it does not mean that every display contains a CRT beam.
What raster scan, scan line, and pixel mean
A raster is the ordered line structure used to build an image. Each scan line is one horizontal pass across the picture. The individual picture elements along those rows are called pixels (or pels); each represents an intensity or color value.
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A frame buffer, sometimes called a refresh buffer, stores image values for screen points. In a raster-style update, those values are retrieved and presented one line at a time. The term therefore describes an ordered way of arranging or presenting picture information, not a particular screen material.
How a CRT raster is drawn
In a CRT, an electron beam moves across the inside of the screen, tracing one horizontal line after another. The beam’s intensity is varied to produce the image by exciting phosphor. After reaching the end of a line, it returns to the left to begin the next one; after completing the frame, it returns to the top.
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These return movements are called horizontal retrace and vertical retrace. The CRT beam is blanked during retrace so that its return path does not appear as picture content. The horizontal and vertical motion is called “scanning,” as described in Sherry Miller Hocking’s 1976 archival account at the Experimental Television Center.
How raster scanning differs from a modern LCD
The row-and-column raster concept applies to pixel images beyond CRTs, but the hardware is different. A CRT draws its image with a moving electron beam and phosphor. An LCD uses electrically updated pixels; it does not contain a raster gun or CRT beam. A raster-style sequence can describe how screen information is updated, but CRT retrace movements should not be treated as LCD mechanisms.
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This distinction matters because “raster display” may refer either to an image’s pixel organization or, in a CRT explanation, to the beam’s path across the screen. The PST display presentation guide explains the distinction between CRT beam scanning and LCD pixel updates.
Interlacing and historical line rates
In an interlaced system, the display presents two fields: one pass covers alternating scan lines and the next covers the lines between them. Together, the fields make one frame. This is a way of dividing the scan, not a universal feature of raster displays.
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Historical line counts and frame or field rates depend on the television system and region. For example, Sherry Miller Hocking’s 1976 account describes the standard US system of its time as 525 horizontal lines per frame, 30 frames per second, and 60 fields per second. Those are figures for that historical US example, not specifications for raster displays generally.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the term is still useful
Raster scan is a useful term when learning how a display image is organized, how a CRT draws a picture, or how a frame buffer’s pixel values relate to screen rows. In computer graphics, “raster” commonly identifies images made from a grid of pixels, even when the screen displaying them is not a CRT.
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For an instructional overview of scan lines, frame buffers, pixels, retrace, and interlacing, see NPTEL’s Raster-Scan Displays lesson. For CRT raster structure and historical context, see the NCBI Bookshelf chapter “Display Characteristics — Video Displays, Work, and Vision.”
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