Raster Scan Display: How Screens Refresh Pixels in Rows

A raster scan display refreshes pixels row by row, from left to right and top to bottom, tracing a fixed pattern across the screen many times per second. This sweeping electron beam or digital signal paints each horizontal line in sequence, then jumps back to start the next line, building a complete frame that your eye perceives as a stable image.

Formula

The calculator above works out the raw pixel throughput of a raster display. The core formula is:

Pixels per second = Horizontal resolution × Vertical resolution × Refresh rate

Where:

• Horizontal resolution (r) – number of pixels in each row, in pixels
• Vertical resolution (c) – number of rows (lines) on the screen, in pixels
• Refresh rate (t) – how many times the whole screen is redrawn per second, in hertz (Hz)

The calculator also gives lines per second (c × t) and the time available for each line (1 / (c × t) in seconds, converted to microseconds). These numbers show why higher resolutions and refresh rates demand faster signal processing and wider bandwidth.

Worked Example

Using the default values – a 1920×1080 display at 60 Hz:

1. Pixels per second = 1920 × 1080 × 60 = 124,416,000 pixels/s (about 124.4 megapixels/s)
2. Lines per second = 1080 × 60 = 64,800 lines/s
3. Time per line = 1 / (1080 × 60) = 1 / 64,800 = 0.00001543 s = 15.43 µs

So each row of 1,920 pixels must be drawn in about 15.4 microseconds. That’s the fundamental timing constraint of a raster display – the beam or data pipeline has to move fast enough to cover the entire screen 60 times each second.

What Is a Raster Scan?

Raster scan is the most common method for displaying images on screens, from old CRT televisions to modern LCD, LED, and OLED monitors. The word “raster� comes from the Latin rastrum (rake), referring to the parallel lines that make up the image. In a raster display, the screen is divided into a grid of pixels (picture elements), and each pixel is turned on or off (or given a color) in a fixed order.

This differs from vector displays, which draw lines and shapes directly by moving the electron beam along arbitrary paths. Vector screens were used in early oscilloscopes and some arcade games, but they couldn’t fill large solid areas efficiently. Raster scanning won because it can render any image, including photographs and video, by simply controlling the brightness of each pixel.

How the Refresh Process Works

In a CRT (cathode-ray tube) display, an electron beam scans the phosphor-coated screen. The beam starts at the top-left corner, moves horizontally to the right, then quickly returns (horizontal retrace) to the left edge, slightly lower, and repeats. When it reaches the bottom-right, it returns to the top-left (vertical retrace) to start the next frame. The beam’s intensity is modulated to control brightness, and color CRTs use three beams (red, green, blue) aimed at phosphor dots.

In modern flat-panel displays, there is no moving beam. Instead, each row of pixels is addressed electronically: the display controller sends pixel data row by row, and thin-film transistors (TFTs) in each pixel hold the charge until the next refresh. The refresh rate (e.g., 60 Hz, 120 Hz, 144 Hz) determines how many times per second the entire array is updated. Even though the process is digital, the fundamental raster order – row by row, left to right, top to bottom – remains.

Key timing elements in raster scanning:

• Horizontal scan – drawing one line of pixels
• Horizontal blanking – short pause while the beam returns to the left edge (or the controller moves to the next row)
• Vertical blanking – longer pause while the beam returns from bottom to top (or the frame buffer is updated)
• Frame refresh – the complete cycle of drawing all lines

Interlaced vs. Progressive Scanning

Early broadcast television used interlaced scanning to save bandwidth. In interlaced mode, the screen is drawn in two passes: first all odd-numbered lines, then all even-numbered lines. Each pass is called a “field,� and two fields make one frame. This effectively doubles the perceived refresh rate without doubling the signal rate, but it can cause flicker on thin horizontal lines and artifacts in fast motion.

Progressive scanning (used in computer monitors, modern TVs, and streaming video) draws every line in order, top to bottom, in a single pass. This provides smoother motion and sharper text, at the cost of requiring higher bandwidth. The “p� in 1080p stands for progressive, while “i� in 1080i means interlaced.

Refresh Rate and Flicker

The refresh rate directly affects perceived flicker. At low rates (below 50 Hz), most people notice a distracting flicker, especially in peripheral vision. Standard rates are 60 Hz for most displays, 120 Hz or 144 Hz for gaming monitors, and even 240 Hz on high-end models. Higher refresh rates reduce flicker and motion blur, but they require the graphics card and cable to support the higher pixel throughput.

For example, a 4K display (3840×2160) at 60 Hz needs about 497 megapixels per second, while the same resolution at 144 Hz needs over 1.19 gigapixels per second. This is why high-refresh 4K monitors require DisplayPort 1.4 or HDMI 2.1 – older connections simply don’t have the bandwidth.

Common Problems and Limitations

Raster displays have several inherent issues:

• Screen tearing – when the display refreshes faster or slower than the graphics card sends frames, causing a visible split. V-Sync and adaptive sync (FreeSync/G-Sync) help align them.
• Motion blur – because each pixel holds its value until the next refresh, fast-moving objects look smeared. Higher refresh rates and faster response times reduce this.
• Burn-in – on CRTs and some OLEDs, static images can permanently damage pixels. This is less common on modern LCDs.
• Viewing angle – LCDs can shift color and contrast when viewed off-center; OLEDs generally have better viewing angles.

Raster vs. Vector Displays

While raster is universal today, vector displays still exist in niche applications like oscilloscopes and some laser light shows. Vector displays draw smooth lines with infinite resolution, but they can’t fill large areas or display photographic images. Raster displays are more versatile and cheaper to manufacture, which is why they dominate everything from smartwatches to cinema projectors.

Frequently Asked Questions

Why does a raster display refresh from top to bottom?
Because that’s the order the electron beam or digital controller was designed to follow – it’s the simplest way to cover a rectangular grid. Starting at the top-left and moving right, then dropping down a line, minimizes retrace distance and makes timing predictable.

What’s the difference between refresh rate and frame rate?
Refresh rate is how often the display redraws the entire screen (in Hz). Frame rate is how often the graphics card produces a new image (frames per second). If they don’t match, you can get tearing or stutter. Many displays now support variable refresh rates to sync the two.

Does a higher refresh rate always look better?
Not always – it depends on the content and your eyes. Most people notice a difference up to 120 Hz, but beyond that it’s subtle. Higher refresh rates also consume more power and require a more powerful GPU.

Why do old CRT monitors flicker at 60 Hz but LCDs don’t?
CRTs emit light in brief pulses as the beam scans, so at 60 Hz the phosphor decays between refreshes, causing visible flicker. LCDs hold the pixel value continuously until the next refresh, so there’s no flicker even at 60 Hz.

Can a raster display show a truly static image without refreshing?
No – all raster displays must refresh continuously to maintain the image. Even a static desktop background is redrawn 60 times per second. The only exception is e-ink displays, which use a different technology and only update when the image changes.

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in t | Refresh rate | Hz | 60 | 1 | 240
in r | Horizontal resolution | pixels | 1920 | 1 | 10000
in c | Vertical resolution | pixels | 1080 | 1 | 10000
out pixels | Pixels per second | pixels/s | r * c * t | 0
out lines | Lines per second | lines/s | c * t | 0
out time | Time per line | µs | (1 / (c * t)) * 1000000 | 2