Displays

How an LCD works

A liquid crystal display (LCD) does not produce light itself; it controls light that comes from behind. The layers, from back to front:

1. A backlight makes even white light across the whole screen.
2. A polarizer lets through only light that vibrates in one direction.
3. A layer of liquid crystals, controlled by a grid of thin-film transistors (TFTs), one per subpixel. The voltage on each transistor sets how much the crystals twist the light.
4. A color filter gives each pixel three subpixels: red, green and blue. Mixing them in different strengths makes every color.
5. A second polarizer, turned 90 degrees. Light only passes to the extent that the crystals have twisted it, so each subpixel acts as a tiny adjustable shutter.

Two consequences follow from this design. Because the crystals never block the backlight completely, black on an LCD is really a dark gray. And because the whole screen depends on the backlight, a failed backlight leaves the image there but invisible.

Older LCDs used a cold cathode fluorescent lamp (CCFL) as backlight, powered by a separate board called an inverter. Almost all current screens use light-emitting diodes (LEDs) instead. That is all an "LED monitor" or "LED TV" is: an LCD with an LED backlight. It is not a different display technology. The same goes for "QLED": an LCD with a quantum-dot layer that improves color, not an OLED.

Panel types

TN, IPS and VA

LCD panels differ in how their liquid crystals are arranged and move. That choice trades speed against viewing angles and contrast. Contrast ratio is the brightness of full white divided by that of full black; a higher number means deeper blacks.

• Twisted nematic (TN): the oldest and cheapest type, with very fast response. Colors shift and fade when you look from the side, and especially from below.
• An in-plane switching (IPS) panel turns its crystals parallel to the screen, which gives the most accurate colors and wide viewing angles. It is the usual choice for office work, photo editing and most laptops. Its black can glow slightly in a dark room ("IPS glow").
• Vertical alignment (VA): crystals stand upright when off and block the light well, so VA has the best contrast of the three, typically around 3000:1 against about 1000:1. Dark transitions are slower, which can show as smearing behind moving objects.

OLED

Organic light-emitting diode (OLED) screens have no backlight: every subpixel is a tiny light source made of organic material that glows when current flows. A black pixel is simply switched off, so black is truly black and contrast is practically unlimited. OLED screens are thin, respond almost instantly, and look good from any angle.

The weakness is aging. Each subpixel wears a little every time it lights up, and blue wears fastest. A static image shown for a very long time (a taskbar, a news channel logo, a dashboard) wears its pixels unevenly and can leave a permanent shadow: burn-in. OLED screens counter it by shifting the image slightly and running refresh cycles, and users can help with dark themes, auto-hiding taskbars and screen savers. OLEDs are also usually less bright than LCDs over the full screen.

Mini-LED

Mini-LED is an LCD with a backlight of thousands of very small LEDs grouped into many local dimming zones. Zones behind dark parts of the image are dimmed or switched off, which brings contrast closer to OLED without the burn-in risk. Around small bright objects on a dark background you may see a faint halo ("blooming"), because a zone is still much larger than a pixel.

Resolution, aspect ratio and pixel density

Resolution is the number of pixels a screen shows, written as width × height: 1920 × 1080 means 1920 columns and 1080 rows. Names like 1080p count the rows (the "p" stands for progressive scan: every line is drawn in each frame). "4K" is looser: it refers to the width of about 4000 pixels, and on monitors and TVs means 4K UHD (ultra high definition), 3840 × 2160.

RESOLUTIONS = {
    "720p (HD)": (1280, 720),
    "1080p (Full HD)": (1920, 1080),
    "1440p (QHD)": (2560, 1440),
    "2160p (4K UHD)": (3840, 2160),
    "5K": (5120, 2880),
    "8K UHD": (7680, 4320),
}

for name, (width, height) in RESOLUTIONS.items():
    pixels = width * height
    print(f"{name:<16} {width:>4} x {height:<4} {pixels:>12,} pixels")

full_hd = 1920 * 1080
uhd = 3840 * 2160
print(f"4K UHD has {uhd // full_hd} times the pixels of 1080p")

720p (HD)        1280 x 720       921,600 pixels
1080p (Full HD)  1920 x 1080    2,073,600 pixels
1440p (QHD)      2560 x 1440    3,686,400 pixels
2160p (4K UHD)   3840 x 2160    8,294,400 pixels
5K               5120 x 2880   14,745,600 pixels
8K UHD           7680 x 4320   33,177,600 pixels
4K UHD has 4 times the pixels of 1080p

Four times the pixels means four times the work for the graphics card per frame, and four times the data per second through the cable. That is why 4K at high refresh rates needs High-Definition Multimedia Interface (HDMI) 2.1 or a recent DisplayPort version.

Native resolution

A flat panel has a fixed grid of physical pixels: its native resolution. Set to anything else, the display must scale the image, spreading one image pixel over parts of several screen pixels, and text turns soft or fuzzy. Always run an LCD or OLED at its native resolution, and use the operating system's scaling setting (125%, 150%) to make text larger instead.

Aspect ratio

The aspect ratio is the ratio of width to height. 16:9 is the standard for TVs, most monitors and video (1920 × 1080, 3840 × 2160). 16:10 is a little taller, which gives more lines of text; many business laptops use it (1920 × 1200). 21:9 is the marketing name for ultrawide monitors such as 3440 × 1440, which replace two side-by-side screens. The older 4:3 ratio still shows up on projectors and old monitors. Showing one ratio on a screen of another leaves black bars (letter- or pillarboxing) or stretches the image.

Pixel density

Pixel density, in pixels per inch (PPI), says how tightly the pixels are packed. The same 1080p looks sharp on a laptop and coarse on a large monitor. To calculate it, take the diagonal in pixels (Pythagoras) and divide it by the diagonal in inches, which is how screens are sold:

import math


def ppi(width, height, diagonal_inches):
    """Pixel density: pixels along the diagonal / diagonal in inches."""
    diagonal_pixels = math.hypot(width, height)
    return diagonal_pixels / diagonal_inches


screens = [
    ("24-inch 1080p office monitor", 1920, 1080, 24),
    ("27-inch 1440p monitor", 2560, 1440, 27),
    ("27-inch 4K monitor", 3840, 2160, 27),
    ("15.6-inch 1080p laptop", 1920, 1080, 15.6),
    ("6.1-inch phone, 2556 x 1179", 2556, 1179, 6.1),
]

for name, width, height, inches in screens:
    print(f"{name:<30} {ppi(width, height, inches):5.0f} PPI")

24-inch 1080p office monitor      92 PPI
27-inch 1440p monitor            109 PPI
27-inch 4K monitor               163 PPI
15.6-inch 1080p laptop           141 PPI
6.1-inch phone, 2556 x 1179      461 PPI

Higher density gives sharper text but smaller interface elements, which is why a 27-inch 4K monitor normally runs at 150% scaling. Viewing distance matters too: a phone is held close, so it needs far more pixels per inch than a TV across the room.

Refresh rate and response time

The refresh rate is how many times per second the display draws a new image, in hertz (Hz). Office monitors run at 60 Hz; gaming monitors at 120, 144, 165, 240 Hz or more. A higher rate makes motion, and even moving the mouse pointer, look smoother. At 60 Hz a new image arrives every 16.7 ms (1000 ÷ 60); at 144 Hz every 6.9 ms; at 240 Hz every 4.2 ms. A high refresh rate only helps if the graphics card produces that many frames per second and the cable and port can carry them; many people buy a 144 Hz monitor and leave it running at 60 Hz because the setting was never changed or the cable is too old.

Response time is how fast a pixel changes from one color to another, usually quoted as gray-to-gray (GtG) in milliseconds. A slow pixel leaves a smear behind moving objects. Makers measure it in their own favorable way, so treat the figure as a rough guide.

Adaptive sync

A graphics card rarely produces frames at exactly the monitor's refresh rate. If the monitor shows a new frame halfway through drawing the old one, the image splits horizontally: screen tearing. Classic vertical sync (V-Sync) makes the card wait for the monitor, which removes tearing but adds stutter and delay. Adaptive sync turns it around: the monitor waits for the card and refreshes the moment a frame is ready. The open standard is VESA (Video Electronics Standards Association) Adaptive-Sync, which AMD markets as FreeSync; NVIDIA's version is G-SYNC, and its "G-SYNC Compatible" monitors use the same open standard. HDMI 2.1 has its own variable refresh rate (VRR).

Brightness, HDR and color

Brightness is measured in nits: one nit is one candela per square meter (cd/m²), the amount of light a screen gives off per area. An office monitor gives about 250 to 350 nits, which is plenty indoors. A laptop used near windows or outside benefits from 400 to 500 nits or more, and HDR screens reach 1000 nits or more in highlights.

High dynamic range (HDR) content uses a wider range from dark to bright, so a sunset can be bright while shadows keep detail. Formats include HDR10 and Dolby Vision. A monitor labeled "HDR" does not necessarily show it well: without high peak brightness and good local dimming (or OLED), HDR can look flat or washed out. VESA's DisplayHDR levels (400, 600, 1000, …) are named after the peak brightness in nits and give a more honest indication.

The color gamut is the range of colors a screen can show, given as a percentage of a standard color space:

• sRGB (standard RGB, where RGB stands for red, green, blue): the standard for the web, Windows and most office work. A typical office monitor covers about all of it.
• DCI-P3: a wider space from the Digital Cinema Initiatives (DCI) standards for film projection, with more saturated reds and greens; used for video and by many phones and laptops.
• Adobe RGB: wide in greens and cyans, used for print photography.

For design work the screen is also calibrated with a colorimeter, a small device hung on the screen that measures what it really shows.

Touchscreens and projectors

Touchscreens

A touchscreen has a touch-sensitive layer, the digitizer, over the display (see Laptops and Mobile Devices for replacing one). Two technologies are common:

• Capacitive: senses the small electrical charge of a finger. It supports multi-touch gestures and is used on phones, tablets and laptops. It does not respond to ordinary gloves or a plain plastic stylus; it needs a conductive tip or an active pen.
• Resistive: two flexible layers that touch when pressed. It works with any object, including gloves, and is found on older point-of-sale terminals and industrial equipment.

When touch input lands in the wrong place, the operating system's calibration tool usually fixes it; when a crack stops touch working in one area, the digitizer needs replacing.

Projectors

A projector throws the image onto a screen or wall. Its brightness is given in lumens (usually ANSI lumens, measured by a method from the American National Standards Institute): the more ambient light in the room and the larger the image, the more lumens you need. The throw ratio is the distance to the screen divided by the image width: with a throw ratio of 1.5, a 2 m wide image needs the projector 3 m away. Short-throw projectors sit close to the wall and keep people out of the beam. Keystone correction straightens the trapezoid shape you get when the projector is not square to the screen.

Lamp-based projectors need a new lamp after some thousands of hours and a clean air filter; let the fan cool the lamp before cutting the power. Laser and LED projectors last much longer. Older projectors often have only VGA or HDMI inputs, which is one reason adapters stay in meeting rooms (see Ports, Connectors and Cables).

Connecting multiple monitors

Most computers can drive two or more displays. Each display needs its own output (or a daisy chain using DisplayPort Multi-Stream Transport, or a dock), and the graphics hardware must support that many screens: see Expansion Cards and Graphics and the video ports. The operating system then offers two main modes:

• Extend: the screens form one large desktop, and you drag windows from one to the other. This is the normal mode for a desk with two monitors.
• Duplicate (mirror): every screen shows the same image, which is what you want for a projector in a presentation. Both screens then use a resolution they share, so one of them may look less sharp.

On Windows, Windows + P switches between "PC screen only", "Duplicate", "Extend" and "Second screen only"; in Settings, System, Display you drag the screens into their real positions, choose the main display and set each screen's resolution and scaling. On Linux, xrandr shows the same layout (illustrative output):

xrandr | grep ' connected'

DP-1 connected primary 2560x1440+0+0 (normal left inverted right x axis y axis) 597mm x 336mm
HDMI-1 connected 1920x1080+2560+0 (normal left inverted right x axis y axis) 527mm x 296mm

The +2560+0 means the HDMI screen starts 2560 pixels to the right of the top-left corner: the desktop is extended, with the second screen to the right of the first. In Duplicate mode both would start at +0+0. The sizes in millimeters are what the monitor reports about itself: 597 × 336 mm is a 27-inch panel.

Common display problems

A quick way to split the problem: if the artifacts or lines also appear on a screenshot, the fault is in the computer (graphics card or driver); if they appear only on the screen, the fault is in the display or its cable. For the full method, see Hardware Troubleshooting.