Monitor

A monitor is an output device that displays a computer's visual output, converting the stream of pixel values produced by a graphics processor into light that a person can see. Its job is simple to state and demanding to do well: it receives frames over a cable, usually HDMI or DisplayPort, and draws a grid of colored dots, redrawing the whole grid many times per second so that images appear steady and motion looks smooth. Monitors are specified by a few headline properties, namely panel technology, resolution, refresh rate and size, and each of them involves its own trade-offs.

Monitor technology has passed through three generations: the cathode ray tube, the liquid crystal display and, more recently, the organic light-emitting diode display. The CRT, descended from the cathode ray tube developed by Karl Ferdinand Braun in 1897, fires an electron beam at a phosphor-coated glass screen and sweeps it across line by line, with a shadow mask separating red, green and blue phosphors; CRTs are bulky and heavy, but they have deep blacks and no fixed pixel grid. The LCD sandwiches liquid crystals between polarizing filters and uses them as tiny light valves in front of a backlight, today almost always LEDs; common panel types are TN, which is fast but has weaker viewing angles, IPS, which has better color and angles, and VA, which has high contrast. An OLED panel has pixels that emit their own light, so a black pixel is simply off, giving near-infinite contrast and fast response, though static images can cause uneven wear known as burn-in. The notation LED monitor usually means an LCD with an LED backlight, not a different display type.

Resolution is the number of pixels along each axis, and it is a different quantity from sharpness. Common values are 1920 by 1080 (Full HD), 2560 by 1440 and 3840 by 2160, the last being the figure called 4K UHD. The same resolution on a larger screen spreads the pixels further apart, so what determines crispness is pixel density, measured in pixels per inch, together with viewing distance; Apple's marketing term Retina names a density high enough that individual pixels are hard to see at normal distance. Higher resolution demands more work and more bandwidth: an uncompressed 3840 by 2160 picture with 24 bits per pixel at 60 frames per second is about 12 gigabits per second before overhead, which is why old cable versions cannot carry it. Running a panel at anything other than its native resolution forces the scaler to interpolate, which blurs text.

Refresh rate is how many times per second the monitor can display a new image, measured in hertz, and it must be told apart from frame rate. A 60 Hz display shows at most 60 distinct frames per second, while a 144 Hz or 240 Hz gaming monitor shows more, giving smoother motion and slightly lower input delay. The graphics card's frame rate is how many frames it actually produces; a monitor cannot show frames that are never rendered, so buying a 240 Hz screen without a machine to feed it gains nothing. Adaptive-sync technologies, including NVIDIA's G-Sync of 2013 and AMD's FreeSync of 2015, let the display change its refresh interval to match the frame rate, preventing tearing. Response time, the speed at which a pixel changes color, is a separate specification, and advertised figures are measured under conditions that vary between makers.

Several practical beliefs about monitors are worth correcting. More pixels do not always help, since a 4K display on a small laptop screen may need scaling to be readable. Color accuracy depends on the gamut it covers, such as sRGB or DCI-P3, and on calibration rather than on resolution. Monitors matter for data work as well: analysts and engineers value additional screen area and accurate color, and a GPU used for machine learning needs no monitor at all, since servers run headless and are used remotely.

Read a monitor as the final stage of the graphics pipeline, a grid of light-emitting or light-controlling dots redrawn many times a second from numbers sent over a cable, whose quality is set jointly by the panel technology, the pixel count and how often the picture is replaced.