An odd state of matter
Liquid crystals are materials that flow like a liquid but have molecules that tend to line up in an ordered way, like a crystal. Their orientation can be changed by a small electric field, and that change can alter how light passes through them. The substance was first described in the late 1800s, and for decades it was mostly a laboratory curiosity. In the 1960s and 1970s, researchers in several countries, including teams at companies and universities in the United States, Britain, Japan and Switzerland, worked on turning it into practical display devices.
How an LCD makes an image
An LCD does not generate light itself. Instead, a typical panel places a layer of liquid crystal between two sheets of glass and two polarizing filters. With no voltage, the crystal twists the light’s polarization so it passes through the second filter. When voltage is applied, the molecules change alignment and the light is blocked. Each tiny region that can be controlled separately is a pixel.
To show color, each pixel is split into three sub-pixels with red, green and blue filters. A light source behind the panel, called a backlight, provides the brightness. Early backlights used small fluorescent lamps, and later panels commonly switched to light-emitting diodes, which reduced thickness and power use.
Watches, calculators and the first uses
The first widespread LCDs were simple. In the 1970s, they appeared in digital watches and pocket calculators, where a few segments could show digits. They used very little power, which suited small battery-powered products, and the displays could be read in bright light because they reflected ambient light instead of competing with it.
Showing a full computer screen was harder. Early panels were small, slow and had low contrast, and addressing many individual pixels was difficult with simple wiring. The solution that mattered most was the active-matrix design, in which every pixel has its own transistor, typically a thin-film transistor, to hold its state. That approach made sharper, faster, more reliable screens and was refined through the 1980s and 1990s.
Laptops and portables
The portable computer was a big driver for flat panels. A cathode ray tube was too deep, heavy and power-hungry to put in a machine meant to be carried. Early laptops of the 1980s used monochrome LCDs, often with limited viewing angles and poor contrast, and some needed good lighting to be readable. Backlit models improved things, and color active-matrix displays became widespread in laptops during the 1990s.
Without a practical flat display, the whole category of laptop computers would have been very different. The same applies to handheld devices, such as personal digital assistants and early handheld consoles, which needed small, low-power screens. Many of these early displays were slow, and moving images could smear or ghost.
Desktop monitors and televisions
By the late 1990s and early 2000s, LCD monitors were available for desktop computers, though at first they were considerably more expensive than tube monitors of comparable size. They offered a sharp, flicker-free image in a thin case, took up less desk space and used less power. As manufacturing improved and prices fell, they began to outsell tube monitors in the first decade of the 2000s.
Flat panels also took over television. The same technology that made computer monitors thin allowed large wall-mounted sets, and the line between computer screens and televisions blurred, with many televisions accepting computer connections and vice versa.
Smartphones and touch
Flat panels also made modern phones possible. A touch-sensitive layer over an LCD, described in the history of touchscreens, allowed the whole front of a device to serve as both display and input. Smartphones of the 2000s and 2010s typically used LCD or OLED screens, with higher pixel density to make text and pictures look smooth. Brightness, color reproduction, response time and power use all improved gradually.
Strengths and weaknesses
LCDs have some well-known characteristics:
- Thin, light and relatively efficient compared with tubes.
- Sharp at their native resolution, but less flexible than tubes when run at other resolutions.
- Viewing angles and contrast depend on the panel type, and dark scenes can look gray because the backlight is always partly on.
- Response time can cause blur in fast motion, though it improved over time.
Different panel designs, with names such as twisted nematic, in-plane switching and vertical alignment, trade off cost, color accuracy, angle and speed. Since the details change quickly, it is wise to treat any specific performance comparison as dated.
Beyond LCD
Newer technologies have added choices. OLED panels create light at each pixel, so black can be truly dark and thinner or flexible designs are possible. Microscopic LED approaches are also under development. LCD remains widely used because it is mature and economical, but it no longer has the field to itself.
The move from tubes to flat panels changed more than the shape of computers. It made them portable, made screens a part of nearly every pocket and desk, and made it practical to put displays into a wide range of devices, from cash registers to car dashboards.