A personal computer is a multi-purpose computer whose size and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or technician. Unlike large costly minicomputer and mainframes, time-sharing by many people at the same time is not used with personal computers. Institutional or corporate computer owners in the 1960s had to write their own programs to do any useful work with the machines. While personal computer users may develop their own applications these systems run commercial software, free-of-charge software or free and open-source software, provided in ready-to-run form. Software for personal computers is developed and distributed independently from the hardware or operating system manufacturers. Many personal computer users no longer need to write their own programs to make any use of a personal computer, although end-user programming is still feasible; this contrasts with mobile systems, where software is only available through a manufacturer-supported channel, end-user program development may be discouraged by lack of support by the manufacturer.
Since the early 1990s, Microsoft operating systems and Intel hardware have dominated much of the personal computer market, first with MS-DOS and with Microsoft Windows. Alternatives to Microsoft's Windows operating systems occupy a minority share of the industry; these include free and open-source Unix-like operating systems such as Linux. Advanced Micro Devices provides the main alternative to Intel's processors; the advent of personal computers and the concurrent Digital Revolution have affected the lives of people in all countries. "PC" is an initialism for "personal computer". The IBM Personal Computer incorporated the designation in its model name, it is sometimes useful to distinguish personal computers of the "IBM Personal Computer" family from personal computers made by other manufacturers. For example, "PC" is used in contrast with "Mac", an Apple Macintosh computer.. Since none of these Apple products were mainframes or time-sharing systems, they were all "personal computers" and not "PC" computers.
The "brain" may one day come down to our level and help with our income-tax and book-keeping calculations. But this is speculation and there is no sign of it so far. In the history of computing, early experimental machines could be operated by a single attendant. For example, ENIAC which became operational in 1946 could be run by a single, albeit trained, person; this mode pre-dated the batch programming, or time-sharing modes with multiple users connected through terminals to mainframe computers. Computers intended for laboratory, instrumentation, or engineering purposes were built, could be operated by one person in an interactive fashion. Examples include such systems as the Bendix G15 and LGP-30of 1956, the Programma 101 introduced in 1964, the Soviet MIR series of computers developed from 1965 to 1969. By the early 1970s, people in academic or research institutions had the opportunity for single-person use of a computer system in interactive mode for extended durations, although these systems would still have been too expensive to be owned by a single person.
In what was to be called the Mother of All Demos, SRI researcher Douglas Engelbart in 1968 gave a preview of what would become the staples of daily working life in the 21st century: e-mail, word processing, video conferencing, the mouse. The demonstration required technical support staff and a mainframe time-sharing computer that were far too costly for individual business use at the time; the development of the microprocessor, with widespread commercial availability starting in the mid 1970's, made computers cheap enough for small businesses and individuals to own. Early personal computers—generally called microcomputers—were sold in a kit form and in limited volumes, were of interest to hobbyists and technicians. Minimal programming was done with toggle switches to enter instructions, output was provided by front panel lamps. Practical use required adding peripherals such as keyboards, computer displays, disk drives, printers. Micral N was the earliest commercial, non-kit microcomputer based on a microprocessor, the Intel 8008.
It was built starting in 1972, few hundred units were sold. This had been preceded by the Datapoint 2200 in 1970, for which the Intel 8008 had been commissioned, though not accepted for use; the CPU design implemented in the Datapoint 2200 became the basis for x86 architecture used in the original IBM PC and its descendants. In 1973, the IBM Los Gatos Scientific Center developed a portable computer prototype called SCAMP based on the IBM PALM processor with a Philips compact cassette drive, small CRT, full function keyboard. SCAMP emulated an IBM 1130 minicomputer in order to run APL/1130. In 1973, APL was available only on mainframe computers, most desktop sized microcomputers such as the Wang 2200 or HP 9800 offered only BASIC; because SCAMP was the first to emulate APL/1130 performance on a portable, single user computer, PC Magazine in 1983 designated SCAMP a "revolutionary concept" and "the world's first personal computer". This seminal, single user portable computer now resides in the Smithsonian Institution, Washington, D.
C.. Successful demonstrations of the 1973 SCAMP prototype led to the IBM 5100 portable microcomputer launched in 1975 with the ability to be programmed in both APL and BASIC for engineers, analysts and other business problem-solvers. In the late 1960s such a machine would have been nearly as large as two desks and would have weigh
The control booth, control room, lighting box, technical booth, tech booth, or just boothor the bio box to theatre or television technicians is the area designated for the operation of technical equipment, is sometimes the location of the deputy stage manager's station as well as the lighting controls and sound board. One or two followspots are located in the booth as well, it is an enclosed space with a large sliding window with a good view of the stage. In a proscenium theater, it is centered in the back of the house, it is sometimes placed at the balcony level. It is designed to allow lighting and sound operators to be able to see the performance, without being in the auditorium itself; this means that they are free to talk to their colleagues in the booth, the Stage management team and other crew members via the communications headset. A booth, sealed to the auditorium allows for noisier equipment to be used, in particular computers and computer-based lighting desks, which require built-in fans in order to work properly.
The downside to having a sealed booth is that it can be difficult for the sound engineer to mix without being able to hear what is happening on stage. In this situation a separate table may be set up in the house for the sound engineer. In some smaller theatres, school halls control booths can sometimes be found above or at the side of the stage; this allows space at the back of the auditorium for a better/larger foyer area. In older theatres, this is because before the advent of thyristor dimming and compact electronic control desks, there was a limit on the distance that lighting controls could be placed away from the dimmers. In some theatres, the control booth is divided into a suite of rooms, allowing each of the technical elements of a production its own customized space; this is likely to be the case where a theatre produces performances which require live sound mixing rather than just pre-recorded effects, as a sound operator needs to be able to hear the sound in the auditorium, so not be sealed from it as a lighting operator, followspot operator or projectionist might be.
In some theatres with one or more balconies, the followspots may be given their own room above the highest balcony, with the lighting booth lower down at the rear of the stalls or the first balcony. In rare cases; this however, is a setup, never used because it hinders the possibility of the technician team to communicate to the backstage area without disrupting the performance. The booth contains a variety of equipment used in the production of theatrical performances. Although booths vary from venue to venue, most booths contain a light board, sometimes a sound board. Most control booths have at least one intercom headset used for communication with the backstage crew during performances; the booth may have equipment racks for the audio equipment. Parts of a theater
Dimmers are devices connected to a light fixture and used to lower the brightness of light. By changing the voltage waveform applied to the lamp, it is possible to lower the intensity of the light output. Although variable-voltage devices are used for various purposes, the term dimmer is reserved for those intended to control light output from resistive incandescent and compact fluorescent lights and light-emitting diodes. More specialized equipment is needed to dim fluorescent, mercury vapor, solid-state, other arc lighting. Dimmers range in size from small units the size of domestic light switches to high-power units used in large theatrical or architectural lighting installations. Small domestic dimmers are directly controlled, although remote control systems are available. Modern professional dimmers are controlled by a digital control system like DMX or DALI. In newer systems, these protocols are used in conjunction with ethernet. In the professional lighting industry, changes in intensity are called "fades" and can be "fade up" or "fade down".
Dimmers with direct manual control had a limit on the speed they could be varied at but this problem has been eliminated with modern digital units. Modern dimmers are built from semiconductors instead of variable resistors, because they have higher efficiency. A variable resistor would dissipate power as heat and acts as a voltage divider. Since semiconductor or solid-state dimmers switch between a low resistance "on" state and a high resistance "off" state, they dissipate little power compared with the controlled load. Early dimmers were directly controlled through the manual manipulation of large dimmer panels; this required all power to come through the lighting control location, which could be inconvenient and dangerous for large or high-powered systems, such as those used for stage lighting. In 1896, Granville Woods patented his "Safety Dimmer", which reduced wasted energy by reducing the amount of energy generated to match desired demand rather than burning off unwanted energy. In 1959, Joel S. Spira, who would found the Lutron Electronics Company in 1961, invented a dimmer based on a diode and a tapped autotransformer, saving energy and allowing the dimmer to be installed in a standard electrical wallbox.
In 1966, Eugene Alessio patented a light bulb socket adapter for adjusting a light level on a single light bulb using a triac. To house this device, he decided on a 2-inch round device with one end capable of being screwed into a light bulb socket and the other end able to receive a light bulb; when solid-state dimmers came into use, analog remote control systems became feasible. The wire for the control systems was much smaller than the heavy power cables of previous lighting systems; each dimmer had its own control wires, resulting in many wires leaving the lighting control location. More recent digital control protocols such as DMX512, DALI, or one of the many Ethernet-based protocols like Art-Net, ETCnet, sACN, ShowNet or KiNET enable the control of a large number of dimmers through a single cable. Dimmers based on rheostats were inefficient since they would dissipate a significant portion of the power rating of the load as heat, they were large and required plenty of cooling air. Because their dimming effect depended a great deal on the total load applied to each rheostat, the load needed to be matched carefully to the power rating of the rheostat.
As they relied on mechanical control they were slow and it was difficult to change many channels at a time. Early examples of a rheostat dimmer include liquid rheostat; the closer the contacts to each other, the more voltage was available for the light. Salt water dimmers required regular addition of maintenance due to corrosion; the coil-rotation transformer used a fixed-position electromagnet coil in conjunction with a variable-position coil to vary the voltage in the line by varying the alignment of the two coils. Rotated 90 degrees apart, the secondary coil is affected by two equal but opposite fields from the primary, which cancel each other out and produce no voltage in the secondary; these coils resembled the standard rotor and stator as used in an electric motor, except that the rotor was held against rotation using brakes and was moved to specific positions using high-torque gearing. Because the rotor did not turn a complete revolution, a commutator was not required and long flexible cables could be used on the rotor instead.
Variable autotransformers were introduced. While they are still nearly as large as rheostat dimmers, which they resemble, they are efficient devices, their voltage output, so their dimming effect, is independent of the load applied so it was far easier to design the lighting that would be attached to each autotransformer channel. Remote control of the dimmers was still unpractical, although some dimmers were equipped with motor drives that could and reduce or increase the brightness of the attached lamps. Autotransformers are used for other applications. Solid-state or semiconductor dimmers were introduced to solve some of these problems. Semiconductor dimmers switch on at an adjustable time after the start of each alternating current half-cycle, thereby
DMX512 is a standard for digital communication networks that are used to control stage lighting and effects. It was intended as a standardized method for controlling light dimmers, prior to DMX512, had employed various incompatible proprietary protocols, it soon became the primary method for linking controllers to dimmers and special effects devices such as fog machines and intelligent lights. DMX has expanded to uses in non-theatrical interior and architectural lighting, at scales ranging from strings of Christmas lights to electronic billboards. DMX can now be used to control anything, reflecting its popularity in theaters and venues. DMX512 employs EIA-485 differential signaling at its physical layer, in conjunction with a variable-size, packet-based communication protocol, it is unidirectional. DMX512 does not include automatic error checking and correction, so is not an appropriate control for hazardous applications, such as pyrotechnics or movement of theatrical rigging. False triggering may be caused by electromagnetic interference, static electricity discharges, improper cable termination, excessively long cables, or poor quality cables.
Developed by the Engineering Commission of United States Institute for Theatre Technology, the DMX512 standard was created in 1986, with subsequent revisions in 1990 leading to USITT DMX512/1990. In 1998 the Entertainment Services and Technology Association began a revision process to develop the standard as an ANSI standard; the resulting revised standard, known as "Entertainment Technology—USITT DMX512-A—Asynchronous Serial Digital Data Transmission Standard for Controlling Lighting Equipment and Accessories", was approved by the American National Standards Institute in November 2004. It was revised again in 2008, is the current standard known as "E1.11 – 2008, USITT DMX512-A", or just "DMX512-A". A DMX512 network employs a multi-drop bus topology with nodes strung together in what is called a daisy chain. A network consists of a single DMX512 controller –, the master of the network — and one or more slave devices. For example, a lighting console is employed as the controller for a network of slave devices such as dimmers, fog machines and intelligent lights.
Each slave device has a DMX512 "IN" connector and an "OUT" connector as well. The controller, which has only an OUT connector, is connected via a DMX512 cable to the IN connector of the first slave. A second cable links the OUT or THRU connector of the first slave to the IN connector of the next slave in the chain, so on. For example, the block diagram below shows a simple network consisting of a controller and three slaves; the specification requires a'terminator' to be connected to the final OUT or THRU connector of the last slave on the daisy chain, which would otherwise be unconnected. A terminator is a stand-alone male connector with an integral 120 Ω resistor connected across the primary data signal pair. If a secondary data pair is used, a termination resistor is connected across it as well. Although simple systems will sometimes function without a terminator, the standard requires its use; some DMX slave devices have built-in terminators that can be manually activated with a mechanical switch or by software, or by automatically sensing the absence of a connected cable.
A DMX512 network is called a "DMX universe". Each OUT connector on a DMX512 controller can control a single universe. Smaller controllers may have a single OUT connector, enabling them to control only one universe, whereas large control desks may have the capacity to control multiple universes, with an OUT connector provided for each universe. DMX512 data is transmitted over a differential pair using EIA-485 voltage levels. DMX512 electrical specifications are identical to those of the EIA-485-A standard, except where stated otherwise in E1.11. DMX512 is a bus network no more than 400 metres long, with not more than 32 unit loads on a single bus. If more than 32 unit loads need to communicate, the network can be expanded across parallel buses using DMX splitters. Network wiring consists of a shielded twisted pair, with a characteristic impedance of 120 Ohms, with a termination resistor at the end of the cable furthest from the controller to absorb signal reflections. DMX512 has two twisted pair data paths, although specification only defines the use of one of the twisted pairs.
The second pair is undefined, but required by the electrical specification. For short cable runs of less than about 45 metres with only a few devices, it is sometimes possible to operate without termination. At short distances, cables with higher capacitance and different characteristic impedance such as microphone cable can be used; as the cable length or number of devices increases, following the specification for termination and correct cable impedance becomes more important. The E1.11 electrical specification addresses the connection of DMX512 signal common to Earth ground. The standard recommends that transmitter ports have a low impedance connection between signal common and ground, it is further recommended that receivers have a high impedance connection between signal common and ground. The standard allows for isolated transmitter ports; the standard allows for non-isolated receivers. The standard recommends that systems ground the signal common at only one p
A joystick is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. A joystick known as the control column, is the principal control device in the cockpit of many civilian and military aircraft, either as a center stick or side-stick, it has supplementary switches to control various aspects of the aircraft's flight. Joysticks are used to control video games, have one or more push-buttons whose state can be read by the computer. A popular variation of the joystick used on modern video game consoles is the analog stick. Joysticks are used for controlling machines such as cranes, underwater unmanned vehicles, surveillance cameras, zero turning radius lawn mowers. Miniature finger-operated joysticks have been adopted as input devices for smaller electronic equipment such as mobile phones. Joysticks originated as controls for aircraft ailerons and elevators, are first known to have been used as such on Louis Bleriot's Bleriot VIII aircraft of 1908, in combination with a foot-operated rudder bar for the yaw control surface on the tail.
The name "joystick" is thought to originate with early 20th century French pilot Robert Esnault-Pelterie. There are competing claims on behalf of fellow pilots Robert Loraine, James Henry Joyce, A. E. George. Loraine is cited by the Oxford English Dictionary for using the term "joystick" in his diary in 1909 when he went to Pau to learn to fly at Bleriot's school. George was a pioneer aviator who with his colleague Jobling built and flew a biplane at Newcastle in England in 1910, he is alleged to have invented the "George Stick". The George and Jobling aircraft control column is in the collection of the Discovery Museum in Newcastle upon Tyne, England. Joysticks were present in early planes; the coining of the term "joystick" may be credited to Loraine, as his is the earliest known usage of the term, although he most did not invent the device. The electrical two-axis joystick was invented by C. B. Mirick at the United States Naval Research Laboratory and patented in 1926". NRL was developing remote controlled aircraft at the time and the joystick was used to support this effort.
In the awarded patent, Mirick writes: "My control system is applicable in maneuvering aircraft without a pilot."The Germans developed an electrical two-axis joystick around 1944. The device was used as part of the Germans' Funkgerät FuG 203 Kehl radio control transmitter system used in certain German bomber aircraft, used to guide both the rocket-boosted anti-ship missile Henschel Hs 293, the unpowered pioneering precision-guided munition Fritz-X, against maritime and other targets. Here, the joystick of the Kehl transmitter was used by an operator to steer the missile towards its target; this joystick had on-off switches rather than analogue sensors. Both the Hs 293 and Fritz-X used FuG 230 Straßburg radio receivers in them to send the Kehl's control signals to the ordnance's control surfaces. A comparable joystick unit was used for the contemporary American Azon steerable munition to laterally steer the munition in the yaw axis only; this German invention was picked up by someone in the team of scientists assembled at the Heeresversuchsanstalt in Peenemünde.
Here a part of the team on the German rocket program was developing the Wasserfall missile, a variant of the V-2 rocket, the first ground-to-air missile. The Wasserfall steering equipment converted the electrical signal to radio signals and transmitted these to the missile. In the 1960s the use of joysticks became widespread in radio-controlled model aircraft systems such as the Kwik Fly produced by Phill Kraft; the now-defunct Kraft Systems firm became an important OEM supplier of joysticks to the computer industry and other users. The first use of joysticks outside the radio-controlled aircraft industry may have been in the control of powered wheelchairs, such as the Permobil. During this time period NASA used joysticks as control devices as part of the Apollo missions. For example, the lunar lander test models were controlled with a joystick. In many modern airliners aircraft, for example all Airbus aircraft developed from the 1980s, the joystick has received a new lease on life for flight control in the form of a "side-stick", a controller similar to a gaming joystick but, used to control the flight, replacing the traditional yoke.
The sidestick saves weight, improves movement and visibility in the cockpit, may be safer in an accident than the traditional "control yoke". Ralph H. Baer, inventor of television video games and the Magnavox Odyssey console, released in 1972, created the first video game joysticks in 1967, they were able to control the vertical position of a spot displayed on a screen. The earliest known electronic game joystick with a fire button was released by Sega as part of their 1969 arcade game Missile, a shooter simulation game that used it as part of an early dual-control scheme, where two directional buttons are used to move a motorized tank and a two-way joystick is used to shoot and steer the missile onto oncoming planes displayed on the screen. In 1970, the game was released in North America as S. A. M. I. by Midway Games. Taito released a four-way joystick as part of their arcade racing video game Astro Race in 1973, while their 1975 run and gun multi-directional shooter game Western Gun introduced dual-stick controls with one eight-way joystick for movement and the other for changing the shooting direction.
In North Americ
A gobo is a stencil or template placed inside or in front of a light source to control the shape of the emitted light. Lighting designers use them with stage lighting instruments to manipulate the shape of the light cast over a space or object—for example to produce a pattern of leaves on a stage floor. Sources The term "gobo" has come to sometimes refer to any device that produces patterns of light and shadow, various pieces of equipment that go before a light. In theatrical lighting, the term more refers to a device placed in'the gate' or at the'point of focus' between the light source and the lenses; this placement is important because it produces design. Gobos placed after the optics do not produce a finely focused image, are more called "flags" or "cucoloris"; the exact derivation of gobo is unclear. It is cited by some lighting professionals as "goes before optics" or, less "goes between optics". An alternative explanation is "graphical optical black out." The term is traced back to the 1930s, originated in reference to a screen or sheet of sound-absorbent material for shielding a microphone from sounds coming from a particular direction, with no application to optics.
The treatment of the word as an acronym is recent and ignores the original definition in favor of popular invention. There are many online examples of acoustic gobos; the word most is a derivative of "goes between." Gobos are used with projectors and simpler light sources to create lighting scenes in theatrical applications. Simple gobos, incorporated into automated lighting systems, are popular at nightclubs and other musical venues to create moving shapes. Gobos may be used for architectural lighting, as well as in interior design, as in projecting a company logo on a wall. Gobos are made of various materials. Common types include steel and plastic. Steel gobos or metal gobos use a metal template from; these are the most sturdy, but require modifications to the original design—called bridging—to display correctly. To represent the letter "O" for example, requires small tabs or bridges to support the opaque center of the letter; these can be visible in the projected image. Glass gobos are made from clear glass with a partial mirror coating to block the light and produce "black" areas in the projected image.
This accommodates more intricate images. Glass gobos can include colored areas, whether by multiple layers of dichroic glass glued on an aluminium or chrome coated black and white gobo, or by newer technologies that vary the thickness of the dichroic coating in a controlled way on a single piece of glass—which makes it possible to turn a color photo into a glass gobo. Glass gobos offer the highest image fidelity, but are the most fragile. Glass gobos are created with laser ablation or photo etching. Plastic gobos or Transparency gobos can be used in LED ellipsoidal spotlights; these "LED Only" plastic gobos are far less delicate. They are new to the market, as are LED lights, their durability and effectiveness vary between brands. In the past, plastic gobos were custom made for when a pattern requires color and glass does not suffice. However, in a "traditional" light fixture, the focus point position of a gobo is hot, so these thin plastic films require special cooling elements to prevent melting.
A lapse in the cooling apparatus for seconds, can ruin a plastic a gobo in a tungsten-halogen lighting instrument. Theatrical and photographic supply companies manufacture many complex stock patterns, they can produce custom gobos from customer artwork. A lighting designer chooses a pattern from a manufacturer's catalog; because of the large number of gobos available, they are referred to by number, not name. Lighting technicians can hand cut custom gobos out of sheet metal stock, or aluminum pie tins. Gobos are used in weddings and corporate events, they can project the couple's names, or just about any artwork. Some companies can turn a custom gobo out in as little as a week. Designers use "stock" gobo patterns for these events—for example for projecting stars or leaves onto the ceiling; the gobo is placed in the focal plane of the lantern. The gobo is inserted back-to-front; the lighting instrument inverts the projected image. The term "gobo" is used to describe black panels of different sizes or shapes placed between a light source and photographic subject to control the modeling effect of the existing light.
It is the opposite of a photographer using a "reflector" to redirect light into a shadow, "additive" lighting and most used. Use of a gobo subtracts light from a portion of an overall shaded subject and creates a contrast between one side of the face and another, it allows the photographer to expose with wider open apertures giving soft natural transitions between the sharp subject and unsharp background, called bokeh. Bat-Signal
A liquid-crystal display is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome. LCDs are available to display arbitrary images or fixed images with low information content, which can be displayed or hidden, such as preset words and seven-segment displays, as in a digital clock, they use the same basic technology, except that arbitrary images are made up of a large number of small pixels, while other displays have larger elements. LCDs can either be on or off, depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background, the color of the backlight, a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.
LCDs are used in a wide range of applications, including LCD televisions, computer monitors, instrument panels, aircraft cockpit displays, indoor and outdoor signage. Small LCD screens are common in portable consumer devices such as digital cameras, watches and mobile telephones, including smartphones. LCD screens are used on consumer electronics products such as DVD players, video game devices and clocks. LCD screens have replaced bulky cathode ray tube displays in nearly all applications. LCD screens are available in a wider range of screen sizes than CRT and plasma displays, with LCD screens available in sizes ranging from tiny digital watches to large television receivers. LCDs are being replaced by OLEDs, which can be made into different shapes, have a lower response time, wider color gamut infinite color contrast and viewing angles, lower weight for a given display size and a slimmer profile and lower power consumption. OLEDs, are more expensive for a given display size due to the expensive electroluminescent materials or phosphors that they use.
Due to the use of phosphors, OLEDs suffer from screen burn-in and there is no way to recycle OLED displays, whereas LCD panels can be recycled, although the technology required to recycle LCDs is not yet widespread. Attempts to increase the lifespan of LCDs are quantum dot displays, which offer similar performance as an OLED display, but the Quantum dot sheet that gives these displays their characteristics can not yet be recycled. Since LCD screens do not use phosphors, they suffer image burn-in when a static image is displayed on a screen for a long time, e.g. the table frame for an airline flight schedule on an indoor sign. LCDs are, susceptible to image persistence; the LCD screen can be disposed of more safely than a CRT can. Its low electrical power consumption enables it to be used in battery-powered electronic equipment more efficiently than CRTs can be. By 2008, annual sales of televisions with LCD screens exceeded sales of CRT units worldwide, the CRT became obsolete for most purposes.
Each pixel of an LCD consists of a layer of molecules aligned between two transparent electrodes, two polarizing filters, the axes of transmission of which are perpendicular to each other. Without the liquid crystal between the polarizing filters, light passing through the first filter would be blocked by the second polarizer. Before an electric field is applied, the orientation of the liquid-crystal molecules is determined by the alignment at the surfaces of electrodes. In a twisted nematic device, the surface alignment directions at the two electrodes are perpendicular to each other, so the molecules arrange themselves in a helical structure, or twist; this induces the rotation of the polarization of the incident light, the device appears gray. If the applied voltage is large enough, the liquid crystal molecules in the center of the layer are completely untwisted and the polarization of the incident light is not rotated as it passes through the liquid crystal layer; this light will be polarized perpendicular to the second filter, thus be blocked and the pixel will appear black.
By controlling the voltage applied across the liquid crystal layer in each pixel, light can be allowed to pass through in varying amounts thus constituting different levels of gray. Color LCD systems use the same technique, with color filters used to generate red and blue pixels; the optical effect of a TN device in the voltage-on state is far less dependent on variations in the device thickness than that in the voltage-off state. Because of this, TN displays with low information content and no backlighting are operated between crossed polarizers such that they appear bright with no voltage; as most of 2010-era LCDs are used in television sets and smartphones, they have high-resolution matrix arrays of pixels to display arbitrary images using backlighting with a dark background. When no image is displayed, different arrangements are used. For this purpose, TN LCDs are operated between parallel polarizers, whereas IPS LCDs feature crossed polarizers. In many applications IPS LCDs have replaced TN LCDs, in particular in smartphones su