A projection screen is an installation consisting of a surface and a support structure used for displaying a projected image for the view of an audience. Projection screens may be permanently installed, as in a movie theater; as in a conference room or other non-dedicated viewing space. Another popular type of portable screens are inflatable screens for outdoor movie screening. Uniformly white or grey screens are used exclusively as to avoid any discoloration to the image, while the most desired brightness of the screen depends on a number of variables, such as the ambient light level and the luminous power of the image source. Flat or curved screens may be used depending on the optics used to project the image and the desired geometrical accuracy of the image production, flat screens being the more common of the two. Screens can be further designed for front or back projection, the more common being front projection systems, which have the image source situated on the same side of the screen as the audience.
Different markets exist for screens targeted for use with digital projectors, movie projectors, overhead projectors and slide projectors, although the basic idea for each of them is much the same: front projection screens work on diffusely reflecting the light projected on to them, whereas back projection screens work by diffusely transmitting the light through them. In the commercial movie theaters, the screen is a reflective surface that may be either aluminized or a white surface with small glass beads; the screen has hundreds of small, evenly spaced holes to allow air to and from the speakers and subwoofer, which are directly behind it. Rigid wall-mounted screens maintain their geometry just like the big movie screens, which makes them suitable for applications that demand exact reproduction of image geometry; such screens are used in home theaters, along with the pull-down screens. Pull-down screens are used in spaces where a permanently installed screen would require too much space; these use painted fabric, rolled in the screen case when not used, making them less obtrusive when the screen is not in use.
Fixed-frame screens provide the greatest level of uniform tension on the screens surface, resulting in the optimal image quality. They are used in home theater and professional environments where the screen does not need to be recessed into the case. Electric screens can be wall ceiling mounted or ceiling recessed; these are larger screens, though electric screens are available for home theater use as well. Electric screens are similar to pull-down screens, but instead of the screen being pulled down manually, an electric motor raises and lowers the screen. Electric screens are raised or lowered using either a remote control or wall-mounted switch, although some projectors are equipped with an interface that connects to the screen and automatically lowers the screen when the projector is switched on and raises it when the projector is switched off. Switchable projection screens can be switched between clear. In the opaque state, projected image on the screen can be viewed from both sides, it is good for advertising on store windows.
Mobile screens use either a pull-down screen on a free stand, or pull up from a weighted base. These can be used when it is impractical to mount the screen to a wall or a ceiling. Both mobile and permanently installed pull-down screens may be of tensioned or not tensioned variety. Tensioned models attempt to keep the fabric flat and immobile, whereas the not tensioned models have the fabric of the screen hanging from their support structures. In the latter screens the fabric can stay immobile if there are currents of air in the room, giving imperfections to the projected image. Specialty screens may not fall into any of these categories; these include inflatable screens and others. See the respective articles for more information. One of the most quoted properties in a home theater screen is the gain; this is a measure of reflectivity of light compared to a screen coated with magnesium carbonate, titanium dioxide, or barium sulfate when the measurement is taken for light targeted and reflected perpendicular to the screen.
Titanium dioxide is a bright white colour, but greater gains can be accomplished with materials that reflect more of the light parallel to projection axis and less off-axis. Quoted gain levels of various materials range from 0.8 of light grey matte screens to 2.5 of the more reflective glass bead screens, some manufacturers claiming higher numbers for their products. High gain levels could be attained by using a mirror surface, although the audience would just see a reflection of the projector, defeating the purpose of using a screen. Many screens with higher gain are semi-glossy, so exhibit more mirror-like properties, namely a bright "hot spot" in the screen—an enlarged reflection of the projector’s lens. Opinions differ as to when this "hot spotting" begins to be distracting, but most viewers do not notice differences as large as 30% in the image luminosity, unless presented with a test image and asked to look for variations in brightness; this is possible because humans have greater sensitivity to contrast in smaller details, but less so in luminosity variations as great as half of the screen.
Other screens with higher gain are semi-retroreflective. Unlike mirrors, retroreflective surfaces reflect light back toward the source. Hot spotting is less of a problem with retroreflectiv
County Londonderry known as County Derry, is one of the six counties of Northern Ireland. Prior to the partition of Ireland, it was one of the counties of the Kingdom of Ireland from 1613 onward and of the United Kingdom after the Acts of Union 1800. Adjoining the north-west shore of Lough Neagh, the county covers an area of 2,074 km² and today has a population of about 247,132. Since 1972, the counties in Northern Ireland, including Londonderry, have no longer been used by the state as part of the local administration. Following further reforms in 2015, the area is now governed under three different districts. Despite no longer being used for local government and administrative purposes, it is sometimes used in a cultural context in All-Ireland sporting and cultural events. Since 1981, it has become one of four counties in Northern Ireland that has a Catholic majority, with 57% of the Catholic population residing within Derry City Council; the county flower is the Purple Saxifrage. The place name Derry is an anglicisation of the old Irish Daire, meaning "oak-grove" or "oak-wood".
As with the city, its name is subject to the Derry/Londonderry name dispute, with the form "Londonderry" preferred by unionists and "Derry" by nationalists. British authorities use the name "Londonderry". Mountsandel located near Coleraine in County Londonderry is "perhaps the oldest recorded settlement within Ireland". At an early period, what became the county of Coleraine was inhabited by the O'Cahans, who were tributary to the O'Neills. Towards the close of the reign of Elizabeth I their territory was seized by England, with the purpose of checking the power of the O'Neills, was made the county of Coleraine, named after the regional capital. A short description of County Coleraine is given in Harris's Hibernica, in Captain Pynnar's Survey of the Escheated Counties of Ulster, Anno 1618: On 2 March 1613, James I granted a charter to The Honourable The Irish Society to undertake the plantation of a new county; this county was named a combination of London and Derry. This charter declared that the "City of Londonderry" and everything contained within the new county: This new county would comprise the County Coleraine—which consisted of the baronies of Tirkeeran and Keenaght—and at the behest of The Irish Society the following additional territory was added: all but the south-west corner of the barony of Loughinsholin a part of County Tyrone, as it had sufficient wood for construction.
The Irish Society was made up of the twelve main livery companies of London, which themselves were composed of various guilds. Whilst The Irish Society as a whole was given possession of the city of Londonderry and Coleraine, the individual companies were each granted an estimated 3,210 acres throughout the county; these companies and the sites of their headquarters were: Clothworkers, based at Killowen and Clothworker's Hall in the barony of Coleraine. As a result of the Local Government Act 1898, the city was detached from the county for administrative purposes, becoming a separate county borough from 1899; the county town of County Londonderry, seat of the Londonderry County Council until its abolition in 1973, was therefore moved to the town of Coleraine. The highest point in the county is the summit of Sawel Mountain on the border with County Tyrone. Sawel is part of the Sperrin Mountains. To the east and west, the land falls into the valleys of the Foyle rivers respectively; the county is home to a number of important buildings and landscapes, including the well-preserved 17th-century city walls of Derry.
A Parclose screen is a screen or railing used to enclose or separate-off a chantry chapel, tomb or manorial chapel, from public areas of a church, for example from the nave or chancel. It should be distinguished from the chancel screen which separates the chancel from the nave, in order to restrict access to the former to clerics and other select persons; as many chantry chapels and manorial chapels were situated at the east end of the north or south aisles, next to the chancel they lay within the area enclosed by the chancel screen. The parclose screen is designed to restrict physical access to those unauthorised to enter, yet still to allow a good view into the restricted area and the entry of sunlight, most to allow for communication with the high altar in the chancel during the elevation of the host at mass; this is achieved by the use of tracery to form the screen. Where a solid masonry wall is used instead of a screen, a hagioscope or squint is required to serve the same purpose. Parclose screens are made of stone or wood and are decoratively carved featuring the coats of arms of the family concerned.
The word derives from the French noun parclose, from the Latin verb claudo, "to close" plus the preposition per, "through, over". In England the use of parclose screens was discontinued in the 16th century after the Reformation, after the Dissolution of the Monasteries when chantries were dissolved. There was therefore no further need to have several altars in the same church, each serving a separate private chantry chapel; the concept of the manorial chapel was discontinued a few centuries after, when burials inside churches and manorial chapels were discontinued. The manorial pew, not screened-off from the congregation, replaced the screened-off manorial chapel. Many fine examples of mediaeval parclose screens survive in the parish churches and cathedrals of England
Screen printing is a printing technique whereby a mesh is used to transfer ink onto a substrate, except in areas made impermeable to the ink by a blocking stencil. A blade or squeegee is moved across the screen to fill the open mesh apertures with ink, a reverse stroke causes the screen to touch the substrate momentarily along a line of contact; this causes the ink to wet the substrate and be pulled out of the mesh apertures as the screen springs back after the blade has passed. One color is printed at a time, so several screens can be used to produce a multicoloured image or design. There are various terms used for what is the same technique. Traditionally the process was called screen printing or silkscreen printing because silk was used in the process, it is known as serigraphy, serigraph printing. Synthetic threads are used in the screen printing process; the most popular mesh in general use is made of polyester. There are special-use mesh materials of nylon and stainless steel available to the screen printer.
There are different types of mesh size which will determine the outcome and look of the finished design on the material. Screen printing first appeared in a recognizable form in China during the Song Dynasty, it was adapted by other Asian countries like Japan, was furthered by creating newer methods. Screen printing was introduced to Western Europe from Asia sometime in the late 18th century, but did not gain large acceptance or use in Europe until silk mesh was more available for trade from the east and a profitable outlet for the medium discovered. Early in the 1910s, several printers experimenting with photo-reactive chemicals used the well-known actinic light–activated cross linking or hardening traits of potassium, sodium or ammonium chromate and dichromate chemicals with glues and gelatin compounds. Roy Beck, Charles Peter and Edward Owens studied and experimented with chromic acid salt sensitized emulsions for photo-reactive stencils; this trio of developers would prove to revolutionize the commercial screen printing industry by introducing photo-imaged stencils to the industry, though the acceptance of this method would take many years.
Commercial screen printing now uses sensitizers less toxic than bichromates. There are large selections of pre-sensitized and "user mixed" sensitized emulsion chemicals for creating photo-reactive stencils. A group of artists who formed the National Serigraph Society, including WPA artists Max Arthur Cohn and Anthony Velonis, coined the word Serigraphy in the 1930s to differentiate the artistic application of screen printing from the industrial use of the process."Serigraphy" is a compound word formed from Latin "sēricum" and Greek "graphein". The Printers' National Environmental Assistance Center says "Screenprinting is arguably the most versatile of all printing processes. Since rudimentary screenprinting materials are so affordable and available, it has been used in underground settings and subcultures, the non-professional look of such DIY culture screenprints have become a significant cultural aesthetic seen on movie posters, record album covers, shirts, commercial fonts in advertising, in artwork and elsewhere.
Credit is given to the artist Andy Warhol for popularising screen printing as an artistic technique. Warhol's silk screens include his 1962 Marilyn Diptych, a portrait of the actress Marilyn Monroe printed in bold colours. Warhol was supported in his production by master screen printer Michel Caza, a founding member of Fespa. Sister Mary Corita Kent, gained international fame for her vibrant serigraphs during the 1960s and 1970s, her works were rainbow colored, contained words that were both political and fostered peace and love and caring. American entrepreneur and inventor Michael Vasilantone started to use and sell a rotatable multicolour garment screen printing machine in 1960. Vasilantone filed for patent on his invention in 1967 granted number 3,427,964 on February 18, 1969; the original machine was manufactured to print logos and team information on bowling garments but soon directed to the new fad of printing on T-shirts. The Vasilantone patent was licensed by multiple manufacturers, the resulting production and boom in printed T-shirts made this garment screen printing machine popular.
Screen printing on garments accounts for over half of the screen printing activity in the United States. Graphic screenprinting is used today to create mass or large batch produced graphics, such as posters or display stands. Full colour prints can be created by printing in CMYK. Screen printing lends itself well to printing on canvas. Andy Warhol, Arthur Okamura, Robert Rauschenberg, Roy Lichtenstein, Harry Gottlieb and many other artists have used screen printing as an expression of creativity and artistic vision. Another variation, digital hybrid screen printing is a union between analog screen printing and traditional digital direct to garment printing, two of the most common textile embellishment technologies in use today. Digital hybrid screen printing is an automatic screen-printing press with a CMYK digital enhancement located on one of the screen print stations. Digital hybrid screen printing is capable of variable data options, creating endless customizations, with the added ability of screen print specific techniques.
A screen is made of a piece of mesh stretched over a frame. The mesh could be made of a synthetic polymer, such as nylon, a finer and smaller aperture for the mesh would be utilized for a design that requires a higher and more delicate degree of detail. For the mesh to be effective, it must be mounted on a frame and it must be unde
An electrical cable is an assembly of one or more wires running side by side or bundled, used to carry electric current. The term cable referred to a nautical line of specific length where multiple ropes are combined to produce a strong thick line, used to anchor large ships; as electric technology developed, people changed from using bare copper wire to using groupings of wires and various sheathing and shackling methods that resembled the mechanical cabling so the term was adopted for electrical wiring. In the 19th century and early 20th century, electrical cable was insulated using cloth, rubber or paper. Plastic materials are used today, except for high-reliability power cables; the term has come to be associated with communications because of its use in electrical communications. Electrical cables are used to connect two or more devices, enabling the transfer of electrical signals or power from one device to the other. Cables are used for a wide range of purposes, each must be tailored for that purpose.
Cables are used extensively in electronic devices for signal circuits. Long-distance communication takes place over undersea cables. Power cables are used for bulk transmission of alternating and direct current power using high-voltage cable. Electrical cables are extensively used in building wiring for lighting and control circuits permanently installed in buildings. Since all the circuit conductors required can be installed in a cable at one time, installation labor is saved compared to certain other wiring methods. Physically, an electrical cable is an assembly consisting of one or more conductors with their own insulations and optional screens, individual covering, assembly protection and protective covering. Electrical cables may be made more flexible by stranding the wires. In this process, smaller individual wires are twisted or braided together to produce larger wires that are more flexible than solid wires of similar size. Bunching small wires before concentric stranding adds the most flexibility.
Copper wires in a cable may be bare, or they may be plated with a thin layer of another metal, most tin but sometimes gold, silver or some other material. Tin and silver are much less prone to oxidation than copper, which may lengthen wire life, makes soldering easier. Tinning is used to provide lubrication between strands. Tinning was used to help removal of rubber insulation. Tight lays during stranding makes the cable extensible. Cables can be securely fastened and organized, such as by using trunking, cable trays, cable ties or cable lacing. Continuous-flex or flexible cables used in moving applications within cable carriers can be secured using strain relief devices or cable ties. At high frequencies, current tends to run along the surface of the conductor; this is known as the skin effect. Any current-carrying conductor, including a cable, radiates an electromagnetic field. Any conductor or cable will pick up energy from any existing electromagnetic field around it; these effects are undesirable, in the first case amounting to unwanted transmission of energy which may adversely affect nearby equipment or other parts of the same piece of equipment.
The first solution to these problems is to keep cable lengths in buildings short, since pick up and transmission are proportional to the length of the cable. The second solution is to route cables away from trouble. Beyond this, there are particular cable designs that minimize electromagnetic pickup and transmission. Three of the principal design techniques are shielding, coaxial geometry, twisted-pair geometry. Shielding makes use of the electrical principle of the Faraday cage; the cable is encased for its entire length in wire mesh. All wires running inside this shielding layer will be to a large extent decoupled from external electrical fields if the shield is connected to a point of constant voltage, such as earth or ground. Simple shielding of this type is not effective against low-frequency magnetic fields, however - such as magnetic "hum" from a nearby power transformer. A grounded shield on cables operating at 2.5 kV or more gathers leakage current and capacitive current, protecting people from electric shock and equalizing stress on the cable insulation.
Coaxial design helps to further reduce low-frequency magnetic pickup. In this design the foil or mesh shield has a circular cross section and the inner conductor is at its center; this causes the voltages induced by a magnetic field between the shield and the core conductor to consist of two nearly equal magnitudes which cancel each other. A twisted pair has two wires of a cable twisted around each other; this can be demonstrated by putting one end of a pair of wires in a hand drill and turning while maintaining moderate tension on the line. Where the interfering signal has a wavelength, long compared to the pitch of the twisted pair, alternate lengths of wires develop opposing voltages, tending to cancel the effect of the interference. In building construction, electrical cable jacket material is a potential source of fuel for fires. To limit the spread of fire along cable jacketing, one may use cable coating materials or one may use cables with jacketing, inherently fire retardant; the plastic covering on some metal clad cables may be stripped off at installation to reduce the fuel source for fires.
Inorganic coatings and boxes around cables safeguard the adjacent areas from the fire thr
A smoke screen is smoke released to mask the movement or location of military units such as infantry, aircraft or ships. Smoke screens are deployed either by a canister or generated by a vehicle. Whereas smoke screens were used to hide movement from enemies' line of sight, modern technology means that they are now available in new forms; these are canister-type grenades used as a ground-to-air signaling device. The body consists of a steel sheet metal cylinder with a few emission holes on top and on the bottom to allow smoke release when the smoke composition inside the grenade is ignited. In those that produce colored smoke, the filler consists of 250 to 350 grams of colored smoke mixture. In those that produce screening smoke, the filler consists of HC smoke mixture or TA smoke mixture. Another type of smoke grenade is filled with white phosphorus, spread by explosive action; the phosphorus catches fire in the presence of air, burns with a brilliant yellow flame, while producing copious amounts of white smoke.
WP grenades double as incendiary grenades. See also: Smoke shellsArtillery and mortars can fire smoke generating munitions, are the main means of generating tactical smokescreens on land; as with grenades, artillery shells are available as both emission type smoke shell, bursting smoke shell. Mortars nearly always use bursting smoke rounds because of the smaller size of mortar bombs and the greater efficiency of bursting rounds. Large or sustained smoke screens are produced by a smoke generator; this machine heats a volatile material to evaporate it mixes the vapor with cool external air at a controlled rate so it condenses to a mist with a controlled droplet size. Cruder designs boiled waste oil over a heater, while more sophisticated ones sprayed a specially formulated oily composition through nozzles onto a heated plate. Choice of a suitable oil, careful control of cooling rate, can produce droplet sizes close to the ideal size for Mie scattering of visible light; this produces a effective obscuration per weight of material used.
This screen can be sustained as long as the generator is supplied with oil, and—especially if a number of generators are used—the screen can build up to a considerable size. One 50 gallon drum of fog oil can obscure 60 miles of land in 15 minutes. Whilst producing large amounts of smoke cheaply, these generators have a number of disadvantages, they are much slower to respond than pyrotechnic sources, require a valuable piece of equipment to be sited at the point of emission of the smoke. They are relatively heavy and not portable, a significant problem if the wind shifts. To overcome this latter problem they may be used in fixed posts dispersed over the battlefield, or else mounted on specially adapted vehicles. An example of the latter is the M56 Coyote generator. Many armoured fighting vehicles can create smoke screens in a similar way by injecting diesel fuel onto the hot exhaust. Warships have sometimes used a simple variation of the smoke generator, by injecting fuel oil directly into the smoke stack.
An simpler method, used in the days of steam-propelled warships was to restrict the supply of air to the boiler. This resulted in incomplete combustion of the oil, which produced a thick black smoke; because the smoke was black, it tended to rise above the water. Therefore, navies turned to various chemicals, such as titanium tetrachloride, that produce a white, low-lying cloud; the proliferation of thermal imaging FLIR systems on the battlefields necessitates the use of obscurant smokes that are opaque in the infrared part of electromagnetic spectrum. This kind of obscurant smoke is sometimes referred to as "Visual and Infrared Screening Smoke". To achieve this, the particle size and composition of the smokes has to be adjusted. One of the approaches is using an aerosol of burning red phosphorus particles and aluminium coated glass fibers. Carbon particles present in the smokes can serve to absorb the beams of laser designators, yet another possibility is a water fog sprayed around the vehicle.
Other materials used as visible/infrared obscurants are micropulverized flakes of brass or graphite, particles of titanium dioxide, or terephthalic acid. Older systems for production of infrared smoke work as generators of aerosol of dust with controlled particle size. Most contemporary vehicle-mounted systems use this approach; however the aerosol stays airborne only for a short time. The brass particles used in some infrared smoke grenades are composed of 70% copper and 30% zinc, they are shaped as irregular flakes with a diameter of about 1.7 thickness of 80-320 nm. Some experimental obscurants work in both millimeter wave region, they include carbon fibers, metal coated fibers or glass particles, metal microwires, particles of iron and of suitable polymers. Zinc c
A rainscreen is an exterior wall detail where the siding stands off from the moisture-resistant surface of an air barrier applied to the sheathing to create a capillary break and to allow drainage and evaporation. The rain screen is the siding itself but the term rainscreen implies a system of building. Ideally the rain screen prevents the wall air/moisture barrier on sheathing from getting wet. In some cases a rainscreen wall is called a pressure-equalized rainscreen wall where the ventilation openings are large enough for the air pressure to nearly equalize on both sides of the rain screen, but this name has been criticized as being redundant and is only useful to scientists and engineers. A screen in general terms is a barrier; the rainscreen in a wall is sometimes defined as the first layer of material on the wall, the siding itself. Rainscreen is defined as the entire system of the siding, drainage plane and a moisture/air barrier. A veneer that does not stand off from the wall sheathing to create a cavity is not a rainscreen.
However, a masonry veneer can be a rainscreen wall. Many terms have been applied to rain screen walls including basic, conventional, pressure-equalized, pressure-moderated rainscreen systems or assemblies; these terms have caused confusion as to what a rain screen is but all reflect the rainscreen principle of a primary and secondary line of defense. One technical difference is between a plane, a gap of 3⁄8 inch or less and a channel, a gap of more than 3⁄8 inch. In general terms a rainscreen wall may be called a drained wall; the two other basic types of exterior walls in terms of water resistance are barrier walls which rely on the one exterior surface to prevent ingress and mass walls which allow but absorb some leakage. In the early 1960s research was conducted in Norway on rain penetration of windows and walls, Øivind Birkeland published a treatise referring to a "rain barrier". In 1963 the Canadian National Research Counsel published a pamphlet titled "Rain Penetration and its Control" using the term "open rain screen".
Rainscreen cladding is a kind of double-wall construction that utilizes a surface to help keep the rain out, as well as an inner layer to offer thermal insulation, prevent excessive air leakage and carry wind loading. The surface breathes. For water to enter a wall first the water must get onto the wall and the wall must have openings. Water can enter the wall by capillary action, gravity and air pressure; the rainscreen system provides for two lines of defense against the water intrusion into the walls: The rainscreen and a means to dissipate leakage referred to as a channel. In a rainscreen the air gap allows the circulation of air on the moisture barrier.. This helps direct water away from the main exterior wall. Keeping the insulation dry helps prevent problems such as mold formation and water leakage; the vapour-permeable air/weather barrier prevents water molecules from entering the insulated cavity but allows the passage of vapour, thus reducing the trapping of moisture within the main wall assembly.
The air gap can be created in several ways. One method is to use furring fastened vertically to the wall. Ventilation openings are made at the bottom and top of the wall so air can rise through the cavity. Wall penetrations including windows and doors require special care to maintain the ventilation. In the pressure-equalized system the ventilation openings must be large enough to allow air-flow to equalize the pressure on both sides of the cladding. A ratio of 10:1 cladding leakage area to ventilation area has been suggested. A water/air resistant membrane is placed between the furring and the sheathing to prevent rain water from entering the wall structure; the membrane directs water away and toward special drip edge flashings which protect other parts of the building. Insulation may be provided beneath the membrane; the thickness of insulation is determined by building code requirements as well as performance requirements set out by the architect. The system is a form of double-wall construction that uses an outer layer to keep out the rain and an inner layer to provide thermal insulation, prevent excessive air leakage and carry wind loading.
The outer layer breathes like a skin. The structural frame of the building is kept dry, as water never reaches it or the thermal insulation. Evaporation and drainage in the cavity removes water. Water droplets are not driven through the panel joints or openings because the rainscreen principle means that wind pressure acting on the outer face of the panel is equalized in the cavity. Therefore, there is no significant pressure differential to drive the rain through joints. During extreme weather, a minimal amount of water may penetrate the outer cladding. This, will run as droplets down the back of the cladding sheets and be dissipated through evaporation and drainage. A rainscreen drainage plane is a separation between the veneer and the weather resistant barrier of a rainscreen, it provides predictable, unobstructed path drainage for liquid moisture to drain from a high point of the wall to a low point of the wall the wall detail. The drainage plane must move the water out of the wall system to prevent absorption and consequential rot and structural degradation.
A drainage plane is designed to shed bulk rainwater and/or condensation downward and o