1.
Airbus Defence and Space
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Airbus Defence and Space is a division of Airbus Group responsible for defence and aerospace products and services. The division was formed in January 2014 during the restructuring of European Aeronautic Defence and Space, and comprises the former Airbus Military, Astrium. It is the second largest space company after Boeing and one of the top ten defence companies in the world. Airbus Defence and Space has its headquarters in Toulouse, France, and is led by Dirk Hoke. The company is divided into three sections, Military Aircraft, Space Systems, and Communication-Intelligence-Security, with its presence in 35 countries, the company employs 40,000 people from 86 nationalities and contributes to 21% of Airbus Group revenues. In 2015 Airbus ranked 100th on the Fortune Global 500 list, the two companies envisaged including the French company Aérospatiale, the other major European aerospace company, but only after its privatisation. The first stage of integration was seen as the transformation of Airbus from a consortium of British Aerospace, DASA, Aérospatiale. The merger of British Aerospace and MES to form BAE Systems was announced on 19 January 1999, DASA and the Spanish aircraft company Construcciones Aeronáuticas SA agreed to merge on 11 June 1999. On 14 October 1999 DASA agreed to merge with Aérospatiale-Matra to create the European Aeronautic Defence and Space Company,10 July 2000 was Day One for the new company which became the worlds second-largest aerospace company after Boeing and the second-largest European arms manufacturer after BAE Systems. In January 2001 Airbus Industrie was transformed from an inherently inefficient consortium structure to a joint stock company, with legal. On 16 June 2003 EADS acquired BAEs 25 % share in Astrium, EADS paid £84 million, however due to the lossmaking status of the company BAE invested an equal amount for restructuring. It was subsequently renamed EADS Astrium, and had the divisions Astrium Satellites, Astrium Space Transportation, on 1 July 2003 EADS Defence & Security Systems was founded with the merger of the activities of missile systems, defence electronics, military aircraft and telecommunications of the EADS Group. Tom Enders became the first CEO of the new division, the predecessor company was established in January 1999 as the Airbus Military Company SAS to manage the Airbus A400M project, taking over from the Euroflag consortium. In May 2003, the company was restructured as Airbus Military Sociedad Limitada prior to the execution of the production contract, the Military Transport Aircraft Division was a division of EADS which designs, manufactures and commercialises EADS-CASA light and medium transport aircraft, and headquartered in Madrid, Spain. In 1999 was Construcciones Aeronáuticas SA in the EADS Group incorporated, in Spain it was still referred to as EADS-CASA. The division manufactured tanker, transport and mission aircraft including Airbus A330 MRTT, Airbus A400M, CASA C-212 Aviocar, CASA/IPTN CN-235, after the merger, it also acquired the production of Eurofighter Typhoon, which was earlier under Cassidian. Eurocopter, which was earlier under Airbus Military, was reorganized as Airbus Helicopters, Astrium was formed in 2000 by the merger of Matra Marconi Space with the space division of DaimlerChrysler Aerospace AG and Computadores Redes e Ingeniería SA. Henceforth Astrium was a joint venture between EADS and BAE Systems, on 16 June 2003 the minority shareholder, BAE Systems, sold its 25% share to EADS, making EADS the sole shareholder
2.
Thales Alenia Space
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The company is Europes largest satellite manufacturer. Alcatel Alenia Space was established on June 1,2005 by the merger of Alcatel Space and Alenia Spazio and was owned by Alcatel-Lucent, the creation of the company was concurrent with the creation of Telespazio Holding. This too was a merger of Finmeccanica and Alcatel businesses, on April 5,2006 Alcatel agreed to sell its share of Alcatel Alenia Space to Thales Group. The European Union agreed in this operation on April 10,2007. The company built the Multi-Purpose Logistics Modules, which was used to transport cargo inside the Space Shuttle orbiters and they also built several modules for the International Space Station, those modules are Cupola, Columbus, Harmony, Tranquility and Leonardo. They currently build the vessels for the Automated Transfer Vehicle. In the mid-1990s, the United States stopped issuing licenses to companies to allow them to launch on Chinese launch vehicles out of fear that this would help Chinas military. In the face of this, Thales Alenia Space built the Chinasat-6B satellite with no components from the United States whatsoever and this allowed it to be launched on a Chinese launch vehicle without violating U. S. ITAR restrictions. The launch, on a Long March 3B rocket, was conducted on July 5,2007. Cannes Mandelieu Space Center Official website
3.
European Space Agency
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The European Space Agency is an intergovernmental organisation of 22 member states, dedicated to the exploration of space. Established in 1975 and headquartered in Paris, France, ESA has a staff of about 2,000. The main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching, after World War II, many European scientists left Western Europe in order to work with the United States. The meeting was attended by representatives from eight countries, including Harrie Massey. The Western European nations decided to have two different agencies, one concerned with developing a system, ELDO, and the precursor of the European Space Agency. The latter was established on 20 March 1964 by an agreement signed on 14 June 1962, from 1968 to 1972, ESRO launched seven research satellites. ESA in its current form was founded with the ESA Convention in 1975, ESA has 10 founding member states, Belgium, Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, Switzerland and the United Kingdom. These signed the ESA Convention in 1975 and deposited the instruments of ratification by 1980, during this interval the agency functioned in a de facto fashion. ESA launched its first major scientific mission in 1975, Cos-B, ESA joined NASA in the IUE, the worlds first high-orbit telescope, which was launched in 1978 and operated very successfully for 18 years. A number of successful Earth-orbit projects followed, and in 1986 ESA began Giotto, its first deep-space mission, to study the comets Halley and Grigg–Skjellerup. Hipparcos, a mission, was launched in 1989 and in the 1990s SOHO, Ulysses. Recent scientific missions in co-operation with NASA include the Cassini–Huygens space probe, as the successor of ELDO, ESA has also constructed rockets for scientific and commercial payloads. Ariane 1, launched in 1979, brought mostly commercial payloads into orbit from 1984 onward, the successor launch vehicle of Ariane 5, the Ariane 6 is already in the definition stage and is envisioned to enter service in the 2020s. The beginning of the new millennium saw ESA become, along with agencies like NASA, JAXA, ISRO, CSA and Roscosmos, one of the major participants in scientific space research. Although ESA had relied on co-operation with NASA in previous decades, especially the 1990s, changed circumstances led to decisions to rely more on itself, a 2011 press issue thus stated, Russia is ESAs first partner in its efforts to ensure long-term access to space. ESA maintains its scientific and research projects mainly for astronomy-space missions such as Corot, launched on 27 December 2006 and this is the reason space exploration is an integral part of overall space activities. It has always been so, and it will be more important in the future. ESA describes its work in two overlapping ways, For the general public the various fields of work are described as Activities, Member states participate to varying degrees in the mandatory and optional space programmes
4.
Sentinel-1
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Sentinel-1 is a space mission funded by the European Union and carried out by the ESA within the Copernicus Programme, consisting of a constellation of two satellites. The payload of Sentinel-1 is a Synthetic Aperture Radar in C band that provides continuous imagery, on 12 March 2010, the European Space Agency and Thales Alenia Space signed a contract worth €270 million to build the second satellite of the Sentinel-1 pair. First Sentinel 1A arrived at launch site in Kourou, French Guiana on 25 February, sentinel-1A satellite was launched on 3 April 2014, by a Soyuz Rocket. Sentinel-1 will provide continuity of data from ERS and Envisat missions, with enhancements in terms of revisit, coverage, timeliness. A summary of applications are, Monitoring sea ice zones and the Arctic environment, the Sentinel-1 satellites are expected to make analysis of earthquakes using InSAR techniques quicker and simpler. Prime contractor of mission is Thales Alenia Space Italy, with whole system integration and also production of platform Spacecraft Management Unit and payload Data Storage. SAR instrument on payload is the responsibility of Astrium Gmbh, the Ground Segment prime contractor is Astrium with subcontractors Telespazio, WERUM, Advanced Computer Systems and Aresys
5.
Mediterranean Sea
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The sea is sometimes considered a part of the Atlantic Ocean, although it is usually identified as a separate body of water. The name Mediterranean is derived from the Latin mediterraneus, meaning inland or in the middle of land and it covers an approximate area of 2.5 million km2, but its connection to the Atlantic is only 14 km wide. The Strait of Gibraltar is a strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Gibraltar. In oceanography, it is called the Eurafrican Mediterranean Sea or the European Mediterranean Sea to distinguish it from mediterranean seas elsewhere. The Mediterranean Sea has a depth of 1,500 m. The sea is bordered on the north by Europe, the east by Asia and it is located between latitudes 30° and 46° N and longitudes 6° W and 36° E. Its west-east length, from the Strait of Gibraltar to the Gulf of Iskenderun, the seas average north-south length, from Croatia’s southern shore to Libya, is approximately 800 km. The Mediterranean Sea, including the Sea of Marmara, has an area of approximately 2,510,000 square km. The sea was an important route for merchants and travelers of ancient times that allowed for trade, the history of the Mediterranean region is crucial to understanding the origins and development of many modern societies. In addition, the Gaza Strip and the British Overseas Territories of Gibraltar and Akrotiri, the term Mediterranean derives from the Latin word mediterraneus, meaning amid the earth or between land, as it is between the continents of Africa, Asia and Europe. The Ancient Greek name Mesogeios, is similarly from μέσο, between + γη, land, earth) and it can be compared with the Ancient Greek name Mesopotamia, meaning between rivers. The Mediterranean Sea has historically had several names, for example, the Carthaginians called it the Syrian Sea and latter Romans commonly called it Mare Nostrum, and occasionally Mare Internum. Another name was the Sea of the Philistines, from the people inhabiting a large portion of its shores near the Israelites, the sea is also called the Great Sea in the General Prologue by Geoffrey Chaucer. In Ottoman Turkish, it has also been called Bahr-i Sefid, in Modern Hebrew, it has been called HaYam HaTikhon, the Middle Sea, reflecting the Seas name in ancient Greek, Latin, and modern languages in both Europe and the Middle East. Similarly, in Modern Arabic, it is known as al-Baḥr al-Mutawassiṭ, in Turkish, it is known as Akdeniz, the White Sea since among Turks the white colour represents the west. Several ancient civilisations were located around the Mediterranean shores, and were influenced by their proximity to the sea. It provided routes for trade, colonisation, and war, as well as food for numerous communities throughout the ages, due to the shared climate, geology, and access to the sea, cultures centered on the Mediterranean tended to have some extent of intertwined culture and history. Two of the most notable Mediterranean civilisations in classical antiquity were the Greek city states, later, when Augustus founded the Roman Empire, the Romans referred to the Mediterranean as Mare Nostrum
6.
Landsat program
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The Landsat program is the longest-running enterprise for acquisition of satellite imagery of Earth. On July 23,1972 the Earth Resources Technology Satellite was launched and this was eventually renamed to Landsat. The most recent, Landsat 8, was launched on February 11,2013, the instruments on the Landsat satellites have acquired millions of images. Landsat 7 data has eight spectral bands with spatial resolutions ranging from 15 to 60 meters, Landsat images are usually divided into scenes for easy downloading. Each Landsat scene is about 115 miles long and 115 miles wide, Hughes Santa Barbara Research Center initiated, designed, and fabricated the first three Multispectral Scanners in 1969, the same year man landed on the moon. The first prototype MSS was completed nine months, in the fall of 1970. It was tested by scanning Half Dome at Yosemite National Park, the program was initially called the Earth Resources Technology Satellites Program, which was used from 1966 to 1975. In 1975, the name was changed to Landsat and this occurred in 1985 when the Earth Observation Satellite Company, a partnership of Hughes Aircraft and RCA, was selected by NOAA to operate the Landsat system with a ten-year contract. EOSAT operated Landsat 4 and Landsat 5, had rights to market Landsat data. In 1989, this transition had not been completed when NOAAs funding for the Landsat program was due to run out. The head of the newly formed National Space Council, Vice President Dan Quayle, noted the situation and arranged emergency funding that allowed the program to continue with the data archives intact. Again in 1990 and 1991, Congress provided only half of the funding to NOAA. In 1992, various efforts were made to procure funding for follow on Landsats and continued operations, Landsat 6 was finally launched on October 5,1993, but was lost in a launch failure. Processing of Landsat 4 and 5 data was resumed by EOSAT in 1994, NASA finally launched Landsat 7 on April 15,1999. Timeline Landsat missions 1 through 5 carried the Landsat Multispectral Scanner and this construct ensured the secondary mirror would simply oscillate about the primary optic axis to maintain focus despite vibration inherent from the 360 mm beryllium scan mirror. This engineering solution allowed the United States to develop LANDSAT at least five years ahead of the French SPOT and this was a direct result of the commercialization efforts begun under the Carter administration, though finally completed under the Reagan administration. This main plate was assembled on a frame, then attached to the magnesium housing with helicoil fasteners. Key to the success of the multi spectral scanner was the scan monitor mounted on the underbelly of the magnesium housing, the beam struck the beryllium scan mirror seven times as it reflected seven times off the flat mirrors
7.
CNES
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The Centre national détudes spatiales is the French government space agency. Its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and it operates from the Toulouse Space Center and Guiana Space Centre, but also has payloads launched from space centres operated by other countries. The president of CNES is Jean-Yves Le Gall, CNES is member of Institute of Space, its Applications and Technologies. CNES benefits from the second largest national budget allocated to space, CNES was established under President Charles de Gaulle in 1961. CNES was responsible for the training of French astronauts, until the last active CNES astronauts transferred to the European Space Agency in 2001. As of January 2015, CNES is working with Germany and a few governments to start a modest research effort with the hope to propose a LOX/methane reusable launch vehicle by mid-2015. If built, flight testing would not start before approximately 2026. Hammaguir missile range built in Algeria,1963 Cats launched into space via Véronique rockets. 1965 First French satellite put in orbit,1969 French Guiana Space Centre completed. Commercial competition in space is fierce, so launch services must be tailored to space operators’ needs, the new versions of Ariane 5 can launch large satellites or perform dual launches while the Vega and Soyuz-2 are small and medium-lift launchers. CNES is taking part in the Galileo navigation programme alongside the European Union, in addition to Spot and the future Pleiades satellites, CNES is working for the defence community as prime contractor for the Helios satellites. France’s contribution to the International Space Station is giving French scientists the opportunity to perform experiments in microgravity. The CNES is studying formation flying, a technique whereby several satellites fly components of a much heavier, CNES is collaborating with other space agencies in a number of projects. Joint missions with NASA result in PARASOL and CALIPSO satellites, CNES plays a major role in the construction and operation of the Soil Moisture and Ocean Salinity satellite. In December 2006, CNES announced that it would publish its UFO archive online by late January or mid-February, most of the 6,000 reports have been filed by the public and airline professionals. Jacques Arnould, an official for the French Space Agency, said that the data had accumulated over a 30-year period and that they were often reported to the Gendarmerie. In the last two decades of the 20th century, France was the country whose government paid UFO investigators, employed by CNESs UFO section GEPAN, later known as SEPRA. On March 22,2007, CNES released its UFO files to the public through its website, the CNES has several tracking stations
8.
Synthetic-aperture radar
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Synthetic aperture radar is a form of radar that is used to create two- or 3-dimensional images of objects, such as landscapes. SAR uses the motion of the radar antenna over a region to provide finer spatial resolution than conventional beam-scanning radars. SAR is typically mounted on a platform such as an aircraft or spacecraft. The distance the SAR device travels over a target in the time taken for the pulses to return to the antenna creates the large synthetic antenna aperture. To create a SAR image, successive pulses of waves are transmitted to illuminate a target scene. The pulses are transmitted and the echoes received using a single beam-forming antenna, as the SAR device on board the aircraft or spacecraft moves, the antenna location relative to the target changes with time. The properties of SAR can be described as having high resolution capability which is independent of flight altitude, SAR also have a great day and night imaging capability considering their own illumination. SAR images have wide applications in sensing and mapping of the surfaces of both the Earth and other planets. Volcano and earthquake monitoring is a part of differential interferometry and it is also useful in environment monitoring like oil spills, flooding, urban growth, global change and military surveillance which includes strategic policy and tactical assessment. SAR can also be implemented as inverse SAR by observing a moving target over a time with a stationary antenna. A Synthetic Aperture Radar is a radar mounted on a moving platform. Electromagnetic waves are transmitted and reflected echoes are collected, digitized and stored by the radar antenna for later processing. As transmission and reception occur at different time, they map to different positions, the well ordered combination of the received signals builds a virtual aperture that is much longer than the physical antenna length. This is why it is named synthetic aperture, giving it the property of being an imaging radar, the 3D processing is done in two steps, the azimuth and range direction are focussed for the generation of 2D high resolution images. After which a digital model is used to measure the phase differences between complex images which is determined from different look angles to recover the height information. This height information, along with the coordinates provided by 2-D SAR focusing. The first step requires only standard processing algorithms, for the step a additional pre-processing stage such as image co-registration. In addition to this, multiple baselines can be used to extend 3D imaging to the time dimension, the SAR algorithm, as given here, applies to phased arrays generally
9.
Infrared
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It extends from the nominal red edge of the visible spectrum at 700 nanometers, to 1000000 nm. Most of the radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon, slightly more than half of the total energy from the Sun was eventually found to arrive on Earth in the form of infrared. The balance between absorbed and emitted infrared radiation has an effect on Earths climate. Infrared radiation is emitted or absorbed by molecules when they change their rotational-vibrational movements and it excites vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states for molecules of the proper symmetry. Infrared spectroscopy examines absorption and transmission of photons in the infrared range, Infrared radiation is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected, Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatuses. Thermal-infrared imaging is used extensively for military and civilian purposes, military applications include target acquisition, surveillance, night vision, homing, and tracking. Humans at normal body temperature radiate chiefly at wavelengths around 10 μm, Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers to 1 mm. This range of wavelengths corresponds to a range of approximately 430 THz down to 300 GHz. Below infrared is the portion of the electromagnetic spectrum. Sunlight, at a temperature of 5,780 kelvins, is composed of near thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level, of this energy,527 watts is infrared radiation,445 watts is visible light, and 32 watts is ultraviolet radiation. Nearly all the radiation in sunlight is near infrared, shorter than 4 micrometers. On the surface of Earth, at far lower temperatures than the surface of the Sun, almost all thermal radiation consists of infrared in mid-infrared region, much longer than in sunlight. Of these natural thermal radiation processes only lightning and natural fires are hot enough to produce much visible energy, thermal infrared radiation also has a maximum emission wavelength, which is inversely proportional to the absolute temperature of object, in accordance with Wiens displacement law. Therefore, the band is often subdivided into smaller sections. Due to the nature of the blackbody radiation curves, typical hot objects, such as exhaust pipes, the three regions are used for observation of different temperature ranges, and hence different environments in space
10.
Ground segment
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A ground segment consists of all the ground-based control elements of a spacecraft system, as opposed to the space segment and user segment. The ground segment serves to control of a spacecraft, and distribution of payload data. They may be located together or separated geographically, and may be operated by different parties, certain elements may support multiple spacecraft simultaneously. Mission control centers process, distribute, and perform some analysis of telemetry, they also issue commands, data uploads, for manned spacecraft, mission control manages voice and video communications with the crew. Control centers may also be responsible for management and data archival. Control centers may be continuously or regularly staffed by flight controllers, staffing is typically greatest during the early phases of a mission, and during critical procedures and periods. Increasingly commonly, control centers for unmanned spacecraft may be set up for lights-out operation, flight control software will typically generate alerts regarding significant events – both planned and unplanned – in the ground or space segment that may require operator action. Control centers use received telemetry to determine the status of a spacecraft, housekeeping, diagnostic, science, and other types of telemetry may be carried on separate virtual channels. Spacecraft commands are formatted according to the database, and are validated against the database before being transmitted. Commands may be issued manually in real time, or they may be part of automated or semi-automated procedures, typically, received commands are acknowledged in telemetry. In certain cases, closed-loop control may be performed, commanded activities may pertain directly to mission objectives, or they may be part of housekeeping. Commands may be encrypted to prevent unauthorized access to the spacecraft or its data, spacecraft procedures are often developed and tested against a spacecraft simulator prior to use with the actual spacecraft. As on-board computing power and flight software complexity have increased, there is a trend toward performing more automated data processing on board the spacecraft. Ground stations provide radio interfaces between the space and ground segments for telemetry, tracking, and command, as well as data transmission and reception. As with control centers, there are typically backup ground stations available to support continuity of operations, tracking networks, such as NASAs Near Earth Network and Space Network, may handle communications with multiple spacecraft through time-sharing. Ground station equipment may be monitored and controlled remotely, often via serial and/or IP interfaces, the timing of passes is determined by the location of ground stations, and by the characteristics of the spacecraft orbit. The Space Network uses geostationary relay satellites to extend pass opportunities over the horizon, the RF signal is then amplified to high power and carried via waveguide to an antenna for transmission. In colder climates, electric heaters or hot air blowers may be necessary to prevent ice or snow buildup on the parabolic dish, received signals are passed through a low-noise amplifier before being down-converted to IF, these two functions may be combined in a low-noise block downconverter
11.
Sentinel-3
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Sentinel-3 is an Earth observation satellite constellation developed by the European Space Agency as part of the Copernicus Programme. On 14 April 2008, the European Space Agency and Thales Alenia Space signed a million contract to build the first GMES Sentinel-3 in its Cannes Mandelieu Space Center. Satellite platform was delivered to France for final assembly in 2013. Communications systems were completed by Thales Alenia Space España in early 2014, Sentinel-3 builds directly on the heritage pioneered by ERS-2 and Envisat satellites. A pair of Sentinel-3 satellites will enable a short time of less than two days for OLCI instrument and less than one day for SLSTR at the equator. The satellite orbit provides a 27-day repeat for the topography package,1,250 kg Orbit, Sun-synchronous Altitude,814 km Inclination,98. 6° Local time of Descending Node,10,00 a. m. It measures in nine spectral channels and two additional bands optimised for fire monitoring, OLCI is a medium-resolution imaging spectrometer that uses five cameras to provide a wide field of view. SLSTR and OLCI are optical instruments with an overlap of their swath path, SRAL is the main topographic instrument to provide accurate topography measurements over sea ice, ice sheets, rivers and lakes. It uses dual-frequency Ku and C band and is supported by a microwave radiometer for atmospheric correction, DORIS is a receiver for orbit positioning. MWR will measure water vapour and cloud water content and the radiation emitted by the Earth. LRR will be used to locate the satellite in orbit using a laser ranging system. When used in combination with SRAL, DORIS, MWR, they will acquire detailed topographic measurements of the ocean, GNSS will provide precise orbit determination and can track multiple satellites simultaneously
12.
Visible spectrum
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The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm, in terms of frequency, this corresponds to a band in the vicinity of 430–770 THz. The spectrum does not, however, contain all the colors that the human eyes, unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Colors containing only one wavelength are called pure colors or spectral colors. Visible wavelengths pass through the window, the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earths atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, the optical window is also referred to as the visible window because it overlaps the human visible response spectrum. The near infrared window lies just out of the vision, as well as the Medium Wavelength IR window. In the 13th century, Roger Bacon theorized that rainbows were produced by a process to the passage of light through glass or crystal. In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light and he was the first to use the word spectrum in this sense in print in 1671 in describing his experiments in optics. The result is red light is bent less sharply than violet as it passes through the prism. Newton divided the spectrum into seven named colors, red, orange, yellow, green, blue, indigo, the human eye is relatively insensitive to indigos frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right. However, the evidence indicates that what Newton meant by indigo, comparing Newtons observation of prismatic colors to a color image of the visible light spectrum shows that indigo corresponds to what is today called blue, whereas blue corresponds to cyan. In the 18th century, Goethe wrote about optical spectra in his Theory of Colours, Goethe used the word spectrum to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow, the spectrum appears only when these edges are close enough to overlap. Young was the first to measure the wavelengths of different colors of light, the connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century
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