An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a higher resolving power than light microscopes and can reveal the structure of smaller objects. A scanning transmission electron microscope has achieved better than 50pm resolution in annular dark-field imaging mode[1] and magnifications of up to about 10,000,000x whereas most light microscopes are limited by diffraction to about 200nm resolution and useful magnifications below 2000x.
Electron microscopes have electron optical lens systems that are analogous to the glass lenses of an optical light microscope.
Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, electron microscopes are often used for quality control and failure analysis. Modern electron microscopes produce electron micrographs using specialized digital cameras and frame grabbers to capture the images.
History

In 1926 Hans Busch developed the electromagnetic lens.
According to Dennis Gabor, the physicist Leó Szilárd tried in 1928 to convince him to build an electron microscope, for which he had filed a patent.[2] The first prototype electron microscope, capable of four-hundred-power magnification, was developed in 1931 by the physicist Ernst Ruska and the electrical engineer Max Knoll.[3] The apparatus was the first practical demonstration of the principles of electron microscopy.[4] In May of the same year, Reinhold Rudenberg, the scientific director of Siemens-Schuckertwerke, obtained a patent for an electron microscope. In 1932, Ernst Lubcke of Siemens & Halske built and obtained images from a prototype electron microscope, applying the concepts described in Rudenberg's patent.[5]
In the following year, 1933, Ruska built the first electron microscope that exceeded the resolution attainable with an optical (light) microscope.[4]  Four years later, in 1937, Siemens financed the work of Ernst Ruska and Bodo von Borries, and employed Helmut Ruska, Ernst's brother, to develop applications for the microscope, especially with biological specimens.[4][6] Also in 1937, Manfred von Ardenne pioneered the scanning electron microscope.[7] Siemens produced the first commercial electron microscope in 1938.[8] The first North American electron microscope was constructed in 1938, at the University of Toronto, by Eli Franklin Burton and students Cecil Hall, James Hillier, and Albert Prebus. Siemens produced a transmission electron microscope (TEM) in 1939.[9] Although current transmission electron microscopes are capable of two million-power magnification, as scientific instruments, they remain based upon Ruska’s prototype.
Types
Transmission electron microscope (TEM)

The original form of the electron microscope, the transmission electron microscope (TEM), uses a high voltage electron beam to illuminate the specimen and create an image. The electron beam is produced by an electron gun, commonly fitted with a tungsten filament cathode as the electron source. The electron beam is accelerated by an anode typically at +100 keV (40 to 400 keV) with respect to the cathode, focused by electrostatic and electromagnetic lenses, and transmitted through the specimen that is in part transparent to electrons and in part scatters them out of the beam. When it emerges from the specimen, the electron beam carries information about the structure of the specimen that is magnified by the objective lens system of the microscope. The spatial variation in this information (the "image") may be viewed by projecting the magnified electron image onto a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide. Alternatively, the image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution phosphor may be coupled by means of a lens optical system or a fibre optic light-guide to the sensor of a digital camera. The image detected by the digital camera may be displayed on a monitor or computer.
The resolution of TEMs is limited primarily by spherical aberration, but a new generation of hardware correctors can reduce spherical aberration to increase the resolution in high-resolution transmission electron microscopy (HRTEM) to below 0.5 angstrom (50 picometres),[1] enabling magnifications above 50 million times.[10] The ability of HRTEM to determine the positions of atoms within materials is useful for nano-technologies research and development.[11]
Transmission electron microscopes are often used in electron diffraction mode. The advantages of electron diffraction over X-ray crystallography are that the specimen need not be a single crystal or even a polycrystalline powder, and also that the Fourier transform reconstruction of the object's magnified structure occurs physically and thus avoids the need for solving the phase problem faced by the X-ray crystallographers after obtaining their X-ray diffraction patterns of a single crystal or polycrystalline powder.
One major disadvantage of the transmission electron microscope is the need for extremely thin sections of the specimens, typically about 100 nanometers. Creating these thin sections for biological and materials specimens is technically very challenging. Semiconductor thin sections can be made using a focused ion beam. Biological tissue specimens are chemically fixed, dehydrated and embedded in a polymer resin to stabilize them sufficiently to allow ultrathin sectioning. Sections of biological specimens, organic polymers, and similar materials may require staining with heavy atom labels in order to achieve the required image contrast.
Serial-section electron microscopy (ssEM)
One application of TEM is serial-section electron microscopy (ssEM), for example in analyzing the connectivity in volumetric samples of brain tissue by imaging many thin sections in sequence.[12]
Scanning electron microscope (SEM)

The SEM produces images by probing the specimen with a focused electron beam that is scanned across a rectangular area of the specimen (raster scanning). When the electron beam interacts with the specimen, it loses energy by a variety of mechanisms. The lost energy is converted into alternative forms such as heat, emission of low-energy secondary electrons and high-energy backscattered electrons, light emission (cathodoluminescence) or X-ray emission, all of which provide signals carrying information about the properties of the specimen surface, such as its topography and composition. The image displayed by an SEM maps the varying intensity of any of these signals into the image in a position corresponding to the position of the beam on the specimen when the signal was generated. In the SEM image of an ant shown below and to the right, the image was constructed from signals produced by a secondary electron detector, the normal or conventional imaging mode in most SEMs.

Generally, the image resolution of an SEM is lower than that of a TEM. However, because the SEM images the surface of a sample rather than its interior, the electrons do not have to travel through the sample. This reduces the need for extensive sample preparation to thin the specimen to electron transparency. The SEM is able to image bulk samples that can fit on its stage and still be maneuvered, including a height less than the working distance being used, often 4 millimeters for high-resolution images. The SEM also has a great depth of field, and so can produce images that are good representations of the three-dimensional surface shape of the sample. Another advantage of SEMs comes with environmental scanning electron microscopes (ESEM) that can produce images of good quality and resolution with hydrated samples or in low, rather than high, vacuum or under chamber gases. This facilitates imaging unfixed biological samples that are unstable in the high vacuum of conventional electron microscopes.
Color
In their most common configurations, electron microscopes produce images with a single brightness value per pixel, with the results usually rendered in grayscale.[13] However, often these images are then colorized through the use of feature-detection software, or simply by hand-editing using a graphics editor. This may be done to clarify structure or for aesthetic effect and generally does not add new information about the specimen.[14]
In some configurations information about several specimen properties is gathered per pixel, usually by the use of multiple detectors.[15] In SEM, the attributes of topography and material contrast can be obtained by a pair of backscattered electron detectors and such attributes can be superimposed in a single color image by assigning a different primary color to each attribute.[16] Similarly, a combination of backscattered and secondary electron signals can be assigned to different colors and superimposed on a single color micrograph displaying simultaneously the properties of the specimen.[17]
Some types of detectors used in SEM have analytical capabilities, and can provide several items of data at each pixel. Examples are the Energy-dispersive X-ray spectroscopy (EDS) detectors used in elemental analysis and Cathodoluminescence microscope (CL) systems that analyse the intensity and spectrum of electron-induced luminescence in (for example) geological specimens.  In SEM systems using these detectors, it is common to color code the signals and superimpose them in a single color image, so that differences in the distribution of the various components of the specimen can be seen clearly and compared. Optionally, the standard secondary electron image can be merged with the one or more compositional channels, so that the specimen's structure and composition can be compared.  Such images can be made while maintaining the full integrity of the original signal, which is not modified in any way.
Reflection electron microscope (REM)
In the reflection electron microscope (REM) as in the TEM, an electron beam is incident on a surface but instead of using the transmission (TEM) or secondary electrons (SEM), the reflected beam of elastically scattered electrons is detected. This technique is typically coupled with reflection high energy electron diffraction (RHEED) and reflection high-energy loss spectroscopy (RHELS). Another variation is spin-polarized low-energy electron microscopy (SPLEEM), which is used for looking at the microstructure of magnetic domains.[18]
Scanning transmission electron microscope (STEM)

The STEM rasters a focused incident probe across a specimen that (as with the TEM) has been thinned to facilitate detection of electrons scattered through the specimen. The high resolution of the TEM is thus possible in STEM. The focusing action (and aberrations) occur before the electrons hit the specimen in the STEM, but afterward in the TEM. The STEMs use of SEM-like beam rastering simplifies annular dark-field imaging, and other analytical techniques, but also means that image data is acquired in serial rather than in parallel fashion. Often TEM can be equipped with the scanning option and then it can function both as TEM and STEM.
Sample preparation

Materials to be viewed under an electron microscope may require processing to produce a suitable sample. The technique required varies depending on the specimen and the analysis required:
Chemical fixation – for biological specimens aims to stabilize the specimen's mobile macromolecular structure by chemical crosslinking of proteins with aldehydes such as formaldehyde and glutaraldehyde, and lipids with osmium tetroxide.
Negative stain – suspensions containing nanoparticles or fine biological material (such as viruses and bacteria) are briefly mixed with a dilute solution of an electron-opaque solution such as ammonium molybdate, uranyl acetate (or formate), or phosphotungstic acid. This mixture is applied to a suitably coated EM grid, blotted, then allowed to dry. Viewing of this preparation in the TEM should be carried out without delay for best results. The method is important in microbiology for fast but crude morphological identification, but can also be used as the basis for high-resolution 3D reconstruction using EM tomography methodology when carbon films are used for support. Negative staining is also used for observation of nanoparticles.
Cryofixation – freezing a specimen so rapidly, in liquid ethane, and maintained at liquid nitrogen or even liquid helium temperatures, so that the water forms vitreous (non-crystalline) ice. This preserves the specimen in a snapshot of its solution state. An entire field called cryo-electron microscopy has branched from this technique. With the development of cryo-electron microscopy of vitreous sections (CEMOVIS), it is now possible to observe samples from virtually any biological specimen close to its native state.
Dehydration – or replacement of water with organic solvents such as ethanol or acetone, followed by critical point drying or infiltration with embedding resins. Also freeze drying.
Embedding, biological specimens – after dehydration, tissue for observation in the transmission electron microscope is embedded so it can be sectioned ready for viewing. To do this the tissue is passed through a 'transition solvent' such as propylene oxide (epoxypropane) or acetone and then infiltrated with an epoxy resin such as Araldite, Epon, or Durcupan;[19] tissues may also be embedded directly in water-miscible acrylic resin. After the resin has been polymerized (hardened) the sample is thin sectioned (ultrathin sections) and stained – it is then ready for viewing.
Embedding, materials – after embedding in resin, the specimen is usually ground and polished to a mirror-like finish using ultra-fine abrasives. The polishing process must be performed carefully to minimize scratches and other polishing artifacts that reduce image quality.
Metal shadowing – Metal (e.g. platinum) is evaporated from an overhead electrode and applied to the surface of a biological sample at an angle.[20] The surface topography results in variations in the thickness of the metal that are seen as variations in brightness and contrast in the electron microscope image.
Replication – A surface shadowed with metal (e.g. platinum, or a mixture of carbon and platinum) at an angle is coated with pure carbon evaporated from carbon electrodes at right angles to the surface. This is followed by removal of the specimen material (e.g. in an acid bath, using enzymes or by mechanical separation[21]) to produce a surface replica that records the surface ultrastructure and can be examined using transmission electron microscopy.
Sectioning – produces thin slices of the specimen, semitransparent to electrons. These can be cut on an ultramicrotome with a glass or diamond knife to produce ultra-thin sections about 60–90nm thick. Disposable glass knives are also used because they can be made in the lab and are much cheaper.
Staining – uses heavy metals such as lead, uranium or tungsten to scatter imaging electrons and thus give contrast between different structures, since many (especially biological) materials are nearly "transparent" to electrons (weak phase objects). In biology, specimens can be stained "en bloc" before embedding and also later after sectioning. Typically thin sections are stained for several minutes with an aqueous or alcoholic solution of uranyl acetate followed by aqueous lead citrate.[22]
Freeze-fracture or freeze-etch – a preparation method[23][24] particularly useful for examining lipid membranes and their incorporated proteins in "face on" view.[25] The fresh tissue or cell suspension is frozen rapidly (cryofixation), then fractured by breaking[26] or by using a microtome while maintained at liquid nitrogen temperature. The cold fractured surface (sometimes "etched" by increasing the temperature to about −100°C for several minutes to let some ice sublime) is then shadowed with evaporated platinum or gold at an average angle of 45° in a high vacuum evaporator. The second coat of carbon, evaporated perpendicular to the average surface plane is often performed to improve the stability of the replica coating. The specimen is returned to room temperature and pressure, then the extremely fragile "pre-shadowed" metal replica of the fracture surface is released from the underlying biological material by careful chemical digestion with acids, hypochlorite solution or SDS detergent. The still-floating replica is thoroughly washed free from residual chemicals, carefully fished up on fine grids, dried then viewed in the TEM.
Freeze-fracture replica immunogold labeling (FRIL) – the freeze-fracture method has been modified to allow the identification of the components of the fracture face by immunogold labeling. Instead of removing all the underlying tissue of the thawed replica as the final step before viewing in the microscope the tissue thickness is minimized during or after the fracture process. The thin layer of tissue remains bound to the metal replica so it can be immunogold labeled with antibodies to the structures of choice. The thin layer of the original specimen on the replica with gold attached allows the identification of structures in the fracture plane.[27] There are also related methods which label the surface of etched cells[28] and other replica labeling variations.[29]
Ion beam milling – thins samples until they are transparent to electrons by firing ions (typically argon) at the surface from an angle and sputtering material from the surface. A subclass of this is focused ion beam milling, where gallium ions are used to produce an electron transparent membrane in a specific region of the sample, for example through a device within a microprocessor. Ion beam milling may also be used for cross-section polishing prior to SEM analysis of materials that are difficult to prepare using mechanical polishing.
Conductive coating – an ultrathin coating of electrically conducting material, deposited either by high vacuum evaporation or by low vacuum sputter coating of the sample. This is done to prevent the accumulation of static electric fields at the specimen due to the electron irradiation required during imaging. The coating materials include gold, gold/palladium, platinum, tungsten, graphite, etc.
Earthing – to avoid electrical charge accumulation on a conductively coated sample, it is usually electrically connected to the metal sample holder. Often an electrically conductive adhesive is used for this purpose.
Disadvantages
Electron microscopes are expensive to build and maintain, on the order of other complex machines such as airplanes. Microscopes designed to achieve high resolutions must be housed in stable buildings (sometimes underground) with special services such as magnetic field canceling systems.  Operating the electron microscope requires  specialized training and continuing practice and education.
The samples largely have to be viewed in vacuum, as the molecules that make up air would scatter the electrons. An exception is liquid-phase electron microscopy [30] using either a closed liquid cell or an environmental chamber, for example, in the environmental scanning electron microscope, which allows hydrated samples to be viewed in a low-pressure (up to 20Torr or 2.7kPa) wet environment. Various techniques for in situ electron microscopy of gaseous samples have been developed as well.[31]
Scanning electron microscopes operating in conventional high-vacuum mode usually image conductive specimens; therefore non-conductive materials require conductive coating (gold/palladium alloy, carbon, osmium, etc.). The low-voltage mode of modern microscopes makes possible the observation of non-conductive specimens without coating.  Non-conductive materials can be imaged also by a variable pressure (or environmental) scanning electron microscope.
Small, stable specimens such as carbon nanotubes, diatom frustules and small mineral crystals (asbestos fibres, for example) require no special treatment before being examined in the electron microscope. Samples of hydrated materials, including almost all biological specimens have to be prepared in various ways to stabilize them, reduce their thickness (ultrathin sectioning) and increase their electron optical contrast (staining). These processes may result in artifacts, but these can usually be identified by comparing the results obtained by using radically different specimen preparation methods. Since the 1980s, analysis of cryofixed, vitrified specimens has also become increasingly used by scientists, further confirming the validity of this technique.[32][33][34]
Applications

Semiconductor and data storage
Circuit edit[35]
Defect analysis[36]
Failure analysis[37]
Biology and life sciences
Cryobiology[38]
Cryo-electron microscopy[39]
Diagnostic electron microscopy[40]
Drug research (e.g. antibiotics)[41]
Electron tomography[42]
Particle analysis[43]
Particle detection[44]
Protein localization[45]
Structural biology[39]
Tissue imaging[46]
Toxicology[47]
Virology (e.g. viral load monitoring)[48]

Materials research
Device testing and characterization[49]
Dynamic materials experiments[50]
Electron beam-induced deposition[51]
In-situ characterisation[52]
Materials qualification[53]
Medical research[41]
Nanometrology[54]
Nanoprototyping[55]
Industry
Chemical/Petrochemical[56]
Direct beam-writing fabrication[57]
Food science[58]
Forensics[59]
Fractography[60]
Micro-characterization[61]
Mining (mineral liberation analysis)[62]
Pharmaceutical QC[63]

See also

Acronyms in microscopy
Electron diffraction
Electron energy loss spectroscopy (EELS)
Electron microscope images
Energy filtered transmission electron microscopy (EFTEM)
Environmental scanning electron microscope (ESEM)
Field emission microscope
HiRISE
Immune electron microscopy
In situ electron microscopy
Microscope image processing
Microscopy
Nanoscience
Nanotechnology
Neutron microscope
Scanning confocal electron microscopy
Scanning electron microscope (SEM)
Scanning tunneling microscope
Surface science
Transmission Electron Aberration-Corrected Microscope
X-ray diffraction
X-ray microscope

Library resources about
 Electron microscopy

: resources for teachers and students

: high school (GCSE, A Level) resource
General
 (Université Paris Sud)

: graphics and images illustrating various procedures

: FEI has had a microscopy image contest every year since 2008
 by David Szondy
: images generated by electron microscopy
: information portal with X-ray microanalysis and EDX contents
History

 (Archives Center, National Museum of American History, Smithsonian Institution)
Other


The Redwood National and State Parks (RNSP) are a complex of several state and national parks located in the United States, along the coast of northern California. Comprising Redwood National Park (established 1968) and California's Del Norte Coast, Jedediah Smith, and Prairie Creek Redwoods State Parks (dating from the 1920s), the combined RNSP contain 139,000 acres (560km2),[3] and feature old-growth temperate rainforests. Located entirely within Del Norte and Humboldt Counties, the four parks, together, protect 45% of all remaining coast redwood (Sequoia sempervirens) old-growth forests, totaling at least 38,982 acres (157.75km2). These trees are the tallest and one of the most massive tree species on Earth. In addition to the redwood forests, the parks preserve other indigenous flora, fauna, grassland prairie, cultural resources, portions of rivers and other streams, and 37 miles (60km) of pristine coastline.
In 1850, old-growth redwood forest covered more than 2,000,000 acres (8,100km2) of the California coast. The northern portion of that area, originally inhabited by Native Americans, attracted many lumbermen and others turned gold miners when a minor gold rush brought them to the region. Failing in efforts to strike it rich in gold, these men turned toward harvesting the giant trees[4] for booming development in San Francisco and other places on the West Coast. After many decades of unrestricted clear-cut logging, serious efforts toward conservation began. By the 1920s the work of the Save the Redwoods League, founded in 1918 to preserve remaining old-growth redwoods, resulted in the establishment of Prairie Creek, Del Norte Coast, and Jedediah Smith Redwoods State Parks among others. Redwood National Park was created in 1968, by which time nearly 90% of the original redwood trees had been logged. The National Park Service (NPS) and the California Department of Parks and Recreation (CDPR) administratively combined Redwood National Park with the three abutting Redwood State Parks in 1994 for the purpose of cooperative forest management and stabilization of forests and watersheds as a single unit.[5]
The ecosystem of the RNSP preserves a number of threatened animal species such as the tidewater goby, Chinook salmon, northern spotted owl, and Steller's sea lion.[6] In recognition of the rare ecosystem and cultural history found in the parks, the United Nations designated them a World Heritage Site on September 5, 1980 and part of the California Coast Ranges International Biosphere Reserve on June 30, 1983.[7][8]
History

Modern day native groups such as the Yurok, Tolowa, Karok, Chilula, and Wiyot all have historical ties to the region, and some Native American groups still live in the park area today. Archaeological study shows they arrived in the area as far back as 3,000 years ago. An 1852 census determined that the Yurok were the most numerous, with 55 villages and an estimated population of 2,500.[9] They used the abundant redwood, which with its linear grain was easily split into planks, as a building material for boats, houses, and small villages.[10] For buildings, the planks would be erected side by side in a narrow trench, with the upper portions bound with leather strapping and held by notches cut into the supporting roof beams. Redwood boards were used to form a shallow sloping roof.[11]
Previous to Jedediah Smith in 1828, no other explorer of European descent is known to have thoroughly investigated the inland region away from the immediate coast. The discovery of gold along the Trinity River in 1850 led to a minor secondary rush in California. This brought miners into the area and many stayed on at the coast after failing to strike it rich. This quickly led to conflicts wherein native peoples were placed under great strain, if not forcibly removed or massacred.[12] By 1895, only one third of the Yurok in one group of villages remained; by 1919, virtually all members of the Chilula tribe had either died or been assimilated into other tribes.[13] The miners logged redwoods for building; when this minor gold rush ended, some of them turned again to logging, cutting down the giant redwood trees. Initially, over 2,000,000 acres (8,100km2) of the California and southwestern coast of Oregon were old-growth redwood forest, but by 1910, extensive logging led conservationists and concerned citizens to begin seeking ways to preserve the remaining trees, which they saw being logged at an alarming rate.[14] In 1911, U.S. Representative John E. Raker, of California, became the first politician to introduce legislation for the creation of a redwood national park. However, no further action was taken by Congress at that time.

Preservation of the redwood stands in California is considered one of the most substantial conservation contributions of the Boone and Crockett Club. The Save the Redwoods League was founded in 1918 by Boone and Crockett Club members Madison Grant, John C. Merriam, Henry Fairfield Osborn, and future member, Frederick Russell Burnham. The initial purchases of land were made by club member Stephen Mather and William Kent. In 1921, Boone and Crockett Club member John C. Phillips donated $32,000 to purchase land and create the Raynal Bolling Memorial Grove in the Humboldt Redwoods State Park.[15] This was timely as U.S. Route 101, which would soon provide nearly unfettered access to the trees, was under construction. Using matching funds provided initially by the County of Humboldt and later by the State of California, the Save the Redwoods League] managed to protect areas of concentrated or multiple redwood groves and a few entire forests in the 1920s. As California created a state park system, beginning in 1927, three of the preserved redwood areas became Prairie Creek Redwoods, Del Norte Coast Redwoods, and Jedediah Smith Redwoods State Parks. A fourth became Humboldt Redwoods State Park, by far the largest of the individual Redwood State Parks, but not in the Redwood National and State Park system. Because of the high demand for lumber during World War II and the construction boom that followed in the 1950s, the creation of a national park was delayed. Efforts by the Save the Redwoods League, the Sierra Club, and the National Geographic Society to create a national park began in the early 1960s.[16]  After intense lobbying of Congress, the bill creating Redwood National Park was signed by President Lyndon Johnson on October 2, 1968. The Save the Redwoods League and other entities purchased over 100,000 acres (400km2), which were added to existing state parks. Amidst both local support of environmentalists and opposition from local loggers and logging companies, 48,000 acres (190km2) were added to Redwood National Park in a major expansion in 1978.[16] However, only a fifth of that land was old-growth forest, the rest having been logged. This expansion protected the watershed along Redwood Creek from being adversely affected by logging operations outside the park. The federal and state parks were administratively combined in 1994.[17]
The United Nations designated Redwood National and State Parks a World Heritage Site on September 5, 1980. The evaluation committee noted 50 prehistoric archaeological sites, spanning 4,500years. It also cited ongoing research in the park by Humboldt State University researchers, among others.[18] The park is part of a much larger region designated the California Coast Ranges International Biosphere Reserve on June 30, 1983.[19] The California Coast Ranges biosphere is overseen by the University of California Natural Reserve System.
The park has served as a filming location for numerous films. Scenes set on the forest moon Endor in Star Wars Episode VI: Return of the Jedi were filmed in the Tall Trees Redwood Grove in the northern part of Humboldt County, though the majority of filming was in private and public forests near the town of Smith River, California. Scenes for The Lost World: Jurassic Park as well as the movie Outbreak were filmed at the nearby Prairie Creek Redwoods State Park and at Patrick's Point State Park.[20]
Park management

Redwood National Park headquarters is located in Crescent City, California, with a service office located in Arcata and an operations center located in Orick, California.
Lack of money has precluded major improvements, however, and timber companies have replanted much of the logged area with non-native tree species. Coastline areas, including dunes and coastal prairie, have been invaded by exotic species, partly due to the suppression of forest fires until the 1980s. A fire management plan now allows controlled burning as one method to return the parkland to its original state. Since the redwoods were logged on the basis of accessibility, with inaccessible areas being cut last, large old-growth forest sections were isolated from one another, sometimes by many miles. In these cases it will be decades more before mature forest can return, regardless of the amount of money used to rehabilitate the ecosystem.[21]
The park has transformed a few logging roads into scenic public drives. These do not meet current safety standards, but funding to improve them is not available at present. Park structures such as visitor centers and employee housing also need updating to meet increasing demands. Park employees perform air and water quality surveys, monitor endangered and threatened species, and work closely with the California Coastal National Monument, which is managed by the U.S. Bureau of Land Management.[22] In 2005, the parks were authorized to expand another 25,000 acres (100km2) to include the Mill Creek watershed.[23]
Natural resources
The Redwood National and State Parks form one of the most significant protected areas of the Northern California coastal forests ecoregion.
Flora
It is estimated that old-growth redwood forest once covered close to 2,000,000 acres (8,100km2) of coastal northern California.[24] 96% of all old-growth redwoods have been logged, and almost half (45%) of the redwoods remaining are found in Redwood National and State Parks. The parks protect 38,982 acres (157.75km2) of old-growth forest almost equally divided between federal 19,640 acres (79.5km2) and state 19,342 acres (78.27km2) management.[23] Redwoods have existed along the coast of northern California for at least 20million years and are related to tree species that existed 160million years ago.[25]
The native range of coast redwood is from the northern California coast north to the southern Oregon Coast. The tree is closely related to the giant sequoia of central California, and more distantly to the dawn redwood which is indigenous to the Sichuan–Hubei region of China. Coast redwoods are the tallest trees on Earth; as of September 2006, the tallest tree in the park was Hyperion at 379.1 feet (115.5m), followed by Helios and Icarus which were 376.3 feet (114.7m) and 371.2 feet (113.1m) respectively.[26]

Before September 2006, the tallest living specimen known was the Stratosphere Giant, outside the park in Humboldt Redwoods State Park, which was 370 feet (110m) in 2004. For many years, one specimen simply named Tall Tree in Prairie Creek Redwoods State Park and within the RNSP was measured at 367.8 feet (112.1m), but the top 10 feet (3.0m) of the tree was reported to have died in the 1990s.[27] One tree that fell in 1991 was reported to be 372.04 feet (113.40m). Only the giant sequoia has more mass. The largest redwood by volume is the 42,500cubic foot (1,205m³) Lost Monarch, located in Jedediah Smith Redwoods State Park. Mature Coast redwoods live an average of 500–700years and a few are documented to be 2,000years old, making them some of the longest-living organisms on earth.[23][28] They are highly resistant to disease, due to a thick protective bark and high tannin content. Redwoods prefer sheltered slopes, slightly inland and near water sources such as rivers and streams.
Redwood trees develop enormous limbs that accumulate deep organic soils and can support tree-sized trunks growing on them. This typically occurs above 150 feet (46m). Scientists have recently discovered that plants which normally grow on the forest floor also grow in these soils, well above ground. The soil mats provide homes to invertebrates, mollusks, earthworms, and salamanders. During drought seasons, some treetops die back, but the trees do not die outright. Instead, redwoods have developed mechanisms to regrow new trunks from other limbs. These secondary trunks, called reiterations, also develop root systems in the accumulated soils at their bases. This helps transport water to the highest reaches of the trees. Coastal fog also provides up to one-third of their annual water needs.[29]
Another large tree commonly found in the forest is the coast Douglas-fir, which has been measured at heights of over 300 feet (91m). Sitka spruce are plentiful along the coast and are better adapted to salty air than other species. The evergreen hardwood tanoak produces a nut similar to the acorns produced by the related genus Quercus (oak). Both tanoaks and oaks are members of the beech family. Trees such as the Pacific madrone, bigleaf maple, California laurel, and red alder are also widespread throughout the parks.[30]
Huckleberry, blackberry, and salmonberry are part of the forest understory and provide food for many animal species. The California rhododendron and azalea are flowering shrubs common in the park, especially in old-growth forest.[30] Plants such as the sword fern are prolific, particularly near ample water sources. In Prairie Creek Redwoods State Park, Fern Canyon is a well-known ravine 30 to 50 feet (9.1 to 15.2m) deep, with walls completely covered in ferns.
Fauna

The ecosystems of RNSP preserve a number of rare animal species. Numerous ecosystems exist, with seacoast, river, prairie, and densely forested zones all within the park. The  tidewater goby is a federally listed endangered species that live near the Pacific coastline. The bald eagle, which usually nests near a water source, is listed as a state of California endangered species. The Chinook salmon, northern spotted owl, and Steller's sea lion are a few of the other animal species that are threatened.[6][31]
Over 40 species of mammals have been documented, including the black bear, coyote, cougar, bobcat, beaver, river otter, black-tailed deer, and elk. Along the coastline, California sea lions, Steller sea lions and harbor seals live near the shore and on seastacks, rocky outcroppings forming small islands just off the coast. Dolphins and Pacific gray whales are occasionally seen offshore. Roosevelt elk are the most readily observed of the large mammals in the park. Successful herds, brought back from the verge of extinction in the region, are now common in park areas south of the Klamath River. Many smaller mammals live in the high forest canopy. Different species of bats, such as the big brown bat, and other smaller mammals including the red squirrel and northern flying squirrel spend most of their lives well above the forest floor.[29]
Brown pelicans and double-crested cormorants are mainly found on cliffs along the coast and on seastacks, while sandpipers and gulls inhabit the seacoast and inland areas. Inland, freshwater-dependent birds such as the common merganser, osprey, red-shouldered hawk, great blue heron, and Steller's jay are a few of the bird species that have been documented. At least 400 bird species have been documented in the forestlands.[32]
Reptiles and amphibians can also be found in the parks, with the northwestern ringneck snake, northern red-legged frog, Pacific giant salamander, and the rough-skinned newt most commonly seen.[31]
Invasive species
Currently, there are over 200 exotic species known to live in Redwood National and State Parks. Of these thirty have been identified as invasive species, ten of these are considered threats to local species and ecosystems. Exotic species currently account for about a quarter of the total flora in the parks. Only about one percent of plant growth in old-growth areas are of exotic species, while areas such as the Bald Hills prairies have a relative cover of fifty to seventy-five percent exotic.[33] The type of foreign vegetation also varies, with plants such as the English Ivy (Hedera helix), Spotted Knapweed (Centaurea maculosa), and the Poison Hemlock (Conium maculatum). The spotted knapweed and poison hemlock are both under consideration for addition to a high priority watch list maintained by the park system.[34]
Geology

The northern coastal region of California, which includes RNSP and the adjacent offshore area, is the most seismically active in the U.S.[35] Frequent minor earthquakes in the park and offshore under the Pacific Ocean have resulted in shifting river channels, landslides, and erosion of seaside cliffs. The North American, Pacific, and Gorda Plates are tectonic plates that all meet at the Mendocino Triple Junction, only 100 miles (160km) southwest of the parks. During the 1990s, more than nine magnitude6.0 earthquakes occurred along this fault zone resulting in 1 death and major financial loss, and there is always potential for a major earthquake.[36] The park ensures that visitors are aware of the potential for a major earthquake through the use of pamphlets and information posted throughout the parks. The threat of a tsunami is of particular concern, and visitors to the seacoast are told to seek higher ground immediately after any significant earthquake.[35]
Both coastline and the Coast Ranges can be found within park boundaries. The majority of the rocks in the parks are part of the Franciscan Assemblage, uplifted from the ocean floor millions of years ago. These sedimentary rocks are primarily sandstone, siltstone, shale, and chert, with lesser amounts of metamorphic rocks such as greenstone. For the most part, these rocks are easily eroded, and can be viewed along the seacoast and where rivers and streams have cut small gorges. Formed during the Cretaceous Period, they are highly deformed from uplift and folding processes. In some areas, river systems have created fluvial deposits of sand, mud, and gravel, which are transported into the park from upstream. Redwood Creek follows the Grogan Fault; along the west bank of the creek, schist and other metamorphic rocks can be found, while sedimentary rocks of the Franciscan Assemblage are located on the east bank.[18]
Climate

The Redwood National and State Parks have an oceanic temperate rainforest climate, with cool-summer Mediterranean characteristics. Weather in RNSP is greatly influenced by the Pacific Ocean. Coastal temperatures generally range between 40 and 60degrees Fahrenheit (4–15°C) all year round, while further from the coast summers are hotter and drier, and winters are colder.[39] Redwoods mostly grow a mile or two (1.5–3km) from the coast, and never more than 50 miles (80km) from it. In this temperate but humid coastal zone, the trees receive moisture from both heavy winter rains and persistent summer fog. The presence and consistency of the summer fog is actually more important to overall health of the trees than heavy precipitation. This fact is born out in annual precipitation totals, which range between 25 and 122 inches (64 and 310cm) annually, with healthy redwood forests throughout the areas of less precipitation because excessive needs for water are mitigated by the ever-present summer fog and the cooler temperatures it ensures. Snow is uncommon even on peaks above 1,500 feet (460m), further exemplifying the mild, temperate nature of this northern latitude;[40] however, light snow mixed with rain is common during the winter months.
Fire management
Wildfires are a natural part of most terrestrial ecosystems. In many ways nature has adapted to fire, and the absence of fire can often be disadvantageous.[41]  Wildfire eliminates dead and decayed plant and tree matter, enriching the soil and ensuring that healthier trees have less competition for limited nutrients. Prescribed fire is currently part of the fire management plan and helps to eliminate exotic species of plants and allows a more fertile and natural ecosystem. Fire is also used to protect prairie grasslands and to keep out forest encroachment, ensuring sufficient rangeland for elk and deer. The oak forest regions also benefit from controlled burns, as Douglas fir would otherwise eventually take over and decrease biodiversity. The use of fire in old-growth redwood zones reduces dead and decaying material, and lessens the mortality of larger redwoods by eliminating competing vegetation. In the park, a fire management plan monitors all fires, weather patterns and the fuel load (dead and decaying plant material). This fuel load is removed from areas near structures and where fire poses high risk to the public, and controlled burns are used elsewhere.[42] The National Interagency Fire Center provides additional firefighters and equipment in the event of a large fire.
Recreation

The DeMartin Redwood Youth Hostel, a low-amenities shared lodging facility (near Klamath), has now closed. There are no hotels or motels within the parks boundaries. However, nearby towns such as Klamath, Requa, and Orick provide small hotels and inns, with extensive lodging options available in the regional trading centers of Crescent City on the northern end of the park and Arcata and Eureka located to the south. The park is about 260 miles (420km) north of San Francisco, and 300 miles (480km) south of Portland, Oregon; U.S. Route 101 passes through it from north to south. The Smith River National Recreation Area, part of the Six Rivers National Forest, is adjacent to the north end of RNSP.
While the state parks have front country campsites that can be driven to, the federal sections of the park do not, and hiking is the only way to reach back country campsites. These are at Mill Creek campground in Del Norte Coast Redwoods State Park and Jedediah Smith campground in Jedediah Smith Redwoods State Park, which together have 251 campsites; the Elk Prairie campground in Prairie Creek Redwoods State Park which has 75; and the Gold Bluffs Beach campground which has 25 campsites. Other nearby state parks have additional front country camping.[43]
Back country camping is by permit only and is only allowed in designated sites, except on gravel bars along Redwood Creek.  Access to the back country is highly regulated to prevent overuse while permitting as many groups as possible to explore the forest. Camping in the back country is therefore limited to five consecutive nights, and 15 nights in any one year. Proper food storage to minimize encounters with bears is strongly enforced, and hikers and backpackers are required to take out any trash they generate.[44]

Almost 200 miles (320km) of hiking trails exist in the parks, but during the rainy season some temporary footbridges are removed, as they would be destroyed by high streams. Throughout the year, trails are often wet and hikers need to be well prepared for rainy weather and consult information centers for updates on trail conditions.[44]
Horseback riding and mountain biking are popular but are only allowed on certain trails. Kayaking is popular along the seacoast and in the various rivers and streams. Kayakers and canoeists frequently travel the Smith River, which is the longest undammed river remaining in California. Fishing for salmon and steelhead, a highly prized anadromous form of rainbow trout over 16 inches (41cm), is best in the Smith and Klamath rivers. A California sport fishing license is required to fish any of the rivers and streams.[45] Hunting is not permitted anywhere in the parks, but is allowed in nearby National Forests.
The park has three visitor centers, where guided nature walks and general information are available, along with two additional information points.[46] Each campground offers campfire talks during the summer months as well as guided tours.[43] The parks have many picnic areas, which are all easily accessed by vehicle.
See also
The following three California State Parks are the only State Parks co-managed within the RNSP system:
Del Norte Coast Redwoods State Park
Jedediah Smith Redwoods State Park
Prairie Creek Redwoods State Park
List of national parks of the United States
Notes

 Retrieved in 2006.
 Retrieved in 2006.

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 by Willard E. Pratt. Forest History Society website.
The
 at the Forest History Society archives.


In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is encipherment. To encipher or encode is to convert information into cipher or code. In common parlance, "cipher" is synonymous with "code", as they are both a set of steps that encrypt a message; however, the concepts are distinct in cryptography, especially classical cryptography.
Codes generally substitute different length strings of characters in the output, while ciphers generally substitute the same number of characters as are input.  There are exceptions and some cipher systems may use slightly more, or fewer, characters when output versus the number that were input.
Codes operated by substituting according to a large codebook which linked a random string of characters or numbers to a word or phrase.  For example, "UQJHSE" could be the code for "Proceed to the following coordinates." When using a cipher the original information is known as plaintext, and the encrypted form as ciphertext. The ciphertext message contains all the information of the plaintext message, but is not in a format readable by a human or computer without the proper mechanism to decrypt it.
The operation of a cipher usually depends on a piece of auxiliary information, called a key (or, in traditional NSA parlance, a cryptovariable). The encrypting procedure is varied depending on the key, which changes the detailed operation of the algorithm. A key must be selected before using a cipher to encrypt a message. Without knowledge of the key, it should be extremely difficult, if not impossible, to decrypt the resulting ciphertext into readable plaintext.
Most modern ciphers can be categorized in several ways
By whether they work on blocks of symbols usually of a fixed size (block ciphers), or on a continuous stream of symbols (stream ciphers).
By whether the same key is used for both encryption and decryption (symmetric key algorithms), or if a different key is used for each (asymmetric key algorithms). If the algorithm is symmetric, the key must be known to the recipient and sender and to no one else. If the algorithm is an asymmetric one, the enciphering key is different from, but closely related to, the deciphering key. If one key cannot be deduced from the other, the asymmetric key algorithm has the public/private key property and one of the keys may be made public without loss of confidentiality.
Etymology
The word "cipher" (minority spelling "cypher") in former times meant "zero" and had the same origin: Middle French as cifre and Medieval Latin as cifra, from the Arabic صفر sifr = zero (see Zero—Etymology).  "Cipher" was later used for any decimal digit, even any number.  There are many theories about how the word "cipher" may have come to mean "encoding".
Encoding often involved numbers.
The Roman number system was very cumbersome because there was no concept of zero (or empty space).  The concept of zero (which was also called "cipher"), which is now common knowledge, was alien to medieval Europe, so confusing and ambiguous to common Europeans that in arguments people would say "talk clearly and not so far fetched as a cipher".  Cipher came to mean concealment of clear messages or encryption.
The French formed the word "chiffre" and adopted the Italian word "zero".
The English used "zero" for "0", and "cipher" from the word "ciphering" as a means of computing.
The Germans used the words "Ziffer" (digit) and "Chiffre".
The Dutch still use the word "cijfer" to refer to a numerical digit.
The Slovaks, similarly, also use the word "cifra" to refer to a numerical digit.
The Bosnians, Croats and Serbians use the word "cifra", which refers to a digit, or in some cases, any number. Besides "cifra", they use word "broj" for a number.
The Italians and the Spanish also use the word "cifra" to refer to a number.
The Swedes use the word "siffra" which refers to a digit.
The Greeks use the word "τζίφρα" ("tzifra") to refer to a hard-to-read signature, especially one written with a single stroke of the pen.
Ibrahim Al-Kadi concluded that the Arabic word sifr, for the digit zero, developed into the European technical term for encryption.[1]
As the decimal zero and its new mathematics spread from the Arabic world to Europe in the Middle Ages, words derived from sifr and zephyrus came to refer to calculation, as well as to privileged knowledge and secret codes. According to Ifrah, "in thirteenth-century Paris, a 'worthless fellow' was called a '... cifre en algorisme', i.e., an 'arithmetical nothing'."[2] Cipher was the European pronunciation of sifr, and cipher came to mean a message or communication not easily understood.[3]
Versus codes

In non-technical usage, a "(secret) code" typically means a "cipher". Within technical discussions, however, the words "code" and "cipher" refer to two different concepts. Codes work at the level of meaning—that is, words or phrases are converted into something else and this chunking generally shortens the message.
An example of this is the Commercial Telegraph Code which was used to shorten long telegraph messages which resulted from entering into commercial contracts using exchanges of Telegrams.
Another example is given by whole word ciphers, which allow the user to replace an entire word with a symbol or character, much like the way Japanese utilize Kanji (Japanese) characters to supplement their language. ex "The quick brown fox jumps over the lazy dog" becomes "The quick brown 狐 jumps 过 the lazy 狗".
Ciphers, on the other hand, work at a lower level: the level of individual letters, small groups of letters, or, in modern schemes, individual bits and blocks of bits. Some systems used both codes and ciphers in one system, using superencipherment to increase the security. In some cases the terms codes and ciphers are also used synonymously to substitution and transposition.
Historically, cryptography was split into a dichotomy of codes and ciphers; and coding had its own terminology, analogous to that for ciphers: "encoding, codetext, decoding" and so on.
However, codes have a variety of drawbacks, including susceptibility to cryptanalysis and the difficulty of managing a cumbersome codebook. Because of this, codes have fallen into disuse in modern cryptography, and ciphers are the dominant technique.
Types
There are a variety of different types of encryption. Algorithms used earlier in the history of cryptography are substantially different from modern methods, and modern ciphers can be classified according to how they operate and whether they use one or two keys.
Historical
Historical pen and paper ciphers used in the past are sometimes known as classical ciphers. They include simple substitution ciphers (such as Rot 13) and transposition ciphers (such as a Rail Fence Cipher). For example, "GOOD DOG" can be encrypted as "PLLX XLP" where "L" substitutes for "O", "P" for "G", and "X" for "D" in the message.  Transposition of the letters "GOOD DOG" can result in "DGOGDOO". These simple ciphers and examples are easy to crack, even without plaintext-ciphertext pairs.[4]
Simple ciphers were replaced by polyalphabetic substitution ciphers (such as the Vigenère) which changed the substitution alphabet for every letter.  For example, "GOOD DOG" can be encrypted as "PLSX TWF" where "L", "S", and "W" substitute for "O".  With even a small amount of known or estimated plaintext, simple polyalphabetic substitution ciphers and letter transposition ciphers designed for pen and paper encryption are easy to crack.[5] It is possible to create a secure pen and paper cipher based on a one-time pad though, but the usual disadvantages of one-time pads apply.
During the early twentieth century, electro-mechanical machines were invented to do encryption and decryption using transposition, polyalphabetic substitution, and a kind of "additive" substitution.  In rotor machines, several rotor disks provided polyalphabetic substitution, while plug boards provided another substitution. Keys were easily changed by changing the rotor disks and the plugboard wires. Although these encryption methods were more complex than previous schemes and required machines to encrypt and decrypt, other machines such as the British Bombe were invented to crack these encryption methods.
Modern
Modern encryption methods can be divided by two criteria: by type of key used, and by type of input data.
By type of key used ciphers are divided into:
symmetric key algorithms (Private-key cryptography), where the same key is used for encryption and decryption, and
asymmetric key algorithms (Public-key cryptography), where two different keys are used for encryption and decryption.
In a symmetric key algorithm (e.g., DES and AES), the sender and receiver must have a shared key set up in advance and kept secret from all other parties; the sender uses this key for encryption, and the receiver uses the same key for decryption.  The Feistel cipher uses a combination of substitution and transposition techniques. Most block cipher algorithms are based on this structure.  In an asymmetric key algorithm (e.g., RSA), there are two separate keys: a public key is published and enables any sender to perform encryption, while a private key is kept secret by the receiver and enables only him to perform correct decryption.
Ciphers can be distinguished into two types by the type of input data:
block ciphers, which encrypt block of data of fixed size, and
stream ciphers, which encrypt continuous streams of data
Key size and vulnerability
In a pure mathematical attack, (i.e., lacking any other information to help break a cipher) two factors above all count:
Computational power available, i.e., the computing power which can be brought to bear on the problem. It is important to note that average performance/capacity of a single computer is not the only factor to consider. An adversary can use multiple computers at once, for instance, to increase the speed of exhaustive search for a key (i.e., "brute force" attack) substantially.
Key size, i.e., the size of key used to encrypt a message. As the key size increases, so does the complexity of exhaustive search to the point where it becomes impractical to crack encryption directly.
Since the desired effect is computational difficulty, in theory one would choose an algorithm and desired difficulty level, thus decide the key length accordingly.
An example of this process can be found at  which uses multiple reports to suggest that a symmetric cipher with 128 bits, an asymmetric cipher with 3072 bit keys, and an elliptic curve cipher with 512 bits, all have similar difficulty at present.
Claude Shannon proved, using information theory considerations, that any theoretically unbreakable cipher must have keys which are at least as long as the plaintext, and used only once: one-time pad.
See also

Autokey cipher
Cover-coding
Cryptography Classification
Encryption software
Famous ciphertexts
Pretty Good Privacy
Steganography
Telegraph code

Notes

Richard J. Aldrich, GCHQ: The Uncensored Story of Britain's Most Secret Intelligence Agency, HarperCollins July 2010.
Helen Fouché Gaines, "Cryptanalysis", 1939, Dover. ISBN0-486-20097-3
Ibrahim A. Al-Kadi, "The origins of cryptology: The Arab contributions", Cryptologia, 16(2) (April 1992) pp.97–126.
David Kahn, The Codebreakers - The Story of Secret Writing (ISBN0-684-83130-9) (1967)
David A. King, The ciphers of the monks - A forgotten number notation of the Middle Ages, Stuttgart: Franz Steiner, 2001 (ISBN3-515-07640-9)
Abraham Sinkov, Elementary Cryptanalysis: A Mathematical Approach, Mathematical Association of America, 1966. ISBN0-88385-622-0
William Stallings, Cryptography and Network Security, principles and practices, 4th Edition

What is the difference between ciphers and codes?