Photography is the art and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employed in many fields of science and business, as well as its more direct uses for art and video production, recreational purposes and mass communication. A lens is used to focus the light reflected or emitted from objects into a real image on the light-sensitive surface inside a camera during a timed exposure. With an electronic image sensor, this produces an electrical charge at each pixel, electronically processed and stored in a digital image file for subsequent display or processing; the result with photographic emulsion is an invisible latent image, chemically "developed" into a visible image, either negative or positive depending on the purpose of the photographic material and the method of processing. A negative image on film is traditionally used to photographically create a positive image on a paper base, known as a print, either by using an enlarger or by contact printing.
The word "photography" was created from the Greek roots φωτός, genitive of φῶς, "light" and γραφή "representation by means of lines" or "drawing", together meaning "drawing with light". Several people may have coined the same new term from these roots independently. Hercules Florence, a French painter and inventor living in Campinas, used the French form of the word, photographie, in private notes which a Brazilian historian believes were written in 1834; this claim is reported but has never been independently confirmed as beyond reasonable doubt. The German newspaper Vossische Zeitung of 25 February 1839 contained an article entitled Photographie, discussing several priority claims – Henry Fox Talbot's – regarding Daguerre's claim of invention; the article is the earliest known occurrence of the word in public print. It was signed "J. M.", believed to have been Berlin astronomer Johann von Maedler. The inventors Nicéphore Niépce, Henry Fox Talbot and Louis Daguerre seem not to have known or used the word "photography", but referred to their processes as "Heliography", "Photogenic Drawing"/"Talbotype"/"Calotype" and "Daguerreotype".
Photography is the result of combining several technical discoveries, relating to seeing an image and capturing the image. The discovery of the camera obscura that provides an image of a scene dates back to ancient China. Greek mathematicians Aristotle and Euclid independently described a pinhole camera in the 5th and 4th centuries BCE. In the 6th century CE, Byzantine mathematician Anthemius of Tralles used a type of camera obscura in his experiments; the Arab physicist Ibn al-Haytham invented a camera obscura and pinhole camera. Leonardo da Vinci mentions natural camera obscura that are formed by dark caves on the edge of a sunlit valley. A hole in the cave wall will act as a pinhole camera and project a laterally reversed, upside down image on a piece of paper. Renaissance painters used the camera obscura which, in fact, gives the optical rendering in color that dominates Western Art, it is a box with a hole in it which allows light to go through and create an image onto the piece of paper.
The birth of photography was concerned with inventing means to capture and keep the image produced by the camera obscura. Albertus Magnus discovered silver nitrate, Georg Fabricius discovered silver chloride, the techniques described in Ibn al-Haytham's Book of Optics are capable of producing primitive photographs using medieval materials. Daniele Barbaro described a diaphragm in 1566. Wilhelm Homberg described how light darkened some chemicals in 1694; the fiction book Giphantie, published in 1760, by French author Tiphaigne de la Roche, described what can be interpreted as photography. Around the year 1800, British inventor Thomas Wedgwood made the first known attempt to capture the image in a camera obscura by means of a light-sensitive substance, he used paper or white leather treated with silver nitrate. Although he succeeded in capturing the shadows of objects placed on the surface in direct sunlight, made shadow copies of paintings on glass, it was reported in 1802 that "the images formed by means of a camera obscura have been found too faint to produce, in any moderate time, an effect upon the nitrate of silver."
The shadow images darkened all over. The first permanent photoetching was an image produced in 1822 by the French inventor Nicéphore Niépce, but it was destroyed in a attempt to make prints from it. Niépce was successful again in 1825. In 1826 or 1827, he made the View from the Window at Le Gras, the earliest surviving photograph from nature; because Niépce's camera photographs required an long exposure, he sought to improve his bitumen process or replace it with one, more practical. In partnership with Louis Daguerre, he worked out post-exposure processing methods that produced visually superior results and replaced the bitumen with a more light-sensitive resin, but hours of exposure in the camera were still required. With an eye to eventual commercial exploitation, the partners opted for total secrecy. Niépce died in 1833 and Daguerre redirected the experiments toward the light-sensitive silver halides, which Niépce had abandoned many years earlier because of his inability to make the images he captured with them light-fast and permanent.
A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses arranged along a common axis. Lenses are made from materials such as glass or plastic, are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism. Devices that focus or disperse waves and radiation other than visible light are called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses; the word lens comes from lēns, the Latin name of the lentil, because a double-convex lens is lentil-shaped. The lentil plant gives its name to a geometric figure; some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass.
Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds mentioning a burning-glass. Pliny the Elder confirms. Pliny has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald. Both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. Ptolemy wrote a book on Optics, which however survives only in the Latin translation of an incomplete and poor Arabic translation; the book was, received, by medieval scholars in the Islamic world, commented upon by Ibn Sahl, in turn improved upon by Alhazen. The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century. Between the 11th and 13th century "reading stones" were invented; these were primitive plano-convex lenses made by cutting a glass sphere in half. The medieval rock cystal Visby lenses may not have been intended for use as burning glasses.
Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of the 13th century. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses; the practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands. With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries by those trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces.
Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention claimed by fellow Englishman John Dollond in a 1758 patent. Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres; each surface can be concave, or planar. The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens; the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may ground after manufacturing to give them a different shape or size; the lens axis may not pass through the physical centre of the lens. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes, they have a different focal power in different meridians. This forms an astigmatic lens. An example is eyeglass lenses. More complex are aspheric lenses.
These are lenses where one or both surfaces have a shape, neither spherical nor cylindrical. The more complicated shapes allow such lenses to form images with less aberration than standard simple lenses, but they are more difficult and expensive to produce. Lenses are classified by the curvature of the two optical surfaces. A lens is biconvex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave. If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is meniscus, it is this type of lens, most used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot behind the lens. In this case, the lens is called a
Photographic film is a strip or sheet of transparent plastic film base coated on one side with a gelatin emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the crystals determine the sensitivity and resolution of the film; the emulsion will darken if left exposed to light, but the process is too slow and incomplete to be of any practical use. Instead, a short exposure to the image formed by a camera lens is used to produce only a slight chemical change, proportional to the amount of light absorbed by each crystal; this creates an invisible latent image in the emulsion, which can be chemically developed into a visible photograph. In addition to visible light, all films are sensitive to ultraviolet, X-rays and high-energy particles. Unmodified silver halide crystals are sensitive only to the blue part of the visible spectrum, producing unnatural-looking renditions of some colored subjects; this problem was resolved with the discovery that certain dyes, called sensitizing dyes, when adsorbed onto the silver halide crystals made them respond to other colors as well.
First orthochromatic and panchromatic films were developed. Panchromatic film renders all colors in shades of gray matching their subjective brightness. By similar techniques, special-purpose films can be made sensitive to the infrared region of the spectrum. In black-and-white photographic film, there is one layer of silver halide crystals; when the exposed silver halide grains are developed, the silver halide crystals are converted to metallic silver, which blocks light and appears as the black part of the film negative. Color film has at least three sensitive layers, incorporating different combinations of sensitizing dyes; the blue-sensitive layer is on top, followed by a yellow filter layer to stop any remaining blue light from affecting the layers below. Next comes a green-and-blue sensitive layer, a red-and-blue sensitive layer, which record the green and red images respectively. During development, the exposed silver halide crystals are converted to metallic silver, just as with black-and-white film.
But in a color film, the by-products of the development reaction combine with chemicals known as color couplers that are included either in the film itself or in the developer solution to form colored dyes. Because the by-products are created in direct proportion to the amount of exposure and development, the dye clouds formed are in proportion to the exposure and development. Following development, the silver is converted back to silver halide crystals in the bleach step, it is removed from the film during the process of fixing the image on the film with a solution of ammonium thiosulfate or sodium thiosulfate. Fixing leaves behind only the formed color dyes, which combine to make up the colored visible image. Color films, like Kodacolor II, have as many as 12 emulsion layers, with upwards of 20 different chemicals in each layer; the earliest practical photographic process was the daguerreotype. The light-sensitive chemicals were formed on the surface of a silver-plated copper sheet; the calotype process produced paper negatives.
Beginning in the 1850s, thin glass plates coated with photographic emulsion became the standard material for use in the camera. Although fragile and heavy, the glass used for photographic plates was of better optical quality than early transparent plastics and was, at first, less expensive. Glass plates continued to be used long after the introduction of film, were used for astrophotography and electron micrography until the early 2000s, when they were supplanted by digital recording methods. Ilford continues to manufacture glass plates for special scientific applications; the first flexible photographic roll film was sold by George Eastman in 1885, but this original "film" was a coating on a paper base. As part of the processing, the image-bearing layer was stripped from the paper and attached to a sheet of hardened clear gelatin; the first transparent plastic roll film followed in 1889. It was made from flammable nitrocellulose, now called "nitrate film". Although cellulose acetate or "safety film" had been introduced by Kodak in 1908, at first it found only a few special applications as an alternative to the hazardous nitrate film, which had the advantages of being tougher more transparent, cheaper.
The changeover was completed for X-ray films in 1933, but although safety film was always used for 16 mm and 8 mm home movies, nitrate film remained standard for theatrical 35 mm films until it was discontinued in 1951. Hurter and Driffield began pioneering work on the light sensitivity of photographic emulsions in 1876, their work enabled the first quantitative measure of film speed to be devised. They developed H&D curves, which are specific for each paper; these curves plot the photographic density against the log of the exposure, to determine sensitivity or speed of the emulsion and enabling correct exposure. Early photographic plates and films were usefully sensitive only to blue and ultraviolet light; as a result, the relative tonal values in a scene registered as they would appear if viewed through a piece of deep blue glass. Blue skies with interesting cloud formations photographed as a white blank. Any detail visible in masses of green foliage was due to the colorless surface gloss. Bright yellows and reds appeared nearly black.
Most skin tones came out unnaturally dark, uneven or freckled complexions were exaggerated. Photographers sometimes compensated by adding in skies from
In photography and image processing, color balance is the global adjustment of the intensities of the colors. An important goal of this adjustment is to render specific colors – neutral colors – correctly. Hence, the general method is sometimes called neutral balance, or white balance. Color balance is used for color correction. Generalized versions of color balance are used to correct colors other than neutrals or to deliberately change them for effect. Image data acquired by sensors – either film or electronic image sensors – must be transformed from the acquired values to new values that are appropriate for color reproduction or display. Several aspects of the acquisition and display process make such color correction essential – including that the acquisition sensors do not match the sensors in the human eye, that the properties of the display medium must be accounted for, that the ambient viewing conditions of the acquisition differ from the display viewing conditions; the color balance operations in popular image editing applications operate directly on the red and blue channel pixel values, without respect to any color sensing or reproduction model.
In film photography, color balance is achieved by using color correction filters over the lights or on the camera lens. Sometimes the adjustment to keep neutrals neutral is called white balance, the phrase color balance refers to the adjustment that in addition makes other colors in a displayed image appear to have the same general appearance as the colors in an original scene, it is important that neutral colors in a scene appear neutral in the reproduction. Humans relate to flesh tones more critically than other colors. Trees and sky can all be off without concern, but if human flesh tones are'off' the human subject can look sick or dead. To address this critical color balance issue, the tri-color primaries themselves are formulated to not balance as a true neutral color; the purpose of this color primary imbalance is to more faithfully reproduce the flesh tones through the entire brightness range. US Patent 2,850,563 states "...effecting a non-distorted transfer of a flesh tone through the system over its entire brightness range, notwithstanding the inherent non-linear transfer characteristic of said system...".
Viewing of the flesh altered primaries is available with a /Color Separation Guide or Color Control Patch. There are 18 color swatches on the guide; the 3-color swatch at 50% density thereon shows the result of the flesh tone primaries, at a 1:1:1 ratio, directly. Most digital cameras have means to select color correction based on the type of scene lighting, using either manual lighting selection, automatic white balance, or custom white balance; the algorithms for these processes perform generalized chromatic adaptation. Many methods exist for color balancing. Setting a button on a camera is a way for the user to indicate to the processor the nature of the scene lighting. Another option on some cameras is a button which one may press when the camera is pointed at a gray card or other neutral colored object; this captures an image of the ambient light, which enables a digital camera to set the correct color balance for that light. There is a large literature on how one might estimate the ambient lighting from the camera data and use this information to transform the image data.
A variety of algorithms have been proposed, the quality of these has been debated. A few examples and examination of the references therein will lead the reader to many others. Examples are an artificial neural network or a Bayesian method. Color balancing an image affects not only other colors as well. An image, not color balanced is said to have a color cast, as everything in the image appears to have been shifted towards one color. Color balancing may be thought in terms of removing this color cast. Color balance is related to color constancy. Algorithms and techniques used to attain color constancy are used for color balancing, as well. Color constancy is, in turn, related to chromatic adaptation. Conceptually, color balancing consists of two steps: first, determining the illuminant under which an image was captured. Viggiano found that white balancing in the camera's native RGB color model tended to produce less color inconstancy than in monitor RGB for over 4000 hypothetical sets of camera sensitivities.
This difference amounted to a factor of more than two in favor of camera RGB. This means that it is advantageous to get color balance right at the time an image is captured, rather than edit on a monitor. If one must color balance balancing the raw image data will tend to produce less distortion of chromatic colors than balancing in monitor RGB. Color balancing is sometimes performed on a three-component image using a 3x3 matrix; this type of transformation is appropriate if the image was captured using the wrong white balance setting on a digital camera, or through a color filter. In principle, one wants to scale all relative luminances in an image so that objects which are believed to be neutral appear so. If, say, a surface with R = 240 was believed to be a white object, if 255 is the count which corresponds to white, one could multiply all red values by 255/240. Doing analogously for green and blue would result, at least in theory
Rack and pinion
A rack and pinion is a type of linear actuator that comprises a circular gear engaging a linear gear, which operate to translate rotational motion into linear motion. Driving the pinion into rotation causes the rack to be driven linearly. Driving the rack linearly will cause the pinion to be driven into a rotation. For example, in a rack railway, the rotation of a pinion mounted on a locomotive or a railcar engages a rack between the rails and forces a train up a steep slope. For every pair of conjugate involute profile, there is a basic rack; this basic rack is the profile of the conjugate gear of infinite pitch radius. A generating rack is a rack outline used to indicate tooth details and dimensions for the design of a generating tool, such as a hob or a gear shaper cutter. Rack and pinion combinations are used as part of a simple linear actuator, where the rotation of a shaft powered by hand or by a motor is converted to linear motion; the rack carries the full load of the actuator directly and so the driving pinion is small, so that the gear ratio reduces the torque required.
This force, thus torque, may still be substantial and so it is common for there to be a reduction gear before this by either a gear or worm gear reduction. Rack gears have a higher ratio, thus require a greater driving torque, than screw actuators. Most stairlifts today operate using the pinion system. A rack and pinion is found in the steering mechanism of cars or other wheeled, steered vehicles. Rack and pinion provides less mechanical advantage than other mechanisms such as recirculating ball, but less backlash and greater feedback, or steering "feel"; the mechanism may be power-assisted by hydraulic or electrical means. The use of a variable rack was invented by Arthur Ernest Bishop in the 1970s, so as to improve vehicle response and steering "feel," at high speeds, he created a low cost press forging process to manufacture the racks, eliminating the need to machine the gear teeth. Rack railways are mountain railways that use a rack built into the center of the track and a pinion on their locomotives.
This allows them to work on steep gradients, up to 45 degrees, as opposed to conventional railways which rely on friction alone for locomotion. Additionally, the rack and pinion addition provides these trains with controlled brakes, reduces the effects of snow or ice on the rails. A rack and pinion with two racks and one pinion is used in actuators. An example is pneumatic rack and pinion actuators that can be used to control valves in pipeline transport; the actuators in the picture on the right are used to control the valves of large water pipeline. In the top actuator, a gray control signal line can be seen connecting to a solenoid valve, used as the pilot for the actuator; the solenoid valve controls the air pressure coming from the input air line. The output air from the solenoid valve is fed to the chamber in the middle of the actuator, increasing the pressure; the pressure in the actuator's chamber pushes the pistons away. While the pistons are moving apart from each other, the attached racks are moved along the pistons in the opposite directions of the two racks.
The two racks are meshed to a pinion at the direct opposite teeth of the pinion. When the two racks move, the pinion is turned, causing the attached main valve of the water pipe to turn. A rack gear, curved is called an arcuate rack; the term appears in many patent applications. List of gear nomenclature Sprocket
A billboard is a large outdoor advertising structure found in high-traffic areas such as alongside busy roads. Billboards present large advertisements to passing drivers. Showing witty slogans and distinctive visuals, billboards are visible in the top designated market areas; the largest ordinary-sized billboards are located on major highways, expressways or principal arterials, command high-density consumer exposure. These afford greatest visibility due not only to their size, but because they allow creative "customizing" through extensions and embellishments. Posters are the other common form of billboard advertising, located along primary and secondary arterial roads. Posters are a smaller format and are viewed principally by residents and commuter traffic, with some pedestrian exposure. Billboard advertisements are designed to catch a person's attention and create a memorable impression quickly, leaving the reader thinking about the advertisement after they have driven past it, they have to be readable in a short time because they are read while being passed at high speeds.
Thus there are only a few words, in large print, a humorous or arresting image in brilliant color. Some billboard designs spill outside the actual space given to them by the billboard, with parts of figures hanging off the billboard edges or jutting out of the billboard in three dimensions. An example in the United States around the turn of the 21st century was the Chick-fil-A billboards, which had three-dimensional cow figures in the act of painting the billboards with misspelled anti-beef slogans such as "frendz don't let frendz eat beef." The first "scented billboard", an outdoor sign emitting the odors of black pepper and charcoal to suggest a grilled steak, was erected on NC 150 near Mooresville, North Carolina by the Bloom grocery chain. The sign depicted a giant cube of beef being pierced by a large fork; the scents were emitted between 7–10 am and 4–7 pm from 28 May 2010 through 18 June 2010. All these billboards were painted in large studios; the image was projected on the series of paper panels.
Line drawings were done traced with a pounce wheel that created perforated lines. The patterns were "pounced" onto the board with a chalk filled pounce bag, marking the outlines of the figures or objects. Using oil paints, artists would use large brushes to paint the image. Once the panels were installed using hydraulic cranes, artists would go up on the installed billboard and touch up the edges between panels; these large, painted billboards were popular in Los Angeles where historic firms such as Foster & Kleiser and Pacific Outdoor Advertising dominated the industry. These painted billboards gave way to graphic reproduction, but hand-painted billboards are still in use in some areas where only a single board or two is required; the "Sunset Strip" in Los Angeles is one area where hand-painted billboards can still be found to advertise upcoming films or albums. A digital billboard is a billboard that shows varying imagery and text created from computer programs and software. Digital billboards can be designed to display running text, display several different displays from the same company, provide several companies a certain time slot during the day.
The changing texts ensure maximum impact and wide exposure to target audiences. The ability to schedule advertisements remotely, in combination with flexible real-time scheduling, has allowed for a decrease in traditional upkeep and maintenance costs. Additionally, digital billboards are continually integrating with real-time advertisement technologies to measure audiences or serve dynamic content. In January 2015, Ooh! Media launched a campaign with Porsche that detected incoming Porsche cars and displayed a dynamic piece of relevant content to Porsche drivers. In May 2014, Beck's Beer released a billboard poster. Conductive ink linked to sensors and speakers means that when touched, the poster begins to play music; the beer company claim it to be'the world's first playable music poster'. However, Agency Republic released the Spotify Powered Interactive Music Poster in April 2012. Creative agency, Grey London collaborated on a interactive poster using touch sensitive inks in April 2014. Outdoor Advertising, such as a mobile billboard, is effective.
According to a UK national survey, it is memorable. Capitol Communications Group found that 81.7% of those polled recalled images they saw on a moving multi-image sign. This is compared to a 19% retention rate for static signs. Unlike a typical billboard, mobile billboards are able to go directly to their target audience, they can be placed wherever there is heavy foot traffic due to an event – including convention centers, train stations and sports arenas. They can repeat routes, ensuring that an advertiser's message is not only noticed, but that information is retained through repetition. Billboards may be multi-purpose. An advertising sign can integrate its main purpose with telecommunications antenna or public lighting support; the structure has a steel pole with a coupling flange on the above-fitted advertising billboard structure that can contain telecommunications antennas. The lighting and any antennas are placed inside the structure. Common along highways are free-standing two-sided as well as three-sided billboards.
Other types of billboards include the billboard bicycle attached to the back of a bicycle or the mobile billboard, a special advertising
A computer is a device that can be instructed to carry out sequences of arithmetic or logical operations automatically via computer programming. Modern computers have the ability to follow generalized sets of called programs; these programs enable computers to perform an wide range of tasks. A "complete" computer including the hardware, the operating system, peripheral equipment required and used for "full" operation can be referred to as a computer system; this term may as well be used for a group of computers that are connected and work together, in particular a computer network or computer cluster. Computers are used as control systems for a wide variety of industrial and consumer devices; this includes simple special purpose devices like microwave ovens and remote controls, factory devices such as industrial robots and computer-aided design, general purpose devices like personal computers and mobile devices such as smartphones. The Internet is run on computers and it connects hundreds of millions of other computers and their users.
Early computers were only conceived as calculating devices. Since ancient times, simple manual devices like the abacus aided people in doing calculations. Early in the Industrial Revolution, some mechanical devices were built to automate long tedious tasks, such as guiding patterns for looms. More sophisticated electrical machines did specialized analog calculations in the early 20th century; the first digital electronic calculating machines were developed during World War II. The speed and versatility of computers have been increasing ever since then. Conventionally, a modern computer consists of at least one processing element a central processing unit, some form of memory; the processing element carries out arithmetic and logical operations, a sequencing and control unit can change the order of operations in response to stored information. Peripheral devices include input devices, output devices, input/output devices that perform both functions. Peripheral devices allow information to be retrieved from an external source and they enable the result of operations to be saved and retrieved.
According to the Oxford English Dictionary, the first known use of the word "computer" was in 1613 in a book called The Yong Mans Gleanings by English writer Richard Braithwait: "I haue read the truest computer of Times, the best Arithmetician that euer breathed, he reduceth thy dayes into a short number." This usage of the term referred to a human computer, a person who carried out calculations or computations. The word continued with the same meaning until the middle of the 20th century. During the latter part of this period women were hired as computers because they could be paid less than their male counterparts. By 1943, most human computers were women. From the end of the 19th century the word began to take on its more familiar meaning, a machine that carries out computations; the Online Etymology Dictionary gives the first attested use of "computer" in the 1640s, meaning "one who calculates". The Online Etymology Dictionary states that the use of the term to mean "'calculating machine' is from 1897."
The Online Etymology Dictionary indicates that the "modern use" of the term, to mean "programmable digital electronic computer" dates from "1945 under this name. Devices have been used to aid computation for thousands of years using one-to-one correspondence with fingers; the earliest counting device was a form of tally stick. Record keeping aids throughout the Fertile Crescent included calculi which represented counts of items livestock or grains, sealed in hollow unbaked clay containers; the use of counting rods is one example. The abacus was used for arithmetic tasks; the Roman abacus was developed from devices used in Babylonia as early as 2400 BC. Since many other forms of reckoning boards or tables have been invented. In a medieval European counting house, a checkered cloth would be placed on a table, markers moved around on it according to certain rules, as an aid to calculating sums of money; the Antikythera mechanism is believed to be the earliest mechanical analog "computer", according to Derek J. de Solla Price.
It was designed to calculate astronomical positions. It was discovered in 1901 in the Antikythera wreck off the Greek island of Antikythera, between Kythera and Crete, has been dated to c. 100 BC. Devices of a level of complexity comparable to that of the Antikythera mechanism would not reappear until a thousand years later. Many mechanical aids to calculation and measurement were constructed for astronomical and navigation use; the planisphere was a star chart invented by Abū Rayhān al-Bīrūnī in the early 11th century. The astrolabe was invented in the Hellenistic world in either the 1st or 2nd centuries BC and is attributed to Hipparchus. A combination of the planisphere and dioptra, the astrolabe was an analog computer capable of working out several different kinds of problems in spherical astronomy. An astrolabe incorporating a mechanical calendar computer and gear-wheels was invented by Abi Bakr of Isfahan, Persia in 1235. Abū Rayhān al-Bīrūnī invented the first mechanical geared lunisolar calendar astrolabe, an early fixed-wired knowledge processing machine with a gear train and gear-wheels, c. 1000 AD.
The sector, a calculating instrument used for solving problems in proportion, trigonometry and division, for various functions, such as squares and cube roots, was developed in