One of the most common and practical applications of polarization is the liquid crystal display (LCD) used in numerous devices including wristwatches, computer screens, timers, clocks, and a host of others. These display systems are based upon the interaction of rod-like liquid crystalline molecules with an electric field and polarized light waves. The liquid crystalline phase exists in a ground state that is termed cholesteric, in which the molecules are oriented in layers, and each successive layer is slightly twisted to form a spiral pattern (Figure 9). When polarized light waves interact with the liquid crystalline phase the wave is "twisted" by an angle of approximately 90 degrees with respect to the incident wave. The exact magnitude of this angle is a function of the helical pitch of the cholesteric liquid crystalline phase, which is dependent upon the chemical composition of the molecules (it can be fine-tuned by small changes to the molecular structure).

The principle behind Brewster's angle is illustrated Figure 3 for a single ray of light reflecting from the flat surface of a transparent medium having a higher refractive index than air. The incident ray is drawn with only two electric vector vibration planes, but is intended to represent light having vibrations in all planes perpendicular to the direction of propagation. When the beam arrives on the surface at a critical angle (Brewster's angle; represented by the variable θ in Figure 3), the polarization degree of the reflected beam is 100 percent, with the orientation of the electric vectors lying perpendicular to the plane of incidence and parallel to the reflecting surface. The incidence plane is defined by the incident, refracted, and reflected waves. The refracted ray is oriented at a 90-degree angle from the reflected ray and is only partially polarized.

Illumination was a complex and costly process, and was therefore usually reserved for special books such as altar bibles, or books for royalty. Heavily illuminated manuscripts are often called "luxury manuscripts" for this reason. In the early Middle Ages, most books were produced in monasteries, whether for their own use, for presentation, or for a commission. These monks would work as a collective group to sponsor the patronage of a manuscript, but that in turn shielded their identites somewhat from history: there are more numerous surviving signatures on works from the scibe and less from the illustrations, but often there is simply the signature of the patron monastery.[1] However, commercial scriptoria grew up in large cities, especially Paris, and in Italy and the Netherlands, and by the late 14th century there was a significant industry producing manuscripts, including agents who would take long-distance commissions, with details of the heraldry of the buyer and the saints of personal interest to him (for the calendar of a book of hours). By the end of the period, many of the painters were women, especially painting the elaborate borders, and perhaps especially in Paris.

Another feature of illuminating manuscripts of the Middle Ages was the use of Marginalia. These additions were typically found within and around decorative borders of the text. Marginalia found within medieval manuscripts were often unique special messages and details indicative of the precision and careful consideration involved in their production. Marginalia shaped the way the text was read and influenced the reader’s interaction with it. Placement of these decorations and messages prompted the reader to scrutinize beyond the physical book to interpret the text from multiple perspectives.[33]

The phenomenon of optical activity in certain chemicals derives from their ability to rotate the plane of polarized light. Included in this category are many sugars, amino acids, organic natural products, certain crystals, and some drugs. Rotation is measured by placing a solution of the target chemical between crossed polarizers in an instrument termed a polariscope. First observed in 1811 by French physicist Dominique Arago, optical activity plays an important role in a variety of biochemical processes where the structural geometry of molecules governs their interactions. Chemicals that rotate the vibrational plane of polarized light in a clockwise direction are termed dextrorotatory, while those that rotate the light in a counterclockwise direction are referred to as levorotatory. Two chemicals having the same molecular formula but different optical properties are termed optical isomers, which rotate the plane of polarized light in different directions.

On the strictest definition, a manuscript is not considered "illuminated" unless one or many illuminations contained metal, normally gold leaf or shell gold paint, or at least was brushed with gold specks. Gold leaf was from the 12th century usually polished, a process known as burnishing. The inclusion of gold alludes to many different possibilities for the text. If the text is of religious nature, lettering in gold is a sign of exalting the text. In the early centuries of Christianity, Gospel manuscripts were sometimes written entirely in gold.[7] The gold ground style, with all or most of the background in gold, was taken from Byzantine mosaics and icons. Aside from adding rich decoration to the text, scribes during the time considered themselves to be praising God with their use of gold. Furthermore, gold was used if a patron who had commissioned a book to be written wished to display the vastness of their riches. Eventually, the addition of gold to manuscripts became so frequent "that its value as a barometer of status with the manuscript was degraded".[29] During this time period the price of gold had become so cheap that its inclusion in an illuminated manuscript accounted for only a tenth of the cost of production.[41] By adding richness and depth to the manuscript, the use of gold in illuminations created pieces of art that are still valued today.

Several versions of prism-based polarizing devices were once widely available, and these were usually named after their designers. The most common polarizing prism (illustrated in Figure 5) was named after William Nicol, who first cleaved and cemented together two crystals of Iceland spar with Canada balsam in 1829. Nicol prisms were first used to measure the polarization angle of birefringent compounds, leading to new developments in the understanding of interactions between polarized light and crystalline substances.

An excellent example of the basic application of liquid crystals to display devices can be found in the seven-segment liquid crystal numerical display (illustrated in Figure 9). Here, the liquid crystalline phase is sandwiched between two glass plates that have electrodes attached, similar to those depicted in the illustration. In Figure 9, the glass plates are configured with seven black electrodes that can be individually charged (these electrodes are transparent to light in real devices). Light passing through polarizer 1 is polarized in the vertical direction and, when no current is applied to the electrodes, the liquid crystalline phase induces a 90 degree "twist" of the light that enables it to pass through polarizer 2, which is polarized horizontally and is oriented perpendicular to polarizer 1. This light can then form one of the seven segments on the display.

At all times, most manuscripts did not have images in them. In the early Middle Ages, manuscripts tend to either be display books with very full illumination, or manuscripts for study with at most a few decorated initials and flourishes. By the Romanesque period many more manuscripts had decorated or historiated initials, and manuscripts essentially for study often contained some images, often not in color. This trend intensified in the Gothic period, when most manuscripts had at least decorative flourishes in places, and a much larger proportion had images of some sort. Display books of the Gothic period in particular had very elaborate decorated borders of foliate patterns, often with small drolleries. A Gothic page might contain several areas and types of decoration: a miniature in a frame, a historiated initial beginning a passage of text, and a border with drolleries. Often different artists worked on the different parts of the decoration.

Marginalia ranged from intricate decorative illustrations to those considered extremely unusual. Some examples of marginalia found within medieval manuscripts included drawings of centaurs, snail and knight combat, warrior women, battles between cats and mice, parables from biblical texts, personified foxes, rabbits, and monkeys, and hidden words and messages buried within the border decorations.[33][34][35] The added drawings and messages of the 13th to 14th centuries were typically devoted to recurring themes and often patterned after other types of popular medieval art such as stained-glass windows, stone carvings, and wall paintings.[35]

The introduction of printing rapidly led to the decline of illumination. Illuminated manuscripts continued to be produced in the early 16th century but in much smaller numbers, mostly for the very wealthy. They are among the most common items to survive from the Middle Ages; many thousands survive. They are also the best surviving specimens of medieval painting, and the best preserved. Indeed, for many areas and time periods, they are the only surviving examples of painting.

The earliest surviving illuminated manuscripts are a small number from late antiquity, and date from between 400 and 600 CE. Examples include the Vergilius Romanus, Vergilius Vaticanus, and the Rossano Gospels.[3] The majority of extant manuscripts are from the Middle Ages, although many survive from the Renaissance. While Islamic manuscripts can also be called illuminated and use essentially the same techniques, comparable Far Eastern and Mesoamerican works are described as painted.[4]

Over a century later, French physicist Etienne Malus examined images made with light reflected through calcite crystals and noticed that, under certain circumstances, one of the images will disappear. He incorrectly speculated that ordinary daylight is composed of two different light forms that were passed through the calcite crystal in separate paths. It was later determined that the difference occurs due to the polarity of the light passing through the crystal. Daylight is composed of light vibrating in all planes, whereas reflected light is often restricted to a single plane that is parallel to the surface from which the light is reflected.

Black was used for inking text as well as for outlining facial features and gilded aspects like halos in order to create further depth and visual emphasis.[32] Black would also be used for "sketching" the illumination before eventually filling it in with color.[32]

Other applications for polarized light include the Polaroid sunglasses discussed above, as well as the use of special polarizing filters for camera lenses. A variety of scientific instruments utilize polarized light, either emitted by lasers, or through polarization of incandescent and fluorescent sources by a host of techniques. Polarizers are sometimes used in room and stage lighting to reduce glare and produce a more even degree of illumination, and are worn as glasses to bestow an apparent sense of depth to three-dimensional movies. Crossed polarizers are even utilized in space suits to dramatically reduce the chances of light from the sun entering the astronaut's eyes during naps.

By the 14th century, the cloisters of monks writing in the scriptorium had almost fully given way to commercial urban scriptoria, especially in Paris, Rome and the Netherlands.[7] While the process of creating an illuminated manuscript did not change, the move from monasteries to commercial settings was a radical step. Demand for manuscripts grew to an extent that monastic libraries began to employ secular scribes and illuminators.[1] These individuals often lived close to the monastery and, in instances, dressed as monks whenever they entered the monastery, but were allowed to leave at the end of the day. Illuminators were often well known and acclaimed and many of their identities have survived.

An explosion of artistic production in Arabic manuscripts occurred in the 12th and especially the 13th century.[21] Thus various Syriac manuscripts of the twelfth and thirteenth centuries, such as Syriac Gospels, Vatican Library, Syr. 559 or Syriac Gospels, British Library, Add. 7170, were derived from the Byzantine tradition, yet stylistically have a lot in common with Islamic illustrated manuscripts such as the Maqāmāt al-Ḥarīrī, pointing to a common pictorial tradition that existed since circa 1180 in Syria and Iraq which was highly influenced by Byzantine art.[23][24][25] Some of the illustrations of these manuscript have been characterized as "illustration byzantine traitée à la manière arabe" ("Byzantine illustration treated in the Arab style").[24][26]

The ordinary and extraordinary light waves generated when a beam of light traverses a birefringent crystal have plane-polarized electric vectors that are mutually perpendicular to each other. In addition, due to differences in electronic interaction that each component experiences during its journey through the crystal, a phase shift usually occurs between the two waves. Although the ordinary and extraordinary waves follow separate trajectories and are widely separated in the calcite crystal described previously, this is not usually the case for crystalline materials having an optical axis that is perpendicular to the plane of incident illumination.

Reports have surfaced that certain species of insects and animals are able to detect polarized light, including ants, fruit flies, and certain fish, although the list may actually be much longer. For example, several insect species (primarily honeybees) are thought to employ polarized light in navigating to their destinations. It is also widely believed that some individuals are sensitive to polarized light, and are able to observe a yellow horizontal line superimposed on the blue sky when staring in a direction perpendicular to the sun's direction (a phenomenon termed Haidinger's brush). Yellow pigment proteins, termed macula lutea, which are dichroic crystals residing in the fovea of the human eye, are credited with enabling a person to view polarized light.

Styles and techniques of manuscript illumination varied by region, and there were distinct differences in aspects like color palette, decoration style, and peak periods of output. Certain places like the Celtic regions specialized in more ornamental details in contrast to the Byzantine pictorial designs, and regions such as Flanders were more prolific in manuscript production much later than other places.[6]

Other illuminated liturgical books appeared during and after the Romanesque period. These included psalters, which usually contained all 150 canonical psalms,[10] and small, personal devotional books made for lay people known as books of hours that would separate one's day into eight hours of devotion.[11] These were often richly illuminated with miniatures, decorated initials and floral borders. They were costly and therefore only owned by wealthy patrons, often women.

Elliptical polarization, unlike plane-polarized and non-polarized light, has a rotational "sense" that refers to the direction of electric vector rotation around the propagation (incident) axis of the light beam. When viewed end-on, the direction of polarization can be either left-handed or right-handed, a property that is termed the handedness of the elliptical polarization. Clockwise rotational sweeps of the vector are referred to as right-handed polarization, and counterclockwise rotational sweeps represent left-handed polarization.

The color red was often associated with imagery like blood, fire, and godly power.[37] It was the most common and inexpensive color and as such was frequently used for initials, lettering, and borders and well as general imagery.[38][32]

The Byzantine world produced manuscripts in its own style, versions of which spread to other Orthodox and Eastern Christian areas. This distinct Byzantine style of illumination had a characteristic color palette along with different ways of preparing pigments and ink and a unique finish to the vellum writing surface which was not as conducive to long term preservation as the more texture Western style.[19] With their traditions of literacy uninterrupted by the Middle Ages, the Muslim world, especially on the Iberian Peninsula, was instrumental in delivering ancient classic works to the growing intellectual circles and universities of Western Europe throughout the 12th century. Books were produced there in large numbers and on paper for the first time in Europe, and with them full treatises on the sciences, especially astrology and medicine where illumination was required to have profuse and accurate representations with the text.[citation needed]

The Gothic period, which generally saw an increase in the production of illuminated books, also saw more secular works such as chronicles and works of literature illuminated. Wealthy people began to build up personal libraries; Philip the Bold probably had the largest personal library of his time in the mid-15th century, is estimated to have had about 600 illuminated manuscripts, whilst a number of his friends and relations had several dozen. Wealthy patrons, however, could have personal prayer books made especially for them, usually in the form of richly illuminated "books of hours", which set down prayers appropriate for various times in the liturgical day. One of the best known examples is the extravagant Très Riches Heures du Duc de Berry for a French prince.

Polarization of light is very useful in many aspects of optical microscopy. The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. Anisotropic materials have optical properties that vary with the propagation direction of light passing through them. In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port.

As discussed above, bright reflections originating from horizontal surfaces, such as the highway or the water in a pool, are partially polarized with the electric field vectors vibrating in a direction that is parallel to the ground. This light can be blocked by polarizing filters oriented in a vertical direction, as illustrated in Figure 4, with a pair of polarized sunglasses. The lenses of the sunglasses have polarizing filters that are oriented vertically with respect to the frames. In the figure, the blue light waves have their electric field vectors oriented in the same direction as the polarizing lenses and, thus, are passed through. In contrast, the red light wave vibration orientation is perpendicular to the filter orientation and is blocked by the lenses. Polarizing sunglasses are very useful when driving in the sun or at the beach where sunlight is reflected from the surface of the road or water, leading to glare that can be almost blinding. Polarizing filters are also quite useful in photography, where they can be attached to the front of a camera lens to reduce glare and increase overall image contrast in photographs or digital images. Polarizers utilized on cameras are generally designed with a mounting ring that allows them to be rotated in use to achieve the desired effect under various lighting conditions.

where n is the refractive index of the medium from which the light is reflected, θ(i) is the angle of incidence, and θ(r) is the angle of refraction. By examining the equation, it becomes obvious that the refractive index of an unknown specimen can be determined by the Brewster angle. This feature is particularly useful in the case of opaque materials that have high absorption coefficients for transmitted light, rendering the usual Snell's Law formula inapplicable. Determining the amount of polarization through reflection techniques also eases the search for the polarizing axis on a sheet of polarizing film that is not marked.

Asymmetric crystals can be utilized to produce polarized light when an electric field is applied to the surface. A common scientific device that employs this concept is termed a Pockels cell, which can be utilized in conjunction with polarized light to change the polarization direction by 90 degrees. Pockels cells can be switched on and off very rapidly by electrical currents and are often used as fast shutters that allow light to pass for very brief periods of time (ranging in nanoseconds). Presented in Figure 10 is a diagrammatic representation of polarized light passing through a Pockels cell (yellow wave). The green and red sinusoidal light waves emanating from the central region of the cell represent light that is polarized either vertically or horizontally. When the cell is turned off, the polarized light is unaffected as it passes through (green wave), but when activated or turned on, the electric vector of the light beam is shifted by 90-degrees (red wave). In situations where extremely large electric fields are available, molecules of certain liquids and gases can behave as anisotropic crystals and be aligned in the same manner. A Kerr cell, designed to house liquids and gases instead of crystals, also operates to change the angle of polarized light.

The type of script depended on local customs and tastes. In England, for example, Textura was widely used from the 12th to 16th centuries, while a cursive hand known as Anglicana emerged around 1260 for business documents.[27] In the Frankish Empire, Carolingian minuscule emerged under the vast educational program of Charlemagne.[28]

Polarized light can be produced from the common physical processes that deviate light beams, including absorption, refraction, reflection, diffraction (or scattering), and the process known as birefringence (the property of double refraction). Light that is reflected from the flat surface of a dielectric (or insulating) material is often partially polarized, with the electric vectors of the reflected light vibrating in a plane that is parallel to the surface of the material. Common examples of surfaces that reflect polarized light are undisturbed water, glass, sheet plastics, and highways. In these instances, light waves that have the electric field vectors parallel to the surface are reflected to a greater degree than those with different orientations. The optical properties of the insulating surface determine the exact amount of reflected light that is polarized. Mirrors are not good polarizers, although a wide spectrum of transparent materials act as very good polarizers, but only if the incident light angle is oriented within certain limits. An important property of reflected polarized light is that the degree of polarization is dependent upon the incident angle of the light, with the increasing amounts of polarization being observed for decreasing incident angles.

One of the light rays emerging from a birefringent crystal is termed the ordinary ray, while the other is called the extraordinary ray. The ordinary ray is refracted to a greater degree by electrostatic forces in the crystal and impacts the cemented surface at the critical angle of total internal reflection. As a result, this ray is reflected out of the prism and eliminated by absorption in the optical mount. The extraordinary ray traverses the prism and emerges as a beam of linearly-polarized light that is passed directly through the condenser and to the specimen (positioned on the microscope stage).

This concept is illustrated in Figure 8, where the resultant electric vector does not vibrate in a single plane, but progressively rotates around the axis of light wave propagation, sweeping out an elliptical trajectory that appears as a spiral when the wave is viewed at an angle. The size of the phase difference between the ordinary and extraordinary waves (of equal amplitude) determines whether the vector sweeps an elliptical or circular pathway when the wave is viewed end-on from the direction of propagation. If the phase shift is either one-quarter or three-quarters of a wavelength, then a circular spiral is scribed by the resultant vector. However, phase shifts of one-half or a full wavelength produce linearly polarized light, and all other phase shifts produce sweeps having various degrees of ellipticity.

Spiritualilluminationmeaning

The sturdy Roman letters of the early Middle Ages gradually gave way to scripts such as Uncial and half-Uncial, especially in the British Isles, where distinctive scripts such as insular majuscule and insular minuscule developed. Stocky, richly textured blackletter was first seen around the 13th century and was particularly popular in the later Middle Ages. Prior to the days of such careful planning, "A typical black-letter page of these Gothic years would show a page in which the lettering was cramped and crowded into a format dominated by huge ornamented capitals that descended from uncial forms or by illustrations".[30] To prevent such poorly made manuscripts and illuminations from occurring, a script was typically supplied first, "and blank spaces were left for the decoration. This presupposes very careful planning by the scribe even before he put pen to parchment."

Illuminationmeaning wikipedia

Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.

The first clues to the existence of polarized light surfaced around 1669 when Erasmus Bartholin discovered that crystals of the mineral Iceland spar (a transparent, colorless variety of calcite) produce a double image when objects are viewed through the crystals in transmitted light. During his experiments, Bartholin also observed a quite unusual phenomenon. When the calcite crystals are rotated about a particular axis, one of the images moves in a circle around the other, providing strong evidence that the crystals are somehow splitting the light into two different beams.

Gas and water molecules in the atmosphere scatter light from the sun in all directions, an effect that is responsible for blue skies, white clouds, red sunsets, and a phenomenon termed atmospheric polarization. The amount of light scattered (termed Rayleigh scattering) depends upon the size of the molecules (hydrogen, oxygen, water) and the wavelength of light, as demonstrated by Lord Rayleigh in 1871. Longer wavelengths, such as red, orange, and yellow, are not scattered as effectively as are the shorter wavelengths, such as violet and blue.

At least in earlier periods, monasteries were the biggest manufacturers of illuminated manuscripts. They produced manuscripts for their own use; heavily illuminated ones tended to be reserved for liturgical use in the early period, while the monastery library held plainer texts. In the early period manuscripts were often commissioned by rulers for their own personal use or as diplomatic gifts, and many old manuscripts continued to be given in this way, even into the Early Modern period.[1] Especially after the book of hours became popular, wealthy individuals commissioned works as a sign of status within the community, sometimes including donor portraits or heraldry: "In a scene from the New Testament, Christ would be shown larger than an apostle, who would be bigger than a mere bystander in the picture, while the humble donor of the painting or the artist himself might appear as a tiny figure in the corner."[7] The calendar was also personalized, recording the feast days of local or family saints.

As the production of manuscripts shifted from monasteries to the public sector during the High Middle Ages, illuminated books began to reflect secular interests.[1] These included short stories, legends of the saints, tales of chivalry, mythological stories, and even accounts of criminal, social or miraculous occurrences. Some of these were also freely used by storytellers and itinerant actors to support their plays. One of the most popular secular texts of the time were bestiaries. These books contained illuminated depictions of various animals, both real and fictional, and often focused on their religious symbolism and significance, as it was a widespread belief in post-classical Europe that animals, and all other organisms on Earth, were manifestations of God. These manuscripts served as both devotional guidance and entertainment for the working class of the Middle Ages.[12][13]

Art historians classify illuminated manuscripts into their historic periods and types, including (but not limited to) Late Antique, Insular, Carolingian, Ottonian, Romanesque, Gothic, and Renaissance manuscripts. There are a few examples from later periods. Books that are heavily and richly illuminated are sometimes known as "display books" in church contexts, or "luxury manuscripts", especially if secular works. In the first millennium, these were most likely to be Gospel Books, such as the Lindisfarne Gospels and the Book of Kells. The Book of Kells is the most widely recognized illuminated manuscript in the Anglosphere, and is famous for its insular designs.[9] The Romanesque and Gothic periods saw the creation of many large illuminated complete bibles. The largest surviving example of these is The Codex Gigas in Sweden; it is so massive that it takes three librarians to lift it.

Pink was considered a fashionable color and was often found in clothing depictions of aristocrats and in filigree detail work.[39] It also was used to color illuminated manuscript depictions of walls, lakes, and skies.[39]

By the end of the Middle Ages even many religious manuscripts were produced in secular commercial workshops, such as that of William de Brailes in 13th-century Oxford, for distribution through a network of agents, and blank spaces might be reserved for the appropriate heraldry to be added locally by the buyer. The growing genre of luxury illuminated manuscripts of secular works was very largely produced in commercial workshops, mostly in cities such as Paris, Ghent, Bruges and north Italy.

Atmospheric polarization is a direct result of the Rayleigh scattering of sunlight by gas molecules in the atmosphere. Upon impact between a photon from the sun and a gas molecule, the electric field from the photon induces a vibration and subsequent re-radiation of polarized light from the molecule (illustrated in Figure 7). The radiated light is scattered at right angles to the direction of sunlight propagation, and is polarized either vertically or horizontally, depending upon the direction of scatter. A majority of the polarized light impacting the Earth is polarized horizontally (over 50 percent), a fact that can be confirmed by viewing the sky through a Polaroid filter.

The first step was to send the manuscript to a rubricator, "who added (in red or other colors) the titles, headlines, the initials of chapters and sections, the notes and so on; and then – if the book was to be illustrated – it was sent to the illuminator".[15] These letters and notes would be applied using an ink-pot and either a sharpened quill feather or a reed pen. In the case of manuscripts that were sold commercially, the writing would "undoubtedly have been discussed initially between the patron and the scribe (or the scribe's agent, but by the time the written gathering were sent off to the illuminator, there was no longer any scope for innovation.)"[29]

Most manuscripts, illuminated or not, were written on parchment until the 2nd century BCE,[5] when a more refined material called vellum, made from stretched calf skin, was supposedly introduced by King Eumenes II of Pergamum. This gradually became the standard for luxury illuminated manuscripts,[6] although modern scholars are often reluctant to distinguish between parchment and vellum, and the skins of various animals might be used. The pages were then normally bound into codices (singular: codex), that is the usual modern book format, although sometimes the older scroll format was used, for various reasons. A very few illuminated fragments also survive on papyrus. Books ranged in size from ones smaller than a modern paperback, such as the pocket gospel, to very large ones such as choirbooks for choirs to sing from, and Atlantic bibles, requiring more than one person to lift them.[7]

A special class of materials, known as compensation or retardation plates, are quite useful in producing elliptically and circularly polarized light for a number of applications, including polarized optical microscopy. These birefringent substances are chosen because, when their optical axis is positioned perpendicular to the incident light beam, the ordinary and extraordinary light rays follow identical trajectories and exhibit a phase difference that is dependent upon the degree of birefringence. Because the pair of orthogonal waves is superimposed, it can be considered a single wave having mutually perpendicular electrical vector components separated by a small difference in phase. When the vectors are combined by simple addition in three-dimensional space, the resulting wave becomes elliptically polarized.

What isthe meaning ofillumination

In cases where the major and minor vectorial axes of the polarization ellipse are equal, then the light wave falls into the category of circularly polarized light, and can be either right-handed or left-handed in sense. Another case often occurs in which the minor axis of the electric vector component in elliptically polarized light goes to zero, and the light becomes linearly polarized. Although each of these polarization motifs can be achieved in the laboratory with the appropriate optical instrumentation, they also occur (to varying, but minor, degrees) in natural non-polarized light.

A number of applications, most notably polarized optical microscopy, rely on crossed polarizers to examine birefringent or doubly refracting specimens. When two polarizers are crossed, their transmission axes are oriented perpendicular to each other and light passing through the first polarizer is completely extinguished, or absorbed, by the second polarizer, which is typically termed an analyzer. The light-absorbing quality of a dichroic polarizing filter determines exactly how much random light is extinguished when the polarizer is utilized in a crossed pair, and is referred to as the extinction factor of the polarizer. Quantitatively, the extinction factor is determined by the ratio of light that is passed by a pair of polarizers when their transmission axes are oriented parallel versus the amount passed when they are positioned perpendicular to each other. In general, extinction factors between 10,000 and 100,000 are required to produce jet-black backgrounds and maximum observable specimen birefringence (and contrast) in polarized optical microscopy.

An illuminated manuscript is a formally prepared document where the text is decorated with flourishes such as borders and miniature illustrations. Often used in the Roman Catholic Church for prayers and liturgical books such as psalters and courtly literature, the practice continued into secular texts from the 13th century onward and typically include proclamations, enrolled bills, laws, charters, inventories, and deeds.[1][2]

For water (refractive index of 1.333), glass (refractive index of 1.515), and diamond (refractive index of 2.417), the critical (Brewster) angles are 53, 57, and 67.5 degrees, respectively. Light reflected from a highway surface at the Brewster angle often produces annoying and distracting glare, which can be demonstrated quite easily by viewing the distant part of a highway or the surface of a swimming pool on a hot, sunny day. Modern lasers commonly take advantage of Brewster's angle to produce linearly polarized light from reflections at the mirrored surfaces positioned near the ends of the laser cavity.

The amount of light passing through a crossed pair of high-quality polarizers is determined by the orientation of the analyzer with respect to the polarizer. When the polarizers are oriented perpendicular to each other, they display a maximum level of extinction. However, at other angles, varying degrees of extinction are obtained, as illustrated by the vector diagrams presented in Figure 6. The analyzer is utilized to control the amount of light passing through the crossed pair, and can be rotated in the light path to enable various amplitudes of polarized light to pass through. In Figure 6(a), the polarizer and analyzer have parallel transmission axes and the electric vectors of light passing through the polarizer and analyzer are of equal magnitude and parallel to each other.

The human eye lacks the ability to distinguish between randomly oriented and polarized light, and plane-polarized light can only be detected through an intensity or color effect, for example, by reduced glare when wearing polarized sun glasses. In effect, humans cannot differentiate between the high contrast real images observed in a polarized light microscope and identical images of the same specimens captured digitally (or on film), and then projected onto a screen with light that is not polarized. The basic concept of polarized light is illustrated in Figure 1 for a non-polarized beam of light incident on two linear polarizers. Electric field vectors are depicted in the incident light beam as sinusoidal waves vibrating in all directions (360 degrees; although only six waves, spaced at 60-degree intervals, are included in the figure). In reality, the incident light electric field vectors are vibrating perpendicular to the direction of propagation with an equal distribution in all planes before encountering the first polarizer.

Presented in Figure 5 is an illustration of the construction of a typical Nicol prism. A crystal of doubly refracting (birefringent) material, usually calcite, is cut along the plane labeled a-b-c-dand the two halves are then cemented together to reproduce the original crystal shape. A beam of non-polarized white light enters the crystal from the left and is split into two components that are polarized in mutually perpendicular directions. One of these beams (labeled the ordinary ray) is refracted to a greater degree and impacts the cemented boundary at an angle that results in its total reflection out of the prism through the uppermost crystal face. The other beam (extraordinary ray) is refracted to a lesser degree and passes through the prism to exit as a plane-polarized beam of light.

When current is applied to the electrodes, the liquid crystalline phase aligns with the current and loses the cholesteric spiral pattern. Light passing through a charged electrode is not twisted and is blocked by polarizer 2. By coordinating the voltage on the seven positive and negative electrodes, the display is capable of rendering the numbers 0 through 9. In this example the upper right and lower left electrodes are charged and block light passing through them, allowing formation of the number "2" by the display device (seen reversed in the figure).

Sunlight and almost every other form of natural and artificial illumination produces light waves whose electric field vectors vibrate in all planes that are perpendicular with respect to the direction of propagation. If the electric field vectors are restricted to a single plane by filtration of the beam with specialized materials, then the light is referred to as plane or linearly polarized with respect to the direction of propagation, and all waves vibrating in a single plane are termed plane parallel or plane-polarized.

Rotating the analyzer transmission axis by 30-degrees with respect to that of the polarizer reduces the amplitude of a light wave passing through the pair, as illustrated in Figure 6(b). In this case, the polarized light transmitted through the polarizer can be resolved into horizontal and vertical components by vector mathematics to determine the amplitude of polarized light that is able to pass through the analyzer. The amplitude of the ray transmitted through the analyzer is equal to the vertical vector component (illustrated as the yellow arrow in Figure 6(b)).

Continued rotation of the analyzer transmission axis, to a 60-degree angle with respect to the transmission axis of the polarizer, further reduces the magnitude of the vector component that is transmitted through the analyzer (Figure 6(c)). When the analyzer and polarizer are completely crossed (90-degree angle), the vertical component becomes negligible (Figure 6(d)) and the polarizers have achieved their maximum extinction value.

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A majority of the polarizing materials used today are derived from synthetic films invented by Dr. Edwin H. Land in 1932, which soon overtook all other materials as the medium of choice for production of plane-polarized light. To produce the films, tiny crystallites of iodoquinine sulfate, oriented in the same direction, are embedded in a transparent polymeric film to prevent migration and reorientation of the crystals. Land developed sheets containing polarizing films that are marketed under the trade name of Polaroid (a registered trademark), which has become the accepted generic term for these sheets. Any device capable of selecting plane-polarized light from natural (non-polarized) white light is now referred to as a polar or polarizer, a name first introduced in 1948 by A. F. Hallimond. Because these filters are capable of differentially transmitting light rays, depending upon their orientation with respect to the polarizer axis, they exhibit a form of dichroism, and are often termed dichroic filters.

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Blue, especially the pigment ultramarine, was a valuable and rare color and was commonly used in depictions of the Virgin Mary and for the clothing of important religious figures.[40] Less expensive or poorer quality blue pigments were sometimes used for initials, lettering, and borders.[38][32]

where I is the intensity of light passing through the analyzer (and the total amount of light passed through the pair of crossed polarizers), I(o) is the intensity of light that is incident upon the polarizer, and θ is the angle between the transmission axes of the polarizer and analyzer. By examining the equation, it can be determined that when the two polarizers are crossed (θ = 90 degrees), the intensity is zero. In this case, light passed by the polarizer is completely extinguished by the analyzer. When the polarizers are partially crossed at 30 and 60 degrees, the light transmitted by the analyzer is reduced by 25 percent and 75 percent, respectively.

The amount of light passing through a pair of polarizers can be quantitatively described by applying Malus' cosine-squared law, as a function of the angles between the polarizer transmission axes, utilizing the equation:

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One of the first polarizing filters was constructed in the early nineteenth century by French scientist François Arago, who was an active investigator into the nature of polarized light. Arago investigated the polarity of light originating from various sources in the sky and proposed a theory that predicted the velocity of light should decrease as it passes into a denser medium. He also worked with Augustin Fresnel to investigate interference in polarized light and discovered that two beams of light polarized with their vibration directions oriented perpendicular to each other will not undergo interference. Arago's polarizing filters, designed and built in 1812, were made from a stack of glass sheets pressed together.

While the use of gold is by far one of the most captivating features of illuminated manuscripts, the bold use of varying colors provided multiple layers of dimension to the illumination. From a religious perspective, "the diverse colors wherewith the book is illustrated, not unworthily represent the multiple grace of heavenly wisdom."[15] There is evidence of illustratiors planning out color choice in advance, which indicates purposeful choice and design in the finished product.[1] There is also a great deal of nuance when it comes to the colors and painting of manuscripts. Illuminators would be trained in color combinations and stylistic distinctions by a form of apprenticeship, so the limited number of primary literary sources discussing colors and techniques may not be accurate to what the actual illuminators learned and followed.[32]

The origins of the pictorial tradition of Arabic illustrated manuscripts are uncertain. The first known decorated manuscripts are some Qur'ans from the 9th century.[20] They were not illustrated, but were "illuminated" with decorations of the frontispieces or headings.[20] The tradition of illustrated manuscripts started with the Graeco-Arabic translation movement and the creation of scientific and technical treatises often based on Greek scientific knowledge, such as the Arabic versions of The Book of Fixed Stars (965 CE), De materia medica or Book of the Ten Treatises of the Eye.[21] The translators were most often Arab Syriac Christians, such as Hunayn ibn Ishaq or Yahya ibn Adi, and their work is known to have been sponsored by local rulers, such as the Artuqids.[22]

When the ordinary and extraordinary waves emerge from a birefringent crystal, they are vibrating in mutually perpendicular planes having a total intensity that is the sum of their individual intensities. Because the polarized waves have electric vectors that vibrate in perpendicular planes, the waves are not capable of undergoing interference. This fact has consequences in the ability of birefringent substances to produce an image. Interference can only occur when the electric vectors of two waves vibrate in the same plane during intersection to produce a change in amplitude of the resultant wave (a requirement for image formation). Therefore, transparent specimens that are birefringent will remain invisible unless they are examined between crossed polarizers, which pass only the components of the elliptically and circularly polarized waves that are parallel to the axis of the polarizer closest to the observer. These components are able to produce amplitude fluctuations to generate contrast and emerge from the polarizer as linearly polarized light.

Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are polarized in mutually perpendicular planes. The velocities of these components are different and vary with the propagation direction through the specimen. After exiting the specimen, the light components are out of phase and sweep an elliptical geometry that is perpendicular to the direction of propagation, but are recombined through constructive and destructive interference when they pass through the analyzer. Polarized light microscopy is a contrast-enhancing technique that improves the quality of the image obtained with birefringent materials when compared to other techniques such as darkfield and brightfield illumination, differential interference contrast, phase contrast, Hoffman modulation contrast, and fluorescence. In addition, use of polarized light allows the measurement of optical properties of minerals and similar materials and can aid in the classification and identification of unknown substances.

Green was a relatively rare pigment on the illuminator's palette.[19] It was used for landscapes and was often associated with visuals related to the Garden of Eden and rebirth.[37] Verdigris Green was a specific shade almost exclusively used in cross imagery, and Green Earth was used under other pigments in order to create depth to skin tones.[40]

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The application of gold leaf or dust to an illumination is a very detailed process that only the most skilled illuminators can undertake and successfully achieve. The first detail an illuminator considered when dealing with gold was whether to use gold leaf or specks of gold that could be applied with a brush. When working with gold leaf, the pieces would be hammered and thinned.[41] The use of this type of leaf allowed for numerous areas of the text to be outlined in gold. There were several ways of applying gold to an illumination. One of the most popular included mixing the gold with stag's glue and then "pour it into water and dissolve it with your finger."[42] Once the gold was soft and malleable in the water, it was ready to be applied to the page. Illuminators had to be very careful when applying gold leaf to the manuscript because gold leaf is able to "adhere to any pigment which had already been laid, ruining the design, and secondly the action of burnishing it is vigorous and runs the risk of smudging any painting already around it."

The Persian miniature tradition mostly began in whole books, rather than single pages for muraqqas or albums, as later became more common. The Great Mongol Shahnameh, probably from the 1330s, is a very early manuscript of one of the most common works for grand illustrated books in Persian courts.

Polarized light microscopy was first introduced during the nineteenth century, but instead of employing transmission-polarizing materials, light was polarized by reflection from a stack of glass plates set at a 57-degree angle to the plane of incidence. Later, more advanced instruments relied on a crystal of doubly refracting material (such as calcite) specially cut and cemented together to form a prism. A beam of white non-polarized light entering a crystal of this type is separated into two components that are polarized in mutually perpendicular (orthogonal) directions.

Up to the 12th century, most manuscripts were produced in monasteries in order to add to the library or after receiving a commission from a wealthy patron. Larger monasteries often contained separate areas for the monks who specialized in the production of manuscripts called a scriptorium.[14] Within the walls of a scriptorium were individualized areas where a monk could sit and work on a manuscript without being disturbed by his fellow brethren. If no scriptorium was available, then "separate little rooms were assigned to book copying; they were situated in such a way that each scribe had to himself a window open to the cloister walk."[15]

White was used often in association with religious objects or figures, and was also used as an underpigment as to provide a base for other colors and provide depth, notably in instances of combination with blues to create skies and with reds to create different skin tones.[40] White was also used, especially in the Gothic period, to outline figures and to create layered highlights.[1][32]

Yellow was often blended with other pigments in order to create natural earth tones, of which were common in medieval manuscript illumination.[19] Yellow paint would also be layered underneath gold paint in order to create a multilayered gold effect.[19]

In modern polarizers, incident light waves having electric vector vibrations that are parallel to the crystal axis of the polarizer are absorbed. Many of the incident waves will have a vector orientation that is oblique, but not perpendicular to the crystal axis, and will only be partially absorbed. The degree of absorption for oblique light waves is dependent upon the vibration angle at which they impact the polarizer. Those rays that have angles close to parallel with respect to the crystal axis will be adsorbed to a much greater degree than those having angles close to the perpendicular. The most common Polaroid filters (termed the H-series) transmit only about 25 percent of the incident light beam, but the degree of polarization of the transmitted rays exceeds 99 percent.

Douglas B. Murphy - Department of Cell Biology and Microscope Facility, Johns Hopkins University School of Medicine, 725 N. Wolfe Street, 107 WBSB, Baltimore, Maryland 21205.

The polarizers illustrated in Figure 1 are actually filters containing long-chain polymer molecules that are oriented in a single direction. Only the incident light that is vibrating in the same plane as the oriented polymer molecules is absorbed, while light vibrating at right angles to the polymer plane is passed through the first polarizing filter. The polarizing direction of the first polarizer is oriented vertically to the incident beam so it will pass only the waves having vertical electric field vectors. The wave passing through the first polarizer is subsequently blocked by the second polarizer, because this polarizer is oriented horizontally with respect to the electric field vector in the light wave. The concept of using two polarizers oriented at right angles with respect to each other is commonly termed crossed polarization and is fundamental to the concept of polarized light microscopy.

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The illumination and decoration was normally planned at the inception of the work, and space reserved for it.[32] However, the text was usually written before illumination began. In the early medieval period the text and illumination were often done by the same people, normally monks, but by the High Middle Ages the roles were typically separated, except for routine initials and flourishes, and by at least the 14th century there were secular workshops producing manuscripts, and by the beginning of the 15th century these were producing most of the best work, and were commissioned even by monasteries. When the text was complete, the illustrator set to work. Complex designs were planned out beforehand, probably on wax tablets, the sketch pad of the era. The design was then traced or drawn onto the vellum (possibly with the aid of pinpricks or other markings, as in the case of the Lindisfarne Gospels). Many incomplete manuscripts survive from most periods, giving us a good idea of working methods.

Other prism configurations were suggested and constructed during the nineteenth and early twentieth centuries, but are currently no longer utilized for producing polarized light in modern applications. Nicol prisms are very expensive and bulky, and have a very limited aperture, which restricts their use at high magnifications. Instead, polarized light is now most commonly produced by absorption of light having a set of specific vibration directions in a filter medium (such as polarizing sheets) where the transmission axis of the filter is perpendicular to the orientation of the linear polymers and crystals that comprise the polarizing material.

When considering the incidence of non-polarized light on a flat insulating surface, there is a unique angle at which the reflected light waves are all polarized into a single plane. This angle is commonly referred to as Brewster's angle, and can be easily calculated utilizing the following equation for a beam of light traveling through air:

In linearly polarized light, the electric vector is vibrating in a plane that is perpendicular to the direction of propagation, as discussed above. Natural light sources, such as sunlight, and artificial sources, including incandescent and fluorescent light, all emit light with orientations of the electric vector that are random in space and time. Light of this type is termed non-polarized. In addition, there exist several states of elliptically polarized light that lie between linear and non-polarized, in which the electric field vector transcribes the shape of an ellipse in all planes perpendicular to the direction of light wave propagation.

Paper manuscripts appeared during the Late Middle Ages. The untypically early 11th century Missal of Silos is from Spain, near to Muslim paper manufacturing centres in Al-Andalus. Textual manuscripts on paper become increasingly common, but the more expensive parchment was mostly used for illuminated manuscripts until the end of the period. Very early printed books left spaces for red text, known as rubrics, miniature illustrations and illuminated initials, all of which would have been added later by hand. Drawings in the margins (known as marginalia) would also allow scribes to add their own notes, diagrams, translations, and even comic flourishes.[8]