The Edgeless & Ever-Shifting Gradient: An Encyclopaedic and Evolving Spectrum of Gradient Knowledge
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The Gradient

The Edgeless & Ever-Shifting Gradient: An Encyclopaedic and Evolving Spectrum of Gradient Knowledge

A gradient, without restriction, is edgeless and ever-shifting. A gradient moves, transitions, progresses, defies being defined as one thing. It formalizes difference across a distance. It’s a spectrum. It’s a spectral smearing. It’s an optical phenomenon occurring in nature. It can be the gradual process of acquiring knowledge. It can be a concept. It can be a graphic expression. It can be all of the above, but likely it’s somewhere in between.

A gradient, in all of it’s varied forms, becomes a catalyst in it’s ability to seamlessly blend one distinct thing/idea/color, to the next distinct thing/idea/color, to the next, etc.

In this sense, it is the gradient and the way it performs that has become a model and an underlying ethos, naturally, for this online publishing initiative that we call The Gradient.

Similarly, it’s our hope that this post—an attempt to survey gradients of all forms and to expand our own understanding of gradients—will also be edgeless and ever-shifting. This post will evolve and be progressively added to in an effort to create, as the subtitle says, an encyclopaedic and evolving spectrum of gradient knowledge.

 


 

Index

 

A

Al Ruppersberg’s Colby Printing Co. Posters

B

Blend roll

Blue hour

Bryce Wilner’s Gradient Puzzle

C

Circumhorizontal arc

Color banding

Color gradient

Color-shifting paint

D

Diffuse sky radiation

E

Electromagnetic spectrum boundaries/bands

F

G

Gender fluidity

Germans Ermičs’ gradient colored glass works

Glissando

Goniochromism

Graded tempering

H

I

Ignaz Schiffermüller’s Versuch eines Farbensystems

Infrared thermography

Iridescence

Iridescent clouds

Iridescence in Aphrodita aculeata setae

Iridescence in birds

Iridescence in insects

Iridescence in snakes

J

K

L

Leptothrix discophora

Liz West’s Our Colour

M

Mark Hagen’s A parliament of some things…

Metachrosis

N

Nature’s abhorrence of gradients

O

Ombré

P

Photodegradation

Photoelasticity

Physiological color change

Physiological color change in cephalopods

Physiological color change in chameleons

Q

R

Rainbow

Rainbow roll

Raw Color’s The Fans

Rayleigh scattering

S

Sam Falls’ Untitled (Thermochromic bench)

Sky coloration

Slope

Spectrum colors

Split fountain

T

Tauba Auerbach’s RGB Colorspace Atlas

Thin-film interference

Tomás Saraceno’s Poetic Cosmos of the Breath

U

V

Visible spectrum

W

X

Y

Z




 

Al Ruppersberg’s Colby Printing Co. Posters

Gradient -- Al Ruppersberg -- Colby Printing Company
Allen Ruppersberg   Are You Crying? from Preview (1988)

It all begins with the end of a story, the one about the Colby Poster Printing Company that shut down in December 2012, taking with itself an emblematical graphic identity into history. A Colby poster can be easily distinguished from others and bear the stamp from L.A. Multicolored posters with unexpected gradients of flashy, typically Californian colors—the yellow of the sun or the beach, the green of the lush vegetation in this “West Coast” Eden, the blue of the ocean, the red or the pink of all the other wonders of this heavenly place on earth—the Colby posters, covered with outrageously bold characters, do not respect any typographical rules. These rules are mistreated, possibly by ignorance, probably on purpose; in either case it is a certain rule of the West not to follow the rules of the East. […]

Individuals and professionals of Los Angeles entrusted the production of their communication media to the Colby printing house: announcements for school fairs, concerts, political meetings, posters for films, performances, and much more. The posters were displayed on the wooden utility poles, becoming true marks of identity in the cityscape of Los Angeles. Many artists—Eve Fowler, Anthony Burrill, Ed Ruscha, Kathryn Andrews, and Peter Coffin, among many others—also turned to the particular aesthetic of the Colby Poster Printing Compnay. Los Angeles-based artist Al Ruppersberg was one of their most faithful and regular customers.

(Source: edited/adapted from micheledidier.com)

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Blend roll (also known as Rainbow roll and Split fountain)

Gradient -- blend roll -- split fountain -- rainbow roll
Photo by Instagram user @oldorchardpress

A technique of simultaneously blending two or more ink colors onto a single inking cylinder, lithographic stone, or relief surface in order to achieve a multiple color-blended print. The objective is for the colors have a soft, blended gradient transition from one color to the next. A blend roll is also known as a rainbow roll or a split fountain.

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Blue hour

Gradient -- blue hour -- sky coloration
Photo by Luis Argerich

The blue hour (from French l’heure bleue) is the period of twilight early in the dawn each morning and late in the dusk each evening, when the sun is at a significant depth below the horizon and when the residual, indirect sunlight takes on a predominantly blue shade. This effect is caused by the relative diffusibility of short, blue wavelengths of light versus the longer, red wavelengths. During the blue “hour” (typically a period about 40 minutes in length), red light passes straight into space while blue light is scattered in the atmosphere and therefore reaches the earth’s surface.

(Source: edited/adapted from Wikipedia)

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Bryce Wilner’s Gradient Puzzle for Areaware (2015)

 
Gradient -- gradient puzzle -- Bryce Willner

Gradient -- gradient puzzle -- Bryce Willner
Gradient Puzzle “Black/White”   Areaware / areaware.com

Bryce Wilner’s Gradient Puzzle is a vibrant way to meditate on color. The act of putting it together is slow and deliberate, where the color of each piece is used to locate its proper position. […] The colors in this puzzle have been arranged to suggest that there exists no precise point on the chromatic scale at which one color becomes another. The puzzle can be considered complete even when its pieces are disassembled and mixed together in the box.

(Source: edited/adapted from Areaware)

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Circumhorizontal arc

Gradient -- circumhorizontal arc
Photographer unknown

A circumhorizontal arc (not to be confused with cloud iridescence) is an optical phenomenon that belongs to the family of ice halos formed by the refraction of sun- or moonlight in plate-shaped ice crystals suspended in the atmosphere, typically in cirrus or cirrostratus clouds. In its full form, the arc has the appearance of a large, brightly spectrum-colored band (red being the topmost color) running parallel to the horizon, located far below the Sun or Moon. The distance between the arc and the Sun or Moon is twice as far as the common 22° halo. Often, when the halo-forming cloud is small or patchy, only fragments of the arc are seen. As with all halos, it can be caused by the Sun as well as (but much more rarely) by the Moon.

Other currently accepted names for the circumhorizontal arc are circumhorizon arc or lower symmetric 46° plate arc. The misleading term “fire rainbow” is sometimes used to describe this phenomenon, although it is neither a rainbow, nor related in any way to fire. The term, apparently coined in 2006, may originate in the occasional appearance of the arc as “flames” in the sky, when it occurs in fragmentary cirrus clouds.

The halo is formed by sunlight entering horizontally-oriented, flat, hexagonal ice crystals through a vertical side face and leaving through the near horizontal bottom face (plate thickness does not affect the formation of the halo). In principle, Parry oriented column crystals may also produce the arc, although this is rare. The 90° inclination between the ray entrance and exit faces produce the well-separated spectral colors. The arc has a considerable angular extent and thus, rarely is complete. When only fragments of a cirrus cloud are in the appropriate sky and sun position, they may appear to shine with spectral colors.

(Source: edited/adapted from Wikipedia)

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Color banding

Gradient -- color banding
Intentionally color-banded image (at 8-bit setting)

Color banding is a problem of inaccurate color presentation in computer graphics. In 24-bit color modes, 8 bits per channel is usually considered sufficient to render images in Rec. 709 or sRGB. However, in some cases there is a risk of producing abrupt changes between shades of the same color. For instance, displaying natural gradients (like sunsets, dawns, or clear blue skies) can show color banding.

Color banding is more noticeable with fewer bits per pixel (BPP) at 16–256 colors (4–8 BPP), where not every shade can be shown because there are insufficient bits to represent them.

Possible solutions include the introduction of dithering and increasing the number of bits per color channel.

(Source: edited/adapted from Wikipedia)

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Color gradient

Gradient -- linear gradient -- radial gradient

In computer graphics, a color gradient (sometimes called a color ramp or color progression) specifies a range of position-dependent colors, usually used to fill a region. For example, many window managers allow the screen background to be specified as a gradient. The colors produced by a gradient vary continuously with position, producing smooth color transitions.

An axial color gradient (sometimes also called a linear color gradient) is specified by two points, and a color at each point. The colors along the line through those points are calculated using linear interpolation, then extended perpendicular to that line. In digital imaging systems, colors are typically interpolated in an RGB color space, often using gamma compressed RGB color values, as opposed to linear.

A radial gradient is specified as a circle that has one color at the edge and another at the center. Colors are calculated by linear interpolation based on distance from the center.

(Source: edited/adapted from Wikipedia)

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Color-shifting paint

Gradient -- color shifting paint -- chameleon paint
Photo courtesy Forgiato Wheels

ChromaFlair (officially known as ChromaFlair® Light-Interference Pigments) is a pigment used in the industry of color-shifting paints, primarily for use on automobiles. When the paint is applied, it changes color depending on the light source and viewing angle. It was created at JDS Uniphase and is used by DuPont and PPG Industries.

The paint’s spectral and shifting gradient-effect is achieved by interfering with the reflection and refraction of light from the painted object’s surface. The paint contains tiny synthetic flakes about one micrometre thick. The flakes are constructed of aluminium coated with glass-like magnesium fluoride embedded in semi-translucent chromium. The aluminium and chrome give the paint a vibrant metallic sparkle, while the glass-like coating acts like a refracting prism, changing the apparent color of the surface as the observer moves.

ChromaFlair paints contain no conventional absorbing pigments; rather, the pigment is a light interference pigment. The color observed is created entirely by the refractive properties of the flakes, analogous to the perception of rainbow colors in oil slicks. This phenomenon, known as thin-film interference, is a process whereby white light is separated into its components through selective, constructive, and destructive interference. This phenomenon is also called structural color: the raw materials are essentially colorless by themselves but, when ordered in the proper sequence, they work together to create color.

ChromaFlair paint has also been used as a substitute for optically variable ink in the use of counterfeiting the currency of the United States. Counterfeiter Art Williams stamped green-silver ChromaFlair paint onto counterfeit bills to replicate the color-shifting ink on the 1996-issued $100 bill.

(Source: edited/adapted from Wikipedia)

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Diffuse sky radiation (see also: Rayleigh scattering)

Gradient -- diffuse sky radiation -- Rayleigh scattering
Photographer unknown

Diffuse sky radiation is solar radiation reaching the Earth’s surface after having been scattered from the direct solar beam by molecules or suspensoids in the atmosphere. It is also called skylight, diffuse skylight, or sky radiation and is the reason for changes in the color and gradient tonality of the sky. Of the total light removed from the direct solar beam by scattering in the atmosphere, about two-thirds ultimately reaches the earth as diffuse sky radiation.

The dominant radiative scattering processes in the atmosphere (Rayleigh scattering and Mie scattering) are elastic in nature and allow light to be deviated from its path without being absorbed and with no change in wavelength.

The sunlit sky is blue because air scatters short-wavelength light more than longer wavelengths. Since blue light is at the short-wavelength end of the visible spectrum, it is more strongly scattered in the atmosphere than long-wavelength red light. The result is that when looking toward parts of the sky other than the sun, the human eye perceives them to be blue.

Near sunrise and sunset, most of the sunlight arrives nearly tangentially to the Earth’s surface; thus, the light’s path through the atmosphere is so long that much of the blue and even green light is scattered out along the way, leaving the sun rays and the clouds it illuminates red. Therefore, when looking at the sunset and sunrise, we see the color red more than the other colors.

(Source: edited/adapted from Wikipedia)

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Electromagnetic spectrum boundaries/bands

Gradient -- electromagnetic spectrum

There are no precisely defined boundaries between the bands of the electromagnetic spectrum. Rather, they fade into each other like the gradiated bands in a rainbow (which is the sub-spectrum of visible light). Radiation of each frequency and wavelength (or in each band) has a mix of properties of the two regions of the spectrum that bound it.

(Source: edited/adapted from Wikipedia)

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Gender fluidity

Gradient -- gender fluidity
Genderfluid symbol   Designed by Meredith Espinosa

Gender fluid is a gender identity and refers to a gender which is not exclusively masculine or feminine‍ and which moves between or fluctuates over time. A gender fluid person may identify as having an overlap of, or indefinite lines between their gender identity. At any given time, they may identify as male, female, neutrois (being agender, nongendered, genderless, or genderfree), or any other non-binary identity, or some combination of identities (being bigender, trigender, or pangender). Their gender, existing more flexibly and on more of a spectrum of gender expression, can also vary at random or vary in response to different circumstances. Gender fluid people may also identify as multigender, non-binary and/or transgender.

(Source: edited/adapted from Gender Wiki)

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Germans Ermičs’ gradient-colored glass works

Gradient -- Germans Ermičs -- gradient glass
Blue Dawn Mirror (2016)   Photo by Lonneke van der Palen

Amsterdam-based Germans Ermičs creates multiple series of color-infused glass- and mirror-based works. The vibrant hues selected by Ermičs dissolve, in gradients, and transition to other colors or levels of transparency. His vast palette of blues, greens, peaches, and violets, often deriving from the natural world, bring to mind the mingling of colors seen at dawn, dusk, and twilight.

(Source: edited/adapted from Surface)

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Glissando

Gradient -- glissando -- continuous glissando
Example musical notation for a glissando

In music, a glissando (plural: glissandi, abbreviated gliss.) is a continuous glide (gradient) from one pitch to another. It is an Italianized musical term derived from the French glisser, to glide. Some colloquial equivalents are slide, sweep (referring to the ‘discrete glissando’ effects on guitar and harp respectively), bend, smear, rip (for a loud, violent gliss to the beginning of a note), lip, plop, or falling hail.

From the standpoint of musical acoustics and scientific terminology, some instruments can change the frequency of their notes with continuously variable pitch over a substantial range. These continuous glissando-capable instruments include unfretted stringed instruments (such as the violin, viola, cello and double bass, and fretless guitars), stringed instruments with a way of stretching the strings (such as the guitar, veena, or sitar), a fretted guitar or lap steel guitar when accompanied with the use of a slide, wind instruments without valves or stops (such as the trombone or slide whistle), timpani (kettledrums), electronic instruments (such as the theremin, the ondes Martenot, synthesizers and keytars), the water organ, and the human voice.

Brass and woodwind instruments such as the trumpet or flute can effect a similar limited slide by altering the lip pressure (trumpet) or a combination of embouchure and rolling the head joint (flute), while the clarinet and some models of flute can achieve this by slowly dragging fingers off tone holes or changing the oral cavity’s resonance by manipulating tongue position, embouchure, and throat shaping.

Other instruments, notably acoustic keyboard instruments, are restricted to quantized (stepped) changes in pitch. Some instruments, such as the clarinet and saxophone, can produce a continuous pitch (frequency) change, although their characteristic design is to provide distinct pitches.

(Source: edited/adapted from Wikipedia)

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Goniochromism (see also: Iridescence)

 
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Graded tempering (also known as Differential tempering)

Gradient -- graded tempering -- differential tempering

Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product.

In graded/differential tempering there is no distinct boundary between the harder and softer metals, but the change from hard to soft is very gradual, forming a continuum, or “grade” (gradient), of hardness. However, higher heating temperatures cause the colors to spread less, creating a much steeper grade, while lower temperatures can make the change more gradual, using a smaller portion of the entire continuum. The tempering colors only represent a fraction of the entire grade, because the metal turns grey above 650 °F (343 °C), making it difficult to judge the temperature, but the hardness will continue to decrease as the temperature rises.

(Source: edited/adapted from Wikipedia)

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Ignaz Schiffermüller’s Versuch eines Farbensystems (1772)

Gradient -- Ignaz Schiffermüller -- color wheel
Ignaz Schiffermüller’s color wheel on page 15 of Versuch eines Farbensystems (1772)

Ignaz Schiffermüller (d. 1806, Linz) was an Austrian naturalist and a teacher at the Theresianum College in Vienna. Schiffermüller, among other accomplishments, was noteworthy for his work in developing a scientifically based color nomenclature.

In his Versuch eines Farbensystems (1772), Schiffermüller addressed the need for a standardised nomenclature with which to describe the countless colors of nature. In addition to his presentation of numerous tables and matrices classifying and sub-classifying shades of color, Versuch eines Farbensystems also contains an attractive full-page engraving with a color circle, inspired by the optical theory of Father Louis Bertrand Castel (1688–1757) and is hand-tinted with twelve colors transition from one to the next in a fluid gradient. Evident throughout this pioneering work is a subtle response to the nuances of color and their accurate rendition.

(Source: edited/adapted from Wikipedia)

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Infrared thermography

Gradient -- infrared thermal imaging
An electric pump motor captured with FLIR T640 Thermal Camera

Infrared thermography, thermal imaging, and thermal video are examples of infrared imaging science. Thermographic cameras usually detect radiation in the long-infrared range of the electromagnetic spectrum (roughly 9,000–14,000 nanometers) and produce images of that radiation, called thermograms. Since infrared radiation is emitted by all objects with a temperature above absolute zero according to the black body radiation law, thermography makes it possible to see one’s environment with or without visible illumination. The amount of radiation emitted by an object increases with temperature; therefore, thermography allows one to see variations and gradiations in temperature. When viewed through a thermal imaging camera, warm objects stand out well against cooler backgrounds; humans and other warm-blooded animals become easily visible against the environment, day or night. As a result, thermography is particularly useful to the military and other users of surveillance cameras.

(Source: edited/adapted from Wikipedia)

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Iridescence (also known as Goniochromism)

Gradient -- iridescence -- goniochromism
Photographer unknown

Iridescence (Goniochromism) is the optical phenomenon of certain surfaces that appear to gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, butterfly wings, and sea shells, as well as certain minerals. It is often created by structural coloration (microstructures that interfere with light).

The word iridescence is derived in part from the Greek word îris, meaning rainbow, and is combined with the Latin suffix -escent, meaning “having a tendency toward.” Iris in turn derives from the goddess Iris of Greek mythology, who is the personification of the rainbow and acted as a messenger of the gods. Goniochromism is derived from the Greek words gonia, meaning “angle,” and chroma, meaning “color.”

Iridescence is often caused by multiple reflections from two or more semi-transparent surfaces in which phase shift and interference of the reflections modulates the incidental light (by amplifying or attenuating some frequencies more than others). The thickness of the layers of the material determines the interference pattern. Iridescence can for example be due to thin-film interference, the functional analogue of selective wavelength attenuation as seen with the Fabry–Pérot interferometer, and can be seen in oil films on water and soap bubbles. Iridescence is also found in plants, animals, and many other items. The range of colors of natural iridescent objects can be narrow, for example shifting between two or three colors as the viewing angle changes, or a wide range of colors can be observed.

Iridescence can also be created by diffraction. This is found in items like CDs, DVDs, or cloud iridescence. In the case of diffraction, the entire rainbow of colors will typically be observed as the viewing angle changes. In biology, this type of iridescence results from the formation of diffraction gratings on the surface, such as the long rows of cells in striated muscle. Some types of flower petals can also generate a diffraction grating, but the iridescence is not visible to humans and flower visiting insects as the diffraction signal is masked by the coloration due to plant pigments.

In biological (and biomimetic) uses, colors produced other than with pigments or dyes are called structural coloration. Microstructures, often multilayered, are used to produce bright but sometimes non-iridescent colors: quite elaborate arrangements are needed to avoid reflecting different colors in different directions. Structural coloration has been understood in general terms since Robert Hooke’s 1665 book Micrographia, where Hooke correctly noted that since the iridescence of a peacock’s feather was lost when it was plunged into water, but reappeared when it was returned to the air, pigments could not be responsible. It was later found that iridescence in the peacock is due to a complex photonic crystal.

(Source: edited/adapted from Wikipedia)

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Iridescent clouds

Gradient -- Iridescent clouds
Photo by Ed Kreminski

A relatively rare phenomenon known as iridescent clouds (not to be confused with a circumhorizontal arc) can show unusual colors vividly or a whole spectrum of gradiating colors simultaneously. These clouds are formed of small water droplets of nearly uniform size. When the Sun is in the right position and mostly hidden by thick clouds, these thinner clouds significantly diffract sunlight in a nearly coherent manner, with different colors being deflected by different amounts. Therefore, different colors will come to the observer from slightly different directions. Many clouds start with uniform regions that could show iridescence but quickly become too thick, too mixed, or too far from the Sun to exhibit striking colors.

(Source: edited/adapted from NASA’s “Astronomy Picture of the Day” series)

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Iridescence in Aphrodita aculeata setae

Gradient -- iridescent aphrodita setae
Detail of spines (setae) of Aphrodita aculeata (sea mouse)   Photo by David Linstead

The spines, or setae, on the scaled back of the Aphrodita aculeata (sea mouse) are one of its unique features. Normally, these have a deep red sheen, warning off predators, but when the light shines on them perpendicularly, they flush green, blue, and red in an iridescent and gradiating manner—a “remarkable example of photonic engineering by a living organism.” This structural coloration is a defense mechanism, giving a warning signal to potential predators. The effect is produced by many hexagonal cylinders within the spines, which “perform much more efficiently than man-made optical fibres.”

(Source: edited/adapted from Wikipedia)

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Iridescence in birds

Gradient -- iridescence in birds
The Nicobar pigeon (Caloenas nicobarica)   Photographer unknown

Iridescence in birds is a result of “structural coloration.” Structural coloration is the production of color by microscopically structured surfaces fine enough to interfere with visible light, sometimes in combination with pigments. For example, peacock tail feathers are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and so are often seen as iridescent.

Structural coloration was first observed by English scientists Robert Hooke and Isaac Newton, and its principle (wave interference) explained by Thomas Young a century later. Young described iridescence as the result of interference between reflections from two or more surfaces of thin films, combined with refraction as light enters and leaves such films. The geometry then determines that at certain angles, the light reflected from both surfaces interferes constructively, while at other angles, the light interferes destructively. Different colors therefore appear at different angles.

In regards to the feathers of birds, interference is created by a range of photonic mechanisms, including diffraction gratings, selective mirrors, photonic crystals, crystal fibres, matrices of nanochannels and proteins that can vary their configuration.

(Source: edited/adapted from Wikipedia)

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Iridescence in insects

Gradient -- iridescence in insects
Chrysis ignita, also known as the ruby-tailed wasp   Photo by André De Kesel

Iridescence in insects is a result of structural coloration and not pigment. Structural coloration is caused by microscopically structured surfaces fine enough to interfere with visible light. In some occurences, the visible colors may be enhanced by underlying pigments. Colors are produced when a surface is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the visible color’s wavelength.

Structural coloration is responsible for the blues and greens of the feathers of many birds (the bee-eater, kingfisher, and roller, for example), as well as many butterfly wings and beetle wing-cases (elytra). The iridescent appearance occurs because the reflected color depends on the viewing angle, which in turn governs the apparent spacing of the structures responsible.

(Source: edited/adapted from Wikipedia)

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Iridescence in snakes

Gradient -- iridescence in snakes
Photographer unknown

Of all the iridescent snakes in the world, the rainbow boa is the most famous, but the sunbeam snake is often considered the most spectacular. Their iridescent shimmer is a result of special structures called iridophores.

Iridophores are often identified as the pigment cells that squids and other famous color-changers possess. For these animals, iridophores lie under the skin, under the chromatophores which control pigment, and are dynamic cells that are under the conscious control of the squid.

Iridophores on snakes are very different. They are found on the top of the scales, a kind of gloss over the layer of melanin which gives the snake its dominant color. These iridophores don’t move. They consist of layers of tiny crystal-like cells which lie on top of each other. Light gets refracted between the layers, causing these light waves to interfere with each other, heightening peaks through constructive interference or flattening them through destructive interference. When looknig at the snake you see different colors which shift depending on the angle from which you view them. The mechanics are similar to that of a soap bubble.

(Source: edited/adapted from Stéphanie M. Doucet and Melissa G. Meadows, “Iridescence: a functional perspective,” Journal of Royal Society Interface 6 (2009), accessed June 14, 2017, doi: 10.1098/rsif.2008.0395.focus)

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Leptothrix discophora

Gradient -- iridescence -- Leptothrix discophora
Views of Leptothrix discophora   Photos by Ed Kreminski

Leptothrix discophora is one of several species of Leptothrix, known for its sheath, and iron-, manganese-, and recently arsenic-removing properties. L. discophora embeds itself in an iridescent film of its own making. The film is visible to the naked eye and, as a result of thin-film interference, often appears like an oil slick.

(Source: edited/adapted from Wikipedia)

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Liz West’s Our Colour (2016)

Gradient -- Liz West -- light installation

For the 2016 edition of the Bristol Biennial, British artist Liz West invited visitors to drench themselves in a color spectrum. West transformed architectural space and turned color into an immersive and embodied experience by refracting light through carefully arranged colored theatre gels. This formed a gradient of color hues that subtly shifted through the spectrum from red to violet within the space, wrapping around window frames and architectural columns. The installation enabled visitors to explore visual perception and how color affects our emotions and our bodies. The installation, designed as an experiment in human perception, utilized light as a sculptural medium in order to investigate the physical, emotional, and psychological responses to color within a specific space.

(Source: edited/adapted from designboom)

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Mark Hagen’s A parliament of some things (Additive and Subtractive Sculpture, Titanium Screen, Panels 6, 7, 8) (2014)

Gradient -- Mark Hagen

Los Angeles-based artist Mark Hagen’s sculpture series A parliament of some things… consists of many screen-like panels structured from honeycomb aluminum. The architecturally-arranged, prismatic panels of gradiating colors are skinned with volcanic glass and titanium sheeting that is rainbow-anodized through a mystifying method of conducting electrical current onto the surfaces of the sheets which are treated with Diet Coke.

(Source: edited/adapted from Wall Street International)

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Metachrosis

Gradient -- Chromataphores -- Cephalopods -- Physiological color change

Chromatophores are special pigment-containing cells that can change their size, thus varying the color and pattern of the animal. The voluntary control of chromatophores is known as metachrosis. For example, cephalopods and chameleons can rapidly change their appearance, both for camouflage and for signalling, as Aristotle first noted over 2000 years ago:

The octopus … seeks its prey by so changing its color as to render it like the color of the stones adjacent to it; it does so also when alarmed.
—Aristotle

When cephalopod molluscs like squid and cuttlefish find themselves against a light background, they contract many of their chromatophores, concentrating the pigment into a smaller area, resulting in a pattern of tiny, dense, but widely spaced dots, appearing light. When they enter a darker environment, they allow their chromatophores to expand, creating a pattern of larger dark spots, and making their bodies appear dark. Amphibians such as frogs have three kinds of star-shaped chromatophore cells in separate layers of their skin. The top layer contains xanthophores with orange, red, or yellow pigments; the middle layer contains iridophores with a silvery light-reflecting pigment; while the bottom layer contains melanophores with dark melanin.

(Source: edited/adapted from Wikipedia)

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Nature’s abhorrence of gradients

Gradient -- Nature abhors a gradient -- gradient reduction theory
Barometric pressure recordings during the week of the Great Hurricane of 1938 (showing a clear and dramatic drop in barometric pressure when the hurricane hit on Wednesday, September 21, 1938) taken at the Ladd Observatory at Brown University in Providence, RI.

The second law [of thermodynamics] says that energy delocalizes—or, as Eric D. Schneider puts it, nature abhors a gradient. From this point of view, life, not just in its matter but also in the essence of its evolutionary process, is a particular historically developed thermodynamic system whose trends from planetary expansion and prokaryotic metabolic innovation to increasingly efficient energy use and even the rise of animal intelligence are all in harmonious keeping with the second law mandate to reduce gradients.

[…] The notion that “nature abhors and gradient” […] applies to open systems […] and focuses our attention on the flows that sustain and help organize them.

Complex systems […] tend to appear spontaneously in nature under the influence of appropriate gradients when and where dynamic conditions permit. A gradient is a measurable difference across a distance of temperature, […] pressure, chemical concentrations, or other variables. […]

From primordial differences gravitationally manifesting into the major distinction between stars and space to temperature and pressure gradients within the protosolar nebula organizing the distributions of chemical elements and compounds, differences are exploited by complex systems, generating further differences […].

That nature abhors a gradient restates thermodynamics’ second law […]. Differences in barometric pressure, for example, lead to hurricanes and tornadoes, complex and cyclical processes that dissipate such gradients and vanish when done. Although such gradient-driven, nonrandom, cyclically complex processes may be chemical, biological, and economic (as well as purely physical), their appearance is not assured merely by the existence of a gradient. Kinetic, chemical, and thermodynamic constraints—an appropriate infrastructure—must also be in place before they “pop into existence” […]. Moreover, the pressure, temperature, electron potential, semiotic, or mathematical gradient must be “just right”: if it is too steep, or not steep enough, no complex system will form. Finally, that complex behaviors “eat” gradients of a certain steepness, temporarily and cyclically reducing them (and thus the source of organization for the complex systems themselves), may be at the root of the cyclical (and fundamentally thermodynamic) processes of physiology.

(Source: edited/adapted from Dorion Sagan and Jessica Hope Whiteside, “Gradient Reduction Theory: Thermodynamics and the Purpose of Life,” in Scientists Debate Gaia: The Next Century, eds. Stephen H. Schneider, James R. Miller, Eileen Crist and Penelope J. Boston (Cambridge: MIT Press, 2004), 174–175.)

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Ombré

Gradient -- Ombré
Raf Simons Men’s Black and White Speckled Ombre Trousers

Ombré (literally “shaded” in French) is the gradual blending of one color hue to another, usually from light to dark. Ombré has become a popular treatment within hair coloring, fashion, nail art, cakes and baked goods, home decorating, and graphic design.

Using shading or creating an ombré effect has been used extensively in fabric printing. For instance, the use of a special printing block called a “rainbowed” block was used in the early 19th century to produce textiles with gradiating color designs. Ombré as a textile treatment came back into fashion in around 1840 and was used throughout the 19th century. In machine embroidery, an ombré effect was achieved by dyeing the threads in graded colors beforehand.

Ombré as a contemporary hair-coloring technique is believed to have been popularised in 2000 when the artist Aaliyah had her hair dyed in a subtle gradual fade from black at the roots to lighter towards the hair tips.

Following the early 21st-century trend, many popular home decorators have incorporated ombré into their home decorating styles. Ombré can be used in many products from textiles to glassware, and as a wall-painting technique. Martha Stewart describes the gentle progression of color in ombré as a transition from wakefulness to slumber.

(Source: edited/adapted from Wikipedia)

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Photodegradation

Gradient -- Photodegradation -- color fading
Amie Dicke   Fade (2008)   photographed in artist’s studio   Photo by Jos Kraaijeveld

Visible Light is produced within the spectrum of electromagnetic energy that includes radio waves, microwaves, X-rays, and ultraviolet (UV) rays. The cause of photodegradation (also known as color fading) is due to a photochemical reaction involving UV rays and visible light. UV wavelengths are very damaging to library materials, dyes, papers, wood, paintings, photographs, and even the foods we eat. UV rays (found either in sunlight or artificial light such as fluorescent lighting) act as a bleaching agent. High energy photons of light, typically found in the ultraviolet or violet spectrum, can disrupt the bonds in the chromophore (a chromophore is the part of a molecule that is responsible for its color), leaving the resulting material colorless. Extended exposure to UV and visible light often leads to widespread discoloration.

(Source: edited/adapted from naturalux.com)

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Photoelasticity

Gradient -- Photoelasticity
View of a plastic protractor using the method of photoelasticity   Photo by Nevit Dilmen

Photoelasticity is a method to visualize and determine the stress distribution in a material. Unlike other analytical methods of stress determination, photoelasticity gives a fairly accurate picture of stress distribution around discontinuities in materials. The method is based on the property of birefringence, as exhibited by certain transparent materials. Birefringence is a phenomenon in which a ray of light passing through a given material experiences two refractive indices. The property of birefringence (or double refraction) is observed in many optical crystals. Upon the application of stresses, photoelastic materials exhibit the property of birefringence, and the magnitude of the refractive indices at each point in the material is directly related to the state of stresses at that point. Information such as maximum shear stress and its orientation are available by analyzing the birefringence with an instrument called a polariscope.

Engineers and physicists, in need of a tool that would help them to more immediately see and understand the areas where a structure might break, have come to widely use photoelasticity. Under polarized light, plastics that can be depicted with photoelasticity will reveal areas of strain within a structure in the form of colorful light fringes. One need only make a representation of that structure out of photoelastic plastic and apply load. Areas of strain from the applied stress will concentrate more where the structure is most vulnerable to breakage and more colorful light fringes will be detected in these areas. The rainbow colored fringes will form much more abundantly around areas of greater strain.

(Source: edited/adapted from Wikipedia)

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Physiological color change (see: Metachrosis)

 
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Physiological color change in cephalopods

Gradient -- Physiological color change in cephalopods
Photographer unknown

Coleoid cephalopods (including octopuses, squids, and cuttlefish) have complex multicellular organs that they use to change color rapidly, producing a wide variety of bright colors and patterns.

Most cephalopods possess an assemblage of skin components that interact with light. These may include iridophores, leucophores, chromatophores, and (in some species) photophores. Chromatophores are colored pigment cells that expand and contract in accordance to produce color and pattern which they can use in a startling array of fashions. As well as providing camouflage with their background, some cephalopods bioluminesce, shining light downwards to disguise their shadows from any predators that may lurk below. Bioluminescence may also be used to entice prey, and some species use colorful displays to impress mates, startle predators, or even communicate with one another.

Each chromatophore unit is composed of a single chromatophore cell and numerous muscle, nerve, glial, and sheath cells. Inside the chromatophore cell, pigment granules are enclosed in an elastic sac, called the cytoelastic sacculus. To change color the animal distorts the sacculus form or size by muscular contraction, changing its translucency, reflectivity, or opacity. This differs from the mechanism used in fish, amphibians, and reptiles in that the shape of the sacculus is changed rather than translocating pigment vesicles within the cell. However, a similar effect is achieved.

Octopuses and most cuttlefish can operate chromatophores in complex, undulating chromatic displays, resulting in a variety of rapidly changing and gradiating color schemata. The nerves that operate the chromatophores are thought to be positioned in the brain in a pattern isomorphic to that of the chromatophores they each control. This means the pattern of color change functionally matches the pattern of neuronal activation. This may explain why, as the neurons are activated in iterative signal cascade, we may observe waves of color changing. Like chameleons, cephalopods use physiological color change for social interaction. They are also among the most skilled at camouflage, having the ability to match both the color distribution and the texture of their local environment with remarkable accuracy.

Cephalopods can change their colors and patterns in milliseconds as their chromatophores are expanded or contracted. Coloration is typically stronger in near-shore species than those living in the open ocean, whose functions tend to be restricted to disruptive camouflage.

(Source: edited/adapted from Wikipedia)

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Physiological color change in chameleons

Gradient -- Physiological color change in chameleons
Panther chameleon (with inset images showing microscopic view of two differing states of the chameleon’s lattice of guanine nanocrystals)   Photo credit: Teyssier, Saenko, van der Marel & Milinkovitch

For a long time it was thought that chameleons changed their color through a dispersion of pigment-containing organelles within their skin. However, research conducted in 2014 on panther chameleons has shown that pigment movement only represents part of the story.

Chameleons have two superimposed layers within their skin that control their color and thermoregulation. The top layer contains a lattice of guanine nanocrystals, and by exciting this lattice the spacing between the nanocrystals can be manipulated, which in turn affects which wavelengths of light are reflected and which are absorbed. Exciting the lattice increases the distance between the nanocrystals, and the skin reflects longer wavelengths of light. Thus, in a relaxed state the crystals reflect blue and green, but in an excited state the longer wavelengths such as yellow, orange, green, and red are reflected.

The skin of a chameleon also contains some yellow pigments, which combined with the blue reflected by a relaxed crystal lattice results in the characteristic green color which is common of many chameleons in their relaxed state. The deeper layer of skins works in a similar fashion but primarily controls the amount of near-infrared light that is absorbed or reflected, and therefore may influence thermoregulation.

(Source: edited/adapted from Wikipedia)

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Rainbow

Gradient -- Rainbow
Rainbow seen at Jasper National Park, Alberta, Canada   Photo by Wing-Chi Poon

A rainbow is not located at a specific distance from the observer, but comes from an optical illusion caused by any water droplets viewed from a certain angle relative to a light source. Thus, a rainbow is not an object and cannot be physically approached. Indeed, it is impossible for an observer to see a rainbow from water droplets at any angle other than the customary one of 42° from the direction opposite the light source. Even if an observer sees another observer who seems “under” or “at the end of” a rainbow, the second observer will see a different rainbow—farther off—at the same angle as seen by the first observer.

Rainbows span a continuous spectrum of colors. Any distinct bands perceived are an artefact of human color vision, and no banding of any type is seen in a black-and-white photo of a rainbow, only a smooth gradation of intensity to a maximum, then fading towards the other side. For colors seen by the human eye, the most commonly cited and remembered sequence is Newton’s sevenfold red, orange, yellow, green, blue, indigo and violet, remembered by the mnemonic, Richard Of York Gave Battle In Vain (ROYGBIV).

Rainbows can be caused by many forms of airborne water. These include not only rain, but also mist, spray, and airborne dew.

(Source: edited/adapted from Wikipedia)

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Rainbow roll (see: Blend roll)

 
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Raw Color’s The Fans (2014)

Gradient -- The Fans -- Raw Color
Photo courtesy Raw Color

The starting point for Raw Color’s installation (exhibited at LYNfabrikken in 2014), The Fans (2014), was the combination of two intriguing phenomena. On the one side it is color and its physical quality and on the other side it is the element of motion and its dynamic nature. To unite these qualities the electric fan has been chosen as a tool. Placed on a podium a group of fans will show their different states by performing a choreography blending color and motion.

(Source: edited/adapted from LYNfabrikken)

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Rayleigh scattering

Gradient -- Rayleigh scattering
Rendering by Thorsten Renk

Except for light that comes directly from the sun, most of the light in the day sky is caused by scattering, which is dominated by a small-particle limit called Rayleigh Scattering. The scattering due to molecule sized particles (as in air) is greater in the forward and backward directions than it is in the lateral direction. Scattering is significant for light at all visible wavelengths but is stronger at the shorter (bluer) end of the visible spectrum, meaning that the scattered light is bluer than its source, the sun. The remaining sunlight, having lost some of its short wavelength components, appears slightly less blue.

Scattering also occurs even more strongly in clouds. Individual water droplets exposed to white light will create a set of colored rings. If a cloud is thick enough, scattering from multiple water droplets will wash out the set of colored rings and create a washed-out white color.

(Source: edited/adapted from Wikipedia)

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Sam Falls’ Untitled (Thermochromic bench) (2014)

Gradient -- Sam Falls -- Thermochromic bench
Photo by Liz Ligon

In his work, Sam Falls examines the relationship between entropy, time, and the artistic process. With Untitled (Thermochromatic Bench), clad with thermochromatic tiles, such elements as shadow, light, and body heat transform the colors of the tiles as they respond to changing temperatures. The bench transitions through a spectrum of color, displaying a gradient of color and allowing for the sizable, minimalist-inspired object to fluctuate and change continually throughout its lifespan.

(Source: edited/adapted from UB Art Gallery Press Release)

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Sky coloration

Gradient -- Sky coloration
Photo by NASA

The sky can turn a multitude of colors such as red, orange, purple and yellow (especially near sunset or sunrise) when the light must pass through a much longer path (or optical depth) through the atmosphere. Scattered light from the sun near the level of the horizon travels through as much as 38 times the atmosphere as does light from the zenith, causing it to lose blue components, causing a blue gradient: vivid/dense at the zenith, and more pale near the horizon. Because red light also scatters if there is enough air between the source and the observer, these longer wavelengths of light will also scatter significantly, making parts of the sky change color during a sunset. As the amount of atmosphere nears infinity, the scattered light appears whiter and whiter.

(Source: edited/adapted from Wikipedia)

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Slope (also known as Gradient)

Gradient -- slope
Slope is calculated by finding the ratio of the “vertical change” to the “horizontal change” between (any) two distinct points on a line.

In mathematics, the slope or gradient of a line is a number that describes both the direction and the steepness of the line. Slope is often denoted by the letter m. The steepness, incline, or grade of a line is measured by the absolute value of the slope. A slope with a greater absolute value indicates a steeper line.

Slope is calculated by finding the ratio of the “vertical change” to the “horizontal change” between (any) two distinct points on a line. Sometimes the ratio is expressed as a quotient (“rise over run”), giving the same number for every two distinct points on the same line. A line that is decreasing has a negative “rise.”

In mathematical language, the slope m of the line is:

Gradient -- slope

The slope of a line in the plane containing the x and y axes is generally represented by the letter m, and is defined as the change in the y coordinate divided by the corresponding change in the x coordinate, between two distinct points on the line. This is described by the following equation:

Gradient -- slope

The Greek letter delta (Δ) is commonly used in mathematics to mean “difference” or “change.”

The concept of slope can be generalized to functions of more than one variable and is more often referred to as gradient.

(Source: edited/adapted from Wikipedia)

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Spectrum colors

Gradient -- Spectrum colors
Light beam through glass prism   Rendering by Matthias Kulka

A spectrum obtained using a glass prism and a point source is a continuum of wavelengths without bands. The number of colors that the human eye is able to distinguish in a spectrum is in the order of 100. Accordingly, the Munsell color system (a 20th-century system for numerically describing colors, based on equal steps for human visual perception) distinguishes 100 hues. The apparent discreteness of main colors is an artefact of human perception and the exact number of main colors is a somewhat arbitrary choice.

Newton, who admitted his eyes were not very critical in distinguishing colors, originally (1672) divided the spectrum into five main colors: red, yellow, green, blue and violet. Later he included orange and indigo, giving seven main colors by analogy to the number of notes in a musical scale. Newton chose to divide the visible spectrum into seven colors out of a belief derived from the beliefs of the ancient Greek sophists, who thought there was a connection between the colors, the musical notes, the known objects in the Solar System, and the days of the week.

According to Isaac Asimov, “It is customary to list indigo as a color lying between blue and violet, but it has never seemed to me that indigo is worth the dignity of being considered a separate color. To my eyes it seems merely deep blue.”

The color pattern of a rainbow is different from a spectrum, and the colors are less saturated. There is spectral smearing in a rainbow owing to the fact that for any particular wavelength, there is a distribution of exit angles, rather than a single unvarying angle. In addition, a rainbow is a blurred version of the bow obtained from a point source, because the disk diameter of the sun (0.5°) cannot be neglected compared to the width of a rainbow (2°). The number of color bands of a rainbow may therefore be different from the number of bands in a spectrum, especially if the droplets are particularly large or small. Therefore, the number of colors of a rainbow is variable. If, however, the word rainbow is used inaccurately to mean spectrum, it is the number of main colors in the spectrum.

(Source: edited/adapted from Wikipedia)

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Split fountain (see: Blend roll)

 
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Tauba Auerbach’s RGB Colorspace Atlas (2011)

Gradient -- Tauba Auerbach -- RGB Colorspace Atlas
Tauba Auerbach   RGB Colorspace Atlas (2011)   Photo by Vegard Kleven

Created by Tauba Auerbach in 2011 as a series of three unique, 8 x 8 x 8-inch, 1,670-page, hard-back books, RGB Colorspace Atlas illustrates the RGB color gradient in its entirety in a 3D format.

Daniel E. Kelm, who bound each tome, summarizes Auerbach’s project as a sculptural object as well as a spatialization of color:

Human eyes typically have three types of color receptor on their retinas, each sensitive to a different range of wavelengths of light. The colors associated with these wavelengths are approximately red, green, and blue. Because there are three types of color receptor, it is possible to map the visible spectrum in a three-dimensional spatial model by assigning red, green, and blue each to a dimension. It is then possible to outline a cube in this space, where the values of red (R), green (G), and blue (B) are visible on a gradient scale of 0 to 100% in their respective directions. These gradients combine to create the RGB color space cube, a volume in which any color can be located by a set of three coordinates. RGB Colorspace Atlas, both a sculptural object and spatialization of color, consists of three books. Each volume contains the entire visible spectrum mapped out over 3,632 pages, representing the RGB cube sliced in a different direction: vertically, horizontally, and from front to back.

(Source: edited/adapted from danielkelm.com)

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Thin-film interference

Gradient -- Thin film interference -- iridescence
Soap bubble macro photography   Photo by Wikimedia Commons user “Pashminu”

Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other. Since the wave cannot be reflected, it is completely transmitted instead. When the thickness is a multiple of a half-wavelength of the light, the two reflected waves reinforce each other, increasing the reflection and reducing the transmission. Thus when white light, which consists of a range of wavelengths, is incident on the film, certain wavelengths (colors) are intensified while others are attenuated. Thin-film interference explains the multiple colors seen in light reflected from soap bubbles and oil films on water. It also is the mechanism behind the action of anti-reflection coatings used on glasses and camera lenses.

A thin film is a layer of material with thickness in the sub-nanometer to micron range. As light strikes the surface of a film it is either transmitted or reflected at the upper surface. Light that is transmitted reaches the bottom surface and may once again be transmitted or reflected. The light reflected from the upper and lower surfaces will interfere. The degree of constructive or destructive interference between the two light waves depends on the difference in their phase. This difference in turn depends on the thickness of the film layer, the refractive index of the film, and the angle of incidence of the original light wave on the film. The pattern of light that results from this interference can appear either as light and dark bands or as colorful bands depending upon the source of the incident light.

If the incident light is broadband, or white, such as light from the sun, interference patterns appear as colorful bands. Different wavelengths of light create constructive interference for different film thicknesses. Different regions of the film appear in different colors depending on the local film thickness.

(Source: edited/adapted from Wikipedia)

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Tomás Saraceno’s Poetic Cosmos of the Breath (2007)

Gradient -- Tomás Saraceno -- Poetic Cosmos of the Breath
Tomás Saraceno   Installation view of Poetic Cosmos of the Breath (2013)   Photo by Studio Tomás Saraceno

Inspired by the work of Dominic Michaelis, an English architect and inventor who came up with the technology for a solar-powered hot air balloon, Tomás Saraceno’s Poetic Cosmos of the Breath is a time-based, experimental, solar dome that takes flight only under certain climatic conditions. It uses deceptively simple materials: a paper-thin foil accompanied by a few sandbags, and a handful of participants to produce a startlingly ethereal, shimmering effect. Staged at dawn, as temperature conditions naturally shift, air inside the balloon is heated by a greenhouse effect and the lightweight material slowly lifts off the ground completely unaided by machines or electrical power. At the same time, sunlight casting through the material creates a vibrant, rainbow-tinged, iridescent glow.

(Source: edited/adapted from tomassaraceno.com)

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Visible spectrum

Gradient -- Visible spectrum

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or, simply, light. A typical human eye will respond to wavelengths from about 390 to 700 nanometres.

The spectrum does not, however, contain all of the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations such as magenta, are absent, for example, because they can be made only by a mix of multiple wavelengths. Colors containing only one wavelength are also called pure colors or spectral colors.

Visible wavelengths pass through the “optical window,” the region of the electromagnetic spectrum that allows wavelengths to pass largely unattenuated through the Earth’s atmosphere. An example of this phenomenon is that clean air scatters blue light more than red wavelengths, and so the mid-day sky appears blue. The optical window is also referred to as the “visible window” because it overlaps the human visible response spectrum.

In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. He was the first to use the word spectrum (Latin for “appearance” or “apparition”) in this sense in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of “corpuscles” (particles) of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass. The result is that red light is bent (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors.

Newton divided the spectrum into seven named colors: red, orange, yellow, green, blue, indigo, and violet. He chose seven colors out of a belief, derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the solar system, and the days of the week. The human eye is relatively insensitive to indigo’s frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some later commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet. However, the evidence indicates that what Newton meant by “indigo” and “blue” does not correspond to the modern meanings of those color words.

In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum (Spektrum) to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them. The spectrum appears only when these edges are close enough to overlap.

(Source: edited/adapted from Wikipedia)

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