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a b s t r a c t o: Ar nan The shifting rainbow hues of Franziska Schenk iridescence have, until recently, remained exclusive to nature. and Andrew Parker Now, the latest advances in nanotechnology enable the introduction of novel, bio- inspired color-shifting flakes into painting—thereby affording artists potential access to the full spectacle of iridescence. Unfortunately, existing rules rtists have never captured a color as dazzling selves of two distinct types: those of easel painting do not apply A that are stable attributes of material to the new medium; but, as and dynamic as the metallic blue of the Morpho butterfly, which nature inspired the technol- is visible for up to a quarter of a mile. Now, with rapid ad- substances, and those that are “acci- dental,” such as the evanescent col- ogy, an exploration of natural vances in nanoscience and technology, we are beginning to ors of the rainbow and the colors of phenomena can best inform unravel nature’s ingenious manipulation of the flow of light. some birds’ feathers, which change how to overcome this hurdle. Scientific research into natural nanoscale architectures, ca- according to the viewpoint of the Thus, by adopting a biomimetic pable of producing eye-catching optical effects, has led to the spectator [2]. approach, this paper outlines the optical principles underly- development of an ever-expanding range of comparable syn- ing iridescence and provides thetic structures. Particularly notable are the latest iridescent The “stable” colors, associated technical ground rules for its flakes. Although industry has exploited the novel properties with chemical pigments, have pre- incorporation into painting. of these flakes for almost a decade, fine-art painting has been occupied painters for millennia. slower to assimilate them. The major apparent hindrance is The rainbow, by contrast, remained the incompatibility with—and resulting confusion caused by mysterious until the 17th century, the material’s non-adherence to—color theory as applied in when Newton famously united light and color through his painting. Iridescent flakes, in themselves colorless, are “opti- prism experiment that proved white light consists of all the cal devices”—entirely different from the chemical pigments colors of the spectrum. The changeable hues of bird feathers traditionally found in paint. Consequently their application kept their secrets much longer. It was only in the mid-20th presents major challenges for artists. However, since the flakes century that science verified what the ancients had intuitively mimic nature’s technology, systematic analysis of the mecha- believed: that the colors of the rainbow and iridescence (a nisms that cause iridescent effects in animals can inspire analo- term evoking Iris, personification of the rainbow) are inextri- gous artistic methods. We can therefore establish, via studying cably linked. Both phenomena are caused by light interacting nature, some ground rules for adapting color-variable technol- with transparent, colorless matter. A rainbow is created when ogy to fine art painting. water droplets, like Newton’s prism, split white light into its As demonstrated by Schenk [1], the latest iridescent tech- components—the colors of the spectrum. Newton concluded nology offers manifold innovative artistic possibilities, together that the angle-dependent colors of birds’ feathers must equally with a unique potential aesthetics. result from light splitters (i.e. thin films) but did not compre- hend the precise color-producing mechanism [3]. In the 1950s electron microscopy, enabling nanoscale observations, finally wo istinctly ifferent ypes of olor T D D T C ascertained that iridescence in hummingbirds, for example, Gage’s Color in Art opens insightfully: is indeed produced by what effectively equates to a stack of thin films [4]. Here spectral colors are made visible via the Any European account of color in art must begin with the be- optical phenomenon of constructive interference, resulting lief, which dominated Western culture for many centuries, that light and color are distinct entities, and that colors are them- in color that changes with the direction of illumination and viewing angle. In the last decade, alternative iridescence-inducing ar- chitectures have been discovered in animals, and initial Franziska Schenk (artist, educator, researcher), Bournville Centre for Visual Arts, attempts to replicate them have been made by manufactur- Birmingham City University, Linden Road, Bournville, Birmingham, B30 1JX, U.K. ers [5,6]. Thin-film interference is the most common form E-mail:
©2011 ISAST LEONARDO, Vol. 44, No. 2, pp. 108–115, 2011 109
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t temporary painters still operate within the confines of subtractive color theory,
ce, Fig. 1. Achieving silver-white.
n derived from practical experience with
e (a) Chaotic layers, each tuned to i
c a color of the spectrum. absorption pigments. Here the primaries s
, (b) Three Stacks, each tuned to
t are red, yellow and blue. A mixture of a primary color. (© A. Parker) these colors becomes darker with each
o: Ar primary added until black results, in an
N complete reversal to what occurs in “sil- ver” fish. Natural iridescence adheres to the rules of additive/light mixture, based on the primaries red, green and blue, and so does iridescent “pigment.” Con- sequently, for iridescence ever to become a fixture on the painter’s palette, addi- tive and subtractive methods have to be mixed—a concept alien to artists. fades over time. Centuries of practical ex- [10]. In the centuries that followed, his In nature, on the other hand, chemical perience with absorption pigments have technique of grinding up the scales of pigments and structural colors are often led to firm rules for colorant mixing. silver fish such as bleak and herring re- combined. The composition of cuttlefish With the recent arrival of an altogether mained the only way of obtaining pearl skin is a prime example, its individually different type of “pigment” that imitates luster effects. adjustable color cells being arranged in natural thin-film structures, it is instruc- layers (Article Frontispiece). A bottom tive to re-examine certain doctrines. Subtractive versus Additive layer of silver “mirror” cells reflects colors Fueled by rapid advancements in na- Color Theory from the surroundings. The cells above, noscience and manufacturing, the evolu- How can silver coloration in fish be ex- containing chemical pigments, switch tion of iridescent “pigment” technology plained? While the color appears identi- on (expand) and switch off (contract) is gaining momentum. While offering cal to that of the precious metal, no silver accordingly, thus (in combination with both non-toxic and fade-resistant prop- traces are found in fish scales/skin. The the “mirror” cells) assuming any color erties, the ever-expanding flake choices metallic color results from transparent desired via optical mixing. Studying such are most notable for their outstanding multi-layered architectures, known as color mechanisms can be instructive to purity, brilliance and innovative optics. broadband reflectors [11]. In a variation the painter. These qualities have enticed those in the of the thin-film formula, a number of lay- auto, cosmetics and plastics industries. ers of varying thickness are stacked (Fig. A First Generation Unfortunately the technology seems to 1a). Each layer reflects a different wave- of Pearlescent Flakes have bypassed fine art painting. The art length/color of the spectrum, depend- From the 1920s onward, sustained field’s lack of awareness of the medium’s ing on its particular optical thickness. attempts were made by industry to potentials is due, in part, to difficulties One layer reflects red; another slightly replicate natural pearl essence with a syn- in sourcing the product. Although paints thinner layer reflects orange; an even thetic alternative, because natural pearl based on first-generation flakes can now thinner one, yellow; then green, then essence is costly and limited in supply. be bought from specialist art suppliers, blue, etc., until all the colors of the rain- This eventually led to the conception of recent industrial color-variable flakes bow are reflected and bright white light “pearlescent pigments,” based on “a trick remain prohibitively expensive [8] and is observed. This process not only dem- copied from nature” [13], namely light as yet unavailable in paints for artists. onstrates what happens when differently interference. Ever since, (semi-)transpar- The greater stumbling block, however, colored reflections/lights are combined ent flakes generating “metal-like effects is the challenge the creative application but also illustrates the laws of additive . . . without using metals” [14] have been presents. One fundamental problem is color theory. classed as pearlescent effect pigments. how to create tonality, shading and a Notably, in the herring the layers of The synthesis of first-generation flakes range of color when intermixing with silver-generating reflectors are organized was demonstrated in 1942. Commercial conventional paints without compromis- differently (Fig. 1b): use, however, only began in the 1970s ing the desired iridescent effect. This is [15]. Reminiscent of pearl essence, the something that perhaps only nature can individual blue, green and red multi- first pearlescent “pigments” were silver- teach us [9]. layer reflectors may simply overlap . . . at white. The platelets consist of colorless any position on the fish the red, green and blue parts of three different scales components: a very thin layer of tita- nium dioxide (TiO ) covers thin plate- In Pursuit of Fish will lie on top of each other [12]. 2 lets of natural mineral mica [16]. Based ilver rom atural S : F N This process perfectly illustrates Young’s on the same principle, by thickening the Pearl Essence to seminal 1801 discoveries that by over- TiO2 layer, a range of interference col- Pearlescent Flakes layering/mixing only three so-called ors in gold, red, blue, violet and green Before we venture into a discussion of primaries, at equal intensity, white is were subsequently developed [17] (Fig. full color, an investigation of “white”— created, and that simply combining dif- 2). Mica-based flakes remain the most- or rather silver, a specular “white”—will ferent quantities of these three colors requested special effect pigment today provide the necessary context. In 1656, can produce any color. In correlation to [18].
Jaquin isolated a silvery substance from the herring’s reflectors, Young’s prima- TiO2-mica platelets (Fig. 3a) approxi- fish scales and turned it into “Essence ries are red, green and blue—a set well mate the effect of a thin film found in d’Orient,” the first pearl-like “pigment” known to today’s computer artists. Con- nature (Fig. 3b), making pure spectral
110 Schenk and Parker, Iridescent Color
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color visible via constructive interfer- ter with a metallic luster; others resemble blue light-waves, which are reflected, t ence. Light traveling at different speeds silky satin; and still others display a sub- and all remaining light-waves, which pass ce,
in different media effectively “recog- dued pearly sheen [25]. How can this through the transparent structure. On n e i
nizes” a boundary at the interface be- diversity be explained, however, when reaching the white ground, the transmit- c s ,
tween air and the TiO2 layer. Certain different Morpho species share virtually ted light (yellow) is reflected back. Thus t light waves, for instance those we per- identical mechanisms for color genera- both light components become visible:
ceive as blue, are reflected at the flake’s tion? Answering this question may pro- At face angle there appears a blue reflec- o: Ar an
upper surface. The remainder of white vide the solution to a major problem: tion and at oblique angle its complemen- N
light is transmitted through the TiO2- Intermixing interference flakes with tary color—yellow. If, however, the blue layer and meets the surface of the mica, opaque white or black absorption pig- is applied on black, a pure, intense blue where again only blue is reflected. This ment (a method commonly adopted results, because all transmitted light is ab- blue component emerges from the flake by painters to adjust tonality) inevitably sorbed and only blue light waves survive. parallel to the first blue reflection. Since compromises iridescence. Comparing Again, this is consistent with what scien- it traveled further, through a layer of a the color mechanisms of three Morpho tists have discovered in M. didius. Impor- certain thickness (tuned to this precise butterflies, M. sulkowskyi, M. didius and tantly, depending on the background’s wavelength), the second component is M. rhetenor, aids alternative artistic strate- tonal value, the same narrowband struc- synchronized with the first; that is, the gies for tonality, purity and luster. ture can produce vivid metallic effects two blue light waves “constructively in- and subtle two-color opalescence. terfere” [19]. The blue appears intensi- Adjusting Tonal Value fied when viewed perpendicular to the Morpho sulkowskyi and M. didius share Adjusting Intensity, Purity painted surface (at face angle). At more virtually identical light-reflecting struc- and Luster oblique (glancing) angles, a hardly no- tures. Yet there are key differences. M. Additional factors have a bearing on the ticeable “few degrees of a color shift” sulkowskyi displays a transient pearly blue visual effect. Morpho rhetenor’s glittering [20] toward the adjacent shorter wave- that shifts from blue-green to violet. At blue, despite comparable amounts of length (violet) occurs—before the color oblique angles, when the blue all but dis- melanin, is much brighter and more disappears at near 45°. Such narrowband appears, a faint yellow background and angle dependent than that of M. didius reflectors, reinforcing a predominant brown markings become visible. Morpho [27]. The reason becomes apparent un- color, are also found in nature. didius, by contrast, features a deeper, less der the microscope. Inspecting a wing of angle-dependent version of the same M. didius, one notices rows of translucent blue. Scientific studies attribute this scales (called “glass scales”) covering the In Pursuit of Morpho marked difference mainly to the varying iridescence-producing ground scales. lue rom irst to B : F F - amounts of black absorption pigment Acting as an optical diffuser, the ridged Second-Generation (melanin) in the two species. Melanin surface of the former strongly refracts Pearlescent Flakes is distributed in the scales of M. didius the incident light, spreading the blue re- Extremely efficient narrowband reflec- (beneath the blue reflectors), while only flection over a wider angle. This spread, tors have evolved in the wing scales of a negligible amount is present in M. in turn, results in a reduction of light certain butterfly families, giving rise to sulkowskyi [26]. intensity [28]. Morpho rhetenor possesses some of the most spectacularly bright The crucial role base pigment plays in negligibly small “glass scales”; hence its iridescent displays found in nature. adjusting color tone is confirmed when glossy brightness and more distinct color The stunningly blue coloration of many working with interference flakes. If a blue shift. This contrast illustrates how surface Morpho butterflies, for example, has at- interference color is coated on white, the texture is adjusted to fine-tune the level tracted and sustained much attention. resulting effect is analogous to what can of sheen. Manufacturers of iridescent flakes have be observed in M. sulkowskyi: The angle- What can be advantageous in nature pinpointed this iconic butterfly as a ma- dependent blue reflection flashes on might be seen as a disadvantage in iri- jor inspiration [21], and scientists have and off to reveal a muted yellow. The descent flakes. Over the last few decades, studied the intricate nano-architectures flake’s layered structure effectively splits special-effect pigment manufacturers of this genus most comprehensively the incident light into two components: have realized that mica-based technol- [22]. The reflectors—consisting of up to 12 alternating layers of chitin and air
[23]—far outshine TiO2-mica. As all lay- ers share the same optical thickness, a certain wavelength, one we see as blue, Fig. 2. Achieving a range is reflected at each interface between of interference colors. (© Franziska Schenk) the media, ultimately yielding a much- The reflection’s color is
intensified blue reflection (which shifts determined by the TiO2- noticeably toward violet at oblique view- layer’s thickness. ing angles) (Fig. 3c). The multilayer reflectors of Morpho rhetenor, one of the brightest species, reflect 70–80% of all incoming blue light [24]. Notably, among various species of the Morpho family the entire range of blue color tones, ranging from a pale moon- beam-blue to a deep, intense violet-blue, is represented. Some of these colors glit-
Schenk and Parker, Iridescent Color 111
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t ogy has limitations. The mica’s ridged purple to green, with certain flakes fluc- are ways to arrive at iridescent color surface (Fig. 4a), by default, invites light tuating between green-blue, blue and mixes, as demonstrated by the plumage ce,
n scattering—an undesirable feature when violet—presenting a pale imitation of of parrots. e i
c maximum purity and intensity is the M. rhetenor’s hues. The color usually associated with par- s ,
t objective. With iron impurities further However, the pearlescent “pigments” rots is green. Yet parrots do not possess diluting its transparency, the mica core most suitable for replicating the inten- green pigment; a camouflaging pigment
o: Ar contributes little to the iridescent ef- sity of M. didius and M. sulkowskyi’s hues simply has never evolved [34]. How has an
N fect [29]. Consequently, first-generation are based on borosilicate glass (Fig. this potential obstacle to the parrot’s mica-based flakes simply cannot match 4c). They combine unique color purity long-term survival been circumvented? Morpho blue. with high transparency, intensive light Color mixing seems the obvious answer. Enhancing the unsaturated color of reflection and noticeable narrowband Green feathers consist of a central shaft mica flakes by tightening particle size color travel. To re-create both versions with lateral branches on either side distribution has led to considerable ad- of Morpho blue, Schenk overlaid the called barbs. Under the microscope a vancements in the last decade [30]. Mica- same blue-reflecting glass flake on back- crosscut of a barb shows three oval areas: based multilayer reflectors, introduced grounds differing in tonal value (Color in the late 1990s [31], ushered in a new Plate A Nos. a–b). To our knowledge, 1. the outer layer made of colorless era, while much recent research activity no pearlescent “pigment” matching the keratin containing yellow pigment has been geared toward substituting the purity and intensity of M. rhetenor exists 2. the intermediary layer (spongy mica core to create ever more efficient to date. Despite 50 years of research, the zone), consisting of keratin rods multi-layer reflectors. latest pearlescent generation cannot rival and air vacuoles nature’s iridescent “genius.” 3. the center, harboring melanin. Second-Generation Pearlescent Flakes Still, with only melanin and yellow pig- olor ixing echanisms Many second-generation pearlescent C M M ment present, what gives rise to green? “pigments” are based on synthetic sub- in Nature as an Inspiration Although not immediately obvious, strates coated with highly refractive metal for Painters there is in fact a blue component present oxides. Pearlescent lines with divergent Parrot Green: Color Mixing here. For decades, scientists thought that optical characteristics, each based on dif- via Layering nanoparticles in the spongy zone scatter ferent synthetic substrates, have emerged Intermixing iridescent flakes presents light at blue wavelengths, in a process [32]. Their perfectly plain, transparent considerable challenges. For example, similar to how the sky is made blue (Tyn- synthetic cores act as optical layers, thus when blue and yellow absorption pig- dall effect). Raman was first to dissent, making them true multilayer systems ments are combined, green results. suggesting constructive interference as that give rise to purer, more intense in- However, a mixture of iridescent blue the blue color’s cause [35]—a proposal terference colors and, in some instances, and gold is white. The components into subsequently supported by Dyck [36] distinct color change. Silica flakes (Fig. which the incoming white light is sepa- and Prum et al. [37]. The latter showed 4b), for example, reflect a broad range of rated by the interference phenomenon that the nanostructured tissue in the wavelengths. Layers of varying thickness are recombined. Mixing interference spongy zone indeed produces blue iri- give rise to pronounced color changes flakes “of different reflection colors is descence in the manner of multi-layer (or “color travel”), for example from therefore undesirable” [33]. Still, there reflectors rather than scattering. A blue-
Fig. 3. Schematic diagrams of thin-film reflectors. (a) TiO2-mica platelet (© Franziska Schenk). (b) Single layer reflector (solid lines indicate incoming light; dashed lines, reflection) (© A. Parker). (c) Narrowband multi-layer reflector (© A. Parker).
112 Schenk and Parker, Iridescent Color
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Fig. 4. Generic pearlescent “pigments.” (© Franziska Schenk)
inducing structure between a yellow pig- ment deposited beneath, together with Layering and pointillist color mixing ment above and an absorbing screen of the blue iridescence, produces a purple, combine to achieve the complicated pat- melanin below generates parrot green. but only when viewed at certain angles. terns of butterflies. Schenk’s artistic sim- To test the theory, Schenk overlaid inter- From other angles the specular blue ulation of Inachis io’s eyespot combines ference blue (on a dark ground) with a either dominates or switches off to ex- both principles (Color Plate A No. e). yellow colorant to successfully simulate pose the red base pigment. Schenk suc- Useful comparisons between “pig- the green feather of the Senegal par- cessfully replicated this multi-color (or ment” flakes and butterfly scales can be rot, Poicephalus senegalus (Color Plate A color-flop) effect in her simulation of the made. Both are components of modular No. c). The colorant’s level of transpar- Purple Tip by coating a blue interference systems and therefore present possibili- ency, together with the amount used, color over a red pigment (Color Plate A ties for additive mixing. In a loose anal- are important considerations. A semi- No. d). ogy to scales, the platelets are oblong, transparent topcoat of yellow results in Importantly, if a red pigment is sus- thin and flat (with the latest generation “parrot” green—almost indistinguish- pended in the same paint vehicle as also featuring rounded edges) to mini- able from a chemical pigment. blue-reflecting flakes, the flop effect is mize unwanted scattering. Orientation, In summary, the appearance of green lessened, and more of an optical blend- alignment and tilt of the scales in rela- feathers is determined by the amount ing occurs: The presence of interspersed tion to the wing membrane are impor- of melanin and yellow pigment and by red particles shifts the reflection color tant factors to consider in studies on feather ultra-structure. These few ele- to a purple [40]. This is analogous to butterflies [44]. Similarly, randomly ments cause all color variation in green what happens in butterflies. Layering oriented interference flakes amount to parrots, from green to bright yellow (no is one contributing factor, but there nothing but a grayish powder; only when blue), from blue (no yellow) to white (no are additional mechanisms. Each wing- properly aligned in the paint vehicle do pigment or iridescence). layer is constructed of a vast collage of they show their true colors—spray ap- Industry has developed so-called com- individual scales [41]. As in a pointillist plication, roller coating and dipping are bination pigments that, in parrot fash- painting, each scale/dot is a particular therefore favored [45]. Like their natu- ion, combine chemistry and structure. A color and can be of an entirely different ral counterparts, interference platelets single platelet combines interference-in- hue than its neighbors. Any perception come in different sizes/grades and can ducing layers with a top layer of transpar- of blending or intermediate shades is a be distributed more densely or sparsely. ent absorption pigment [38]. For most visual effect of the spatial arrangement of The coarser the “pigment,” the greater applications, however, transparent ab- scales of discrete color so small that our its transparency and brilliance [46]. sorption colorants are simply layered or eye cannot distinguish them [42]. As a Also, crucially, with larger, more sparsely intermixed with true interference flakes, result additive color mixing occurs [43] distributed flakes the opportunity for a multitude of color effects being derived (e.g. the eye perceives a mixture of tiny pointillist color mixing increases consid- with less logistical effort. red and blue dots as purple). erably. In wing patterns, all markings are Butterfly Purple: Layering formed by concentrations of scales of the and Pointillist Color Mixing same or different colors. For example, Conclusions In nature it is more common for ab- the eyespots adorning the forewings of In both nature and art a myriad of com- sorption pigments to be deposited un- the peacock butterfly Inachis( io) bear a plex interrelated factors contribute to derneath the reflectors, thus retaining half-moon–shaped patch of semi-trans- the diversity of iridescent effects. As dem- maximum purity and intensity. Butter- parent blue-reflecting cover scales across onstrated in this paper, understanding flies of the genera Colotis, for instance, two differently colored areas of ground the fundamentals of natural iridescence possess a red pigment that is combined scales (one red and one black). Where can aid artistic application. Each of the with a structural blue to produce orange- the blue cover scales are superimposed cases presented here features a highly pinks and magentas. In the wing tip of and interspersed with pigmented red efficient color system that illuminates the Purple Tip (Colotis regina), a blue scales, a muted purple results; where nature’s great economy of means [47]. reflectance peak is due to Morpho-type the structural blue interacts with black Iridescent paint technology has yet to multi-layer reflectors [39]. The red pig- ground scales, an intense blue appears. evolve to a comparable point.
Schenk and Parker, Iridescent Color 113
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n the absence of ready-made paints and 23. P. Vukusic, “Structural Color Effects in Lepidop- e i tera,” in Kinoshita and Yoshioka [22] pp. 95–112. c rules of application, half-forgotten skills s Acknowledgments ,
t could regain currency. Preparing paints 24. Vukusic [23]. and formulating methods, together with The Arts and Humanities Research Council (AHRC) and the Arts Council of England funded this re- 25. S. Berthier, Iridescences: The Physical Colors of Insects o: Ar certain “old-masterly” techniques (e.g. search. Thanks also to Dr. Michael Rösler and Helga (London: Springer, 2007).
an Wolf from Merck KGaA, Germany.
N a tonal “under-painting” overlaid with 26. S. Kinoshita and S. Yoshioka, “Photophysical Ap- semi-transparent glazes) could help us proach to Blue Coloring in the Morpho Butterflies,” realize this new medium’s full potential, References and Notes in Kinoshita and Yoshioka [22] pp. 138–139. bringing us back to the exemplar of the 27. Vukusic [23]. Renaissance painter as chemist, material Unedited references as provided by the authors. scientist and, in this case, physicist. Inti- 1. F. Schenk, “Nature’s Fluctuating Color Captured 28. Pfaff and Reynders [15]. on Canvas?” International Journal of Design & Nature mate familiarity with the materials’ opti- and Ecodynamics Vol. 4, No. 3, 274–284 (2009). 29. S. Yoshioka and S. Kinoshita, “Inter-scale Mecha- cal properties, together with meticulous, nisms in Blue-light Reflection of aMorpho Butterfly,” 2. J. Gage, Color in Art (London: Thames & Hudson, in Kinoshita and Yoshioka [22] pp. 141–152. time-consuming experimentation, are 2006) p. 15. prerequisites to developing these tools. 30. Pfaff and Reynders [15]. The creation of new color mixes, the 3. I. Newton, Opticks (1730), reprint (New York: Do- ver Publications Inc., 1952). 31. Reisch [8]. kind that result from many years of trial- and-error tests, is now largely a project of 4. C.H. Greenewalt, W. Brandt, D.D. Friel, “The Iri- 32. G. Pfaff, “Optical Principles, Manufacture, Prop- descent Colors of Hummingbird Feathers,” Proceed- erties and Types of Special Effect Pigments,” in Pfaff industry, where formulae are protected ings of the American Philosophical Society Vol. 104, No. [6] pp. 16–91. by confidentiality agreements [48]. 3, 249–253 (June 1960). 33. Pfaff [32] pp. 72–79. The development of a simplified 5. A. Parker and H.E. Townley, “Biomimetics of Pho- “primary” system that allows both the tonic Nanostructures,” Nature Nanotechnology Vol. 2, 34. Greenstein [10] p. 840. No. 6, 347–353 (2007). intermixing of interference flakes with 35. Parker [7]. each other, as well as more convenient 6. G. Pfaff (ed.), Special Effect Pigments (Hannover: 36. C.V. Raman, “The Origin of the Colors in the Vincentz Network, 2008). integration with transparent pigments, Plumage of Birds,” Proceedings of the Indian Academy is essential. This in turn demands a 7. A. Parker, Seven Deadly Colors: The Genius of Nature’s of Sciences, Sect. A, 1–7 (1935). shift to a more holistic color theory that Palette and How It Eluded Darwin (London: Free Press, 2005) p. 58. 37. J. Dyck, “Structure and Spectral Reflectance of overcomes the artificial divide between Green and Blue Feathers of the Rose-Faced Love- additive and subtractive color mixing. 8. M.S. Reisch, “Rainbow in a Can,” Chemical & En- bird (Agapornis Roseicollis),” Biologiske Skrifter Vol. 18, No. 2, 1–65 (1971). The recent introduction of pearlescent gineering News Vol. 81, No. 443, 25–27 (Nov 2003). flakes, developed as a systematic group 9. F. Schenk and J. Harvey, “Reflections on the Nat- 38. R.O. Prum, R. Torres, S. Williamson, J. Dyck, [49], is a step in the right direction. ural History Museum: The Art of Iridescence and “Two-dimensional Fourier Analysis of the Spongy Nature’s Jewels,” The International Journal of the Arts Medullary Keratin of Structurally Colored Feather Predetermining each pigment’s optical in Society Vol. 3, Nr. 5, 133–144 (2009). Barbs,” Proceedings of the Royal Society of London B 266, hue angle (via colorimetric calculations) 13–22 (1999). 10. L.M. Greenstein: “Nacreous (Pearlescent) Pig- makes them more readily intermixable ments and Interference Pigments,” The Pigment 39. Maisch [17] p. 39. [50]—a major breakthrough and sure in- Handbook 2nd edition, Vol. 1, 829–857 (New York: Wiley, 1988). 40. M.A. Giraldo, Butterfly Wing Scales: Pigmentation dication that as the technology advances and Structural Properties, Ph.D. dissertation (The its application will become easier. Still, 11. E.J. Denton, “Reflectors in Fishes,”Scientific Ameri- Netherlands: Print Partners Ipskamp, 2007) p. 8. can 224, 64–72 (1971). much remains to be resolved: Color- 41. Greenstein [10] p. 842. variable flakes present huge challenges 12. Parker [7] p. 244. for analysis, understanding and control- 42. H.F. Nijhout, The Development and Evolution of But- 13. Merck KGaA, “Effects—More Than Just Color,” terfly Wing Patterns (Washington, D.C.: Smithsonian ling color mixing. With the range of ef-
114 Schenk and Parker, Iridescent Color
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Glossary prism, split white light into its components, thereby on adapting iridescent color technology from t making pure spectral color visible its inception, Schenk has initiated a series of
color mixing—combining two or more colors result- ce, ing in a third color pearlescence—industrial term referring to (semi-) related arts and science projects, including n e transparent, non-metallic special effect pigments i
the collaboration with Andrew Parker that c
color-variable flakes—synonymous with iridescent s
resulted in this article. ,
“pigment” t
interference—iridescence-generating optical effect Manuscript received 29 June 2009. Andrew Parker is a Research Leader at the associated with thin films Natural History Museum (London), an Hon- o: Ar an
Franziska Schenk is an artist and lecturer in orary Research Fellow at Green Templeton Col- N iridescence—metallic, angle-dependent color gen- fine art at Birmingham City University, and erated by transparent, colorless nano-architectures lege (Oxford) and Professor at Shanghai Jiao Honorary Research Associate at the Univer- Tong University. He researches structural color light splitters—clear structures that, like Newton’s sity of Birmingham (U.K.). Having worked in nature and the evolution of vision.
CALL FOR PAPERS
Leonardo Music Journal 22 (2012) Acoustics
Immersed as we are in electronically mediated sound, at the end of the day—whether it’s coming from ukuleles or earbuds—sound reaches us through acoustic pressure. The sheer physicality of sound, and its quirky interaction with our sense of hearing, has driven many a composer and sound artist to go back to the “year zero” in music—before the codification of melody, rhythm and harmony—and explore funda- mental aspects of the physics and perception of sound.
For Volume 22 of LMJ we solicit articles and artist’s statements on the role of acoustics and psychoacoustics in music and audio art.
DEADLINES
15 October 2011: Rough proposals, queries 1 January 2012: Submission of finished articles Address inquiries to Nicolas Collins, Editor-in-Chief, at:
Schenk and Parker, Iridescent Color 115
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