Thursday, February 29, 2024

Blue Morphos Have a Cool Color

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Lately I’ve noticed a presence that has accompanied me from room to room, fluttering over the pages I read and landing on a surprising number of them. This presence is the blue morpho, a large butterfly (8-inch wingspan) that inhabits Central and South America and is celebrated for its beautiful electric-blue color. That color is structural: it is caused by the interaction of incident light with the shape of a light-receiving surface, not by wavelength-dependent light absorption by that surface.


I first encountered the blue morpho in Kai Kupferschmidt’s book, Blue: In Search of Nature’s Rarest Color, which I reviewed in Issue 504 of ISCC News. There I learned about a tricky problem that the butterfly appears to have solved through natural selection. Interference patterns can lead to brilliant structural colors, but the color you see generally depends on the angles of illumination and viewing. As the viewing geometry changes, it is difficult to preserve the color of the butterfly’s wing at all angles. Kupferschmidt notes that scientists have only recently understood the angle-independence of that color. The proof of their understanding was their effectiveness in imitating the butterfly’s color and is the subject of several U.S. patents (see U.S. Patent Nos. 11,428,854 and 11,200,583, and also Patent Application No. 20210018659). The inventor is none other than Andrew Parker, who gave one of the ISCC presentations on color and evolution last September, a month or so after I had encountered Kupferschmidt’s book.


Parker’s solution is a multi-layer composite of scales similar to the microscopic scales on the wings of the morpho butterfly. Normally, a set of nearly identical features with even spacing would cause an interference pattern. The trick inspired by the morpho butterfly is to add a constrained randomness in the orientation of the scales. Besides being randomly oriented, Parker’s scales are convex, and that convexity causes the incident light to diverge into a variety of directions while keeping a sufficient intensity and wavelength selectivity to stay electric blue. This convexity seems to be the main mechanism of the solution: Parker designed his multi-layers so as to minimize the effects of interference in the light scatter. To this end, he forced the refractive index to have a smooth dependence on spatial position within the layer, which encourages a ray-optic analysis. (This could be a big surprise to those of us who assume that iridescence is a property intrinsic to a structural color---but remember Newton’s prism can serve as a light-dispersing element.) You have seen the ray-spreading behavior before, in the convex side mirrors on your car (“objects in mirror are closer than they appear”). When you look at this mirror’s reflection of a localized light source, it indeed seems farther away than a flat-mirror reflection. The source retains its intensity but occupies less visual angle and (in compensation) visibility in a wider range of scattering directions.


At about the same time I learned of Parker’s invention, I received my August issue of Optics and Photonics News, and there was our blue friend again, now in the context of “Radiative cooling in living color.” The research was done by Wanlin Wang et al. in Shenzhen University in China. The primary research publication is W. Wang et al., Cooling colors below ambient temperature, Optica 10(2023), 1059-1068.

As did Parker, the Chinese group claimed a blue-morpho-inspired composite layer that achieves the viewing-angle insensitive blue color. For angle diversity, they found that frosted glass provided a satisfactory solution. But the Chinese group simultaneously strove for a different objective. Their angle-constant blue layer also provides substantial radiative cooling-- up to 2 degrees C lower than the ambient and better than using a white reflector. To achieve this cooling performance, they adjusted the spectral properties of the layer over all visible through mid-IR electromagnetic wavelengths. In particular, the absorbed part of the incident solar radiation heats the surface and converts the heat to black-body radiation with a mid-IR component that is much stronger than the passive cooling emission that would otherwise be present in mid-IR. It turns out that the Earth’s atmosphere is completely transparent to mid-IR radiation (8000 to 13000 nm). Because of that transparency, radiation in the mid-IR window promptly leaves the Earth and never returns. For us it might be a window of escape from global warming.


Having read about the Shenzhen group’s radiative-cooling layer, I tried to tie the design back to the morpho butterfly but found the analogy wanting. It seemed to me that their device comprised two independent inventions---one for angle-invariant color and one for radiative cooling---only the first of which pertains to structural colors and thereby to the blue morpho. The main emphasis of the Optica article was radiative cooling, which is not a property I’ve ever seen attributed to the blue morpho. How, then, can one convincingly assert that this research is related to the blue morpho?


The plausibility seems to hinge on the choice between two alternative words: “imitate” versus “inspire.”   The Shenzhen group used both words to describe the connection. On one hand, they imitated the structure of the blue morpho’s wing (presumably in solving the color-versus-angle problem), but they were not imitating it anymore when they were solving the cooling problem. On the other hand, they could have been inspired by the blue morpho to do the entire project including the cooling problem. Unlike an object of imitation, an object of inspiration does not imply technical transfer to the project from the source of inspiration. Imitation is hard, as implied by Yogi Berra: “If you can’t imitate him, don’t copy him.” So, the blue morpho as inspiration is more inclusive (perhaps all-inclusive: anything can be inspired by anything else).


I tried to be tolerant of the looser connection implied by “inspired,” but ultimately its all-inclusiveness turned me off and I remained unswayed.   


Suddenly, the butterflies that were fluttering around me disappeared.


The blue morpho is indeed a beautiful creature and a wonderful muse. It’s not often I see a single actor participating in so many shows at the same time. But the radiative-cooling show seems far-fetched for this actor. I question the relevance, other than to say that the blue morpho has a cool color.


 Figure 1. Blue Morpho, Female, Dorsal view.

Source: Wikipedia. Copyright


Michael H. Brill

Wednesday, November 1, 2023

Blue Book

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After 12 years of waiting, I have found the answer to the color-realism question I asked in a poem and kept on my Hue Angles blog site ( Does color lie in the world or in the mind? Answer: “Blue is not out there, and it is not inside us either. The radiant blue of a cornflower is a kind of collaboration between us and the plant.” The origin of this quote is Kai Kupferschmidt’s book, Blue: In Search of Nature’s Rarest Color [The Experiment, 2021], p. 7. Kupferschmidt holds a degree in molecular biomedicine from the University of Bonn, writes for Science magazine, and lives in Berlin. The pertinence of the sentence I quote is in no way constrained by his restrictions of subject matter to the color blue or to the reciprocal communication between humans and plants. I hope it might quiet the din of debate between realists and subjectivists in philosophy.


Heartened by the author’s simple resolution of the realist question in color, I backed away, for a moment, from the book—which a close friend had just given to me. It is a small but physically dense 216 pages, adorned on the inside with color layout (mostly blue) with full-page completely blue separators between the chapters. The hard cover is entirely colored in subtly different shades of blue (except for a peacock on the front), and even the page edges are a dark blue. Receiving this object, I felt as if I were receiving a mystical talisman like David Lynch’s blue key in the movie Mulholland Drive.


Hoping to understand this object better than I understand Mulholland Drive, I thought, what’s the big deal about blue? Kupferschmidt made some assertions about blue that seem to be oxymorons—that blue is ubiquitous (e.g., the blue sky) and rare (e.g., blue lobsters). Then I checked how many Hue-Angles articles have “blue” in the title. The answer is four, not including the present one. In comparison, green appears in three Hue-Angles titles, seven colors appear in one or two titles, and no other color is mentioned at all. From the example of Hue Angles itself, I was forced to acknowledge the plausibility of Kupferschmidt’s assertions, even if they seem self-contradictory.


So, I started to read. The five simply titled chapters are logical, clear, and interesting.


STONES deals with the development of inorganic colorants, including chemistry and historical connections. Kupferschmidt makes clear that the search for the perfect blue led not only to Prussian blue as an artist’s tool, but also to discovery of hydrocyanic acid—used to kill millions of people. There is more than one side to the color blue.


SEEING discusses the anatomy, physiology, and psychophysics of color perception. The author covers a vast domain and makes the subject look easy by his clear language and organization. For example, he explains the Blue Dress by invoking color constancy. (I would have said “failure of color constancy,” but that is a quibble.)


PLANTS discusses color as a collaborative language to communicate with animals (and with us). Also, the chapter emphasizes that most plants have colors that are determined by molecular reactions to light, not by diffraction off structures (as is more common for animals). The author observes that green light is reflected from plants, and we see the light because it is available and useless to the plants. From a plant’s point of view, the green part of the spectrum is a “green gap.” But still in Kupferschmidt’s text, the color blue predominates, from indigo to the coal-tar blues, from cornflowers to the unattained grail of the blue rose.


SPEAKING emphasizes the cultural dependence of color-name boundaries, as studied by Berlin and Kay. The chapter begins with William Gladstone and ends with political correctness.


ANIMALS talks about structural colors, and most of the examples are birds. The chapter argues convincingly that the success of structural colors lies in their invariance with respect to lighting/viewing geometry. This is ensured if the precise diffractive structures are mixed with apparently random perturbations that have a hidden regularity. We only recently came to understand this subtlety.[1]


The book has two main themes. (1) blue is a special color; and (2) one should give blue the attention, appreciation, and awe that it deserves. Much of what the author finds special in the color blue is anecdotal or otherwise not amenable to quantitative study; it certainly lacks a specific point-by-point comparison of blue with other colors. Nonetheless, he has devoted his non-working life to compiling an impressive array of historical and scientific matter that will have wide appeal. He travelled far to acquire these things.


Early in the book, the author uses a cryptic comment to command a reader’s thoughtful attention (much as does a Zen koan[2]), The comment is this (p. 34): “It is […] no accident that accidents have played such an important role in the history of color. The reason? So far it has been nearly impossible to predict with any certainty what color a particular substance will have without making it.” For example, the same process and raw material can give rise to either ruby or emerald. The color can be considered an accident. But the fact is that many such accidents happen in pigment preparation; that fact by itself is no accident. It is the sensitivity of the coloration mechanism to detailed orbital energies. The author applies similar logic to explain the many years of exploration preceding the discovery of Prussian blue and later blue pigments such as YInMn as late as 2009. The author uses the device of non-accidental accidents to fortify his argument for the specialness of blue.


At the start of the SEEING chapter, Kupferschmidt delivers his over-arching message through reference to a scene from the TV show The Simpsons. Homer Simpson, after declaring himself “not easily impressed,” lets his attention wander and exclaims, “Whoa! A blue car.”  Although Kupferschmidt acknowledges the humor, he muses that “even the sight of a blue car ought to amaze us” (p. 53). To end the SEEING chapter, he echoes “Whoa! A blue sky.” The substitution of “sky” for “car” recalls the opening lines of the book, recounting the Apollo 17 astronauts’ awe upon seeing the Earth from space. The ensuing photograph of the Earth (called The Blue Marble) is famous and is often a point of departure for a lecture warning us that we have a clear responsibility to save our planet. Kupferschmidt does not give this lecture, but instead determines “to look out into the world more often, as if I were seeing all its colors for the first time, and to say to myself, ‘Whoa! A blue sky.’” (p. 82)


Having solved one philosophical problem to my satisfaction, Kupferschmidt’s book raises other questions: How can one sustain the enthusiasm of seeing colors as if for the first time? Why is blue so special? 

I see this book as a report of and homage to Kupferschmidt’s personal spiritual journey in search of blue. For more about this journey, please read the final section, HERE WAS BLUE. I won’t spoil it for you. I also wish him well.


Michael H. Brill

[1] The rainbow colors that we see when we look at a diffraction grating (like a CD) are one type of simple structural color. But they are very sensitive to the angles of lighting and viewing. Peacocks and Morphos butterflies pull some three-dimensional tricks with their structural color so that the color is constant over some range of angles. [John Seymour]

[2] A paradox to be meditated upon that is used to train Zen Buddhist monks to abandon ultimate dependence on reason and to force them into gaining sudden intuitive enlightenment.

“Koan.” Dictionary, Merriam-Webster,