Light-reflection Strategies

Andrew R Parker. American Scientist. Volume 87, Issue 3. May/Jun 1999.

When the Stealth bomber first appeared in 1988, it was billed as possessing some of the most advanced aircraft designs ever built. The idea of the Stealth was simple: a plane whose design allowed it to evade radar detection because its surfaces could absorb or otherwise slip through the electromagnetic radiation meant to detect it. The Stealth used specialized coatings and a shape that represented the cutting edge of human technology. Since then advances in nanotechnology and other areas have allowed further improvements in antireflective materials and coatings designed for other optical effects. Antireflection, however, is hardly a human invention. A wealth of natural structures have evolved that can perform similar feats.

Beginning early in the history of life on earth, organisms have evolved to interact with the electromagnetic radiation coming from the sun, both as an energy source and as an information carrier. Because interaction with ambient light begins at an organism’s surface, the optical properties of surfaces are extremely important. Optical surfaces and coatings in nature are often finely tuned to the character and abundance of the light they interact with, and to the function they serve in the organism. These natural structures often rival the ones people have designed to perform similar functions. There is a long tradition of work that compares human designs to the structures produced by the blind work of natural selection. This article continues that tradition by comparing the reflective properties of naturally occurring structures to more recent, human designs.

Reflection in Black and White The Stealth bomber tries to eliminate the reflection of radar wavelengths completely. Although the wavelengths targeted are different, this strategy is analogous to the way the black leopard’s coat provides camouflage. The pigment in the leopard’s coat absorbs most optical light, so little is reflected back or scattered. Since leopards are most active during the night, when there is little ambient light in the optical range, this allows them to blend into the background and lower their chance of detection by potential observers.

At first blush, it might seem that if you want to avoid detection, the best way is to prevent a would-be detector from seeing any of the light that reflects off you-in other words, to eliminate reflected light completely. More often than not, however, this “men-in-black” approach is apt to make one more conspicuous, rather than less. Commercial trap fisherman in Tasmania, for example, hang black flags on their buoys to make them easier to find, because black is the most conspicuous pigment on the shimmering surface of the sea. Organisms trying to conceal themselves during the day are therefore not particularly well served by a total antireflection strategy. In some cases, in fact, the best strategy may be to do just the opposite, and reflect all incident light.

The Oleander butterfly (Euploea core), for example, has a chrysalis designed to reflect most incident optical light. The reflection from the surface is so complete that it gives the chrysalis a metallic sheen. The sheen is produced by alternating layers of high- and lowdensity chitin (the material of insect exoskeletons). Although the chitin is transparent, the boundary between stacked layers behaves like a boundary between air and glass. The difference between one layer’s refractive index (which measures how much light is bent when it enters a particular medium) and the index of the layer below it causes some light to be reflected, with the rest passing through slightly bent from its original path. Layering the chitin coatings creates a succession of refractive index changes, resulting in the reflection of a large proportion of the incident light.

This reflection is aided by variations in the thickness of the chitin layers (in fact, it is the optical thickness, or actual thickness multiplied by the refractive index, that is important here). When light passes from a low-index medium into a high-index one, the reflected light undergoes what is known as a phase inversion. In effect, this means that the waves making up the light are displaced by half a wavelength. In a multilayer reflector like the chrysalis, if the high- and low-index layers are each a quarter-wavelength thick, then light reflected from the bottom of a layer has traveled back and forth in that layer just enough for its wave to be aligned with the light reflected from the top of the layer. The net effect, when combined with the phase inversion, is that reflection from each layer constructively interferes with reflection from the layer below it.

Layer thicknesses correspond very closely to the wavelengths they have evolved to reflect. If these thicknesses are modified the optical properties change substantially. For example, freezing an Oleander chrysalis causes the layers to become transparent, an effect that can be reversed if the chrysalis is warmed. By stacking more than 30 layers with thicknesses of 150-250 nanometers, a high degree of reflectivity can be achieved over the entire optical range. In the diffuse and uniform light environment created by the tree canopy where the chrysalis hangs, this is an effective camouflage strategy that protects the chrysalis from predation.

Many species of fish living in water with diffuse light also have shiny metallic surfaces produced by alternating layers of transparent materials. As with the Oleander chrysalis, the reflections from such surfaces mimic the lighting effects found in the organism’s environment for example, shallow water. Although the fishes’ mirrored surfaces are quite effective at reproducing the intensity and chromatic properties of ambient lighting, they do modify at least one characteristic of the reflected light-its polarization. Polarization refers to the orientation of the electric and magnetic waves that constitute light (the electric component of the wave can be vertical, horizontal, or somewhere in between, with the magnetic component always perpendicular to it).

Transparent surfaces reflect light with an electric component that is polarized perpendicular to the plane of incidence more than they reflect light with a parallel-polarized electric component. The light reflected by mirrored fish therefore has significantly less parallel-polarized light than does the ambient light it mimics. Other fish and squid that feed on mirrored fish have evolved light detectors specifically designed to foil the smoke-and-mirrors strategy of their prey by detecting this difference in polarization. The exact mechanism of these polarization detectors is still under investigation but, unlike most human polarization-detection designs, seems not to involve polarization filters. Instead, the natural versions seem to involve arranging the light-detecting components of the eye in different orientations, each of which is designed to interact more with light polarized in a particular direction.

Although nature provides examples of both high- and low-reflectivity camouflage solutions, these extreme strategies are more the exception than the rule. If an organism’s goal is to avoid notice, the best strategy is clearly to blend into the surroundings. Given the variability of the optical light environment on earth, this generally involves a particular surface color or pattern. One way to achieve the variation necessary is to try to be invisible by interacting as little as possible with incident light. Few organisms are transparent enough to make this strategy effective. Other organisms, such as chameleons and cuttlefish, change their own colors to match the background. Human designers, however, who have so far been unable to develop Wonder Woman’s invisible plane technology or alternative futuristic cloaking devices, have taken their cue from some of nature’s more prosaic designs.

Various Shades of Gray

Nearly all natural earthly camouflage is tailored to the optical region of the spectrum, in large part because the sun’s peak wavelength intensity falls in this region. Squirrels that blend into the woods, moths that disappear on tree trunks and lions that melt into the grass all have evolved to match the visual colors of their surrounding environments. Apart from special environments such as those discussed above, natural camouflage usually involves the addition of pigment to moderately reflective surfaces that scatter incident light in all directions. This is the same approach the military uses when it disguises its equipment in camouflage patterns tuned to the hue of the surrounding landscape.

Although matching wavelengths is an important factor in designing appropriate camouflage, it is just as important to ensure that the reflected wavelengths match the intensity of the surrounding light. Birds in flight, for example, interact with two very different light intensities from above and below. A bird viewed from above is lit by the direct light of the sun, or “skyshine.” For a bird to be camouflaged, its color must match the darker ground over which it flies, so many birds have a evolved a relatively dark dorsal coloration.

From below birds are lit by “earthshine,” which is much dimmer than the light overhead. To avoid contrasting too much with the sky, birds’ undersides often contain much less pigment, so that they reflect as much earthshine as possible. Their rounded bodies also allow for some of the skyshine to be reflected down, so they do not appear as dim against the bright sky.

Fish and other marine animals such as dolphins face a lighting environment similar to that confronting birds. Since the light coming from above is much brighter than the light from below, they too have developed a twotone coloration.

Some fish, many of which are very ancient, have evolved a novel strategy that seems to help them match the ambient light intensity to avoid detection from below. These marine animals among them the lantern shark, the midshipman fish, and species of the myctophid fish Tarletonbeania-have bioluminescent ventral photophores that emit light. They also possess sensors on their dorsal surfaces, enabling them to judge the intensity of the light coming from above. By lighting their undersides they can match this intensity and minimize the shadow they cast.

This time-tested strategy for recreating ambient light intensity was actually used on B-24 bombers during the Second World War, although whether or not they were inspired by such fish is anyone’s guess. Plane coloration sometimes features the kind of countershading found in nature, but in this case (known at the time as “Project Yahudi”), the undersides of the bombers were fitted with lights. The planes were also equipped with photosensors to detect the intensity of the background light, so the lights could be set to match it. Although the design was relatively effective, the advent of radar (to which I return below) has made visual camouflage a secondary concern.

As important as camouflage is, it carries with it an important caveat: Camouflage that is too good might prevent an organism from being detected not just by predators but also by its conspecifics. This would be a bad strategy for any organism hoping to mate. Certain members of the genus Ovalipes, which includes 11 species of large swimming crabs, have developed a specially adapted optical surface that gets around this problem. Some of the species of this crab possess a large reflecting layer in their surface cuticle, which appears iridescent in direct white light. This reflector consists of some 20 corrugated layers of chitin that alternate with hollow regions filled with water.

My colleagues and I analyzed the reflective properties of this structure, and determined that it reflects each wavelength in the visual spectrum in a particular direction. When incident white light is perpendicular to the surface, yellow light is reflected back. Transmission electron micrographs revealed that the layers are just the right thickness to produce constructive interference of yellow light on reflection. When light strikes the surface at a larger angle, however, the layers selectively reinforce blue light.

Ovalipes apparently uses these reflectors to signal conspecifics. What is particularly interesting is that only deep-water Ovalipes species possess iridescent coatings over most of their bodies. Ovalipes living in shallow waters exhibit little if any iridescence, and instead signal each other using welldeveloped “stridulatory structures” that produce sounds. Deep-dwelling Ovalipes species possess similar structures, but they are poorly developed. This difference seems to be the result of the distribution of optical wavelengths in sea water. At shallow depths, light from across the optical spectrum is relatively abundant. If shallow-water Ovalipes possessed the corrugated reflector, the yellow light they reflected vertically would be detected easily by predators. In deeper waters, however, the only optical wavelength present is blue light, which is reflected parallel to the ocean floor. In such an optical environment, Ovalipes with the corrugated coating are safe from predators.

Antireflection

Because the deep-dwelling crabs face a light and detection environment different from their shallow-water cousins, they do not need to worry about interacting with many kinds of electromagnetic radiation. For similar reasons, most organisms on earth have little need to worry about the interaction of their surfaces with radio and microwaves. Because the sun is not nearly as powerful a source of radiowaves as it is of optical waves, the ambient radiowave intensity on earth is relatively low. Natural radio camouflage to avoid predation is therefore unnecessary.

With the rise of radar in the 1930s, however, selection pressures on human designs have favored such camouflage. Detection issues involving radar are somewhat different from those involving optical wavelengths. The absence of intense radiation in the range from the sun makes radio sources, and objects that reflect radiowaves, relatively easy to detect. For this reason, radar camouflage actually does demand total external antireflection-our “men-inblack” strategy.

One of the better-known features of Stealth planes, for example, is that they are extremely angular and have few curved surfaces. Although this makes the planes much more difficult to keep aloft, it makes back-reflection of microwaves and radiowaves from radar sources much less likely. It does not, however, eliminate reflection in other directions. To minimize all reflection, the Stealth must employ a variety of coatings that, in the future, may draw on natural antireflective structures.

We have already seen that pressures favoring complete antireflection do not confront most organisms, since optical wavelengths are so abundant in the environment that black is fairly conspicuous. Nevertheless, environmental pressures do favor wide-spectrum antireflection for other important biological functions. Plants and other organisms have developed a variety of surfaces that minimize reflection. Unmodified, these structures would only be effective for optical and near-optical wavelengths. Even so, because many aspects of the behavior of light are scalable with respect to wavelength, such structures can-and have-inspired human solutions to nonoptical design challenges.

Photosynthesis is one function for which completely antireflective surfaces are best. Before light reaches the plant pigments that actually absorb light and convert it into useful energy, it must first pass through the plant’s surface with a minimal loss of intensity. The larger the difference in refractive index between the material the light is traveling through and the material it is entering (in this case, air and the plant), the more reflective the surface boundary in between. One way to reduce this reflection is to match the refractive index of the leaf with that of air. Coatings with intermediate refractive indices can also ease the transition from one medium to another, reducing reflection.

The most effective antireflective coating of this sort is one that has a one-quarter-wavelength optical thickness. Such coatings work in a way similar to the layers on the mirrored fish or the Oleander butterfly, except that in this case, because of the refractive indices of the layers involved, there is no net phase shift when the lightwaves are reflected. Light reaching the coating’s bottom that would in other circumstances be reflected back has traveled just far enough to be exactly out of phase with light that would be reflected at the coating’s top surface. The destructive interference of the two light rays effectively cancels the reflection and the light that would have been reflected joins the transmitted light (see “Iridescent Blue Plants,” January-February 1997).

Some of my own work has focused on other biological structures for which minimization of reflection is crucial: those developed for the perception of light stimuli. As in photosynthesis, the goal here is to allow light-sensitive pigments to harvest as much light with as little distortion as possible. Many animals, such as cats, have a reflective coating known as a tapetum behind their retinas, which reflects back photons that the retinal membrane fails to absorb on the light’s first pass. The tapetum consists of a series of thin layers that works by the same principles as do the Oleander butterfly chrysalis coatings. It gives the animals a second chance to harvest all available light, which is particularly useful at night, when photons are scarce. Some photon detectors of human design employ a similar “second-chance” technique.

Another way of increasing the efficiency of vision is to reduce reflection from the corneal surface, the outer boundary of the eye. Usually, the lightdetecting apparatus is shielded behind a protective coating, which must allow light to pass without modifying it. Coatings used in cameras, microscopes, glasses and other lenses take advantage of the same antireflective tricks exploited by plants.

My research in this area has concentrated on surface structures found in insects that ensure maximum light harvesting. Insects have compound eyes consisting of multiple ommatidia, or facets. Each ommatidium detects light over a narrow angular range, with comprehensive vision achieved in a mosaic-like manner.

Moth Eyes

The ommatidia of many insects have smooth surfaces, but some, such as those of moths and butterflies, are covered with tiny, slightly tapered protuberances. These structures are approximately 200 nanometers in both height and diameter at their base, and are arrayed across the surface of the ommatidia in a regular hexagonal pattern. These structures were first observed in nocturnal moths by W. H. Miller and colleagues in 1962. They were also recently identified in the eyes of the silverfish, a fairly primitive insect that evolved around 300 million years ago.

Because the species that possess these structures tend to be active at night or in the dark, it is important that they absorb as much of the available light as possible. The function of the “moth-eye protuberances” seems to be to minimize reflection from the surface of the ommatidia and thereby maximize the possibility that light is absorbed by the receptor cells underneath. Like much of the exoskeleton of insects, the surface of each ommatidium is made of chitin, which has a refractive index significantly higher than that of air (1.55 compared to 1.00).

The protuberances work by providing a gradual transition in refractive index from air to ommatidium. Each individual photon that is incident on the ommatidia first encounters the thinner tops of the protuberances, making the effective refractive index only slightly higher than air’s. As the protuberance widens closer to the bottom, the refractive index of the surface approaches that of pure chitin. Because the size and periodicity of the protuberances are smaller than those of the optical wavelengths absorbed, each individual photon encounters this gradual transition, and reflection from the surface is minimized.

The antireflective properties of these structures have been known for some time-in 1956 R. W. Klopfenstein proposed a similar structure that provides a high degree of antireflection with the shortest protuberances possible. Although it has advantages over the moth-eye structure, including a wider angular range, the Klopfenstein taper is less durable than the moth protuberances, which have fewer sharp corners. This may explain why natural selection has favored the latter. In any event, the technology for producing either of these structures on the scale of the moth eye-the scale required if the structures are to be effective for optical wavelengths-has only been available to humans since 1973. Human designers have put this technology to good use since then in their own antireflective applications.

The moth-eye structure has been applied to double- and triple-paned windows, for example. Glass in air reflects about 4 percent of incident light perpendicular to the glass, and generally more if the incident light is not perpendicular. For triple-paned windows, this reflection is compounded and becomes a significant nuisance. A person in a room with such a window would only see 78 percent of the light coming from the other side, superimposed with 22 percent of the light from their own side. By coating each pane with a moth eye-patterned plastic film with the same refractive index as glass, reflection on each side of the window can be reduced to 4 percent. Moth-eye structures have also been used to reduce reflection from the surfaces of optical disks, improving resolution and thereby increasing capacity.

The moth-eye structure is well suited to many antireflection tasks, but it has its limitations. When incident light is perpendicular to a moth-eye surface, reflection is nearly zero. But if incident light deviates more than 10 degrees from the perpendicular, the antireflection properties start to decrease. It is not yet clear why the moth eye structure is not as good an antireflector as other structures for a wide range of angles.

Better wide-angle antireflectors have been developed by human engineers for solar absorbers that use the sun’s energy to produce heat. Because the direction of the sun’s light changes with the time of day and the season, these absorbers must minimize reflection over as wide an angular range as possible. They are able to do this with the help of a diffraction grating that, like the moth-eye structure, is scaled to specific light wavelengths. The grating structure, like the protuberances and tapers, provides a smooth transition from air to the grating material, reducing the refractive index contrast.

Fly Eyes

My colleagues and I have recently identified a similar diffraction grating in a mummified fly (an Eocene dolichopodid dipteran) preserved in amber resin for 45 million years. This structure seems to have evolved after the motheye structures, and we believe it to be the most efficient known natural antireflector at high angles of incidence. In 1976, Piotr Mierzejewski in Poland ground down the amber surrounding the fly and then painstakingly removed the thin layer that remained to expose the surface of its eye. After the eye was photographed with an electron microscope, the specimen was put on display in Warsaw by the Polish Academy of Sciences, but little attention was paid to the minute structures revealed.

We were particularly interested in what appeared to be a periodic grating structure on the curved surfaces of the fly ommatidia. The grating consists of a series of fine parallel ridges only 145 nanometers high, spaced 240 nanometers apart. We decided to analyze the reflective properties of this structure. Because insect corneas sometimes contain interior color filters (which allow them to achieve a kind of color vision), we could not test the recovered fly eye itself. Instead, we made a model of the fly-eye structure using a bulk material whose refractive index is very close to that of chitin. We embossed the fly-eye grating onto this material using two interfering laser beams. We then constructed an instrument that could shine any wavelength of the visible spectrum on the model at an angle from 0-90 degrees, with the plane of incidence parallel to the grooves of the grating structure. We also compared the structure’s reflection of light polarized both perpendicular and parallel to the plane of incidence.

The fly-eye structure is a very effective antireflector for angles up to 7Q degrees from the perpendicular, as we can see if we compare the reflective properties of our model to those of a smooth surface of the bulk material from which it is made. It also significantly reduced the difference in the reflection of perpendicular- and parallel-polarized light. Although the moth eye is a slightly better antireflector in the narrow range around the perpendicular, the fly eye is superior for larger angles.

The fly seems to have evolved the grating structures because its ommatidia, unlike those of the moths, have a highly curved surface. This means that light coming from the particular direction that each individual ommatidium monitors is more likely to strike that ommatidium’s surface at an angle. Some modern flies, whose ommatidia contain both curved and flattened regions, possess both the protuberance structure on the flattened surface and the grating structure in the curved areas. In these species, the moth-eye structures seem to organize into the fly-eye grating as the surface becomes more curved. This is further evidence that the fly-eye structure evolved as a way to maximize the absorption of light striking the eye’s surface at a high angle of incidence.

Fly-eye structures could be useful in a variety of situations that demand a reduction of angular reflection. Work is currently under way to modify solar absorber and solar panel surfaces to mimic the fly-eye grating. Previous surface designs have used gratings whose ridges have a rectangular profile, but the fly-eye grating, with its sinusoidal profile, seems to have won out. The structure might also be used for improved window coatings similar to moth-eye coatings already in use or to cover automobile dashboards to minimize glare from the sun.

Because the scale of a periodic structure affects the way different wavelengths of light interact with it, it would also be possible to alter the flyeye structure to make it applicable to radar wavelengths. This would involve stretching the distance between ridges and using a higher-index material to avoid ridges so high that they could not fit on an aircraft wing. Since minimizing reflection of radar is one of the key goals of Stealth plane technology, the fly-eye design could be extremely helpful. One advantage the structures may have over the antireflection technology currently in use is their potential applicability to a wide range of weather conditions. Stealth antireflective surface coatings have run into problems in the rain, because the porous material used to create a gradual change in refractive index became clogged with water, significantly altering its optical properties. Given the long period of testing to which natural selection has subjected the insect-eye structures, they may be better suited to inclement weather conditions.

Although human beings pride themselves on being the sole intelligent designers on earth, and it is true that human intelligence has crafted things with no natural parallels, our designs are constantly inspired by structures occurring in nature. Human designs often copy natural ones, and equally common are cases in which human designs are independently developed and then analogous natural structures are “discovered” once we know what it is we are looking at. As we have seen, there are many cases in which natural designs are better than human ones designed to perform similar tasks. Although natural selection is a blind process, it has a tremendous head start on the human designer, and many of the structures we have been discussing have stood the test of time with remarkable resilience. This is reason enough to pay attention to such “natural designs” and to look for further instances of our own best design ideas reflected in the natural mirror.