Department of Ecology and Evolutionary Biology, and Natural History Museum, University of Kansas, Lawrence, Kansas 66045-7561.
Integr Comp Biol. 2003 Aug;43(4):591-602. doi: 10.1093/icb/43.4.591.
The fundamental dichotomy between incoherent (phase independent) and coherent (phase dependent) light scattering provides the best criterion for a classification of biological structural color production mechanisms. Incoherent scattering includes Rayleigh, Tyndall, and Mie scattering. Coherent scattering encompasses interference, reinforcement, thin-film reflection, and diffraction. There are three main classes of coherently scattering nanostructures-laminar, crystal-like, and quasi-ordered. Laminar and crystal-like nanostructures commonly produce iridescence, which is absent or less conspicuous in quasi-ordered nanostructures. Laminar and crystal-like arrays have been analyzed with methods from thin-film optics and Bragg's Law, respectively, but no traditional methods were available for the analysis of color production by quasi-ordered arrays. We have developed a tool using two-dimensional (2D) Fourier analysis of transmission electron micrographs (TEMs) that analyzes the spatial variation in refractive index (available from the authors). This Fourier tool can examine whether light scatterers are spatially independent, and test whether light scattering can be characterized as predominantly incoherent or coherent. The tool also provides a coherent scattering prediction of the back scattering reflectance spectrum of a biological nanostructure. Our applications of the Fourier tool have falsified the century old hypothesis that the non-iridescent structural colors of avian feather barbs and skin are produced by incoherent Rayleigh or Tyndall scattering. 2D Fourier analysis of these quasi-ordered arrays in bird feathers and skin demonstrate that these non-iridescent colors are produced by coherent scattering. No other previous examples of biological structural color production by incoherent scattering have been tested critically with either analysis of scatterer spatial independence or spectrophotometry. The Fourier tool is applied here for the first time to coherent scattering by a laminar array from iridescent bird feather barbules (Nectarinia) to demonstrate the efficacy of the technique on thin films. Unlike previous physical methods, the Fourier tool provides a single method for the analysis of coherent scattering by a diversity of nanostructural classes. This advance will facilitate the study of the evolution of nanostructural classes from one another and the evolution of nanostructure itself. The article concludes with comments on the emerging role of photonics in research on biological structural colors, and the future directions in development of the tool.
非相干（与相位无关）和相干（与相位有关）光散射之间的基本二分法为生物结构色产生机制的分类提供了最佳标准。非相干散射包括瑞利散射、丁达尔散射和米氏散射。相干散射包括干涉、增强、薄膜反射和衍射。相干散射的纳米结构主要有三类——层状、类晶体和准有序。层状和类晶体纳米结构通常产生虹彩，而在准有序纳米结构中则不存在或不太明显。层状和类晶体结构已通过薄膜光学和布拉格定律的方法进行了分析，但对于准有序结构的颜色产生分析还没有传统的方法。我们开发了一种工具，使用二维（2D）电子显微镜（TEM）透射电子显微镜的傅里叶分析（可从作者处获得）。该傅里叶工具可以分析光散射体的空间变化，从而分析光散射是否具有空间独立性，并检验光散射是否可以主要归类为非相干或相干散射。该工具还提供了生物纳米结构背散射反射光谱的相干散射预测。我们对傅里叶工具的应用否定了一个世纪以来的假设，即鸟类羽毛和皮肤的非虹彩结构色是由非相干瑞利或丁达尔散射产生的。对鸟类羽毛和皮肤中的这些准有序结构进行 2D 傅里叶分析表明，这些非虹彩颜色是由相干散射产生的。以前没有其他关于生物结构色产生的非相干散射的例子通过散射体空间独立性分析或分光光度法进行过严格的测试。傅里叶工具首次应用于虹彩鸟类羽毛羽小枝（Nectarinia）的层状阵列的相干散射，以证明该技术在薄膜上的有效性。与以前的物理方法不同，傅里叶工具为分析各种纳米结构类型的相干散射提供了一种单一的方法。这一进展将有助于研究纳米结构类型之间的演变以及纳米结构本身的演变。文章最后讨论了光子学在生物结构色研究中的新兴作用，以及该工具未来的发展方向。
