Color that is not produced by pigment is called structural color. An object is iridescent if its color changes with viewing angle. Structural color and iridescence are ubiquitous. Soap bubbles, compact disks, hair, skin, iris, jewel beetles, pigeons, peacocks, and butterflies all display structural color and iridescence [DCS95] [Par00] [VSLW01] [VS03] [LCRB03] [EC04] [KY05] [LB06] [WBK*07] [KYM08]. These phenomena arise from the interaction of light with micro- or nano-level features.
The male Morpho butterfly displays a particularly brilliant iridescent blue as shown in Fig. 1. This beautiful Morpho blue arises from optical interference in subwavelength structures of nano-meter size on the butterfly scales. A remarkable characteristic due to the sophisticated nano-structure is backscattering which is backward reflection to the light source.
Fig. 1. Photograph and animation of a male Morpho rhetenor and helena, respectively.
It is difficult to reproduce structural colors using geometric optics, because it is invalid for nano-structures. It is necessary to take optical interference effects into account.
The nonstandard finite-difference time-domain (NS-FDTD) algorithm is well suited for this purpose [Col02]. The NS-FDTD algorithm is optimized to monochromatic waves and is accurate for calculating complicated nano-structures [OC10].
Related works
The scale structure of the male Morpho butterfly has been studied intensively, and several groups have attempted to render its structural color [SIK04] [Sun06] [SJ08]. But they fail to reproduce both the realistic brilliant blue and its strong backscattering, because the diffraction superposition model [KYFO02] that they used cannot capture the optical interference effects in the nano-structure of the butterfly's scales.
Although nano-optical effects have not been completely ignored in conventional CG renderings, the structures are spheres, thin films, multilayer films, and regular diffraction gratings, which can be theoretically analyzed. For example, there have been renderings of the atmosphere [JW97], clouds [HL01], soap bubbles [IMN04], iridescent colors [SK05], and compact discs [EC06]; other examples are given in [Sta99] [HKYM00] [LA06] [OKG*10]. Since the subwavelength structures of the Morpho butterfly are much more complicated [GTE01] [SIK04] [Sun06] [SJ08], its blue color cannot be mimicked by simple structural assumptions.
In recent years various artificial subwavelength structures have been fabricated and have become an important topic. The superlens based on surface plasmon resonances [Pen00] [MB05] [Bru09] was an experimental realization of negative refractive index, and overcame the classical optics limitation that the resolving power of a lens is fundamentally constrained by the wavelength of the illumination. The surface plasmon is an electronic subwavelength oscillation on the surface of metals and radiates subwavelength evanescent waves. We can now manipulate light with subwavelength structures, and called "metamaterials" [SS09] [CSL09]. For example, colloidal gold particles in stained-glass constitute a metamaterial and its color arises from the surface plasmon resonances [HD04]. Rendering the optical properties of metamaterials is also difficult using purely analytical approaches, and a simulation coupled with CG rendering may be a fruitful approach.
The nano-optical phenomena of the Morpho butterfly are difficult to understand analytically, and optical measurements are also difficult because the widely-used detector rotation methods cannot capture the backscattering [KZK11]. But the nano-level interaction can be calculated using numerical simulations. Simulating the Morpho butterfly using finite-difference time-domain (FDTD) optical simulations has been attempted in [BD07] [ZKCC09] [LS09], but their computed reflectance spectra are different from experimental measurements [KZK11] due to an oversimplified computation model. Thus, the rendered image [LS09] fails to capture the real iridescent blue and backscattering. The nano-optical rendering of complicated subwavelength structures is a new tool for CG that can produce much more realistic iridescent images. We present a Morpho butterfly rendering based on both a rigorous physical model and a FDTD simulation as a first step towards the subwavelength optical CG imaging.
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