A generalized approach to characterize optical properties of natural objects

Abstract

To understand the diversity of ways in which natural materials interact with light, it is important to consider how their reflectance changes with the angle of illumination or viewing and to consider wavelengths beyond the visible. Efforts to characterize these optical properties, however, have been hampered by heterogeneity in measurement techniques, parameters and terminology. Here, we propose a standardized set of measurements, parameters and terminology to describe the optical properties of natural objects based on spectrometry, including angle-dependent effects, such as iridescence and specularity. We select a set of existing measurements and parameters that are generalizable to any wavelength range and spectral shape, and we highlight which subsets of measures are relevant to different biological questions. As a case study, we have applied these measures to 30 species of Christmas beetles, in which we observed previously unrealized diversity in visible and near-infrared reflectance. As expected, reflection of short wavelengths was associated with high spectral purity and angle dependence. In contrast to simple, artificial structures, iridescence and specularity were not strongly correlated, highlighting the complexity and modularity of natural materials. Species did not cluster according to spectral parameters or genus, suggesting high lability of optical properties. The proposed standardization of measures and parameters will improve our understanding of biological adaptations for manipulating light by facilitating the systematic comparison of complex optical properties, such as glossy or metallic appearances and visible or near-infrared iridescence.

Wide-gamut structural colours on oakblue butterflies by naturally tuned photonic nanoarchitectures

“…The angle-dependence of the wings was explored using spectrogoniometry. As the reflectance of the wings of Arhopala specimens is highly dependent on the illumination and detection angle, three types of measurements (electronic supplementary material, figure S3) were performed [ 13 , 32 , 33 ]: back-scattered with rotating sample (electronic supplementary material, figure S4), forward-scattered with rotating detector (electronic supplementary material, figure S5) and specular under various incidence/detection angles (electronic supplementary material, figure S6). In figure 5 a–c , the results of the three measurement configurations are depicted, respectively, for A. araxes as an example, while in figure 5 d, the peak shift results of the specular reflectance measurement are summarized for the five investigated species.…”

The carotenoid redshift: Physical basis and implications for visual signaling

“…A second metric of perceived hue also showed substantial hue shifts in carotenoids, but little in chlorophyll, as concentration increases (white circles on spectra in Figure 2a,c,e). The wavelength at half‐maximum reflectance is a traditional metric of perceived hue for carotenoids (Montgomerie, 2006), a measure of spectral location in sigmoidal reflectance curves (Andersson et al, 2002; Ospina‐Rozo et al, 2022) that is comparable to the wavelength at maximum reflectance for Gaussian spectra (Maia et al, 2019) such as chlorophyll (Figure 2a,c,e).…”

Exposure to thermal extremes favors higher solar reflectivity in intertidal gastropods

“…The light source and collector probes were set at 10° and −10° to the normal of the sample, and all measurements were relative to a Spectralon 99% white reflectance standard (LabSphere, NH, USA). Whilst an integrating sphere would provide a more accurate measurement of reflection across all angles, 55 most of the shells were too small and curved for the sampling area of an integrating sphere (commonly 4mm or greater). Our set-up allows measurement of a small area (∼1 mm) but assumes that shell surfaces are diffuse.…”

“…Reflectivity is the ratio of incident light to reflected light, integrated over the wavelength range of solar radiation (300–2600 nm). 55 Specifically, reflectivity (R) is calculated using Equation 1 , 14 where is the reflectance of the shell and is the solar irradiance across wavelengths from to . We calculated reflectivity from 300–1700 nm, which accounts for roughly 98.9% of incident solar radiation.…”