Non-invasive measurement of frog skin reflectivity in high spatial resolution using a dual hyperspectral approach.

Francisco Pinto,Michael Mielewczik,Frank Liebisch,Uwe Rascher,Achim Walter,Hartmut Greven

doi:10.1371/journal.pone.0073234

Abstract

BackgroundMost spectral data for the amphibian integument are limited to the visible spectrum of light and have been collected using point measurements with low spatial resolution. In the present study a dual camera setup consisting of two push broom hyperspectral imaging systems was employed, which produces reflectance images between 400 and 2500 nm with high spectral and spatial resolution and a high dynamic range.Methodology/Principal FindingsWe briefly introduce the system and document the high efficiency of this technique analyzing exemplarily the spectral reflectivity of the integument of three arboreal anuran species (Litoria caerulea, Agalychnis callidryas and Hyla arborea), all of which appear green to the human eye. The imaging setup generates a high number of spectral bands within seconds and allows non-invasive characterization of spectral characteristics with relatively high working distance. Despite the comparatively uniform coloration, spectral reflectivity between 700 and 1100 nm differed markedly among the species. In contrast to H. arborea, L. caerulea and A. callidryas showed reflection in this range. For all three species, reflectivity above 1100 nm is primarily defined by water absorption. Furthermore, the high resolution allowed examining even small structures such as fingers and toes, which in A. callidryas showed an increased reflectivity in the near infrared part of the spectrum.Conclusion/SignificanceHyperspectral imaging was found to be a very useful alternative technique combining the spectral resolution of spectrometric measurements with a higher spatial resolution. In addition, we used Digital Infrared/Red-Edge Photography as new simple method to roughly determine the near infrared reflectivity of frog specimens in field, where hyperspectral imaging is typically difficult.

Highlights

Coloration and color patterns of the integument of vertebrates vary considerably
It was suggested that increased reflectivity in the near infrared range (NIR) from 700–1400 nm may provide concealment from hypothetical predators or prey with extended vision in this range in environments of near high infrared reflectivity [10,11,12,14,20] as it is provided by green leaves [21]
In accordance with its coloration the dorsal skin of Hyla arborea showed a maximum peak in its visible range (VIS) reflection spectrum with no marked increase in the NIR region (Fig. 5e)

Summary

Introduction

Coloration and color patterns of the integument of vertebrates vary considerably. Generally, the spectral properties of an integument are mainly defined by absorption and (diffuse) reflection occurring in its different layers, i.e. epidermis, dermis and subcutis, where differences in the pigmentation and in the structural organization of reflecting platelet layers lead to variations in the spectral reflectance [1,2,3,4]. The appearance of species in the visible range (VIS) from 400 to 700 nm is determined by the spectral reflectance characteristics of the integument and pigments, defining colors and patterns [Ibid]. This visible range is referred to human perception. In any case the near-infrared absorption contributes to heat load and may influence thermoregulation [12,13,14,15,16,17,18,19]. In the present study a dual camera setup consisting of two push broom hyperspectral imaging systems was employed, which produces reflectance images between 400 and 2500 nm with high spectral and spatial resolution and a high dynamic range

Methods

Results

Discussion

Conclusion