GoSPo: a goniospectropolarimeter to assess reflectance, transmittance, and surface polarization as related to leaf optical properties

Abstract

Visible-near infrared (VIS-NIR) spectral data are widely used for remotely estimating a number of crop health metrics. In general, these indices and models do not explicitly account for leaf surface characteristics, which themselves can be indicators of plant status or environmental responses. To explicitly include leaf surface characteristics, data are required linking optical properties to surface characteristics. We present the design and experimental validation of a goniospectropolarimeter (GoSPo) that combines the capabilities of a spectrometer, goniometer, and polarimeter. GoSPo was designed with the objective of studying the relationships between leaf surface characteristics and the resulting light reflectance, transmission, and polarization as functions of both direction and VIS-NIR spectra. Using six motors, a pneumatic system, two spectrometers, and a combination of lenses, polarizers, and mirrors, GoSPo can examine a leaf from a particular angle, approximate hemispherical transmittance and reflectance (with root-mean-square error values of 0.0189 and 0.0216 for reflectance and transmittance, respectively, compared to a spectrophotometer and integrating sphere), and obtain spectral polarization measurements without disrupting the sample between measurements. The data collected with GoSPo will aid in model development for remote sensing applications. © The Authors.Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.

Characterization of leaf surface phenotypes based on light interaction

“…To ensure the same location on each leaf was analyzed microscopically and spectrally, the leaves were placed in a sample holder that centered the area of interest to be examined for each measurement as shown in Fig. 1 [31]. In total, 349 leaves and associated measurements were included in the dataset, representing 59 different species.…”

“…Light that does not scatter specularly from the surface is scattered diffusely within the leaf (as reflectance or transmittance) and is depolarized. Spectral measurements were obtained between 400 and 1700 nm using the GoSPo goniospectropolarimeter [31] with illumination and sensor at Brewster's Angle (approximately 55° from nadir using a refractive index of 1.45). A wire grid polarizing filter was placed between the sample and sensor (Edmund Optics, Barrington, NJ, USA).…”