A Multiband Polarimetric Imager for Field Crop Survey: ―Instrumentation and Preliminary Observations of Heading-stage Wheat Canopies―

Abstract

A spectral and polarization image observation technique for detecting multiband polarimetric characteristics of reflected light from field-growing plants under daylight conditions was developed and the potential application of the method to in-situ assessments of wheat-leaf orientation at the heading stage was assessed. The developed digital imaging system corresponded to wavelength bands centered at 470, 550, and 647 nm, each with bandwidths of 10 nm. The instrument was fitted with a glass polarizer, which rotated from 0 to 360º in 15º steps, and polarized images of 1360 ×1024 pixels were captured at heading in wheat plots subjected to different fertilizer regimes at the jointing stage. Degree of polarization (DP) and mean brightness (MB) of the three bands were calculated from images of several pairs of top-dressed and non-top-dressed (non-dressed) plots, with a camera depression angle of 15–20º on two clear days. The relative azimuth angles between the view and insolation were approximately 135º (oblique front) and 180º (right in front), respectively. The mean DP for each plot area in the images varied between 0.3 and 1.4%. Although most of the top-dressed plots had significantly higher DPs than the non-dressed plots in the 550 nm band, few of the MB images in any band showed a clear difference between the top-dressed and non-dressed plots.