Evaluating the performance of a double integrating sphere in measurement of reflectance, transmittance, and albedo of coniferous needles

Abstract

Highlights• Adaptation of a compact double integrating sphere for spectral measurements of coniferous needles. • Double integrating sphere is fast to operate and suitable for monitoring purposes and collection of large spectral databases. • Measured spectra showed negative bias, which could potentially be reduced by building an optimized measurement setup. AbstractLeaf reflectance and transmittance spectra are essential information in many applications such as developing remote sensing methods, computing shortwave energy balance (albedo) of forest canopies, and monitoring health or stress of trees. Measurement of coniferous needle spectra has usually been carried out with single integrating spheres, which has involved a lot of tedious manual work. A small double integrating sphere would make the measurements considerably faster, because of its ease of operation and small sample sizes required. Here we applied a compact double integrating sphere setup, used previously for measurement of broad leaves, for measurement of coniferous needles. Test measurements with the double integrating sphere showed relative underestimation of needle albedo by 5-39% compared to a well-established single integrating sphere setup. A small part of the bias can be explained by the bias of the single sphere. Yet the observed bias is quite significant if absolute accuracy of measurements is required. For relative measurements, e.g. for monitoring development of needle spectra over time, the double sphere system provides notable improvement. Furthermore, it might be possible to reduce the bias by building an optimized measurement setup that minimizes absorption losses in the sample port.Our study indicates that double spheres, after some technical improvement, may provide a new and fast way to collect extensive spectral libraries of tree species.

A dataset composed of multiangular spectral libraries and auxiliary data at tree, leaf, needle, and bark level for three common European tree species

“…Special sample holders (‘needle carriers’, see Fig. 1 in [5] ) were used to attach the needle samples to the integrating sphere. The needle carriers were used also when measuring leaves, to ensure a comparable measurement.…”

Empirical validation of photon recollision probability in single crowns of tree seedlings

“…We used spectrally black needle carriers (see e.g. Hovi et al, 2020 ) of 0.8 mm thickness and placed needles at 0.5–1 needle widths apart. Also the leaves of broadleaved trees were placed in the needle carriers to ensure a comparable measurement.…”

“…For pine, measurements close to the center of needles were made, and for oak a randomly selected spot on the leaf was measured so that major veins in the leaf were avoided. The measurement protocol was the same as used by Hovi et al (2020) . An uncalibrated Spectralon® panel was used as white reference and was calibrated afterwards against a known standard.…”

“…For oak leaves, gap fractions were zero. Gap fraction in a needle sample was obtained by scanning the needle carrier with needles in it, using a digital film scanner (Epson Perfection V550), and by applying a threshold to the obtained 8-bit grayscale images to separate needles from the background ( Hovi et al, 2020 ). For needles of pine and spruce, the threshold value (202 for pine, 187 for spruce) was optimized by forcing the needle transmittance to predetermined values at 410–420 nm.…”

“…In that campaign, the gap fractions in the needle samples were obtained by painting the illuminated side of needles in black, which prevented specular reflection from the illuminated side to contribute to transmittance and thus the observed transmittance was purely caused by the light transmitted through the gaps in the needle sample ( Daughtry et al, 1989 ). Finally, we applied an empirical bias correction to all processed leaf and needle transmittance spectra (adjusting transmittance 5.5% downwards) that was taken from the measurements made against a trusted reference method in Hovi et al (2020) . This ensured that leaf and needle albedo did not exceed unity in any of the measurements.…”