Evaluation of Accuracy and Practical Applicability of Methods for Measuring Leaf Reflectance and Transmittance Spectra

Abstract

Leaf reflectance and transmittance spectra are urgently needed in interpretation of remote sensing data and modeling energy budgets of vegetation. The measurement methods should be fast to operate and preferably portable to enable quick collection of spectral databases and in situ measurements. At the same time, the collected spectra must be comparable across measurement campaigns. We compared three different methods for acquiring leaf reflectance and transmittance spectra. These were a single integrating sphere (ASD RTS-3ZC), a small double integrating sphere (Ocean Optics SpectroClip-TR), and a leaf clip (PP Systems UNI501 Mini Leaf Clip). With all methods, an ASD FieldSpec 4 spectrometer was used to measure white paper and tree leaves. Single and double integrating spheres showed comparable within-method variability in the measurements. Variability with leaf clip was slightly higher. The systematic difference in mean reflectance spectra between single and double integrating spheres was only minor (average relative difference of 1%), whereas a large difference (14%) was observed in transmittance. Reflectance measured with leaf clip was on average 14% higher compared to single integrating sphere. The differences between methods influenced also spectral vegetation indices calculated from the spectra, particularly those that were designed to track small changes in spectra. Measurements with double integrating sphere were four, and with leaf clip six times as fast as with single integrating sphere, if slightly reduced signal level (integration time reduced from optimum) was allowed for the double integrating sphere. Thus, these methods are fast alternatives to a conventional single integrating sphere. However, because the differences between methods depended on the measured target and wavelength, care must be taken when comparing the leaf spectra acquired with different methods.

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

“…In case of leaf-like samples, exactly the same samples were measured with all methods, which eliminated the effects of between-sample variability. The differences between DIS and SIS were similar as observed by Hovi et al (2018), and the overall differences between methods (up to 28%) exceeded the expected measurement uncertainty of SIS ref (2%). In case of needle-like samples and needles, where the same physical samples could not be measured and between-sample variability can have a larger role, the samples were taken from the same population (see Section 2.2.2 for sampling strategy and precautions that minimized the effects of needle aging), and statistical tests were performed to confirm the betweenmethod differences.…”

“…Interesting differences were identified by the comparison of the DIS and SIS setups against the reference method. As shown already by Hovi et al (2018), DIS underestimates transmittance compared to SIS. Comparison to the reference method revealed which of the methods provides results closer to "truth", and thus helped to evaluate potential causes for the differences.…”

“…Our primary measurement setups were SpectroClip-TR double integrating sphere (DIS) and ASD RTS-3ZC single integrating sphere (SIS). Descriptions of the setups were given in Hovi et al (2018), and a more detailed description of DIS was given in Mõttus et al (2017). Technical details of the setups are also explained in the Supplementary file S1, available at https://doi.org/10.14214/ sf.10270.…”

“…These materials are hereafter referred to as "paper" and "silicon". The paper was the same as measured by Hovi et al (2018). The silicon was selected for comparison, because it had high T and low R, whereas the paper had low T and high R. The measurements of spruce needles were made on two days (5th-6th December 2018).…”

Seasonal dynamics of lingonberry and blueberry spectra

“…Lambertian) and lossless reference surface, both illuminated by a single directional light source (Nicodemus et al 1977;Schaepman-Strub et al 2006). To obtain spectral reflectance from SpectroClip-TR measurements, we applied a measurement protocol from previous work by Hovi et al (2018), and further the theoretical algorithms developed by Mõttus et al (2017). The leaf reflectance (and transmittance, not shown) was calculated from several individual spectral readings of white reference, dark current, stray light, empty sphere reflectance and transmittance, and leaf response to incident light.…”