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.

Highlights

  • Information on leaf optical properties is utilized, for example, in interpretation of data from remote sensing instruments, as well as in modeling energy budgets of vegetation canopies

  • Within-Method Variability in the Measurements per leaf), and double integrating sphere (DIS) the slowest (15.0 min), when using the integration times determined in the Previous studies mainly these focused on evaluating theWith accuracy terms ofitsystematic errors optimization

  • In addition to showing between-method differences, we demonstrated that the spectral indices were highly dependent on whether they were calculated from leaf reflectance or transmittance spectra, or from adaxial or abaxial side of the leaf

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Summary

Introduction

Information on leaf optical properties is utilized, for example, in interpretation of data from remote sensing instruments, as well as in modeling energy budgets of vegetation canopies. It is essential that the collected data are comparable across measurement campaigns Another important requirement is that the applied measurement methods should be easy. Measurements of leaf reflectance and transmittance spectra are traditionally obtained with integrating spheres [1,2,3,4,5]. Integrating spheres are relatively large and heavy, and not very easy to transport into remote locations for field measurements. Portable systems such as contact probes [7,8,9,10], can be applied to measure spectra in situ so that storage of the leaf samples can be avoided.

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