Comparison of Reflectance Measurements Acquired with a Contact Probe and an Integration Sphere: Implications for the Spectral Properties of Vegetation at a Leaf Level

Abstract

Laboratory spectroscopy in visible and infrared regions is an important tool for studies dealing with plant ecophysiology and early recognition of plant stress due to changing environmental conditions. Leaf optical properties are typically acquired with a spectroradiometer coupled with an integration sphere (IS) in a laboratory or with a contact probe (CP), which has the advantage of operating flexibility and the provision of repetitive in-situ reflectance measurements. Experiments comparing reflectance spectra measured with different devices and device settings are rarely reported in literature. Thus, in our study we focused on a comparison of spectra collected with two ISs on identical samples ranging from a Spectralon and coloured papers as reference standards to vegetation samples with broadleaved (Nicotiana Rustica L.) and coniferous (Picea abies L. Karst.) leaf types. First, statistical measures such as mean absolute difference, median of differences, standard deviation and paired-sample t-test were applied in order to evaluate differences between collected reflectance values. The possibility of linear transformation between spectra was also tested. Moreover, correlation between normalised differential indexes (NDI) derived for each device and all combinations of wavelengths between 450 nm and 1800 nm were assessed. Finally, relationships between laboratory measured leaf compounds (total chlorophyll, carotenoids and water content), NDI and selected spectral indices often used in remote sensing were studied. The results showed differences between spectra acquired with different devices. While differences were negligible in the case of the Spectralon and they were possible to be modelled with a linear transformation in the case of coloured papers, the spectra collected with the CP and the ISs differed significantly in the case of vegetation samples. Regarding the spectral indices calculated from the reflectance data collected with the three devices, their mean values were in the range of the corresponding standard deviations in the case of broadleaved leaf type. Larger differences in optical leaf properties of spruce needles collected with the CP and ISs are implicated from the different measurement procedure due to needle-like leaf where shoots with spatially oriented needles were measured with the CP and individual needles with the IS. The study shows that a direct comparison between the spectra collected with two devices is not advisable as spectrally dependent offsets may likely exist. We propose that the future studies shall focus on standardisation of measurement procedures so that open access spectral libraries could serve as a reliable input for modelling of optical properties on a leaf level.

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

“…Reflectance contributions originating from areas outside the leaf and multiple scattering from background (i.e., transmitted through the sample, reflected by background and transmitted again) explain only a small portion of the difference (see Section 2.2.3). Previous comparison between ASD contact probe and integrating spheres observed slightly smaller mean absolute differences than we did, indicating that factors such as the measurement geometry and design of the probe may influence the comparison [18]. Because leaf spectral BRDFs may vary between species [30], a contact probe is probably not the best choice for comparing spectral differences between species.…”

Aspects of Invasiveness of Ludwigia and Nelumbo in Shallow Temperate Fluvial Lakes

“…During measurements, macrophyte leaves were placed over a flat neoprene plate (absorbance > 95%) in order to minimize background reflection of light transmitted through the leaves. This approach was chosen because it is easier and faster under challenging field conditions (e.g., boat-based surveys) compared with using an integrating sphere to fully determine reflectance and transmittance, and only introduces minimal distortion in leaf reflectance measurements (Sims and Gamon, 2002; Potučková et al, 2016).…”

Feasibility of hyperspectral vegetation indices for the detection of chlorophyll concentration in three high Arctic plants: Salix polaris, Bistorta vivipara, and Dryas octopetala

“…The field campaign was carried out from August 4th to 7th in 2015 [35]. The following devices were used during the field research: (i) ASD FieldSpec 3 spectrometer connected with a fiber optic contact probe (ASD PlantProbe, which records the reflected electromagnetic radiation in the 350-2,500 nm range from the built-in lamp, providing stable conditions for all tested plants [46]) and (ii) Force-A Dualex Scientific sensor, which allows us to perform real-time and nondestructive measurements of chlorophyll (Chl), anthocyanins (Anth), flavonoids (Flav), and nitrogen (NBI) indices [47]. The chlorophyll index is highly correlated with the chlorophyll extracts measured in laboratory conditions (R 2 oscillated around 0.88-0.96 in hundreds of samples, and errors were not higher than 16%).…”