Reflectance is the fraction of the incident radiation that is reflected by a surface:
Reflectance of a surface varies as a function of wavelength and the reflectance curve of a material is descriptive of its structure and constitution. Such curves are also called spectral signatures and can be used for the recognition and the study of the attributes of multiple elements. The reflectance curves of vegetation depend on the biochemical and biophysical properties of the leaves and the structure of the canopy (Figure 1). In the visible part of the spectrum (VIS: 380 – 720 nm) reflectance is low due to the absorption of leaf pigments. Red and blue radiation in strongly absorbed, mainly by chlorophylls and carotenoids, while green radiation is less absorbed. The reflectance in the Near-Infrared (NIR: 720 - 1300 nm) is higher, contrasting to VIS reflectance, due to intense scattering in the leaf tissues and the lack of absorption by the pigments. The intense spectral difference between the red and the NIR part of the spectrum is a vegetation attribute that is not met in any other natural surface or artificial structure. Thus, is very valuable for the discrimination and recognition of healthy vegetation.
Vegetation reflectance can be used to quantify various biochemical and biophysical attributes without interference with the plant sample. It is therefore a quick, convenient and non-destructive method to study plant physiology. It is also a way to scale-up from leaf to canopy measurements and acquire spatially and temporally extended datasets using remote sensing instruments.
Reflectance can be measured in the field using portable spectroradiometers. The measurements can be focused on leaf scale, using appropriate leaf clips, internal lamps and diffuse reflectance standards (Photo 1). They can also be performed on canopy scale, where suitable structures must be used, depending on the canopy height (Photos 2 and 3).
Photo 2. Calibration of the spectroradiometer with a reflectance standrard for canopy measurements.
Figure 1. Reflectance spectra for 4 forest species.
Photo 1. Measuring leaf reflectance with a leaf clip.
Photo 3. Measuring canopy reflectance in a Pinus nigra forest.