Reflectance of vegetation using 2 channel spectrometer system – Oceanview schematic
Reflectance of sunlight from the ground is critical to understanding remote sensing, hyperspectral imagery, and is the fundamental measurement used in precision agriculture, drought detection, monitoring invasive species and a host of new airborne and land AUV based applications.
The measurement of reflection in a lab setting is straightforward — a stable light source is used to illuminate a white reflection standard (REF), and then the sample (SAM). The ratio of the reflected light from the sample to that of the white standard is the Reflectance (%R).
%R = SAM/REF
In the field, the light source is the sun. The sunlight is not stable over time, and on partly cloudy days it can change second by second. One option is to use 2 spectrometers. One spectrometer is pointing up, and measures the downwelling light (REF). The other points down towards the sample and measures upwelling reflected light (SAM). The basic concept is similar to the lab experiment, except that the signals received by the two systems can be different magnitudes. The downwelling sensor is usually a cosine corrector, which measures the normal component of light all directions in the hemisphere. The upwelling sensor usually has a smaller field of view, depending on how large an area you want to sample.
The setup below shows the basics of this system. The pair of STS-VIS spectrometers are mounted for upwelling and downwelling. A white reflection standard is sitting below the upwelling sensor. Both are exposed to sunlight. The same devices can be easily mounted on a drone, or tower for looking at crops, forest land,bio-fuel ponds etc.
To get reliable %R data, the two spectrometer systems must be “balanced” or normalized. That is done by recording a spectra from the reflected light from the white standard, and the sunlight hitting the cosine corrector at the same time. These are set as working reference spectra, and all subsequent spectra expressed as a ratio to the stored value.
Oceanview software allows the user to create data analysis schemes using a graphical interface. Nodes (boxes) are added, connections indicating the flow of data are added as arrows. The boxes can be opened to set parameters or see the status of the data at that point.
The schematic below was created for this two channel balanced radiometer problem.
Data starts as raw counts from each STS spectrometer. The next green box is where integration period, signal averaging and boxcar smoothing are set.
In each leg, there is a box to capture and store a dark, which is then subtracted from each realtime spectra.
The next step is critical to combine data from two different spectrometers. The interpolate node recalculates the data array from the top STS into the wavelengths used by the bottom STS.
There is a node to capture and store a reference spectra. This is the spectra looking at the white standard or sunlight at the same moment in time
The spectra are now normalized on the same scale, and dividing one by the other yields the reflectance. The ratio is multiplied by 100, given units of % and presented as a graph in the view node.
Command buttons for storing darks and references appear above the graph view, making it easy to update. The project can be saved and reloaded at a later date.
STS spectrometers are ideal for radiometry outdoors. These units have a large dynamic range, essential for working in dim to bright sunlight. The STS-VIS units can be supplemented with STS-NIR units to give full coverage of the VIS-NIR range,.