Drought-stressed detection of coffee plant using a combination of an lctf camera with a polarizer filter

One of the challenges of the space imaging technology is to obtain an accurate spectral signature of an object from an off-nadir angle. The greater the angle from the normal angle, the higher the challenge, especially with the mirror angle between the light source and the sensor. In this particular case, polarization becomes an important variable to be considered, as shown in several studies.

However, polarization on vegetation is still being an underestimated study, looking at only a handful of scientific record has been done. None of these studies have a complete detail observation of a single leaf, and choose to only focus on canopy level of the tree or grass. There is a big chance that failure to understand phenomena on a single leaf level may lead to misunderstood the phenomena on a tree-level or higher. That is why there is an urge to do this experiment, to mend this small gap.

This thesis discusses an empirical study of the polarized-light phenomena on a single leaf in a dark room laboratory using a halogen bulb as the light source and a Liquid Crystal Tunable Filter (LCTF) camera as the sensor. The camera was combined with a linear polarizer filter attached on the front of its lens and was rotated to obtain the degree of polarization (DoP) of the target. The instrument was capable of capturing the spectrum from 460nm to 780nm with the step of 1nm. This camera was chosen because with it, we can capture the detail of the spectral signature of the target, so that we can get more accurate indexes analyses and other spectra related calculations. Moreover, because LCTF camera has an area of the field of view (FOV), we can choose every pixel we want to analyze. This is a novel combination of an LCTF hyperspectral camera
with a polarizer.

The experiment was focused capturing the polarization from the specular reflectance of the surface of the leaf, thus we set the angle between the light source and the sensor always in vertically mirror angle. A type of Coffea Arabica was observed in a dark-room laboratory.

The result shows three distinctive phenomena.
1. The specular noise (sun-glint) on the reflectance of the leaves was having the characteristic of the light source, with the scale linear to the incident angle.
2. The portion of the polarized and non-polarized radiance was successfully separated.
3. A drought-stress plant could be distinguished by the characteristic of the polarization on its leaves. The difference on the DoP was recorded in the wavelength shorter than 520nm, and incident angle higher than 600.

Polarized-light is a key component of the remote sensing probe to understand the cuticular condition of broad-leaf plants, and in a wider range, an evergreen rainforest canopy. The continuation of this project could lead to finding a new method of exploring polarization to understand the complexity of nature around us.