Normalized Difference Vegetation Index (NDVI)
The measurement of NDVI is important to understand the hydration and health status of plants by measuring the greenness using Red and Near Infrared (NIR) light reflectance.
Plants that are actively photosynthesising have a high visible red light absorption and an increased near infrared reflection. The plants appear green, and the cells are turgid! As plants become stressed, Red light reflection increases as the photosynthetic system becomes less efficient and a decreased near Infrared reflection as the cells become less turgid due to water stress. This can be seen in the image with the two trees, where the left is unstressed and the right tree is stressed.
Apogee NDVI Sensors measure the incoming radiation of visible Red and near Infrared light, and the reflectance of visible Red and near Infrared light from the crop canopy or vegetation surface. Â The NDVI value is calculated using the following formula:
NDVI= (NIR-RED)/(NIR+RED)
Frequently measured by remote sensing methods, such as satellite or aerial (drone) imaging, NDVI can be measured at the ground level by sensors to ground truth the remotely sensed data.
This ground truthing of data allows for robust and reliable measurement, and when combined with other ICT International sensors, a holistic approach to the growing conditions. This combined approach can be seen in the Pasture Management case study here.
To calculate NDVI, a pair of sensors are used to measure incoming Near Infra Red (NIR) and Visible Red (RED). These occur in different portions of the electromagnetic spectrum or wavelength.
The Apogee NDVI sensors measure NIR in the 810 nm ± 5 nm spectral band (with 65 full-width half-maximum) and Red in the 650 nm ± 5 nm spectral band (with 65 full-width half-maximum).
From these measurments, NDVI is calculated by subtracting the red channel from the near-infrared (NIR) channel and dividing their difference by the sum of the two channels: NDVI=(NIR-RED)/(NIR+RED).
When installing any field of view sensor the following must be known:
- Field of view angle: this is a fixed value included in the specification of the sensor; for example, the downward looking sensor in the Apogee NDVI pairing has a 35° field of view
- Any areas that are to be avoided, for example the footing at the base of the post that the sensor is being mounted to
- The intended angle to mount the sensor
- The intended height to mount the sensor
With those known, and some trigonometry or the ICT International R script for Field of View sensor installation, the target area can be calculated.
