NDRE vs NDVI: What’s the difference?
At Aerobotics, we’re able to detect stress in orchards at an early stage by using different wavelengths on our cameras that are more sensitive to chlorophyll in the leaf than our eyes.
Since tree health is visible based on chlorophyll levels or leaf color, and the human eye is limited to seeing several shades, in order to generate in-depth data we need a more advanced “eye” to monitor farms.
How the human eye processes color
Light or radiation from the sun travels in different wavelengths (see illustration below). These wavelengths behave differently depending on the surface they hit.
The human eye is only able to respond to a very small portion of the electromagnetic spectrum, known as the “visible region”. The visible region ranges from 400 nm (nanometers) to 700 nm and is comprised of Red, Green, and Blue light, otherwise known as “RGB”.
Why leaves appear green to the human eye
We see tree crop’s foliage as green because the chlorophyll pigment in the leaves absorbs a certain percentage of visual blue light, a certain percentage of visual red light, and reflects a certain percentage of green light back to our eyes.
Different cultivars, varieties, and crop types will exhibit different leaf colors. Leaf color varies depending on the chlorophyll pigment’s percentage of reflectance and absorbance of visible red, green and blue light.
So, every color that the human eye perceives is comprised of RGB (visual light) and it only depends on the percentage of red, green, and blue light that is absorbed/reflected by the object. Therefore, in theory, a black object will absorb 100% red, green, and blue light, and a white object will, therefore, reflect 100% red, green, and blue light.
An illustration of the electromagnetic spectrum and the different wavelengths of each.
Beyond the human eye with NDRE and NDVI
At Aerobotics we make use of the Micasense RedEdge-M sensors on our drones. These multispectral sensors function as “eyes” on the drone and can pick up light that our human eyes cannot respond to.
We’re able to detect stress in orchards at an early stage in orchards by using wavelengths on our cameras that are more sensitive to chlorophyll content than the human eye. These wavelengths we then use to calculate vegetation indices and present them to our growers in a way that is visually more attractive and easier to understand.
The cameras that we attach to drones are able to respond to:
- Red, Green and Blue (RGB) light (400nm — 700nm)
- Near-Infrared light (NIR, ±800nm)
- Red Edge light (±720nm)
Health index (NDRE)
The Health Index on Aeroview gives an estimation of chlorophyll content in leaves and highlights problem trees, ranging from nutrient deficiencies to pest and disease damage. The leaf chlorophyll content is essential in the conversion of light (radiation) to energy via the electron transport chain in the photosynthesis process.
Trees that have a lower Health index value (NDRE), will have a lower concentration of this pigment in their leaves and will have lower photosynthesis capabilities. Lower photosynthesis capabilities will cause a decrease in sugar production and also fewer sugars converted to starch and cellulose. Plant parts/processes will have fewer building blocks allocated to them and will start to appear stressed after a while.
The NDRE index is calculated by using the reflectances of near-infrared and RedEdge light to the sensor and plugging it into the Normalized Difference Red Edge index (NDRE) equation.
NDRE =(NIR — RedEdge)/(NIR + RedEdge)
The Health index not only estimates the chlorophyll content of the outermost layer of the canopy but is able to penetrate the tree canopy deeper than NDVI.
The RedEdge light of the electromagnetic spectrum isn’t as strongly absorbed by chlorophyll pigments as Visual Red light in the leaf, therefore the light penetrates deeper into the tree canopy.
The per tree NDVI index is an estimation of the greenness and density of biomass of the tree. Because the NDVI uses visual red light as one of the bands in the calculation, the light can’t penetrate very deep into the canopy.
The reason for this is that visual Red light is very strongly absorbed by chlorophyll pigment and NIR is strongly reflected by healthy mesophyll cells in the leaves. Thus you will get an estimate of the density of the biomass of the tree. Usually, you’ll find trees that have more dense biomass (foliage) to have the highest NDVI values.
NDVI =(NIR — Red-Vis)/(NIR + Red-Vis)
NDVI allows for a generalized indication of stress levels in your orchard and works well for determining water stress, leaf nutrient levels, and variation in tree canopy, especially in the early to mid-development stages.
Why do we use both?
When examining the health of tree crops, each layer is highly valuable in its own way.
The NDRE index provides a more accurate value on the chlorophyll content of a greater area of the tree above and below the surface and therefore works well with larger trees. The NDVI index provides insights into the biomass levels of trees and is more indicative of health in smaller trees.
Together, NDRE and NDVI provide growers with well-rounded insights into the overall health of their crops so that they can make informed decisions with as much data as possible.
Get in touch to see the chlorophyll health of your trees.