This one is a little tricky, because it involves ideas that do not make much sense initially. You can grasp the idea that a soil monitor can be plunged into the ground and used to reveal soil moisture content. You can see a farmer crushing an arable crop into a handheld monitor, and then getting a digital readout. But when we move toward the edge of a light spectum, things become a little hazy.
Crop sensors come in different forms based on what they measure; wind, rain, moisture, air quality, water turbidity (how opaque or clear it is) all make sense because we can see them in action. Optical Crop Sensors are mounted on tractors, drones, or handheld devices. They shine light (often red and near-infrared) on the crop canopy. This reflection measures reflected light to calculate vegetation health.
How does reflecting a light off of a plant tell you that it is healthy, or not?
Good question. Sunlight contains the full light spectrum, including ultra violet and infrared. Bees and birds can see UV, but we cannot. Some people actually can perceive UV light, but most of us cannot.
At the other end of the spectrum we begin to open a conversation about light spectroscopy. Plants aren’t just green for fun, …they’re green because of chlorophyll, the pigment responsible for photosynthesis. Chlorophyll strongly absorbs red and blue light because those wavelengths provide the energy needed to fuel photosynthesis. Plants drink red light like vitamin drinks. When a plant is healthy and full of chlorophyll, it sucks in red light deeply.
Think about people and their summer wardrobes. When the sun comes out, many immediately go full color. While in winter and Autumn they go for thick dark browns and blacks. When the sun gets too intense you begin to see a deluge of billowing white fabrics. If we keep the same analogy for plants, the “vibrancy” of growth is reflected in a multitude of colors. Too much sunshine or water stress”saps” that energy, turning things yellow and faded.
As you get near Infra Red (NIR) the energy contained is too penetrative and begins to damage the skin of the plant in a similar way to how we get burned by the sun. Near Infrared (~700–1100 nanometers). So what does the plant do? It reflects most NIR to stay “cool.” Imagine you’re wearing infrared goggles: Healthy plants glow in NIR, like they’re lit from within. Sick plants look duller, less reflective. Dead plants? Almost invisible
The practical use of this information is applied by creating what are effectively heat maps. Normalised crop sectors will have a particular reflective pattern and abnormal sections will show up differently. There are many different types of optical sensors and a huge amount of different applications, but the basic agricultural principle stands; plants reflect NIR to survive, and absorb red to grow. NDVI (charting the plants against a standard index) watches that balance and turns it into real-time health information.
You can tell if you have diseased crops, pest infestation (and where), different growing properties and soil types, nutrient deficiencies, water issues, and more.
