Scientists have discovered that the plant health is directly proportional to the rays it reflects which is near-infrared rays. The plant receives different wavelength of rays from the atmosphere.
It uses blue and red light to create food for the plants. The infrared rays received is partially absorbed and reflected back. Our eyes perceive the plants as green because green light is reflected back by the pigments of the leaf. The near-infrared rays is reflected from the chlorophyll layer whose strength actually determines the health of the plant. The better the chlorophyll layer, more is the infrared rays reflected. The camera capturing near-infrared images is used to detect the health of the plant.
The left side image is showing the actual RGB image of the plants. The right side image is showing the amount of IR rays reflected from the plants with better health. Just to make it clear, the green color is not a factor which determines plant health. There are many plants which has a different color other than green. But the appearance color is due to the pigments of the plant.
You can check in the above images the comparison between the amount of IR reflection from a healthy plant and an unhealthy plant.
The first step to start the implementation of precision agriculture is to relate technology to plants. And to do that we need to know what output does a plant give which can be used to understand it. And the IR emission is the first step into knowing a plant.