The following is an excerpt from Utah State University Cooperative Extension. I posted this as a resource reference for FlightRiot users to get a quick understanding of the relationship between NIR and vegetation. Upon reading this post you can easily understand the vast uses of NIR imagery in agriculture, vegetation management, resource management and potentially even highly targeted pesticide application.
NIR is a small portion of the much larger region called infrared (IR), located between the visible and microwave portions of the electromagnetic spectrum. NIR makes up the part of IR closest in wavelength to visible light and occupies the wavelengths between about 700 nanometers and 1500 nanometers (0.7 µm – 1.5 µm). NIR is not to be confused with thermal infrared, which is on the extreme other end of the infrared spectrum and measures radiant (emitted) heat.
What Does NIR Tell US?
Since NIR has longer wavelengths than visible light, it exhibits peculiar properties that can be exploited for remote sensing applications. Some of the information that can be obtained from NIR is crop stress (water and nutrient stress being the most common) and weed/pest infestations.
Chlorophyll pigment absorbs most energy at about 650 nm (red) and around 450 nm (blue). Other pigments absorb more visible wavelengths, but the most absorption occurs in the red and blue portions of the spectrum. This absorption removes these colors from the amount of light that is transmitted and reflected, causing the predominant visible color that reaches our eyes as green. This is the reason healthy vegetation appears as a dark green. Unhealthy vegetation, on the other hand, will have less chlorophyll and thus will appear brighter (visibly) since less is absorbed and more is reflected to our eyes. This increase in red reflectance along with the green is what causes a general yellow appearance of unhealthy plants.
Another feature of vegetation is the strong reflectance within the NIR. Since NIR is not absorbed by any pigments within a plant, it travels through most of the leaf and interacts with the spongy mesophyll cells. This interaction causes about half of the energy to be reflected and the other half to be transmitted through the leaf. In plants with turged and healthy mesophyll cell walls and in dense canopies, more NIR energy will be reflected and less transmitted. This cell wall/air space interaction within these cells causes healthy vegetation to look very bright in the NIR. In fact, much more NIR is reflected than visible.
By monitoring the amount of NIR and visible energy reflected from the plant, it is possible to determine the health of the plant.
High NIR reflectance / Low visible reflectance = Healthy
Low NIR reflectance / High visible reflectance = Unhealthy
Because of the wider range of reflectance between healthy and unhealthy vegetation within the NIR region, sensors that detect within this region are much more sensitive to subtle changes in plant health. These sensors use silicon based arrays to measure the amount of NIR and visible energy reflected. These devices will often convert the result into one of several indicies such as the normalized difference vegetation index (NDVI). In this way, the health of vegetation can be constantly monitored.