Enhanced Normalized Difference Vegetation Index (eNDVI) explained Live green plants absorb solar radiation in the photosynthetically active radiation (PAR) spectral region (between about 450 – 700 nm). Plants use this energy in the process of photosynthesis. Leaf cells have evolved to scatter (i.e., reflect and transmit) solar radiation in the near-infrared spectral region (greater than about 700 nm, which carries approximately half of the total incoming solar energy) because the energy level per photon in that domain is not sufficient to be useful to synthesize organic molecules. In addition, by reflecting the NIR, the plant keeps the leaf cool and does not cook in the sun. In World War II, the US military discovered that by looking at plants with an infrared camera, the plants would look white while a painted green camouflaged object typically looked dark. Thus, while the human eye may see a camouflaged green object as green and look quite similar in colour to a green plant, the infrared camera sees the plant as white while the green paint is dark making detection easy (Figure 1).
Figure 1: An example of healthy vegetation taken with an IR-only camera.
Agribyte Technologies Pty Ltd. ACN: 143 199 446 P.O Box 385 Lenah Valley TAS 7008 Email:
[email protected] Phone: 0407 832 479
As an outgrowth of the military work scientists worked on using visible and infrared cameras to detect when vegetation was stressed or not healthy. They noticed that when plants become stressed, their infrared reflectivity drops faster than their visible green colour. You can see were a plant is having a problem by comparing the ratio of the NIR light to the visible green (Figure 2).
Figure 2: Changes in spectral reflectance of a picked healthy plant leaf shows how light reflectance can be used to estimate plant health.
A variety of mathematical indexes have been developed to quantify the relationship between the amount of reflected NIR and visible light from plants. The oldest one is called the Normalized Difference Vegetation Index (NDVI). The NDVI is calculated from these individual measurements as follows:
Where:
NIR = near infrared reflectance Red = red reflectance
Agribyte Technologies Pty Ltd. ACN: 143 199 446 P.O Box 385 Lenah Valley TAS 7008 Email:
[email protected] Phone: 0407 832 479
These spectral reflectances are themselves ratios of the reflected over the incoming radiation in each spectral band individually; hence they take on values between 0.0 and 1.0. By design, the NDVI itself thus varies between -1.0 and +1.0. We have found that you can get even better results if you use the NIR and green as the reflective channels while using the blue as the absorption channel. Remember that a normal healthy plant will reflect both visible green light and NIR light. We improve on the NDVI formula to produce an enhanced NDVI (eNDVI) by changing the formula:
At Agribyte, our eNDVI imaging service quickly generates plant vigour classes, which are represented by dark red (no plants, roads & tracks, buildings, water bodies), light red, orange, yellow (low density of plants &/or very stressed plants/dead plants), yellow through to light green (young plant growth with a lot of background soil &/or stressed plants), green, dark green through to blue (highly vigorous plant growth or areas of high plant density and functional chlorophyll (i.e. healthy green leaf) (Figure 3).
Agribyte Technologies Pty Ltd. ACN: 143 199 446 P.O Box 385 Lenah Valley TAS 7008 Email:
[email protected] Phone: 0407 832 479
Figure 3: Imaging of irrigated horticultural crops in central and NW Tasmania reveals plant growth and productivity related to water use history, soil type, topography, plant density and health. The human eye cannot easily see some of these variations in the field. Early identification of problem areas allows for more timely & effective management.
Agribyte Technologies Pty Ltd. ACN: 143 199 446 P.O Box 385 Lenah Valley TAS 7008 Email:
[email protected] Phone: 0407 832 479
