Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Chlorophyll allow plants to absorb energy from light. Chlorophylls absorb light most strongly in the blue portion of the electromagnetic spectrum as well as the red portion.Conversely, it is a poor absorber of green and near-green portions of the spectrum. Hence chlorophyll-containing tissues appear green.
Normalized Difference Vegetation Index (NDVI) quantifies vegetation by measuring the difference between near-infrared (which vegetation strongly reflects) and red light (which vegetation absorbs). So, the scientists use NDVI to assess foliage health by measuring the difference between the amount of near-infrared and red light that is reflected off the foliage, using the following equation:
- NDVI always ranges from -1 to +1, and healthy foliage falls within the 0.2 to 0.8 range approximately and according to this tutorial made with Arduino UNO: https://www.sensingthecity.com/interpreting-as7263-nir-spectral-sensor-readings-to-determine-plant-foliage-health-and-turn-on-an-led/
- But, if you have an NDVI value close to +1, there’s a high possibility that it’s dense green leaves. This is according this publication: https://gisgeography.com/ndvi-normalized-difference-vegetation-index/
In my case I used the range between 0.2 to 0.65, because I did tests with my sensor and I found that at values upper 0.7 some leaves were dry. so I lower the upper threshold a bit to ensure good readings.
For this tutorial, I will apply this method to a three plant’s leaves by plugging in readings from the AS7263 NIR sensor into the NDVI equation. The sensor detects wavelengths across 6 channels and puts out readings as counts per μW/cm2, which gives a measure of the amount of incident light that is being reflected at each wavelength. Two of the sensor’s channels are close to the wavelengths that are typically used in the NDVI equation. These channels are S and V, giving out readings for the 680 nm (red range) and 810 nm (near-infrared) wavelengths, respectively. Once the NDVI value is calculate, we can use an "if else" statement to signal whether the foliage is within a healthy range or not.