Multispectral Vs. RGB Cameras
 
Our human eye is sensitive to Red, Green, and Blue (RGB) bands of light. Standard cameras capture the same RGB bands and produce images that are very similar to what our eyes see.
Beside visible bands, multispectral cameras capture lights that are invisible to our human eye, such as near-infrared (NIR) light.
A typical multispectral camera consists of 3 bands: Red, Green and NIR bands. Enhanced multispectral cameras can capture more than 3 bands, such as Red, Green, Blue, Red-Edge, and NIR.
Example of multispectral camera
Parrot Sequoia+. It consists of 4 bands: Green (550 nm), Red (660 nm), Red Edge (735 nm) and Near Infrared (790 nm).
Example of multispectral camera
MicaSense RedEdge-MX. It consists of 5 bands: Blue (475 nm), Green (560 nm), Red (668 nm), Red Edge (717 nm) and Near Infrared (840 nm).
Why use Multispectral for Precision Farming?
A normal, healthy plant will absorb blue and red light and reflect green light, which is why they appear green to our eyes.
It is found that plants also reflect near-infrared (NIR) light. The healthier the plant, the more NIR light is reflected, because NIR light is not used for the photosynthesis process.
By using a multispectral camera and comparing its visible and NIR bands, agronomists can easily differentiate vegetation field from non-vegetation field, and healthy plants from unhealthy plants.
Normalized Difference Vegetation Index (NDVI)
As shown here, a stressed leaf and a healthy leaf reflect nearly the same amount of blue, green, red light, but a healthy leaf reflects more NIR light.
NDVI is a commonly used vegetation index to quantify plant health. Its formula:

NDVI = (NIR - Red) / (NIR + Red)

NDVI values between -1 and 0 correspond to non-vegetation surfaces such as road, water, or soil. Soil's value is close to 0. Plant values range from 0.1 to 1. The higher the NDVI value, the greater their density and health.
NDVI Map of Vegetation Field captured by Parrot Sequoia camera

Typically, a multispectral orthomosaic map is represented in a form of NDVI values from -1 to 1 (-1 is shown as red and 1 as green).
Sample of Multispectral Imagery captured by MicaSense Multispectral Camera
RGB Color
Chlorophyll Map
Applications of Multispectral Imagery in Farming
Identify pests, disease and weeds at early stage. NDVI enables agronomists and farmers to see stressed crops in a field earlier before the human eye would be able to detect.
Provide data on soil fertility and refine fertilization by detecting nutrient deficiencies.
Count plants and determine population or spacing issues.
Measure irrigation, survey farm boundary, classify landuse.
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