Typically, sensors found in a cell phone or point-and-shoot cameras are equipped with three channels for sensing colors. The channels are usually aligned to optimize photography of the visible spectral range (red, green, blue -- RGB) so that your photographs match what you see with your eye. Near-infrared (NIR) light is invisible to the human eye and is where plants do a lot of reflecting. While RGB sensors are capable of detecting light in the NIR range, that range is typically blocked using an optical filter. Otherwise, the NIR light would affect your pictures, making them look poor and washed out. Some companies offer services to remove the NIR filter and substitute different ones inside commercial RGB cameras; however, because the cameras themselves are not designed to capture true NIR wavelengths, the modifications only result in poor and inaccurate data.
Bottom line: you need a true NIR camera to capture true NDVI data.
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