Normalized Difference Vegetation Index (NDVI) sensor?

does know buy or how build ndvi sensor detect between grass , sidewalk or soil robot mower?  know nasa uses ndvi sensors on landsat satellites these might little expensive me  .

this best basic definition of nvdi find nvdi wiki need able measure amount of light in visible spectrum , in near ir spectrum in general need tow calibrated cameras @ minimum.  can find better ways need.

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