To expand a little: chlorophyll absorbs red and blue, but reflects green and NIR (near infrared, just beyond visibility), so basically it measures how "green" t

To expand a little: chlorophyll absorbs red and blue, but reflects green and NIR (near infrared, just beyond visibility), so basically it measures how "green" the plants are, but using NIR instead of green as its a more robust measure. Also infrared photography is cool because plants are bright white.