Understanding NDVI scale (green to red)

[Jeffrey Warren]

Hi, Grace - I'm CC'ing the infrared discussion list, which is a good place to get in touch with other community members for help. Your project sounds really interesting and I hope you'll post more about it on PublicLab.org!

With most settings on Infragram.org, "hotter" colors correlate with stronger photosynthetic activity. You can read a lot of other work on the subject here: http://publiclab.org/tag/ndvi

Jeff

1/18/2015 13:56:17 Question/observation

I am doing a science fair project for school and I am using Infragram to see the effects of UV radiation and fertilizer on algae. When I was making my pictures into Infragram, using a blue filter, I was confused about the scale. I think the blue means it's non-living and the green to red means living. What I am not sure about is what the difference between the red and  green means? Thank you!

PublicLaboratory.org GraceB

[Chris Fastie]

Hi Grace,

I think you are correct that in a proper NDVI image with the color scale at infragram.org, blue means it's non-living and the green to red means living. However, there is more than one color scale that can be applied to an NDVI image at infragram.org, so it would be good to see an example to know for sure. In a proper NDVI image with that green to red gradient, green should represent average plant health and red should represent above average plant health.

To make an NDVI image with the values in the correct range (so the colors have the correct meaning), several things have to be done carefully. 

1. Performing a custom white balance on the camera is important. 
   
2. The color of the light hitting the plants will affect the colors that are reflected off the leaves and captured by the camera. 
3. The exposure of the photo (bright vs dark) will affect the color balance of the photo.

So in order to compare the NDVI results from two separate photos, the lighting and exposure should be identical. That might mean an experiment should be designed so that all the photos for comparison are taken at the same time. Even better, try to get all the plants you want to compare in the same photo.

If you post some of your photos and results here we might be able to help more.

Chris