Amplify NIR
Chris Fastie
Hi Rita,
The custom white balance procedure we always do with single-camera systems is intended to adjust the brightness of the NIR and VIS channels so that NDVI values are in the right range. Without a custom white balance the NDVI values are usually not even close. The white balance process was developed by trial and error and is not quantitative. We just point the camera at brightly colored paper when the custom white balance is done. For a red filtered Canon Powershot I point the camera a bright red origami paper in direct sunlight. This floods the sensor with red light and fools the camera into amplifying the blue channel (which is where the NIR is captured) relative to the red channel.
This research note has a figure with the ratio of NIR:VIS that produces NDVI values in the correct range for healthy plants. I have never been able to modify a photo after the fact so it produces good NDVI values. If the white balance is not correct, the photo will not produce meaningful NDVI values. If raw photo data is captured instead of jpeg images, it should be possible but I have not done it.
Ned Horning has been working on a process to calibrate the NDVI images, which may be the only way to get consistently meaningful NDVI values.
Chris
From: Rita
Sent: Thursday, January 22, 2015 11:44 AM
To: 'Chris Fastie'
Subject: RE: [plots-infrared] Re: [PLOTS] NDVI formula using NGB and NRG
images
Hi Chris sorry to bother you….
I used a red filter on my experiments but I also have a problem with gain on the NIR channel, I need to “amplify” it so my NDVI results are ok.
Has anyone reported this type of problem with red filters? Do you also tweak the NIR channel with white balance?
If so, how many times do you make it brighter?
I need to find some constant to multiply my NIR channel with, I was thinking about calibrating it with a NDVI tetracam we have on our research project but the tetracamera malfunctioned and went back to the store on warranty…
Do you calibrate NDVI with red filters?
Thanks
Rita