The NoIR Raspberry Pi camera comes with a blue filter to do near infrared photography – the blue filter ices the visible red but passes near IR which records as red, apparently.
NDVI (Normalized Difference Vegetation Index) is the near IR plus red divided by near IR minus red. Take a look at this image for the meaning of the colours – red, magenta and white is more photosynthesis, cool colours and black are less. Chlorophyll uses red but doesn’t use near IR which it reflects, hence the difference carries useful information.Lots more at Public Lab.
So I thought I might use this to look for plant health at The Oak Tree. My NoIR Rpi camera was out in the field so I was looking for a different way to do this using a regular camera to take two images. The blue filter starts off bad by throwing out the red channel, but in some of the older Public Lab stuff there is a way of integrating two photos- one a bog-standard RGB pic and the other a picture taken through an IR filter. I have a B&W IR filter that looks almost black so I shot pictures with the filter and without filter, both using a tripod, then used Ned Horning’s photo monitoring plugin for Fiji image processing to do the grunt work of matching the two images up.
The NRG image has its own charm and is reminiscent of IR film days – I think the IR is added to the red but I am not sure
The normal colour image is this
Exposure times through the IR filter are shockingly long, presumably because the camera IR stop filter is fighting what residual gets through. There is a case to be made for using a raspberry Pi or perhaps two to take simultaneous pictures, one with the NoIR and the IR filter and the other with the regular Pi camera.