IR Photography question

Hi,

ok not really photography but this is the most appropriate forum

I'd done a little IR photography before with my unmodded 400d and a cokin 007 filter. After changing the IR blocking filter on a 1000D recently to a baader filter I thought I'd give that a go with the cokin filter. The results were not what I was suspecting.

For astro the 1000d, captures noticeably more red. And on these garden shots it is clearly producing a brighter and redder image:
400D
1000D
All as expected so far.

Then we add the Cokin 007 IR filter.

400D:

As expected the foliage on the plants comes out strongly
1000D:

huh? Darker, no brightness on the leaves.

I've shot the images in raw, used the same camera settings, at least I cant find any setting that is different.

Any ideas as to why the 1000d is giving these results?

So after messing about with what available light sources I have, the answer is the obvious one. The canon filter simply lets more light though at around 750nm than the Baader one.

We're only taking about less than 1% transmission and it's not apparent on any transmission curves for the filters that I can find. However it looks as though the Baader filter does a very good job of blocking light above 700nm, whereas the original canon filter will let just a little light through above 700nm, and a little is all you need with a long exposure and an IR filter that only starts to pass light at 700nm.

Leaves most strongly reflect IR at around 760nm. For the botanists it's the internal layer (the mesophyll layer) that reflects IR not the surfaces (the epidermis). That reflection gives a good indication of the health of the leaves and it's that reflection that the Earth monitoring satellites can use to help agriculture and monitor forests, rather than just seeing how green an area looks.