IR question

doenoe · Feb 28, 2006

Hello

i was wondering, if you shoot IR, you have to make a custom whitebalance. You take a pic a green grass (with only the grass in the frame) and set that as a custom white balance. Right?
Well, cant you just have a piece of green paper or something like that and and take a pic of that? so you can use that as a white balance?
Cause there isnt always a patch if green grass nearby, you know
Thanks for any input.
Greetz Daan.

Rob · Feb 28, 2006

I don't think a bit of green grass is right for WB, surely that'd make the colour shift hugely? I believe that you're meant to use a white/grey card... I'm not entirely sure as I don't bother with custom WB, I leave it on Auto unless it's important when I would shoot in RAW and do it on the computer.

Rob

markc · Feb 28, 2006

I haven't shot much IR at all, but I know that a card won't compare to grass. Think of IR as another color altogether. The card may look green to us, the same as the grass, but there's another color, infrared, that in the mix that we can't see. The grass reflects a lot of IR, so shows up light. The card may or may not, depending on what it's made of.

Imagine that you have a camera that filters out everything but visible blue. So this means that pure blue would be like white, and anything else would taper towards black, depending on how much blue is in it. A blue sky would look white, green grass would be grey, as green has some blue in it, and a red shoe would be black.

Now back to IR. The green grass looks green to us, but it's a mix of green and IR. It has a lot of IR so it shows near white on IR film, which exposes only IR and not visible light. The green itself plays no part. The blue sky looks black. Not because it's blue, but because there's no IR mixed in that particular blue. A green card is likely to show near black if that paint doesn't reflect IR. That green is a different green from the grass green. If we could see IR, it would look like a different color to us, the same way blue looks different from green because there's some yellow in the green as well as blue.

My guess as to using grass as white balance is that it's going to be the closest thing to "white" in the scene since it reflects so much IR comapred to everything else. At least what gets translated to white.