IR white balance?

Mack

Mu-43 All-Pro
Joined
Jan 14, 2018
Messages
1,816
From my web perusing, it appears many who shoot IR photos take their White Balance off green grass or foliage. I assume that the idea behind doing this is to make the foliage "IR white" (in B&W) and not red as what normally happens with a normal WB setting.

If the idea is to make the sky appear darker, I'm thinking the WB should be set from using the complementary color of the blue sky? If one uses the blue sky for WB, then it seems that would make the sky a lighter (white) than taking the complementary blue sky color?

The x-rite ColorChecker Passport shows its "Sky Blue" (Top 3rd color from left.) as RGB=98 122 154. The complement would be an amber/sepia color with RGB=192 108 47. That may make the blue sky a neutral darker black - in theory - and not a lighter neutral white?

Fwiw, I'm getting a used E-1 Mark I model converted to Full Spectrum for IR by Lifepixel.com for sundry IR filters and wishing to make the sky darker than what I get with a red filter. Otherwise. I'd try this WB method out to see if it works.

Tia.
 

Petrochemist

Mu-43 All-Pro
Joined
Mar 21, 2013
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1,387
Location
N Essex, UK
Real Name
Mike
Healthy foliage is quite capable of getting you black skies when shooting IR. the foliage reflects loads of IT so gives a fairly neutral result in IR. The actual visual colour is totally irrelevant, for any IR shot at >720nm. With IR only shooting using a blue sky for CWB will give you much the same as trying to white balance on black.

With shorter wavelength IR+ shots there can be a very wide rage of suitable white balance targets which will give different results. PTFE is the ultimate reflects all target, white paper isn't that far behind. Skin, concrete foliage etc have all been used successfully. If the filter being used allows blur or UV through to the sensor skies (with some cloud) might work out well.

Converted Bayer based cameras see IR in all three colour channels, getting a fairly even distribution between the channels for wavelengths longer than 850nm. Between 700 & 800nm the red channel has a considerable increase in IR sensitivity. Of course the exact performance will depend on the filters used in the Bayer filter but all those I've seen data for are fairly similar.

Foveon cameras with no Bayer filter, calculate colour by how far into the sensor the radiation travels, more energetic wavelengths like UV & blue travel deeper than red & IR. Because of this design Foveon sensors see IR as red, and need to be treated quite differently.
 

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