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How to get rid of IR contamination

The issue of IR contamination of the red and green channels has been discussed here repeatedly. The fact is that it significantly affects the colors of a photo and interferes with achieving an Aerochrome-like look. For example, it causes vegetation to appear paler and the color changes (red -> yellow, car taillights for example) to be off.

I now made an experiment with the green-orange combo to see how much there R and G are “bothered” with IR light and how this can be turned off. To start with: The way to eliminate the contamination coincides well with my previous post-processing. The following picture should illustrate this.

Used were: Iphone flashlight, two IR filters (Hoya 720nm and B+W 830nm) stacked, the full spectrum Nikon D7100, and an older Nikon 50mm f/2, green filter (Hoya X1) and orange filter (Tiffen 16), Kolari Hot Mirror.

Standard setting in-camera, white balance from white card (in post).

IR contamination

Top row. Left: Visible light. Right: Orange-Green (without any changes in PS Raw converter except white balance), all RGB channels together.

Middle row. Orange-Green (without any changes in PS Raw converter except white balance), RGB channels split. You can see that IR is in all channels, strongest in B.

Bottom row. Orange-Green, RGB channels split. Now we tried to find out where the threshold is, i.e. at which adjustments the IR from R and G disappears. The following values were determined: In the Raw Converter -30 at the “blues” in the color mixer plus subsequently in the channel mixer at G (before the channel swap): -30 at blue, compensated in green, so there +30. Now IR can only be found in the blue channel.

Conclusion: I could not remove the IR contamination in G in the raw converter, so the channel mixer was used here. The IR contamination in R, on the other hand, can be eliminated with a value of -30 in the blue tones. The result partly coincides well with my earlier (by naked eye) calculated, suitable values for a good Aerochrome look, above all the -30 for blue. Maybe there are ways to get the values right in the raw converter. Further tests would be necessary.

But be careful: other cameras, lenses, filters can of course influence the threshold. But this method can be used for appropriate calibration.

Processed with the values listed above:

IR contamination

The same method can be done with the TB550/660/850 (link). There the threshold is higher: -50 instead of -30 in each case. And the result is less good for my taste, because the red tones are paler:

IR contamination

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