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I am new to IR photography. I recently converted my Nikon D800 to full spectrum (done at a shop professionally) and got a Kolari 550nm IR filter because I want to capture a lot of false colour in my images but something doesn't seem to be right. I have tried shooting with auto white balance as well as setting my own white balance in camera but once I swap the colour channels in post production the images turn to just one colour (usually blue). I'm not quite sure what I'm doing wrong as I have tried editing differently and playing around with the camera but it always comes out very similar to this. Any opinions in what I could be doing wrong or how to fix this?

Auto WB

Channel Swap Auto WB

Custom WB

Channel Swap Custom WB

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    \$\begingroup\$ Can you please define colours in sense of infrared? \$\endgroup\$ Nov 21, 2023 at 13:57
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    \$\begingroup\$ Please describe what you have done differently for each of the images. BTW, the last image looks pretty close to typical IR photographs. \$\endgroup\$
    – scottbb
    Nov 27, 2023 at 1:08

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Here's what a true color rendering of an image exposed to only infrared light looks like to our eyes:

enter image description here

Any time we create an image using only wavelengths that our human vision does not perceive we must assign false colors to the wavelengths that we do not have the ability to perceive.

How that image will look depends on how we assign different colors we can see to represent differences in the information collected in the image.

Sometimes we assign different colors based on the intensity of infrared light in the image, regardless of what specific wavelength(s) within the infrared spectrum are being captured in any particular spot of the image. We might make the most intense areas of the image red and the least intense areas of the image that still have some signal above the noise floor blue. Or we could make the brightest parts blue and the dimmest parts red (or green, or purple, etc.). This is how many "contact free" thermal imaging devices work. But the colors assigned to various intensity levels are purely arbitrary and do not show differences in various wavelengths of infrared light, only differences in the intensity of infrared light.

In order to assign different colors based on the predominant wavelengths present in any part of the image, we need a sensor with a color filter array that can differentiate between "shorter", "medium", and "longer" wavelengths of the infrared band in the same way our normal cameras can differentiate between shorter, medium, and longer wavelengths of visible light.

Infrared begins at about 750nm and reaches all of the way to 1mm (1,000,000nm!). The infrared portion of the electromagnetic spectrum is far wider than the portion of the electromagnetic spectrum we call "visible light, which is from around 380nm-700nm. The shortest wavelengths we can perceive are only about half as long as the longest wavelengths we can perceive. On the other hand, the longest wavelengths of the infrared band are over 1,300 times as long as the shortest ones. Converted visible light cameras, though, are only sensitive to the shortest infrared wavelengths, around 800nm to 2500nm, which we call near infrared

Even if we leave a traditional Bayer mask on a camera and use a filter which blocks visible light and only allows infrared light into the camera, all three filters of a traditional CFA are about equally transmissive to wavelengths above 800nm. So they can't help discriminate shorter from longer wavelengths of near-infrared light. Taking the CFA off and not replacing it with a CFA that has filters with different "colors" of infrared means our converted camera is purely monochrome. It can only detect intensities and can't tell different wavelengths apart. It can only count the number of photons that fall on each photosite (a/k/a sensel or "pixel well").

If your converted camera had its visible light CFA removed to allow it to be more sensitive to near infrared then you'll only be able to capture monochromatic information with it. Most "infrared" images you see with different colors that are similar to "natural" colors are made with a camera still having a normal CFA with the IR-cut filter removed from in front of the sensor so that IR light is not totally blocked but is allowed to reach the sensor. Adding an "infrared filter"² to the front of your lens will diminish the differences between colors, depending upon how strong the filter is and where it "cuts off" all wavelengths shorter than a certain value.

If your camera was converted by removing the IR-cut filter but leaving the CFA in place, try an infrared filter that cuts visible light below around 720nm. Your 550nm filter is still letting a lot of visible red light into the camera and that light is overpowering the infrared light. Reducing the visible red light will allow you to expose brighter (i.e longer, or with a wider aperture) to allow more infrared light to be captured.

¹ Near infrared is so named because it is the portion of the infrared band of the electromagnetic spectrum which is nearest to visible light, not because it is a portion of the visible light band closest to infrared wavelengths.
² "Infrared filters" do not reduce infrared light, they reduce non-infrared visible light to one degree or another while allowing infrared light to pass relatively unmitigated.

For further reading here at Photography SE:

Can I use on-lens IR filter for a full-spectrum converted camera?
Infrared photography—why are green leaves not appearing as white?
Infrared pics coming too red or with dark leaves (filters are top quality)

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    \$\begingroup\$ Your example image needs a bit of adjustments in the highlights n_n \$\endgroup\$
    – Rafael
    Nov 21, 2023 at 23:50
  • \$\begingroup\$ and maybe a bit more balance between the dodging and burning in the lower-right corner? \$\endgroup\$
    – uhoh
    Nov 27, 2023 at 1:57

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