I’ve had a few people contact me about the old retired film stock known for the reds, pinks, and it’s ability to see into the world of the invisible. Over the years I’ve done some studies and it is possible to get pretty damn close to the original film stock using a combination of filters, a converted camera and a custom color profile.
Most of you who are reading this should be somewhat familiar with infrared photography at this point so I’ll skip over a lot of the basics (see Beyond visible) and focus on what makes Kodak Aerochrome III 1443 unique. With Aerochrome film, the spectrum is shifted into the IR by one primary color. Green becomes blue, red becomes green and infrared becomes red. Foliage reflects a ton of infrared and some green which turns into lots of red and some blue giving it a unique reddish pink color in the developed image. I’m not going for ‘looks like Aerochrome’, I’m going for the real deal. Can’t tell the difference, shot on digital or film, same response, same spectral, same density, I mean really identical.
Now for the fun part –
How do we copy the spectral sensitivity and density curves to the digital process?
Lots of disassembly, reverse engineering, studying of optical filters, math and reading datasheets!
We need to get the spectral and density data from Kodak, which isn’t on their website because the film is discontinued, I would guess. I dug around a bit and found a copy, which I’ve also hosted here. The spectral sensitivity is the most important part as the density can be copied as you would for any other film stock. Here’s the spectral sensitivity of the Kodak Aerochrome 1443 film:
One important thing to note is that the film is very sensitive to blue light (400-500nm). There isn’t any yellow filter to block the blue from reaching the film as it would make the developed image have a yellow color. To counteract this, Kodak recommends using a Wratten #12 yellow filter when shooting. Here is the same response but with the yellow filter applied:
Notice how the blue contamination is eliminated.
Now that we know what sensitivity we are after, it’s time for step 2.
Disassemble your camera, remove the IR cut filter, insert shims to adjust for focus, replace the shutter monitor IR led and photodiode with a SWIR one so it doesn’t affect your exposure and re-assemble. Make sure you have a clean environment and take good notes! There are a couple services out there that offer infrared, full spectrum, etc camera conversions but none of them will replace the internal LED’s with shortwave IR versions. I do a lot of night photography so it’s pretty important to me to not have light leaks.
Make a spectrometer so you can get the real spectral response of your sensor. It’s a bit tricky because if you use a prism the spectrum isn’t linear meaning that the distance from one wavelength to the next varies depending on the glass but the intensity is uniform. If you use diffraction grating the spectrum is uniform but the intensity isn’t. I chose the diffraction grating method because the math is easier in my opinion and you can pick diffraction grating up at Amazon for cheap Link. You also have to do a little more math to correct for your light source, as that isn’t uniform either. You can possibly get your camera sensitivity curves from somewhere online but from my experience the data is difficult or impossible to get and is usually uncorrected.
Here’s the response curves from my converted Sony a7R ii
Again, we have a bunch of blue light being captured, we also have more deep IR than we want (900+nm).
To counteract the blue and unwanted IR contamination we can use a Wratten #12 again and a IR cut filter that starts to block in the middle of the near infrared, as we want to keep the infrared from 700-900 but not beyond. Kopp has a 7093 glass that blocks the IR about where we want which also reduces the flaring you get with most lenses too. Link The optimal thickness I’ve calculated is about 0.9 to 1.0mm. Here is the response curves for the Sony again but with the Wratten #12 and the Kopp 7093 with 1.0mm thickness:
Here’s a link to a handy filter transmission vs thickness calculator i made you may find useful.
Now that we are able to capture the spectral range we want, we need to get the captured sensitivity curves to mach as best as possible the target film stock.
Load the spectral data into Microsoft Excel using LibRaw and Matlab, then solve based on the difference between the matrixed data and the Kodak data. Basically we add and subtract color channels until we get a curve that matches as best as possible our target (Kodak Aerochrome)
Then we get a nice spectral response like this:
which is damn close to the Aerochrome response, although the green is a bit wider than I would like.
Create a profile from the color matrix that we found before. I created a profile and changed the base curves and color primaries to reflect the values we solved before. I started with a linear CIE profile, as most camera manufacturers integrate a 3d LUT into their camera profiles to get the ‘look’ they want which makes it a pain to fiddle with. I’ve also developed a program to inject the proper color profile using the Y52 filter into DNG files. You can get it here. That’s the way I prefer as you can then use Adobe Camera RAW to process the DNG’s.
A couple notes if you’re using the Aerochrome profile injector:
The build is for Windows, if you want it compiled for another OS, definitely send me a message or call me.
It’s command line based, to use it type-
and it’ll inject all of the five color profile tags necessary to properly render the DNG in Photoshop, ACR, Lightroom, Bridge, and other Adobe products.
CaptureOne doesn’t work as it completely ignores the profile and uses its built in profile for the camera :(
You may get mixed results with other DNG reading softwares such as RawTherapee darktable etc.
DNG’s can be created using Adobe DNG Converter from pretty much any RAW file out there.
I’ve tested it with DNG files created from Sony ARW’s shot with a converted A7R2. Let me know if it works with other camera types as they have very similar CFA dyes (color filters)
Because of how Adobe handles the white balance, when you load the modified RAW file into Photoshop, ACR, Lightroom etc. you’ll have to set it back to ‘daylight’ or whatever white balance you want. No need for doing any weird RAW data manipulations or channel swapping, the Aerochrome profile is actually embedded into the DNG.
Once injected you can adjust to taste in Adobe Camera RAW then run the DNG’s through Adobe DNG converter again to re-generate thumbnails (not necessary but looks nice to be able to preview them)
Take your converted camera out, put a couple filters on it and start shooting!
Probably not the easiest thing to do, but it can be done. Definitely contact me if this is something you are interested in, as it is a huge interest of mine and I love to help out fellow photographers, cinematographers and the like.
More photos taken with the digital Aerochrome process: