This is the result of an unintended experiment I conducted recently. A single roll of film I forgot in a bag went through a CT carry-on luggage scanner on the airport. It’s a nice opportunity to see what this single scan does to (in this case) Kodak Vision3 50D film. It’s worse than I had imagined!
So here’s the deal: we went to Crete (a first for me, and I can recommend it to anyone), so we hopped on a plane on our nearest airport. I brought some film to shoot on the island, but there was this single, partially exposed roll that I forgot in my camera bag. This bag stayed in my carry-on backpack, which went through the modern CT scanner at the airport. When we arrived in Crete, I found that roll and realized what had happened. I marked the roll so I wouldn’t mistakenly use it on our trip, and decided to process it at home to see what it would yield.
Most of the film I shot on this trip was Kodak Vision3 50D, as was the affected/experimental roll. I ended up processing this film 2 rolls to a tank (Jobo 1520), which gives a nice control: the CT-affected roll was processed alongside an unaffected roll of film, allowing for a straight comparison. All the film also came from the same 400ft master roll, so it’s the same age, stored under the same conditions, handled in the same way etc.
Well, without further ado, here’s the result of a few snippets of this film, side by side, as scanned on my Epson 4990 flatbed. The CT-affected film is at the top, the control film at the bottom.
A first glance confirms what I thought I saw with the naked eye, handing the negatives to the light – the CT-affected film seems to have a slightly darker, more muddy base color. This makes sense; the base color is essentially magenta + yellow dye, but if overall fog occurs (which is what the powerful x-rays of CT will do), some cyan dye will be added on top of this. Adding cyan to magenta and yellow basically shifts the hue towards grey, so the base color becomes a less saturated – and of course darker. This is indeed visible in the strips above if you look closely.
In my experience, if you see these kinds of differences with the naked eye in color negative film, the differences when printed or inverted and color balanced will be quite severe. So let’s have a go at (crudely) inverting and color balancing this scan, using the (unaffected) control film as the basis for the color balance:
Oops…the differences are quite clear indeed. Note how I based the black point for the inversion on the unaffected film. This results in the black point of the CT-scanned film being quite far above actual black, rendering a muddy brown-green. What’s more, the actual photos on the affected film also are much less crisp and look more bland and lifeless than the ones on the control strip.
Btw, ignore the smudges on the leader of the CT-scanned film. Those are just some remjet stains I didn’t remove carefully enough. They have nothing to do with the CT fogging problem.
Have a look at some of the frames, in this case the bottom 2×2:
Note how e.g. the top right frame (CT-affected) is bland and overall brown, while the bottom frames have normal shadow regions and normal colors (save for the quick & dirty balancing). Contrast is significantly lacking in the CT-scanned film.
Of course, it’s mostly the shadow areas that are affected – that’s how flashing works, after all. The low-density areas will be affected relatively much stronger than the higher densities. This means that bright areas in the negatives will look pretty much the same, but the shadows are all lifted up and compressed on the CT-fogged film. This affects overall contrast and it’s something that cannot really be compensated for effectively, especially when optically printing the film.
I’m actually quite surprised at how severe the effect is, keeping in mind that this is nominally 50 ISO film (which I rated at 64, but that’s more or less inconsequential for the findings). Imagine what the effect will be on much faster film!
Of course, this is nothing new, but it drives home the point that CT scanners for carry-on luggage really are not safe for photographic film – not even for slow film!