This was a joint experiment conducted by Aleksandar Nikolic and Travis Goodspeed, presented at ShmooCon in DC on 13th of January 2024. This writeup serves as a log documenting all the relevant details.
A cabinet x-ray machine is a handy tool for any reverse engineer of electronics, but the price tag keeps convenient x-ray photography beyond the reach of most hobbyists, particularly when the digital sensor works. In this lecture, we'll show two film alternatives to digital photography, generating x-ray pictures first with a proper dark room under a red light and second with polaroid/fujiroid film in the absence of wet chemistry. We'll also explain where film can be a better alternative to a digital sensor, offering better resolution and dynamic range at a much larger surface area.
"Cabinet" means that the imaging area is completely enclosed, shielded. They usually have relatively low x-ray energies (35 kVp) which doesn't require a lot of shielding making them very safe to use. Popular among independent electronics engineers are a few vintage models from Faxitron: mx-20 and dx-50. Most important feature of these machines is their micro-focus x-ray source. This directly translates to high resolving power, essential for studying small features.
When these were originally produced, they were offered with a few different imaging options. The imaging part of these machines is completely independent in operation from the machine itself. Most common options were either 50x50mm or 120x120mm hamamatsu CMOS based detectors. If you think about it, these are basically large format camera sensors which makes them extremely expensive, even to this day. The sensor itself easily constitutes the bulk of the machine cost. It is not uncommon to run into a machine where a sensor is missing or broken but that still produces x-rays.
Missing sensor makes them almost useless, but a crafty individual can still use them to produce x-ray images the old way, with film and chemicals. This sounds like it'd be messy, complicated and like it would require expensive equipment - goal of thIs guide is to show you that it's actually easy to do. We'll go through two recipes. One that produces high quality images but is a bit involved. And another, that sacrifices quality for speed and ease and makes for a fun party trick.
The machine in my possession is a Faxitron DX-50, the smaller model. It's imaging area is roughly limited by the diameter of the chamber which is about 200mm. This is a bigger area than 120x120mm sensor and much bigger than the more common 50x50mm sensor. Coincidentally, standard size 4x5 medium format photographic film fits comfortably with room to spare. This means that, by using large format film, we can get a much larger imaging area.
As already mentioned, digital sensor works independently of the x-ray machine. In simple terms, software that controls them both tells the machine to start shooting x-rays and then separately starts capture on the sensor. Sensor has different gain settings but relatively short maximum exposure times. Software usually makes a couple of exposures with different settings that then get merged. This translates into relatively low dynamic range. In other words, if you are imaging something that has very thin and very thick parts, you might have trouble seeing both in the end result. Modern black and white film has incredibly wide dynamic range which, coupled with long exposure times, can lead to more detailed results.
Additionally, even the bigger sensor (hamamatsu C9732DK) has limited resolution (2400x2400px) for the size of the sensor. A modern, low-speed, fine-grain film can record much finer details.
Of course, using film has drawbacks. It can be a bit messy and the turnaround time is not exactly short…
For my experiments, I've decided to use Ilford Ortho Plus film in 4x5 format.
It's very popular, still produced and easy to find. It's a technical, fine grain, film with ISO rating of 80 which makes it "high resolution". Ortho in the name stands for orthochromatic which means that it's not sensitive to deep red light , meaning that it can be handled safely in a darkroom under safe light. This last bit isn't crucial, as the recipe we will outline doesn't require a dark room, but it's certainly convenient.
Besides the film, you will need a couple of things to develop it. My goto black and white developer is Kodak HC-110 which is extremely inexpensive, versatile and available everywhere. Fixer is fixer and doesn't really matter but my choice is Ilford RapidFix.  
If you have a darkroom, or a particularly dark bathroom or closet, you can get away with most of these, but they are handy and allow you to do film processing anywhere.
- Changing bag
- Development tank or development trays
- Film holder
- Bottles and measuring cups for mixing chemicals.
The imperative in radiography is to minimize the patient x-ray dose as much as possible. This is done through a combination of short exposure times, fast films and intensifier screens. Specialty X-Ray film is designed to be very sensitive at the expense of resolving power. It is usually coated with emulsion on both sides to further increase the speed. Additionally, most x-ray film is made to be very sensitive to green light as it is made to be used in cartridges that have built in intensifying screens. An intensifying screen is simply a piece of coated paper that fluoresces green when hit with x-rays. This further reduces necessary exposure times, but further decreases image quality.
We are concerned with imaging inanimate objects, so we can forgo trying to limit the patient dose. This simplifies our job as we'll be exposing film to x-rays directly, in very close contact with the object being imaged. We just need to keep it safe from all visible light. To do so, we need to fashion a light tight, but x-ray permeable, film holder. I've made a simple 3d printable design that uses a piece of thin, black opaque plastic. You want to make sure that whatever material you are using isn't absorbing too much x-rays. You can get away with few layers of PETG (surprisingly, PLA seems to be more absorbant), but the thinner the better. I used the same material that film bags are made of. The only important part is that the whole film holder is completely light tight.
Now that we can safely keep the film out of light, we need to figure out the exposure time. There isn't really a practical way to estimate this, but I found 2 minutes to be good for most of the subjects. Thicker, multi-layer boards might need more time.
Yes, this is A LOT of exposure compared to relatively short exposures
required by the digital sensor, but so it goes… For my experiment's,
I've been using xray.py script from John McMaster's repository. It
allows you to set the time, set the kVp and fire the machine. Since
35kVp is already at the edge of what's practically usable for imaging
PCBs , I never saw any benefit to using anything lower than that.
You will need to prepare enough chemicals for the size of the development tank that you have, but the ratios stay the same. For developer, HC-110, the default is the so called "Dilution B" which is 1:31 dilution of concentrate with water. That is, 1 part concentrate to 31 parts of water.
For fixer, standard dilution is either 1+4 or 1+9 for 1 part fixer to either 4 or 9 parts of water. There's no downside to using higher dilution, except slightly longer wait times, so I tend to save a bit and use a more diluted solution.
For the steps in-between developing and fixing, so called stop bath, plain water is fine. Clearly mark the bottles, if you mix up the order your development will fail.
Second step is to transfer the film from its original packaging to the slide holder that goes into the x-ray machine without exposing it to light. To do so, you load the following into the changing bag:
- Pack of film
- Film holder
- Film holder clips
Zip up the film bag, push your hands through the sleeves and load the film. Take one sheet out of the film pack, center it into the holder (emulsion side up preferably, but the x-rays don't care that much). Use the clips to make the holder light tight. Before opening the bag, make sure that the film pack is closed and secure, lest you expose all the film.
Carry the prepared film holder into the X-ray machine and position it in the center of the chamber. Put the object-to-be x-rayed on top of the slide holder, trying to make it lie parallel.
Turn on the machine and wait for warmup. Using xray.py set exposure
time to 120 seconds and kVp to 35. When everything is ready, issue the
fire command and wait two minutes until the microwave oven tells you
it's done.
The film is now exposed and we need to safely develop it. Load the following into the dark bag:
- Slide holder
- Development tank
Zip up the bag, pull your hands through the sleeves and carefully transfer the film sheet from the slide holder to the film carrier and into the development thank. Shut the tank close and make it light tight. The rest can be performed in daylight.
Film is now safely loaded into development tank and is ready for processing. The rest of the steps can be performed under regular lighting with no fear. We need to perform the following steps:
- Develop
- Stop
- Fix
- Rinse
Development time depends on your choice of film and developer. It also depends on the exposure, and since we are dealing with x-rays manufacturer recommended development times aren't really helpful. After some trial and error, I've concluded that with hc-110 in dilution b 6 minutes seems to be a good enough developing time.
The thank is light tight. You open the cap and pour in the developer solution. You then proceed to agitate (turn upside-down and back) the tank for first 30 seconds and then for 10 seconds every minute.
After 6 minutes have passed, pour out the developer and pour in water. This acts as a stop bath. Older films had thicker emulsions and required stronger, slightly acidic stop baths, but plain water is usually fine for this purpose. Agitate the tank a couple of times and pour out the stop bath.
Next up is fixing. Pour the fixer into the tank and follow the same procedure as with development, agitating the tank for 10 seconds every minute but for about 5 minutes. Don't stress this part too much as film can be put back into fixing bath again if it was under-fixed. After the initial fixing, it is effectively no longer light sensitive (unless subverted to additional time in developer solution), so it is safe to examine it under regular light.
After fixing, the film should look mostly clear with no milky parts, but should clearly show a negative image. Last step is to rinse it with water again. If you want slightly better drying, adding a drop of mild soap or other wetting agent into the rinse water can help but isn't strictly necessary.
After thorough rinsing, hang the film to dry in (as much as possible) dust free area.
After drying , the developed film negatives can be scanned of photographed with a regular camera against a white light and then post processed with regular photo software. Here's a few examples:
An SmartResponse XE device. Above image shows a wide dynamic range from completely opaque screws to very light, thin, plactic.
Defcon badge from 2022.
An example of imaging a much thicker board. Big one is a thunderbolt dock. The board comtains many layers and has a heavy ground plane. Imaging this on film is a bit of a struggle, but can be acomplished with mdoified chemistry.
Here's a closeup of an SD card that really shows the ammount of details that can be achieved with this method. Even though silicon is mostly invisible, you can see where the bonding wires terminate.
Through experimentation, we've determined few factors that help get better results from challenging subjects such as boards with thicker coper pours or too many layers. First, extending the exposure time to 3 or more minutes can help, but also has limits. X-rays scatter and "fog" the film.
Another option to get higher contrast from dense targets is to modify the development chemistry. Mainly in the way of using a more aggressive developer. Photo paper developers are much stronger and faster acting, which can be to our advantage when trying to lift very light shadow details. A good option in our experimentation was Illford PQ Universal developer in standard dilution. It's fast acting and the time film spends in the developer is very short. From 1:30 to 2 minutes. The results are negatives with higher contrast, and higher grain, but with more details shown in thick boards. My recomendation would be to first try the standard proposed recipe and then experiment with PQ developer if neccessarry.
Fixing step is always the same.
I hope I've shown that film developing isn't all that hard and messy as it might have looked like. Nevertheless, there's another option we've experimented with that's much simpler. Albeit at the expense of quality. Polaroids have been around forever and are loads of fun, even if they can be a bit pricey per photo. My only gripe with polaroids today is that they are mostly very small.
As an alternative to Polaroid, Fuji also manufactures instant photo film (fujiroid) and they have several formats. Biggest of them is "fuji instax wide" which is just slightly smaller than previously shown 4x5 film.
Each instant photo/slide has these 3 pouches at the bottom that contain development chemicals. When a photo is taken, film is ejected from the camera and is passed between rollers that squish the pouches and spread the chemicals over the film. The reactions are timed so that fixing kicks in after development is finished and all is done in a matter of minutes. It's really fascinating achievement!
This basically means that you can develop polaroids on a sturdy countertop with a rolling pin if neccessarry.
In order to avoid wet development (and rolling pins), we can either reuse a regular instax camera or acquire one of these instax printers. These are nifty devices that you can use to print digital images onto instax film from your phone. But, nothing is stopping us from using it to process the film that hasn't been exposed to regular light.
The procedure for our use case will be to take the instax film that's been exposed to x-rays, put it back into instax pack and load it into the printer. When a "new" film pack is loaded, printer first ejects the "dark slide". Since we've just put the exposed film into it, this will effectively push the film through the rollers which develops it.
Into the dark bag go:
- X-ray film holder
- Holder clips
- Instax film pack
- Light tight box for instax film pack
- Small flashlight
In the dark changing bag, open the film pack, take out the dark slide. Take out one piece of film. Quickly turn the flashlight on and off, exposing the film. This is counterintuitive, but crucial for instax film. This pushes the film over its activation threshold which makes it more sensitive to all light and makes it possible to expose it properly with x-rays. A very brief moment with not too bright flashlight is enough.
After this short exposure, load the film into film holder and clip it light tight with clips. The slide is now ready. Still in the bag, safely put the rest of the instal film pack into a dark light tight box to protect it from exposure.
As before, carry the prepared film holder into the X-ray machine and position it in the center of the chamber. Put the object-to-be x-rayed on top of the slide holder, trying to make it lie parallel. Remember that the instax film is smaller than 4x5, so imaging area is smaller, too.
Like before , turn on the machine and wait for warmup. Using xray.py
set exposure time to 180 seconds this time and set kVp to 35. When
everything is ready, issue the fire command and wait 3 minutes until
done.
With the film exposed, into the bag go:
- Film holder with exposed film
- Instax printer or instax camera
- Empty instax pack
With everything in the bag, and the bag light tight, take the film out out the film holder and put it into an empty instax pack, making sure to orient it properly. Next, load the instax pack into the printer/camera. Once you close the printer, it will automatically eject the first, and in this case, only slide. Pushing it out starts the film development which is completed after a couple of minutes. Do not, no matter what silly pop songs tell you, shake it like a polaroid picture. It doesn't actually help.
Results achieved with this method are much quicker but maybe less practically useful for complex PCB reverse engineering. They do, however, make for a very fun party trick.
This one shows different smaller devices. A yubikey, an SD card, Pi Pico and a casio digital watch board. While these are clearly of a lesser quality than the ones made on regular photographic film, they can still be used to identify locations of various components for example.
We hope to have shown that not only is developing film not hard , it really does have merit of its own in certain situations. Especially when imaging larger objects that don't fit the digital sensor.
