Watch out for the giant Timelapse X-ray sensor!


We have seen unique image acquisitions in our time, but this one, using a hospital x-ray sensor, produced amazing results.

At first, two-thirds of an inch chips were manner too small for cinematic images (except when they weren’t). Then, once we replaced them with Super-35mm chips, it quickly became apparent that they or they were not big enough, hence the absolutely irreproachable technical justification of the large format chips, which are absolutely always a good idea (except when they are not). The famous Internet handyman Ben Krasnow, owner of the excellent Applied Science YouTube Channel, seems to want to put that argument to bed once and for all, however, with its time-lapse images taken with – wait – a nearly sixteen-square-inch image sensor.

After you’ve baited enough clicks, let’s be clear, this isn’t exactly what we might call a conventional CMOS camera chip, and it isn’t capable of capturing at video frame rates. At 400mm on the side, that’s a pretty low resolution at 2048 pixels on the side, for a total of four megapixels (although think about it gargantuan pixel size). It takes several seconds to capture an exposure. The sensor itself is stacked behind a phosphor screen, as it is a device designed to detect x-rays as part of a portable medical imaging system. Conventional camera sensors don’t see x-rays, but phosphors glow when exposed to them, and we can photograph the light emitted by the phosphor; that’s how everything works, from x-ray machines in hospitals to baggage scanners.

Often, modern x-ray machines use a fairly conventional DSLR to take a photo of the bright phosphorescent screen, which is faster and cheaper than processing photochemical film. It may also involve a reduced dose of X-rays (which are, after all, potentially dangerous ionizing radiation) due to the higher sensitivity. The Krasnow Sensor comes in a flat plastic package designed to be easily slipped into a convenient location without having to move the patient too much.

In this situation, there is no room for the conventional DSLR setup, and the only solution is a very large area sensor at least the size of the area we want to image. The sensor must be at less the size of the thing we want to see because all x-ray images are effectively shadowgrams. It is very difficult to make a lens that works with x-rays, or at least a lens of serious power, because the high energy of x-ray photons compared to visible light means (for complicated reasons) the refractive index of almost everything is close to unity. Similar problems arise when creating reflective optical elements for x-rays, so they are very difficult to focus; as such, the x-ray glasses popularized by 1950s science fiction have to stay exactly that.

So, to form an x-ray image, we usually create the smallest x-ray point source possible and let it cast a shadow over the subject we are interested in, meaning that the shadow will always be slightly larger than the subject, at a degree controlled by the distances between the subject, the x-ray source, and the sensor.

Krasnow is a popular figure among serious propeller heads, with a huge portfolio of projects involving engineering, electronics, pyrotechnics, and optics. He used his large sensor to create time-lapse images of various mechanical and biological phenomena, especially plant stems soaking up the fluid by placing them in a vase containing potassium iodide for better x-ray contrast.

To do this, he had to operate an x-ray tube, which is not without risk due to both the very high voltages and ionizing radiation, although it can be a risk to some extent with almost any free electron device using five -figure voltages. While Krasnow is a seasoned experimenter and has taken simple steps to reduce his exposure to insignificant levels, this is certainly not something to play with at home, regardless of the fun toys available on eBay this week. No, you couldn’t shoot an entire x-ray film, even if the sensor was doing video frame rates, without risking radiological lesion of the plaster cast.

So, sit back and watch the particularly fascinating x-ray stopping motion of a zoom lens in action, sure that YouTube has done it so you don’t have to. Marvel at the engineering required to make such a large and sensitive device and make it useful, then weep profusely at the abject horror of the bundled software against the backdrop of what a device like this once had to cost.

Krasnow succeeded.

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