Former radiographer/x-ray tech here. In the film days, the cassette in which the film was loaded was more than a light-tight box to hold the film flat. While photographic film is sensitive to x-rays, long ago they started lining the inside of cassettes with radio-luminescent plastic, which converted the x-ray energy into photons (light), providing a much lower dose of x-rays’ ionizing radiation during the medical procedure. No idea if this trick is still used in the digital realm.
Good responses here. Part of it is that a transparent image, lit from behind, can reproduce a *much* finer range of detail, and wider range of brightness, than an opaque image lit from in front…that’s why slide projection is popular in color photography.
Med students spend a *lot* of time learning how to tell one object from another in an X-ray…every faint, subtle line means something.
The clear plastic has a layer of tiny silver halide (commonly bromide) crystals. When you take an xray exposure, you flood the object with xrays, and most of them pass through the object. Some of them get scattered by the object. Dense things, like bones, scatter more than soft things, like fleshy bits, so fewer xrays make it all the way through to the film behind the object. The places where the xrays strike the silver halide crystals on the film, a chemical reaction takes place that activates the crystal (it gets charged). This leaves it susceptible to a process called chemical reduction (you get a bit of black metalic silver) at that spot when it’s placed in the developer. In the places where the xrays didn’t make it through the object, you have less black metalic silver formed, and so it is more transparent to light.
No printing, there’s just less of a blackening chemical reaction in the places where fewer xrays hit the film.
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