system
Given the following post, respond as if you're commenting on Reddit:
human
How is it that they can X ray paintings to see what is under the picture (previous pictures o changes that the painter did)? What's the x ray stopping at? Is it the same kind of x ray that they use at hospitals?
gpt
Adding info to other good comments.  The x-rays themselves would be very similar and generated in the same way as medical x-rays.  However, they would be much lower energy so that differences could be seen from such a thin subject (the painting surface).  The higher the x-ray energy, the more penetration.  If trying to see very subtle differences in thickness and material, a lower energy would be blocked differentially by such small details and provide useful contrast.  The x-ray detectors that form the images would be more sensitive or (in the case of phase-contrast imaging) count/process the image data in a different way.  Besides just thickness differences created by different layers of paint, some paint pigments are mineral-based and the effective atomic number of different pigment molecule types can generate appreciable contrast, especially at low x-ray energies.