Why is the density of perspective different from that of photography?

Their imaging principles are different.

X-ray photography principle: equivalent to optical photography, it uses the reflection principle, that is, after X-rays are sent out, they are received by the film in front of the human body or at a specific reflection position, and its imaging effect is just the opposite of perspective, that is, the part with low density and more penetration has less reflection, and the image on the film is dark, while the reflection with high density and less penetration is more and brighter. Computer digital image processing technology combined with X-ray radiation technology is an advanced X-ray machine. On the basis of the original diagnostic X-ray direct film imaging, the real-time image is digitally processed by A/D conversion and D/A conversion, and then the image is digitized.

X-ray perspective principle: X-rays have strong penetrating power and can penetrate many substances that are opaque to visible light, such as ink paper and wood. This invisible ray can make many solid substances have visible fluorescence, make photographic negatives sensitive and ionize air. When X-rays pass through the human body, they will be absorbed to varying degrees. For example, the amount of X-rays absorbed by bones is more than that absorbed by muscles, so the amount of X-rays passing through the human body is different, which carries the information of the density distribution of various parts of the human body.