The rays name X refers to a radiation electromagnetic, invisible to the eye human, capable of traversing opaque bodies and print photographic films.

The current digital systems allow the collection and display of the radiographic image directly on a computer without having to print it. The wavelength is between 10 to 0.01 nanometers, corresponding to frequencies in the range of 30 to 30000 PHz (from 50 to 5000 times the frequency of visible light).