The X-Radiograph and photograph of the model train engine (from the collection of Bruce Wagner) were taken in the X-ray facility at the Goldwater Center at Arizona State University.

An X-ray image of the internal structure of an object is called a Radiograph. The object is called the Specimen. The Specimen is placed between an X-ray generator, called the Source, and the recording medium, called the Target. In medical or art museum X-Radiography, the recording medium is special photographic film that is sensitive to X-rays. X-ray film becomes exposed wherever X-rays strike the Target. More X-rays at the Target cause greater exposure of the film and fewer X-rays at the Target cause less exposure of the film. Where the Specimen blocks the X-rays by absorbing them, thie Target does not get exposed. On the other hand, where the Specimen does not absorb the X-rays, they pass through the Specimen and the Target becomes very exposed. In most cases, the amount of exposure at the Target can vary continuously from fully exposed to not exposed at all, depending on the internal structure and composition of the Specimen. Figure 1 is a schematic view of the procedure.

Figure 1.

Upon development of the X-ray film, the differences in exposure produce differences in contranst on the film. This pattern of contrast corresponds to the internal structure of the object. Radiographs produced in this way are essentially still photographs of the internal structure of the Specimen. Areas of the film that are not exposed correspond to thick part of the Specimen or to parts of the Specimen that are made from elements with high atomic numbers, such as iron or lead.

Another type of X-radiography uses a fluorescent screen as the Target to produce visible light images of the internal structure of an object. The fluorescent screen is made of materials that emit visible light when X-rays strike the screen. The radiograph produced in visible light corresponds to a radiograph produced by exposure of film. Security systems at airports use fluorescent screens with television cameras to produce radiographs of baggage and other objects for quick inspection of their contents and/or internal structure. The Fluoroscope, a medical device, also uses a fluorescent screen to produce dynamic images that help doctors to diagnose problems that cannot be observed with the static images produced by film.

At the Goldwater Center at ASU we use a fluorescent screen and conventional polaroid film to produce images similar to traditional X-Radiographs (see Fig. 2). The fluorescent screen and polaroid film are assembled together such that the film to be exposed is pressed agaginst the screen and held in place by a cartridge that protects the polaroid film from exposure by ambient visible light. The cartridge, with the screen and film inside, is the Target. It is important to note that the polaroid film is sensitive to visible light only. X-Rays will not expose the polaroid film. When X-rays strike the fluorescent screen, it emits visible light in a pattern that conveys the pattern of X-rays on the screen. Hence, instead of exposing the film directly, the X-rays generate a visible light image on the fluorescent screen which then exposes the polaroid film in the traditional photographic way. The exposed polaroid film is then removed from the cartridge in a manner similar to a modern polaroid camera. The positive print is available in 1 minute.

Figure 2.

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