• A radiographic exposure can be made directly on to photographic film but this requires a relatively long exposure time. It is usual, therefore, for most examinations to use calcium tungstate fluorescent screens which have the property of transforming the invisible X-ray radiant image to a visible blue image.

• Film emulsion is far more sensitive to blue light than to X-rays and thus these screens "intensify" the effect of the X-ray beam and allow the amount of radiation necessary to produce a radiograph to be reduced to one-tenth to one-fifteenth of what would otherwise be required.

• An intensifying screen consists of a uniform homogeneous coating of minute calcium tungstate crystals mounted on a plastic base.

• Two screens are used inside a film cassette to sandwich the special X-ray film, which is coated with emulsion on both sides, in close and uniform contact.

The care of intensifying screens:

• Screens are expensive and easily damaged. An unnoticed splash of developer, or a knock which causes an abrasion of the surface, will permanently damage the screen, and an artifact will be present on all films taken in conjunction with the damaged screen.

• Dust and animal hair are also constant nuisances which should be regularly removed with a soft brush, as they cause extraneous shadows on a radiograph.