Operating Manual

5958

The X-ray film

and its properties

7.1 Structure of the X-ray film

An X-ray film, total thickness approx. 0.5 mm, is made up of seven layers,

see figure 1-7 consisting of :

A transparent cellulose triacetate or polyester base (d). On both sides of this base

are applied:

• a layer of hardened gelatine (a) to protect the emulsion

• emulsion layer (b) which is suspended in gelatine, sensitive to radiation

• a very thin layer called the substratum (c)

which bonds the emulsion layer to the base

The normal X-ray film, therefore, has two coatings of emulsion doubling the speed

compared to a film with a single emulsion layer. Photographic emulsion is a substance

sensitive to ionising radiation and light, and consists of microscopic particles of silver

halide crystals suspended in gelatine.

Note: In the past radiography on paper was not unusual. In this ‘ instant cycle’ process

results became available within 60 seconds. The quality of the images, however, was extre-

mely poor and the life of the film limited to a few months.

The availability of better and faster “instant cycle” techniques such as digital radiography

(see chapter 16), has made radiography on paper obsolete.

7.2 Radiographic image

Latent image

When light or X-radiation strikes a sensitive emulsion, the portions receiving a suffi-

cient quantity of radiation undergo a change; extremely small particles of silver halide

crystals are converted into metallic silver.

These traces of silver are so minute that the sensitive layer remains to all appearances

unchanged. The number of silver particles produced is higher in the portions struck by

a greater quantity of radiation and less high where struck by a lesser quantity.

In this manner a complete, though as yet invisible, image is formed in the light-sensiti-

ve layer when exposure takes place, and this image is called the “ latent image”.

Before and after exposure, but prior to development of the film, the latent image has a

shiny pale green appearance.

a.layer of

hardened gelatine

b. emulsion layer

c. substratum

(bonding layer)

c. substratum

d. cellulose

triacetate or

polyester base

a.layer of

hardened gelatine

Fig. 1-7. Schematic cross-section of an X-ray film, total thickness approx. 0.5 mm.