the final preparation. Ethylene glycol is the other major ingredient. This is commonly
called antifreeze. It is produced from oil and natural gas in a process of oxidizing and
hydrolizing ethylene gas. When DMT and ethylene glycol are mixed in heat for a long
period of time, polyester is formed. The polyester is reheated and given a two-way
stretch until its thickness is .007 inch.
b. Emulsion. When examined under a microscope (figure 1-3), countless tiny
crystals of silver bromide embedded in gelatin can be seen. Upon exposure and
development, these crystals are changed into irregular clumps and strands of black
metallic silver, which all together form the radiographic image. Figure 1-4 shows a
cross-section of an exposed and developed emulsion layer. Each emulsion layer is
about 0.001 inch thick. Since x-rays pass readily through x-ray film--and also through
intensifying screens--an emulsion layer is coated on both sides of the film base. This
provides a greater effect upon exposure to x-rays than would be possible with an
emulsion coated only on one side. Actually, when intensifying screens are used, the
fluorescent light they emit accounts for most of the exposure.
(1) Silver bromide crystal. In the early stages of emulsion manufacturing, a
solution of silver nitrate and potassium bromide is produced. This results in the
formation of silver bromide crystals and potassium nitrate. The potassium nitrate is then
washed out and the result is a silver bromide crystal consisting of bound atoms of silver
(2) Gelatin. Gelatin is a necessary consistent of the x-ray film emulsion. It
is an ideal suspension medium for the silver bromide crystals for three primary reasons:
It provides an even suspension for the crystals; it has the ability to swell, shrink, and
harden; and it increases the sensitivity of the silver bromide crystals.
Figure 1-3. Photomicrograph (2500X) of film emulsion.