After
reading this section you will be able to do the following:
Identify
and explain the three important parts to a radiographic film.
Summarize
the basic process of developing film.
To understand how the image on a radiograph
is formed, we need to first look at the characteristics of the
film itself. There are three important parts to a radiographic
film. These include the base, the emulsion, and the protective
coating.
The
base
All radiographic film
consists of a base for which the other materials are applied.
The film base is usually made from a clear, flexible plastic such
as cellulose acetate. This plastic is similar to what you might
find in a wallet for holding pictures. The principle function
of the base is to provide support for the emulsion. It is not
sensitive to radiation, nor can it record an image.
The clarity or transparency of the
film base is an important feature. Radiographic film must be capable
of transmitting light. Once a film has been processed chemically,
it is subject to interpretation. This is commonly done by using
a film illuminating device, which is usually a high intensity
light source.
The emulsion
The film emulsion and protective
coating comprise the other two components and are essentially
made from the same material. They are applied to the film during
manufacturing and usually take on a pale yellow color with a glassy
appearance. Although they are made from the same material, they
offer two distinct features to the film. These features are separated
into the image layer of the emulsion, and the protective layer.
The protective
layer
The protective layer has the
important function of protecting the softer emulsion layers below.
It is simply a very thin skin of gelatin protecting the film from
scratches during handling. It offers very important properties
to film manufacturers, which include shrinkage (during drying
that forms glassy protective layers) and dissolving in warm water.
It will absorb the water and swell if it is dissolved in cold
water.
The softer layers of the gelatin coating
are technically known as the emulsion. An emulsion holds something
in suspension. It is this material in suspension that is sensitive
to radiation and forms the latent image on the film. During manufacturing
of the film, silver bromide is added to the solution of dissolved
gelatin. When the gelatin hardens the silver bromide crystals
are held in suspension throughout the emulsion. Upon exposure
of the film to radiation, the silver bromide crystals become ionized
in varying degrees forming the latent image. Each grain or crystal
of silver bromide that has become ionized can be reduced or developed
to form a grain of black metallic silver. This is what forms the
visible image on the radiograph. This visible image is made up
of an extremely large number of silver crystals each is individually
exposed to radiation but working together as a unit to form the
image.
Once a film has been exposed to radiation and possesses the latent
image, it requires chemical development. The purpose of developing
the film is to bring the latent image out so that it can be seen
visibly. There are three processing solutions that must be used
to convert an exposed film to a useful radiograph. These are the
developer, stop bath, and the fixer. Each of these solutions is
important in processing the image so that it may be viewed and
stored over a period of time.
The process
of developing film
1.
To begin the process of converting the latent image on the radiograph
to a useful image we first expose the film to the developer solution.
The developer’s purpose is to develop, and to make the latent
image visible. A special chemical within the developer solution
acts on the film by reducing the exposed silver bromide crystals
to black metallic silver. This process of developing is actually
a multi-step process. Recall the characteristics of the film manufacturing
mentioned earlier, they become important in the development process.
Before the developer can change the silver crystals it must penetrate
the protective coating of the film. Keep in mind that the protective
coating of the film is made of gelatin and is sensitive to temperature
and water. The developer solution is comprised of a combination
of chemicals, consisting of alkali and metol or hydroquinone mixed
with water. The purpose of the alkali is to penetrate the protective
coating allowing the metol to reduce the exposed silver bromide
to black metallic oxide.
2. The
second step in the development process is the stop bath.
This bath is comprised of a glacial acetic acid and water. It
is important to recognize that alkali’s and acid’s neutralize
each other. The function of the stop bath is to quickly neutralize
any excessive development of the silver crystals. Over development
of the silver crystals results in a radiographic image that is
virtually impossible to interpret.
3.
The third step in development is the fixer. Its function
is to permanently fix the image on the film. This is also a multi-step
process. The fixer must first remove any unexposed silver crystals
and then harden the remaining crystals in the emulsion. It is
this process that is used to preserve the radiographic image over
time.
4.
Once the film has been properly developed, it is then rinsed in
water and dried so that it may be visually examined.
Review:
The three
main part to radiographic film are the base, the emulsion, and
the protective coating.
Steps
in developing film include developing, stopping the developer,
fixing, rinsing and drying.
