FILM CONSTRUCTION
This film basically has two parts -- the base and the emulsion
Fig. 9.1). Most films have the emulsion coated on both side and therefore are
called double-emulsion film. Between the emulsion and the base is a thin
coating of material called the adhesive layer to ensure uniform adhesion of the
emulsion to the base . The emulsion is enclosed by a protective covering of gelatin,
called the super coating. This protects the emulsion from scratching, pressure,
and contamination during use and processing and allows for relatively rough
manipulation of X-ray film before exposure. The thickness of the 5. sheet of radiographic film ranges
form 200-300 um (about10.25 mm).
The double coating film helps in getting better density and
contrast and also to reduce the exposure required. This also helps to avoid
curling of the film.
The Base -
The base is the foundation for radio graphic film and it provides a rigid
structure into which the emulsion can be coated. It is flexible and unbreakable
and allows easy handling of films. The base of radio graphic film maintains its
size and shape during use and processing so that it does not contribute to
image distortion. This property of the base is known as dimensional stability. The
base is inert so that sensitometric property of the emulsion is not affected.
The base is of uniform lucency for viewing the radio graphs clearly. During
manufacture, dye is added to the base to slightly tint the film blue. Compared
with untinted film, this coloring results in less eye strain and fatigue for
the radiologist.
Two types of bases are available-cellulose triacetate and
polyester. Polyester is more resistant to wrapping with age and stronger than
cellulose triacetate. Its dimensional stability is superior. Polyester bases
are thinner than triacetate bases (approx. 160 um compared with 180 um
respectively) but are just as strong. The polyester base is similar in
composition to the polyester fibres in clothing
The Emulsion - The emulsion is
the heart of the X-ray film. It is the material 2 which X-ray or light photons
from screen interact and transfer information. The emulsion consists of a
homogenous mixture of gelatin and silver halide crystals. Besides above, also
contains various additives like chemical sensitizers, wetting agents
antifoggants, hardeners, etc. which impart the required qualities to the film. Emulsion
should be clear so at it transmits light and is sufficiently porous for the
processing chemicals to penetrate to the crystals of silver halide during processing.
its principal function is to provide mechanical support for the silver halide crystals
by holding them uniformly dispersed.
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