Chapter 2 Film construction

Chapter 2 Film construction

RADIOGRAPHIC IMAGING SCIENCES

FILM CONSTRUCTION

• A film; is a special sheet used to display a radiographic
• image.
• There are two main groups of films
• Direct exposure films (x-ray films)
• Light only films
• Direct exposure films; these are films exposed to x-rays
• alone or to a combination of both x-rays and light.
• e.g. intra-oral films dental films and radiation monitoring
• film
• Light only films; these are films which are sensitive to
• light only. A film is a made up of several layers as shown below.
• (i)Film base; made up of cellulose triacetate or polyester.
• Polyester being the most common.
• Functions of film base
• Provides support and stability to emulsion layer
• Transmits light so that the image can be viewed.
• Desirable characteristics of film base
• Transparent and free of any defect
• Strong but flexible.
• Uniform in thickness
• Dimensionally stable throughout processing.
• Non-flammable.
• Chemically inactive.
• Uniform in colour.
• What is the thickness of film base?
• Thickness of film base depends on type of film and

its use.

• Medical x-ray film needs a film base of 0.18mm.(ii)Subbing/adhesive layer; it is situated between the
• base and emulsion layer.
• Functions
• It ensures that the emulsion layer adheres to the
• base material.
• To prevent any separation of the emulsion from the
• base during processing.
• (iii)The emulsion layer; it is the layer in which a
• radiographic image is formed.
• It consists of photosensitive materials suspended in
• gelatin.
• Types of films according to film construction
• Duplitized films; these are films with an emulsion
• layer on both sides of the base.
• Single sided film; these are films with one

emulsion layer only on one surface of the base.(iv)The supercoat; this is a thin layer of clear gelatin.

• Functions
• To protect the sensitive emulsion.
• To provide suitable surface characteristics.
• (iv)Non-curl backing; this is a layer which prevents
• curling in single-sided emulsion films.
• Since emulsion layer usually swells during
• processing, it gives single-sided emulsion films a
• tendency to curl.→ Duplitized films have no tendency to curl due to
• presence of emulsion layer on each side of the film
• base hence they have no non-curl backing layer.
• (iii)Anti-halation layer; it is the layer which prevents
• reflection of light from the film base back to the
• emulsion.
• When light is reflected from the film base back to
• the emulsion it causes halation.
• Halation refers to production of diffuse image or
• ‘halo’ around a proper image.Manufacturers use the following methods of preventing
• halation.
• Adding a dye to the non-curl backing.
• Adding a dye to the base.
• NB; → Crossover effect; is the type of halation which occurs
• when a film is used with intensifying screen.
• Irradiation; is the sideways scattering of light within the
• emulsion itself.
• Just like halation, irradiation is another source of image
• unsharpness.Light-sensitive and X-ray sensitive emulsions
• There are two fundamentally different conditions of
• exposure for the films used in an x-ray department.
• They may be exposed to visible light emitted by
• fluorescent intensifying screen.
• They may be exposed solely to radiation.
• Light-sensitive film
• Light sensitive films are films in which production of an

image is through exposure to visible light.NB; All films are light sensitive but this group of films refers to those films whose image has been formed after exposure to light.

• The emulsion layer of the film must absorb the energy of
• the visible light photons incident on it in order to form an
• image.
• Silver halide grains in the emulsion layer do absorb
• visible light photons particularly light in the blue-violet
• part of the spectrum.
• But for a photon to be absorbed, there must be a silver
• halide grain in its path.
• Therefore, to ensure efficient absorption, the emulsion
• must contain high concentration of silver halide.
• The following are ways to increase concentration of
• silver halide in order to ensure efficient absorption of
• light photons.
• Close parking of the silver halide grains
• Increasing the size of the silver halide grains
• Increasing the thickness of the emulsion layer
• Modifying the shape of the halide grains However, emulsion thickness must be limited to optimum
• thickness for several reasons;
• Image resolution tends to reduce as emulsion
• thickness increases.
• Thick emulsion layers require longer processing
• times.
• Most absorption of light occurs in the upper layers of
• the emulsion.
• Duplitized films
• Duplitizing of film is a method of gaining the benefits of
• increased emulsion thickness with few of the drawbacks.
• Duplitized film is designed to be used with two
• intensifying screens.
• It is the most commonly used type of x-ray film in a
• radiology department.
• After processing, a duplitized film has two identical
• images, one on each side of the film base.
• When a radiograph is viewed, the two images are
• superimposed and observed as one.
• Only on a very close observation it is possible to detect
• the dual imaged due to parallax effect.Advantages of duplitized films
• Increased sensitivity; adequate image density can
• be achieved from a smaller radiation exposure.
• Increased image contrast; contrast is the ability to
• distinguish between two different densities on the
• image.
• Disadvantages of duplitized films
• Loss of image quality; due to incidences like crossover effect.
• Economic reasons; they require a high coating
• weight of silver than single emulsion films. Silver is

an expensive resource.