Radiographic processing
Principles
Introduction
►A
latent image is formed in the film emulsion when
exposed to different intensities of radiation passed
through an object.
► Chemical processing of the film is necessary to
make the latent image visible and permanent
► Film processing involves a number of complex
chemical reactions whose activity and efficiency
are influenced by various factors such as;
 Temperature
 Acidity or alkalinity (pH)
of the chemicals used
Processing cycle
Dev.
Rinse
Fix
Wash
Dryer
Processing stages
Manual processing
Automatic processing
► Development
► Development
► Rinsing
► Fixing
► Fixing
► Washing
► Washing
► Drying
► Drying
Development
► Primary
purpose is to convert into visible
form the invisible latent image
► The silver halide grains affected by
exposure are reduced to metallic silver
► Silver halide grains unaffected by exposure
remains largely unchanged
Chemical action of developer
► The
emulsion becomes wet
► Chemicals penetrate in to the emulsion
► A process of chemical reduction takes place
► Developer donates electrons to the silver ions in
the exposed silver bromide grains
► Silver ions by obtaining electrons converted into
metallic silver
(The mode of action is not fully known but
explained as follows)
Result of exposure on silver halide
Unexposed AgBr grain
Br
Br
Br
-
Ag+
-
Exposed AgBr grain
Br
-
Br
-
Ag+
Ag+
Br
Ag+
Ag+
Ag+ Ag+
-
Br
Br
-
Br
X ray
photon
Br
-
-
Ag+
-
-
Ag+
Ag
Ag Ag
e
Br
Br
Br
Sensitivity
Spec
Sensitivity specs are deliberately formed during film
emulsion production
When exposed to radiation The bromine ion release
electrons which lodge in the sensitivity area resulting
attracting and neutralizing silver ions
Mode of action of Developer
Unexposed AgBr grain
Br
Br
Br
-
Ag+
-
Exposed AgBr grain
Br
-
Br
-
Ag+
Ag+
Ag+
Ag+ +
Ag+ Ag
Br Br
Br
Br
-
Br
-
-
Ag+
-
-
Ag+
Ag
Ag Ag
Br
e
Br
Br
Sensitivity
Spec
e
Ag
Br
Mode of action
► AgBr
grains are surrounded by a negative
barrier of Br ions
► The charge barriers protect the silver ions
from attack by electrons
► Exposed AgBr grains possess a weakness in
the charge barrier caused by the presence
of neutral silver atoms at the sensitivity
specs (development centre)
► The development centre enables electrons
from the developer to penetrate grain and
reduce all its silver into metallic silver
Result of over development
► If
a film spends too long a time in the
developer, or if the develop is hyperactive
the charge barrier around unexposed grains
may be unable to prevent electron
penetration, and unexposed grains will be
reduced to metallic silver
► Chemical fogging is the result
Effects of the by-products of
development
► With
continuous use, the developer
becomes depleted of electrons and it is said
to be oxidized and reduced in activity
► The remaining negative Br ions react with
positive hydrogen ions in the solution to
form hydrobromic acid.
H
► This causes the p to fall and reduces the
developer activity
► The atmospheric oxygen also cause
developer oxidation and reduce activity
Constitution of developer solution
► The
developer in automatic processing and
manual processing are different in several
aspects
► In automatic processing the solution added
to the working developer in the tank is
called developer replenisher
► When the developer tank is emptied and
fresh solution is added the activity of the
replenisher is modified by adding a chemical
called the starter solution
Developer replenisher
+
Starter
=
Working developer
Constituents of developer replenisher
solution
► Solvent
► Developing
agent
► Accelerator
► Buffers
► Restrainers
► Preservative
► Hardener
► Sequestering
agents
Solvent – water
Functions
► Acts as the carrying medium in which the
developer constituents are dissolved.
► Provides a means of controlling the developer
activity by diluting its effects.
► Has a softening effect on film emulsion to allow
the chemicals to penetrate into the emulsion and
reach AgBr grains
► (The water should be clean enough not to have
any chemical reaction with the developer
constituents
► It should not contain mud & dirt or particles which
may cause damage to the delicate film emulsion)
Developing agents
► Are
reducing agents which supply electrons
► Should exhibit the following properties
 Must be selective and distinguish between
exposed and unexposed grains
 Sufficiently high activity
 As resistant as possible to the presence of
bromine ions in the solution
► No
Single reducing agent which satisfies all
of these requirements
► Therefore combinations are used
► Metol
& Hydroquinone (MQ) were use in
early manual developers
► Phenidone & Hydroquinone (PQ) are used in
modern automatic processing
Phenidone = 1-pheyl-3-pyrazolidone
► Quick
acting
► Selectivity low
► Mildly sensitive to bromine ion in
concentration
► Produces low contrast characteristics
Hydroquinone
► Requires
a strongly alkaline medium in
which to act
► More selective than phenidone
► Slow to start the development action
► But once started acts rapidly on exposed
AgBr grains
► Does not affect lightly exposed or
unexposed grains
► Produces high contrast characteristics
Advantages of PQ developer
► When
phenidone and hydroquinone are
together in one developer their
characteristics complement each other
► Therefore PQ developers have the following
advantages over the other developers
► Tolerance
of increase in bromine ion
concentration
► High selectivity and low chemical fog
► Adequate activity even in low concentration
► Available in liquid concentration form
► Fast acting permitting complete
development in 20-30 seconds
► Adequate contrast characteristics
► Super-additive effect (the reducing effect of
combination is much greater than the sum
of the effects they produce when used
separately)
Super-additive effect
Optical density
The shape of the characteristic curve is
controlled by the two developing agents
Combine
effect
Phenidone
only
Exposure
Hydroquinone
only
Accelerators
► PQ
developers need an alkaline medium to
operate
► It is established by adding a strong alkali
► Usually Potassium carbonate or Potassium
hydroxide is used
► The alkali is known as accelerator since its
function is to accelerate the developing
process
PH and development
pH of the developer has a marked effect
on its activity and on image contrast:
► The
 Low pH – low activity and development sluggish
 High pH – high activity and chemical fog
range of pH values for different
developers will vary from 9.8 to 11.4
H
► However for a particular developer the p
should not vary.
► The
Buffers
►A
buffer is a chemical compound which has
the effect of maintaining the pH of a solution
within close limits
► Potassium sulphite used as the preservative
and the potassium carbonate used as
accelerator provides an adequate buffering
action
Restrainers
► Modifies
the behaviour of developing agents
so that they become more selective
► That is to reduce the tendency to convert
unexposed silver halide grains to metallic
silver and therefore to prevent chemical
fogging
► KBr-The developer by product will act as a
restrainer
► In PQ developer benzotriazole is used as a
restrainer and an antifoggant.
Preservatives
► To
check oxidation of developing agents by
the action of atmospheric oxygen
► Additionally it permits more efficient
regeneration of Phenidone by hydroquinone
► Potassium sulphite is commonly used
Hardeners
► To
prevent excessive swelling of the
emulsion by the absorption of water which
is pronounced in the alkaline developer
► Powerful hardeners like glutaraldehyde may
employed – they are more effective in
alkaline developer
► But manufacturers use alternatives because
of its undesirable allergenic effects
► Manual processing does not need hardeners
Sequestering agents
► These
are chemicals which prevent
precipitation of insoluble mineral salts which
tends to occur in hard water
► They act by combining with the minerals to
form soluble compounds which do not react
with the developer chemicals
► Compounds based on EDTA are commonly
used
Other constituents
► Bactericides
► Fungicides
Starter solution
► The
starter solution is added to the fresh solution in
the developer tank at the start
► To reduce the initial activity of the developer to the
safe working level by bringing down the pH to the
correct value
► It is an acidic restrainer which provides KBr until the
development process produce its self restraining
effect
► Developer replenisher + starter = working developer
► Manual processing does not use starter solution
Efficiency of development & Factors
affecting development
► How
is the efficiency of development
measured? Or assessed?
 Assessing the optical density
 Assessing the contrast
► Factors
influence the quantity and quality of
development are
 Constitution of the developer
 Temperature of the developer
 Development time
How Constitution of the developer affect
development?
The activity of the developer depends on




Type of developer
Concentration
pH
Concentration of restrainer & anti-foggant
The developer activity reduces with use
because,
The concentration of developing agent and
pH drops, while the concentration of
restrainer increases
► Therefore
the developer has to be replenished
continuously if the activity is to be maintained.
► The rate of replenishment depends on
 The area of film processed
 Type of emulsion
 Type of image
 Aerial oxidation
► Not only the activity but the volume of developer
also is reduced with use because of
 The developer carry over by the films
 Evaporation
► The
replenishment compensates for this too
How does developer temperature affect
development?
► The
developer activity increases with temperature
► The temperature has to be controlled and
maintained if development is to be standardized
Methods used – temperature & cycle times
► High
temperature development ; 38 – 42 0 C , 90S
► Low temperature development : 20 – 30 0 C, 7m 90s
► Medium temperature development ; 37 – 33 0 C,
90s-160s
Effect of increase of temperature
►A
slightly increased temperature (0.5 0C)
without compensating reduction of
development time results in:
 Increased image density for the same exposure
 Slightly increased chemical fog
 Increased image contrast
►A
more severe rise in temperature (several
degrees) initially leads to
 Gross increase in density
 Unacceptably increase in chemical fog
 Reduction in contrast
Effect of increase of temperature
Effect of decrease of temperature
►A
slightly lowered temperature will result in:
 Decreased image density for the same exposure
 Slightly reduced chemical fog
 Reduced image contrast
►A
more severe fall in temperature will result
in:
 Gross overall reduction in density
 Loss of contrast and reduced D-max
Effect of decrease of temperature
Developer temperature in manual
processing
► Manufacturers
of manual processing
recommend a temperature of 20 0 C for
optimum result. But they provide acceptable
results two or three degrees high or low
provided the developing time is adjusted
accordingly
Development time
► This
is the time the film is kept in the developer
► In automatic processing it may be defined as the
time interval which elapses between the entry of a
specified part of a film (e.g. leading edge) into the
developing solution and the exit from the
developing solution of the same part of the film.
► It varies with the full cycle time of the processor




7 m -------- 130 s
3 ½ m ------- 68 s
110 s --------- 30 s
90 s ---------- 26 s
► Development




time is determined by:
Developer concentration
Developer temperature
Film type
Agitation of developer solution
► Agitation
of solution in automatic processing
is achieved by
 Film motion
 Rotation of rollers
 Circulation of developer
Summary
► Development
is the most important part of
processing
► The quality of the recorded image greatly
depend on development
► To maintain standard development following
has to be maintained as constant as
possible
 Developer concentration by constant
replenishment
 Constant temperature
 Constant time