Chapter 14 Processing the Latent Image

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Chapter 14 Processing the Latent Image

• The processing of the latent image produces the
  manifest image.
• The exposed silver ions in the silver halide
  crystals are converted to microscopic black
  grains of silver.
• Films can be processed with manual dipping or
  with automatic film processors.

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Six Steps of Processing

• Wetting
• Developing
• Stop bath
• Fixing
• Washing
• Drying
• (manual processing only)

• Swells the emulsion for better chemical
  penetration.
• Produces a visible image from the latent image.
• Terminates development and removes excess
  chemicals from emulsion.
• Removes remaining silver halide and hardens
  gelatin.
• Removes excess chemicals
• Removes water and prepares film for viewing

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Manual Processing

• Film is hung on metal frames and dipped into
  tanks of processing chemicals. Process take about
  1 hour per film.
• Film immersed in developer for 5 minutes at 70F.
• Developer? stop bath? fixer? water?film dryer

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First Automatic Processor

• First automatic processor introduced in 1942 by
  Pako.
• Used film racks.
• First automatic processor reduced processing time
  down to 40 minutes per film.

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First Roller Transport Processor

• Produced by Kodak
• Used rollers to transport film through
  development.
• 10 feet long
• In todays dollars cost was 225,000.

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Automatic Processor

• In 1965 Kodak introduced the 90 second processor.
• Capacity of 215 films per hour.
• 1987 Konica introduced 45 second processing.
• Today processors are available with processing
  times from 45 seconds to 2.5 minutes.

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Developer

• The wetting and developing steps are combined in
  automatic processors.
• Development converts the latent image to the
  manifest image.
• Wetting softens the gelatin to allow the
  developer to come in contact with the silver
  halide crystals.

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Developer Components

• Water wets the emulsion
• Hydroquinone develops the dark areas of the film.
• Phenidone develops the grays.
• Glutaraldehyde hardens the emulsion.
• Sodium Carbonate is a buffer
• Sodium Sulfite is a preservative

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Developer Components

• Potassium Bromide is an antifogging agent that
  keeps the unexposed crystals from being
  developed.
• Sodium Sulfite helps controls oxidation.
  Developer turns brown when it oxides.
• Stored in air tight containers to minimize
  oxidation.

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Importance of Proper Development

• Ideally, proper development implies that all of
  the exposed crystals containing the latent image
  are reduced to metallic silver.
• Development is not perfect so some of the latent
  image is not reduced and some of the unexposed
  crystals are.

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Importance of Proper Development

• Development is a chemical reaction governed by
• Time
• Temperature
• Concentration of the developer
• Long time with low temperature or high
  temperature with short time will work.

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Importance of Proper Development

• With proper concentration, the reducing agents
  are more powerful and can penetrate both exposed
  and unexposed silver halide crystals.
• The film and chemical manufactures have carefully
  determined the proper parameters for proper
  development of the film.

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Importance of Proper Development

• Any deviation from those parameters will result
  in a loss of image quality, usually resulting in
  fog.
• Fog causes an increase in base fog and a drop in
  contrast. A fogged image is gray with poor
  contrast.
• Three ways to fog film.

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Three Ways to Fog Film

• Chemical fog contaminated developer, high
  temperature, slow transport.
• Radiation fog unintentional exposure to
  radiation.
• Improper storage wrong safelight or storage in
  high heat and humidity, expired or out of date
  film.

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Fixing the Image

• Once the image is developed, it must be treated
  so it will not fade but remain permanently.
• This is called fixing the image.
• We must stop development as soon as the film
  leaves the developer tank. Acetic acid is used as
  the stop bath in the fixer. This is referred to
  as the activator.

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Fixing the Image

• Ammonium thiosulfate removes the undeveloped
  silver bromide from the film. This is referred to
  as clearing the film.
• Hypo Retention is the undesired retention of
  fixer on the emulsion. It is caused by improper
  improper washing of the film.
• Fixers slowly oxidizes to form silver sulfide
  which turns the image yellow- brown.

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Fixing the Image

• The undeveloped and unreduced silver bromide is
  removed from the emulsion during fixing. The
  emulsion shrinks.
• A hardener is used to speed this process causing
  the emulsion to become rigid. Potassium alum,
  aluminum chloride or chromium alum are hardeners.

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Fixing the Image

• Hardening of the image is important for proper
  transport of the image through the processor and
  to permanently fix the image.
• Used Fixer will contain silver making it toxic to
  aquatic life. Must be processed as hazardous
  waste.

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Wash

• Once the image is fixed, all remaining chemicals
  must be washed off the film with water.
• The water in the wash tank is used to stabilize
  the developer temperature.
• Inadequate washing results in fixer retention.

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Drying

• Warm and dry air is blown over both sides of the
  emulsion to dry the film as it moves through the
  dryer assembly of the processor.

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Automatic Film Processor
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Components of a Automatic Processor

• Transport system
• Temperature Control
• Circulation
• Replenishment
• Wash
• Dry
• Electrical

• Moves film
• Controls Developer temp
• Agitates chemicals
• Maintains concentration
• Removes chemicals
• Removes moisture vents exhaust
• Fused power

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Transport System Functions

• Moves film through processor at the correct
  speed.
• Entrance Rollers activates replenishment of
  developer and fixer.
• Racks of rollers used to move film.
• Crossover Racks move film from one tank to the
  next tank and remove chemicals from film.

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Transport System Functions

• Turn around or master rollers turn the film
  around at the bottom of the tanks.
• Crossover and Turnaround rollers have guide
  shoes. If out of adjustment, can scratch film.
• Motor drives gears that turn the rollers. Speed
  controlled to within 2.

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Temperature Control

• A heater, thermostat and tubes running through
  the bottom of the wash tank controls the
  developer temperature. In California it must be
  within 0.5F
• Fixer temp controlled by temperature of the wash
  water and developer.
• Water temperature is no longer controlled.

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Circulation System

• Pumps are used to provide agitation of the
  chemicals as they pass over the film. This
  provide even development and mixing of the
  chemicals.
• Filters remove impurities and flecks of gelatin
  that are dislodged from the emulsion in the
  developer.

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Replenishment System

• Each time a film passes through the chemicals,
  fresh chemicals are pumped into the tank.
• This maintains the proper concentration and level
  of chemicals in the tanks.
• Developer replenishment is 60 to 70 ml for each
  14 x 17.

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Replenishment System

• Fixer replenishment is 100 to 110 ml for a 14 x
  17.
• The developer overflow can mix with the water
  overflow and be released down the drain.
• Fixer overflow is captured as hazardous waste.

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Wash

• Cold water flows into the processor when films
  are being processed.
• If flow of water is not adequate or if too much
  fixer gets into the wash water, it becomes
  hazardous waste.
• Water dilutes the overflow developer so it can be
  safely discharged.

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Dryer System

• Dryer removes all of the moisture from the film.
• Consists of a heat coils, thermostat, ducts and
  blower. Heat should be exhausted to the return
  air system of the dark room.
• Some processors used Infrared Heater to dry the
  film.

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Electrical System

• Provides power to the electrical subsystems.
• All circuits are fused or have circuit breakers.

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Alternative Processing

• New processors can process the film in 45
  seconds. This is called Rapid Processing.
• Extended Processing is used in mammography to
  reduce dose and increase contrast. The developer
  temperature is raised and the drive slowed to
  extend development.

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Alternative Processing

• Daylight Processing The processor automatically
  unloads the film from the cassette and feeds it
  into the processor. No darkroom is needed. The
  cassettes are automatically reloaded.
• Takes about 15 seconds compared to two minutes in
  the darkroom.

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Dry Processing

• Dry processing refers to development of the
  images without the use of wet chemicals.
• It continues to advance and replace conventional
  chemical based film processing.
• Used for printing computer based images from
  digital radiography, CT, MRI, Nuclear Medicine
  and Ultrasound.

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Dry Processing Advantages

• Elimination of handling, maintenance and disposal
  of processing chemicals.
• No Darkroom required (space saved)
• No plumbing required
• Less environmental impact
• Reduced capital costs
• Reduced operating costs
• Higher throughput

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Types of Dry Chemical Processors

• Although there are several approaches to dry
  chemical processing, two technologies dominate.
• Photothermography (PTG)
• Thermography (TG)

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Photothermography (PTG)

• PTG uses a low power modulated laser beam to
  record the image signal on the film.
• The latent image so formed on the sensitized
  silver halide is developed by a thermal process
  at 125ºC that take about 15s.
• This is referred to as the dwell time.

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Thermography (TG)

• Thermography uses a modulated thermal print head
  that coverts electrical energy into heat using
  resistive elements.
• No latent image is produced as the organic silver
  salts are developed directly by the heat.

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PTG or TG

• PTG is generally superior because the laser is
  easier to modulate compared to the thermal print
  head.
• PTG images are generally sharper and less
  pixilated.

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