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.

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

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 70°F.

• Developer stop bath fixer waterfilm dryer

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.

First Roller Transport Processor

• Produced by Kodak• Used rollers to

transport film through development.

• 10 feet long• In today’s dollars cost

was $225,000.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Automatic Film Processor

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

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 on tank to the next tank and remove chemicals from film.

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%.

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 ±0.5°F

• Fixer temp controlled by temperature of the wash water and developer.

• Water temperature is no longer controlled.

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.

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.

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.

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.

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.

Electrical System

• Provides power to the electrical subsystems.

• All circuits are fused or have circuit breakers.

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.

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.

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.

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

Types of Dry Chemical Processors

• Although there are several approaches to dry chemical processing, two technologies dominate.

• Photothermography (PTG)

• Thermography (TG)

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.

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.

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.

The End of Lecture