Processing The Latent Image

DMI 50B

Kyle Thornton

Hand Processing

• Films were immersed in tanks containing processing chemicals for a period of time

• The chemicals had to be mixed and stirred by hand

• The total time was about an hour from developer to drying

• Occasionally, a film was read right out of the developer– Thus the term wet read

Automatic Processing• The first automatic processor still immersed film

in tanks– Films were hung, and a conveyor did the rest of the

work– It could process about 120 films/hour– Total time was about 40 minutes

• Eventually, the roller transport system was invented

• This is what is used today in automatic processors

Automated Processing 1940’s Style

An Updated Version

Advantages Of Automatic Processing

• Saves time and manpower

• Processing time is reduced to 90 seconds

• The variability in results caused by hand-dunking is eliminated

• Radiographs are available in minutes

• There is no variance in temperature

• Allows standardization of techniques

• Overall, efficiency is improved

Converting the Latent Image to a Manifest Image

• Step I– Formation of the latent image by exposing the

silver-halide grains to x-radiation

• Step II– Development of only the exposed grains by

reducing agents

• Step III– Removal of the unexposed grains and making the

image permanent by fixing

Latent Image Formation

The Sequence Of Processing Steps

• Wetting– Loosens the emulsion to allow penetration of

developing chemicals

• Development– The stage in which the latent image is converted

to a manifest image

• Stop Bath– An acid solution to stop the development process

The Sequence Of Processing Steps

• Fixing– Removes any silver halide crystals that were not

exposed to radiation or light

• Washing– Removes any chemicals not rinsed off in the

previous steps

• Drying– Removes any water, prepares the film for handling,

storage, and viewing

Sequence of Processing Steps

Wetting Solution

• Water– This is the solvent for all processing chemicals– A solvent is what solids and powders are mixed

into

• In automatic processing, this step is combined with development

Development

• The developer solution is a reducing agent

• It reduces the silver ions of exposed crystals to metallic silver

• The main development agent is hydroquinone– Rapidly produces shades of gray

• The other developing agent phenidone– Produces black tones slowly

The Effect of Development Upon Silver Halide Crystals

Exposure and ReductionOf Silver Halide Crystals

Underdevelopment v. Overdevelopment

Developer Chemicals and Their Functions

Fixing

• The stop bath is combined with fixing in the automatic processor

• Fixing stops the developing process and makes the manifest image permanent

• The activator that neutralizes any residual developer is acetic acid

• The other fixing agents remove any unexposed or undeveloped silver-halide crystals

Fixing Agents And Their Purpose

Hypo Retention

• Occurs when fixing solution (thiosulfate or hypo) is not rinsed sufficiently

• This will cause a processed film to discolor over time

• This is caused by oxidation of the fixing agent• The result is a discolored film of poor quality• This means that the wash cycle is not working

properly

Washing

• The agent in the washing cycle is water

• The film is rinsed to remove any chemical residue

• Water is continuously cycled through the wash tank to ensure that it remains clean

• Poor washing can result in hypo retention or other chemical artifacts

Drying

• The final step of the processing cycle

• Hot air is blown over the film to dry it

• The dryer stage works independently of the processing stages

Safety

• Developer chemicals are caustic and can cause chemical burns

• It will stain fabric permanently

• If chemicals are splashed on skin, or in eyes, rinse immediately, and seek treatment

• Protective eyewear, and aprons should be worn if mixing chemicals

Processor Anatomy

• Transport System

• Temperature Control System

• Circulation System

• Replenishment System

• Dryer System

• Electrical System

Processor Anatomy

Transport System

• A series of rollers moving in precision convey the film through each stage of development at precise intervals

• Begins at the feed tray

• A microswitch is placed here that detects the size film and controls replenishment

• The transport system consists of three subsystems

Subsystems• Rollers

– 1 inch diameter rollers that convey the film through the processor

• Transport racks

– All of the rollers are positioned on a rack assembly

– At each bend there is a curved metal lip called a guideshoe to properly position the film for turning

• Drive subsystem

– Supplied by a motor

– Drives the rollers and a series of gears, belts, and pulleys

Transport System Components

Temperature Control System

• Temperature, time, and chemical concentration control development

• Optimum temperature is app. 95 degrees F

• The temperature is monitored by a thermostat

• This controls a heating element in the developer tank

Circulation System

• Continuously agitates the processing chemicals ensuring that they are properly mixed

• Water is continuously circulated through the processor

Replenishment System

• Monitors film activity and replenishes the solutions as needed

• This prevents oxidation of the solutions in the processor

• A microswitch at the feeding tray monitors film size and regulates appropriate replenishment

Feeding Tray

Dryer System

• A blower sucks in room air and blows it across a heating element in order to dry film

• This also serves as exhaust for the automatic processor

Electrical System

• Supplies power to the processor

• Each electrical component of the processor has its own wiring and fuses

Rapid Processing

• Reduces processing time to about 30 seconds

• The chemicals are more concentrated

• Fixer and developer temperatures are higher

• Rapid processing is very useful in surgery, and emergency where time is valuable

Extended Processing

• Used in mammography

• Allows for lower patient dose

• Film is in the developer longer

• Processing time if about 3 minutes

• Contrast is improved

Daylight processing

• A processor is still used• The means of loading cassettes and putting

films is different• The daylight system allows automatic loading

of cassettes for use• Also automatically unloads them into the

processor• Eliminates time in the darkroom

Daylight Processing Unit

Dry Processing

• Does not use any wet chemicals• Two techniques for dry processing

– Photothermography• Laser light prints image• Formed on silver halide• Image developed by heat

– Thermography• Uses a heat source (print head)• Heat creates the image in organic silver salts

– Photothermography renders more accurate detail– Thermography can look pixely, and dust can accumulate between

print head and film