Film Imaging and Processing Flashcards
Image receptor - film (LOOK AT SLIDES)
Protective coat is transparent layer (usually gelatine) to protect emulsion
Emulsion has silver halide (usually silver bromide) crystals held in gelatine
Adhesive layer sticks emulsion to the base
Base is translucent blue plastic to make structure sturdy
Image receptor - screen (LOOK AT SLIDES)
Protective layer is a transparent layer to protect the phosphor Phosphor can be one of several different materials All convert x rays to light Reflective layer reflects light back towards film Base is a strong plastic sheet to make structure sturdy
Image receptor - screen interaction (LOOK AT SLIDES)
X-ray photon will likely interact in phosphor layer above/ below film converting its energy to many (~1000) light photons Photons are emitted in all directions. Those that head towards film may activate silver halide in emulsion. As photons spread out from interaction point they cause larger area of blackening < resolution of system
Fluorescent materials
Calcium tungstate
Rare earth phosphors e.g. gadolinium & lanthanum
Yttrium (non-rare earth but similar properties)
-diff screens emit diff colours of light &
diff films are most sensitive to diff colours.
Thus films & screens must be matched to work
efficiently
Speed depends on
Thickness of phosphor layer in screen Crystal size in film emulsion Conversion efficiency Light absorbing dyes within screen -rare earth screens up to 50x faster than calcium tungstate
Thickness of phosphor layer in screen
Thicker phosphors have more chance of interaction with x-rays
Crystal size in film emulsion
Larger crystals give more blackening but worse resolution
Conversion efficiency
More light photons out for each x-ray photon interaction
Light absorbing dyes within screen
Prevent the light from spreading too far increasing resolution but
decreasing speed
Latent image formation
Film emulsion made up of silver halide crystals suspended in gelatine (>90% AgBr, <10% AgI)
When x-ray photon hits crystal in emulsion an e- from Br ion gets enough energy to escape
e- moves to defect in crystal or to Ag2S impurity known as sensitivity speck
Latent image formation after e- moves to sensitivity speck
This becomes -ively charged and attracts mobile silver ion from within crystal to form silver atom
Ag atom then acts as trap for a second e- and
process is repeated
In this way small deposits of silver will have formed in those
crystals hit by an x-ray photon
These crystals will then be more sensitive to developer and
whole crystal will be blackened
Developer
An alkali and acts as reducing agent
Converts remaining Ag+ ions to silver atoms by addition of e-
Process begins at sensitivity specks
-crystals which have been sensitised affected first
If film developed for too long all AgBr crystals will be converted to silver (black film)
Intermediate washing
Stops developer working on film
Removes developer from process
Fixing
Fixer is an acid
Removes unreacted silver halide crystals
Fixing duration
About twice the time taken to clear
Final wash
Fixer soln (containing silver and bromine compounds) still slightly light sensitive Must be removed otherwise it slowly reacts with silver image producing AgS (brown stain)
Stages
Latent image formation Developer Intermediate wash Fixing Final wash Dry it
Latent image fading
Silver atoms may, in time, revert to silver ions
Optical density (OD) may fall by 10-20% in 24 hours
Has implications for mobile units
