latent image formation

• crystals of silver halogens: exhibit sensitivity
• Carl Wilhelm Scheele, 1777

  • crystals of silver halogens exhibit sensitivity since the late 18th century
• Examples of halogens:

  • Silver Bromide
  • Silver iodide
  • Silver Chloride
• GURNEY-MOTT THEORY (1938)

  • Widely accepted explanation of formation of latent image in photography
• Gurney-mott theory states how metallic silver is formed at sensistivity centers located at imperfections in structure of silver halide crystals
• X-ray production

  • production of primary beam
  • first step in latent image formation
• PART OF INTEREST

  • Exit beam interacts with the image receptor
  • second step in latent image formation
• LUMINESCENCE

  • X-ray energies (Remnant Radiation) interacts with the IR, causing the outer shell electron of the phosphor material to become excited. Eventually, the excited electron will return to its relaxed state, releasing HEAT in the form of visible light
  • third step in latent image formation
• SECONDARY ELECTRON PRODUCTION

  1. Silver halide grains absorbs the light photons (From I.S)
  2. Light Photons interacts with the Br- Ions resulting to IONIZATION.
  3. This causes the release of secondary electron through Photoelectric effect or Compton effect.
  4. Secondary electrons (photoelectron or Compton electron) wanders about the grain
• Silver halide crystals contains a negatively charged surface
• Positively charged atoms are within the silver halide crystal
• Sensitivity speck/ sensitivity center/ center of defect: Site of latent image formation
• composition of sensistivity speck: allythiourea
• allythiourea is composed of a sulfur-containing compound which is silver sulfide
• Allythiourea causes the cations to lose its original lattice site and occupies an interstitial void (frenkel defect) resulting to a defect
• ELECTRON TRAPPING

  1. The wandering electron (Photoelectron or Compton electron) eventually gets trapped by the sensitivity speck.
• electron trapping gives the sensitivity speck a negative charge
• INTERSTITIAL SILVER ATTRACTION

  • The negatively charged sensitivity speck now attracts the positively charged silver ions.
• DISRUPTION OF CRYSTAL LATTICE

  • When attracted silver ion arrives at the sensitivity speck, its positive charge counteracts the negative charge of sensitivity speck. = NEUTRAL SENSITIVITY SPECK
• the neutral sensitivity speck causes deposition of metallic silver at the silver halide grain
• RECURRENCE OF STEP 4-9

  Cycle reoccurs and more metallic silver is deposited at the crystal until LATENT IMAGE is formed
• The latent image is intensified by the developer agent action and all exposed Silver Halide crystals will be entirely transformed into metallic Silver.
• Since radiation exposure provides few electrons only, developers will provide sufficient electrons to form a manifest image.
• fixer agent dissolves the Silver Halides into a solution. By this mean, now being soluble, they can be flushed out with the fixer agent.
• only the exposed parts are covered with metallic Silver
• unexposed parts of the film have been thoroughly cleaned by the fixer, and contain no more Silver Halides, nor do they contain any metallic Silver