• this is a bleach
• it oxidizes silver metal and forms Silver Ferricyanide
  • this is an insoluble salt so it stays in the gelatin
• the bleach has no effect on the remaining (unexposed) silver chloride
• the bleaching step should be done under low light level to avoid darkening this AgCl

  • We now expose the film to light
    • ordinary room light is fine
  • The put it in a developer again
    • All the silver halide is now exposed and develops as black silver metal
    • (remember, this occurs in the areas where the original scene was dark)
  • Then stop and fix the image
    • The thiosulfate will dissolve the Silver Ferricynanide for the earlier step
    • That was in areas where the scene was well lighted
    • So those areas are now clear
    The result-- a Photographic positive

The reversal process was at the heart of the earliest color photography

PhotoMuseum Web page, reference to Autochrome process

• You start with an ordinary photographic film (actually a glass plate coated with AgCl)
• You glue to the other side a thin layer of potato starch
  • the starch gains we previously dyed red, green and blue and mixed
  • effectively we've got millions of tiny color filters
  • Note: the starch granules and their dyes remain intact throughout the processing
• The plate is exposed through the starch layer
  • the starch below a red granule is only exposed by red regions of the scene
    • likewise for blue and green regions
    • other regions (like yellow) might affect AgCl below two filters
• The film is then developed by the reversal process
  • the area below the red granule was exposed, developed and bleached away
  • when we look through the finished product this region will look red
    • after all, the light still must travel through the red filter
  • the other colors affect their areas in a similar manner
  • the result is a full colored picture, direct from the developing baths
• The picture is a little fuzzy (viewed through tiny granules)
  • the method needs longer exposure (each granule throws away much of the light)
  • up close, the picture shows tiny color dots
    • it's an early version of what we now call pixels
    • it's the way television, monitors and digital prints display color today

• in the 1970's the Polaroid Corporation tried a similar approach to rapid color slides and rapid 8 mm color movies
  • this was in that wasteland of time before everyone had a video camera
    • yes, Virginia, parents did take 8 mm color films of the family at play and vacations
    • they then sent them (the film, not the family) out to be developed and could view their pictures in a week of two
  • Polaroid was in the business of Instant snapshots and they decided the world needed nearly instant movies
    • The film was made on a plastic backing that had tiny colored plastic squares (the film had color stripes)
      • technology advances-- plastic replaces potato
    • The B&W film was exposed through these filters
    • The chemical processing involved reversal development
      • This was done by cranking the film into and out of a developing module
      • The film emerged in about 3 minutes as a full color movie
• The product was not a commercial or an artistic success
  • quality was poor
  • processing was awkward
  • cost was much higher than conventional film

• The Polaroid Corporation pioneered and still dominates the field of Instant Photograph
  • The older black and white version is easier to explain than the newer color films
• The firm was basically an ordinary black and white film with a layer of silver halide in gelatin
  • It was packaged in a different style since it needed to carry a full darkroom with it
• When the picture was taken one pulled on the film and it advanced through steel rollers
  • the newer cameras do likewise, but they sue a small motor
• A small packet of developing gel was broken and smeared across the face of the film
  • At the same time another roll of paper was brought up against the negative and the developer
  • Ordinary developers converted the exposed silver to metal and locked it permanently to the negative
• The real trick came next
  • Another agent, similar to fixer, dissolved the unexposed silver halide
  • This silver diffused across the developing gel and reached the paper
  • The paper was coated with a powerful reducing agent and it converted all of the silver salt into silver metal, depositing it on the paper
• After 60 seconds, the two layers were pulled apart and the paper contained a finished black and white positive print.