After talking about the layers in a photographic film, now I am going to talk about the most important layer – emulsion.

Take the example of color negative film, the emulsion consists of:

  1. Gelatin
  2. Silver halide
  3. Sensitizing dye
  4. Dye coupler
  5. Yellow filter
  6. Orange color mask
  7. DIRs (Developer Inhibitor Releasing couplers)
  8. DIARs (Developer Inhibitor Anchimeric Releasing couplers)
Cross Section of an unexposed color negative film

Gelatin

Gelatin is a material made from the bone of cattle. It is gel liked and all photosensitive chemicals are mixed with it and then coated on the film base. In modern film, multi-layers of coating are needed which I will discuss later.

I heard a joke from an employee from a China photographic film manufacturer in 90s, said that China could not make good quality film because the quality of cow was not as good as the US. The quality of gelatin will affect the coating of film emulsion such as the thickness and the evenness.

Silver Halide

The word halide is derived from a chemical family called Halogen which consists of 4 elements – fluorine, chlorine, bromine and iodine. Their anions are fluoride, chloride, bromide and iodide so collectively called halide. They form compounds with silver ion and have the following photo sensitive characteristics:

HALIDESPEEDAPPLICATION
Silver chlorideSlowPhoto paper
Silver chlorobromideSlowPhoto paper
Silver bromideFastNegative film
Silver bromoiodideFastestNegative film
Silver iodideUnsuitable
Silver fluoride is not photo sensitive so it is not used

They have almost unique properties of light sensitivity and amplification that make them suitable for film. When light (a photon) strike on a silver halide crystal, it will strike off an electron from the halide ion and the free electron will combine with silver ion (Ag+) to form silver metal. This is the property that makes silver halide suitable for recording an image on film.

Silver chloride is the least light sensitive of the silver halides and is used in some photo paper. By adding a little bromide, to make silver chlorobromide, we can increase the speed.

But for films fast enough to be exposed in a camera, we need to use silver bromide or silver bromoiodide crystals for the high-speed negatives. Pure silver iodide is not used as it has other characteristics that make it unsuitable.

I will talk a bit more on the characteristics of silver halide later.

To be continued……

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