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Studying how latent images are developed, I came across the concept of "Photographic Sensitivity".
I imagine it to be related to the behaviour of light-sensitive material, but I couldn't enough material nor information to be certain about it.
I read this Wikpedia paragraph, and some articles online but they suppose a strong background that I don't have. I'd love a simple explanation of this concept.

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There are at least three major accepted usages of the phrase photographic sensitivity.

  • Saying that a particular chemical compound has photographic sensitivity means that a material has a measurable chemical reaction to light.
  • Sometimes we refer to the degree to which that material reacts to a specific amount of light for a specific duration as that material's photographic sensitivity.
  • The academic field of photographic sensitivity is concerned with how the light sensitive chemicals we have been using for photography for over 150 years actually work. Even though we've been making photographs with silver halides for a long, long time, we are still in the process of discovering exactly how it all fits together from a chemistry and physics perspective.

In chemical photography light causes a chemical reaction to materials contained in a photographic emulsion spread on a substrate. That substrate can be glass, metal, paper, or translucent film.

How many photons are needed for each crystal of the chemicals in the emulsion to react and chemically change determines its photographic sensitivity. The crystals are primarily made of a silver halide. That is, a salt that contains silver and one of the group VII halogens. For photographic use the three used are silver iodide (AgI), silver chloride (AgCl), and silver bromide (AgBr).

Emulsions that are more sensitive to light require less exposure to transform bits of the silver halide crystals they contain into small specks of silver we call sensitivity specks. These bits of silver are what we consider to be the latent image. When a chemical reaction to the photons that make up visible light change a bit of a silver halide crystal into a sensitivity speck on the surface of a silver halide crystal that change can not be visually observed. Only when chemical developer is added does the silver in the sensitivity speck react with the developer to transform the entire silver halide crystal of which it is a part into atomic silver that is visible in the negative.

From the Wikipedia article on sensitivity speck:

When a photon is absorbed by a silver halide crystal, a free-carrier (electron in the conduction band) is generated. This free-carrier can migrate through the crystal lattice of silver halide, until captured by the shallow electron trap, where the electron is likely to reduce an interstitial silver ion to form an atomic silver. Subsequent exposure can grow the size of silver cluster through the same mechanism. This forms the latent image where the silver cluster becomes large enough to render the entire crystal developable in developer solution.

As the article on latent image cited in the question says,

A pure, defect-free crystal exhibits poor photographic sensitivity, since it lacks a shallow electron trap that facilitates the formation of a latent image.

Shallow electron traps are created by sulfur sensitization, introduction of a crystalline defect (edge dislocation), and incorporating a trace amount of non-silver salt as a dopant. The location, kind and number of shallow traps have a huge influence on the efficiency by which the photoelectrons create latent image centers, and consequently, on photographic sensitivity.

To increase the sensitivity of a silver halide to light we can add other materials that create what are known as shallow electron traps in each silver halide crystal. The kind and amount of these other materials added to the silver halide crystals increase the number of shallow electron traps by varying amounts and thus affect how sensitive the crystals are to light. The more shallow electron traps we can create, the more sensitive the crystals are to light.

Basically, the more imperfections we can create that will act as shallow electron traps in the silver halide crystals, the fewer photon strikes it will take to create sensitivity specks on the surface of a crystal. These sensitivity specks react with photographic developer to transform the entire crystal of which they are a part to atomic silver, which is what gives a photographic negative optical density.

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    Thank you for this detailed and wonderful explanation. I'd love if at the beginning or the end you could add a little paragraph stating what for you would be a proper definition of photographic sensitivity ( like a small concise summary). Something that goes like "So, you can think of Photographic Sensitivity as" or "So Photographic Sensitivity is". Thank you anyway! – Gabriele Scarlatti Dec 11 '17 at 19:42
  • The reaction of the silver halides in a film emulsion depends on the rate at which the particles are exposed to photons, not the raw number exclusively. That is why reciprocity failure occurs. – user50888 Dec 11 '17 at 21:31
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    "I'd love a simple explanation of this concept." I tried to make it as simple as possible. reciprocity failure has already been well covered on this site. – Michael C Dec 12 '17 at 2:21
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We are talking about how much light (or other radiant energy) is required to expose film or digital image sensor in order to achieve a usable image using a reasonable length of time to expose the material in a camera other perhaps other means. As to film, there are three light sensitive compounds in common usage. In order of sensitivity to light, they are silver bromine, silver chorine, and lest sensitive, silver iodine. If any are chemically pure (hard to attain), they are insensitive to light. Add some impurity and they gain sensitivity to light. The recipe for making film adds some impurity such as gold or cadmium or sulfur to heighten sensitivity to light is gained. In their natural state, they are only sensitive to violet and blue. Their natural color is off white yellowish. By dying the crystals other colors, the become sensitive to green and red.

A pure crystal has a neutral charge. Doping the crystal with some impurity disturbs the neutrality and the net changes. This is proprietary stuff related to how film is made. Once Kodak made a batch of film that had unexpected high sensitivity. Detective work found that the gelatin ingredient, made from animal hide and bones, had a high level of sulfur due to the animals eating mustard seed. Mustard became an additive in many Kodak film recipes.

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