Important Characteristics of Silver Halide (AgX)
- Conventional vs. Tabular grain
- Light Scattering
- Spectral Sensitivity
- Exposure Latitude
Conventional vs. Tabular
Although silver halide crystals generally have a cubic crystal structure with alternating silver and halide ions, their external shape can vary.
So-called conventional silver halide crystals, used for many years, have a 3-dimensional shape, while tabular crystals such as Kodak’s patented T-GRAINS are very flat, and this brings a number of advantages. Other manufacturers also use tabular crystals but with a different ratio of thickness to diameter.
Whatever the kind of crystal, conventional or tabular, the larger the crystal the faster or more sensitive it is (bigger target for the light photons to hit) but at the expense of higher grain too!
Different sizes of crystals are used in different types of film and in different layers within a film.
The above chart shows how larger grains allow faster films, but film has been improving and allowing today’s high-speed negative films to be much finer-grained than their predecessors.
For the same amount of silver halide, tabular grains have a larger surface area to capture light and become more “development centers” afterwards be developed into images. Development centers are the microscopic locations in the silver halide crystal lattice where the latent image specks form by the action of light exposure. More development centers mean higher sensitivity, or smaller grains, for the same sensitivity.
Another of the benefits of tabular T-GRAIN crystals is that they scatter light much less than the conventional or “3D” crystals. They really only have two parallel faces compared with the multiple faces of conventional crystals.
When light strikes on a crystal of silver halide grain, it is either absorbed and form the “development center” as described above, or it will be reflected to other silver halide crystals nearby and causing unwanted exposure. Not only that, but T-GRAIN crystals also lie flatter in the emulsion, giving only vertical, rather than lateral reflections of light rays. Thus provides a sharper image.
I have discussed that visible light contains color of red, green and blue. However, silver halide is only sensitive ultra-voilet and some blue light and not naturally sensitive to green or red light. Because of this, silver halide in film needs to be chemically sensitized to green or red light.
In color negative film, 3 layers of silver halide emulsion are coated on the film base so each layer will be responsible to record one color, so it comes the red, green and blue sensitive layers, or yellow, magenta, cyan imaging layers. In addition, green sensitizing dye is added with silver halide to the green sensitive layer, and red sensitizing dye is added with silver halide to the red sensitive layer.
When light strike on the film, the first layer is UV absorbing layer so ultra-violet ray cannot pass through and expose the imaging layers because UV is not visible to human eyes
After UV being absorbed, visible light containing RGB will pass through and exposes the imaging layer. Blue sensitive layer is the first, and then green sensitive at the 2nd and red sensitive layer at the bottom.
When RGB lights strike on the film, blue light will expose the blue sensitive layer and then goes into the green and red sensitive layers. However, we do not want green and red layers to be exposed by blue light as this will cause incorrect color reproduction. Therefore, a yellow filter is placed after the blue layer to absorb the blue light. As a result, only green light and red light can expose the green layer and red layer respectively.
This yellow color filter will be dissolved and removed during film processing.
Exposure latitude of film will be discussed in the next post, together with basic concept of sensitometry.
To be continued……