An Electron Microscopic Study
Photographic film captures the image formed by light reflecting from the surface being photographed. The sensitive elements in the film are crystals of, most often, silver halide which can change their structure when excited by light (photons). In general less sensitive films (slower films) have finer grains that are closely packed and more sensitive films (faster films) have courser grains. A film may have a distribution of grain sizes to obtain certain desirable properties. The reason for the sensitivity relationship to grain size is related directly to how the grains are converted from a stable non-developable state to another stable state (latent state) from which they can be developed chemically. This happens in something like the following way. When a photon of light strikes a grain it dissipates its energy in the crystal (grain). This energy may or may not be enough to flip the crystal into a latent state. Generally it takes a few photons to flip the grain (depending on its size and sensitivity). In the meantime, thermal energy is jiggling the grain and tending to drop it back into its normal state. If enough photons strike the grain in a given time, the grain flips to a latent state and sticks there. We then have a grain that can be turned opaque chemically. Thus the photons build up a latent image that is later developed. The darkness of the image is more or less proportional to the light striking the film. It takes about the same number of photons to flip a large grain as a small one. Since the larger grain intercepts more light more of the larger grains will be flipped and thus less light is required to create a latent image. This later phenomenon makes coarse grained films faster (more sensitive). All the silver halide salt particles reside in some emulsion layer depositing on the base layer of films. Color film has three layers of emulsion for three kind of silver halide which are sensitive for the light of 3 different light wavelengths. The following diagram shows the basic layered structure of photographic film.
Fig. 1 Film structure
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The focus of this project is to measure the grain size of different kind of photographic films and correlate them with macro-phenomena, which is the quality of developed image. Because on undeveloped film, silver halide particles are surrounded and fixed on the base by gelatin, which is perfect insulator. This is just like hazelnuts (silver halide) embedded in chocolate (gelatin). Therefore, if we take image directly with the unprocessed photographic film samples, what could be seen is just the situation on the very top of the gelatin/silver halide particle mixture. This means very poor image quality and sparsely distributed silver halide particles, which is of course not desirable. Taken these into account, some special sample preparation is necessary: to remove silver halide particles from film base and then redisperse them on SEM sample stub. Actually, this turns out to be the most difficult part of this project.
Fig. 2 Image of silver halide particle embedded in gelatin. (a. SE detector, b. BSE detector) The image quality is constrained by gelatin layer.
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Experiment and Data Analysis
A. Sample preparation
1. Remove remjet with methanol
2. Coupler Removal (do this cycle 2 times. for color film only)
5-10 min 1% Na carbonate soak
Dip in water
5-10 min methanol soak
5 min ultra-pure water soak
3. Grain removal from film support
-suspend sample in test tube
-Add 25-30 drops 2% unfiltered protease. Add enough 40C ultra-pure water to cover sample.
Place test tube in 40C hot water bath 3 hours until grains removed. Discard film support.
-Centrifuge 2 min @ 3000rpm
-Carefully and quickly decant and discard supernatant.
4. Redisperse Grains
-Warm test tube containing "pellet" of grains.
-Sonicate briefly. At this point, the pellet should be broken up and there should be a cloudy dispersion.
-Add 3 ml warmed ultra-pure water. Sonicate.
-Dilute with water to reach desired concentration.
-Spot a droplet of this mixture on a sample stub and dry it at 70C for 20 minutes.
After all these procedure, most of the gelatin should have been desolved by protease solution and removed. The residue on the sample stub is pure silver halide particle from emulsion, which is as shown in Fig. 3
Fig.3 Silver halide particles deposited on sample holder
Fig. 4 SE electron and X-Ray line simulation
B. Measurement and Data Analysis.
Up to 6 types of photographic films are measured and the results are as shown below
Kodak 160VC. Professional portrait negative film . Average grain size: 1um
a. Mix detector image b. In-lens detector image
Kodak TMAX400. Professional B&W negative film. Average grain size: 2-3um
a. Mix detector image b. BSE detector Image
Kodak Gold 100. Consumer color negative film. Average grain size: 2um
a. In lens detector image b. BSE detector image
Konica 160. Professional color negative film. Average grain size: 1.5 um
a. BSE detector image b. MIX detector image
Fujifilm RVP. Professional color positive film. ISO50. Average grain size: 0.8 um
a. SE detector image b. BSE detector image
Fujifilm Superia 100. Consumer color color negative film. Average grain size: 1um
SE detector image
These results meet the macro-pheomena: Professional film, positive film and low ISO film show smoother grain and vice versa.
Besides this, some other interesting effect is observed also: When electron bean is focused on some given silver halide particles, these particles show some deformation with time. We conclude this as the effect of the reduction reaction caused by high energy electron beam. But this assumption is to be proved by some more sound theory.
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In this project, up to 6 different film samples, which includes professional film and consumer film, B&W film and color film, film of different ISO values, negative and positive film, are studied and the measure result could explain the different macro grain they show in image. Also the reaction between high energy electron beam and undeveloped silver halide particles is observed. But further research work is needed to give a complete explaination to this effect.
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