Caracterización Número de Días

Modern films are composed of four (4) important layers (fig. 13). The upper layer is a protective anti-scratch layer which is usually dull/non-reflective in appearance. Below this is the most vital layer to the process of photography, the emulsion layer. In this layer, silver halide (such as silver bromide) crystals/grains are suspended in a gelatin matrix some 25.0µm thick. In this layer, the photochemical reactions required to produce a

permanent, high resolution image take place. Under the emulsion layer is a support layer commonly composed of cellulose acetate (commercially known as “estar”). The final layer is known as the anti-halation layer which prevents photon/electron backscatter to the

emulsion layer. This layer is shiny in appearance, therefore, if you examine a sheet of film you should notice a dull/emulsion side and a shiny side. Most manufacturers (Kodak) will cut a small notch in the film to allow one to determine the emulsion side of the film even in total darkness - the emulsion side is up when the film is held with the notch in the upper right corner (fig. 14). The emulsion side of photographic paper is easy to determine since it is highly glossy under the bright yellow or amber safelight used in printing.

Fig. 13

Fig. 14

anti-scratch - dull side

Emulsion (silver bromide/gelatin) Support/Base (cellulose acetate) anti-halation - shiny side

Film Composition:

Film Notching Guide:

emulsion side is up

✥ Emulsion

The effect of electrons is similar to that of photons on light sensitive emulsions. As stated earlier, the emulsion layer is the active site of the photographic process wherein a

chemical reaction occurs. The emulsion layer is approximately 25.0µm thick and is

composed of usually uniformly sized silver halide (such as silver bromide - Ag⁺Br^-) crystals suspended in a gelatin matrix. Depending on the film, the crystals can range in size from 0.1 to 10µm in diameter. This crystal diameter relates to the film speed as shall be seen.

For film, this emulsion layer is coated on a cellulose acetate support or base. For paper, the emulsion is coated on cellulose-based paper (Kodak Kodabromide paper) or coated on a resin (Kodak RC paper).

When an energy source such as electrons or photons encounter a crystal, the crystal is disturbed/rearranged in various regions. The crystal is said to be in an activated state, however, an actual image cannot be seen at this point. A latent image exists which

theoretically can persist for years. This latent image must be converted to an actual image through processing of the film or paper. The chemical reaction that occurs when electrons/

photons encounter a crystal is reduction, the gain of electrons.

Relative to a gain of electrons, the reduction reaction can be summarized as follows:

Ag⁺ + e^- ➙ Ag˚

In this case, silver ions are reduced to silver atoms. The main difference between electron versus photon effects on the emulsion is that electrons are 100% quantum efficient. This means that a single electron is capable of activating a single silver halide crystal whereas, it takes 10 to 100 photons to activate a single crystal. Therefore, no true film speed exists with respect to electron effects on light sensitive emulsions. Since this is the case, the electron microscopist can take advantage by using very fine-grained films for the highest resolution. This is true only when electrons bombard the film to form the image as in the case of the TEM. In SEM, photons from a high resolution photo-CRT are activating the emulsion. It should be indicated that a single silver halide/bromide crystal is composed of a large number of individual ionic bonded Ag⁺Br^- molecules. During the reduction reaction, the crystalline lattice is rearranged yielding the latent image.

✥ Film Speed (ISO/ASA)

The film speed simply refers to the diameter of the silver halide crystals suspended in the emulsion layer. Fast films (400, 1000) have large crystal diameters. These large crystals are more likely to encounter and react with photons under conditions of low source (photon) density. If you have limited light conditions (indoors without a flash) you will need a fast film in order to record an image. The resultant enlargement prints are very grainy and of low resolution due to the large crystal size. Slow films (25,100) require an adequate photon concentration since they are very fine-grained. The fine-grained films

result in high resolution prints even at high magnifications.

Since electrons are 100% quantum efficient relative to the silver crystals, we can take advantage of the slow, fine-grained, blue-sensitive films available, such as Kodak Electron Image Film 4489 with an estar base and an ASA of 15. This 3.5" by 4.25" sheet film

permits excellent enlargements of high resolution. As it is blue-sensitive, the same safe light used in enlargement printing, yellow or amber, can be used when handling this film.

Light sensitive emulsions, even when used to capture electron generated images, are typically sensitive to certain wavelengths/colors of the visible spectrum. Safelights can be used to take advantage of this fact and allow the individual to handle films and papers under some lighting conditions. When choosing the appropriate safelight filter, the wavelength selected must be below the threshold energy necessary to activate the silver halide crystals of the emulsion. If the film is blue-sensitive, a yellow or amber OA or OC safelight filter is appropriate. If the film is yellow-sensitive, a yellow safelight filter would provide the necessary energy to activate the emulsion. In this case, a red filter could be used. Bear in mind that along with the appropriate filter, a low 10-15watt bulb is necessary at a minimum distance of 4 to 5 feet.

✥ Supports (Base) Film

In the past, emulsions were coated onto breakable glass plates. The advantage of glass is that it contains no water and requires no desiccation prior to insertion into the TEM. This is not important in the SEM since images are taken off the high resolution recording CRT at atmospheric pressure. The modern support used for film is cellulose acetate or “estar”.

Although the estar plates are easier to handle and not subject to breakage, they must be handled carefully to avoid dust, fingerprints and scratches. The cellulose acetate contains water and must be desiccated prior to insertion in the TEM. Most modern TEM’s have a plate dryer which is evacuated by a rotary pump.

Paper

Paper for enlargement printing is manufactured by coating an emulsion layer on either cellulose paper (Kodak Kodabromide) or resin (resin-coated or RC paper). Since the processing chemicals are absorbed by the cellulose paper, it requires longer processing times, especially the final wash in running water of up to 60 minutes. With RC paper, only the emulsion is affected resulting in shorter processing times. By example, the final wash time is reduced to 4 minutes. For drying, the RC paper should be treated for removal of excess water using a sponge or squeegee and allowed to air dry. By contrast, the cellulose paper should be dried in a heated drum or plate dryer, allowing the print surface to contact the shiny ferrotype surface for a high gloss final print.

Papers are designated by both letter and number combinations. The letters refer to the

type of print surface such as matte or glossy. For scientific publications, the glossy surface is most desirable and is noted by the letter “F”. The numbers (usually 1 through 5)

indicate the paper grade or contrast level of the paper. Paper grades will be covered in more detail under the topic of enlargement printing.

✥ Routine Photographic Films and Papers Used For TEM and SEM Films

TEM: Kodak Electron Image Film 4489 (3.25 x 4", ASA=15) - OA or OC safelight.

SEM: Kodak Commercial Film 4127 (4 x 5", ASA=50) - A1 (red) safelight.

Polaroid Type 55 P/N Film (4 x 5" positive and negative, ASA=50) - no safelight required.

Paper

For use in the enlargement printing of any of the negatives listed above:

Kodabromide F, 1-5 (fiber based, single or double weight) - OA or OC safelight

Kodak Polycontrast F - RC (resin coated, requires a Polycontrast Filter Kit) - OA or OC safelight

Chapter 9 - Processing (Films and Papers)

Processing involves a series of steps that will transform the latent image, produced via exposure of the film or paper, into an actual image made up of dark, light, and gray (halftones) regions, namely, contrast. The image will also be fixed and preserved on the film or paper by these procedures which include: Development, Stop Bath, Fixation, Washing and Drying.

The ideal way to process negatives involves loading the exposed film into the appropriate sized open frames/holders and pass them through the required solutions which are stored in tanks with floating lids (fig. 15). Papers are usually processed in various sized trays with washing carried out in a unit which has an attachment for circulating water, such as a large tub which houses a rotating drum (fig. 16). It is important that the films and papers be agitated throughout the entire process. Agitation reduces the chance that air bells (bubbles) on the surface will prevent necessary contact with the various processing solutions.

Figs. 15 & 16 – Film & Print Processing Stations Illustrations

Running

NOTE: Times and dilutions indicated pertain to Kodak 4489 TEM Film

H C

NOTE: Times and dilutions indicated pertain to Kodak RC Paper