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54 The PCB Magazine • February 2014 frame. The emulsion layer contains the light- sensitive silver halide crystals. This layer is where the visible image is formed during exposure and processing. It consists of a uniform dispersion of silver halide crystals in gelatin. The sub-coat is a series of very thin coatings which allow the gelatin to adhere to the base. The base is usually a 7-mil thick polyester film, though glass has been chosen for use in some applications. It is the base which gives the film its strength, dura- bility, flexibility, clarity, and dimensional stabil- ity characteristics. Another sub-coat is required to provide adhesion to the backing gelatin. The backing is a layer of gelatin containing a colored anti-halation dye and anti-static agents. Its pur- pose is to improve image quality, help control curl, and minimize dirt pick-up. The drawing you see here is not to scale. Typically, the base is 7 mils (175 μm). The coated layers, including the backing, add only 0.2–0.3 mils (5–7μm) to the total thickness. Figure 3 shows just one crystal that has formed some metallic silver. The first processing step is called develop- ment (Figure 4). Here, the exposed silver halide crystals are converted to metallic silver. The la- tent image acts as a catalyst for this reduction reaction, thus providing a means to distinguish between exposed and unexposed crystals. Once the reduction of the crystal starts, the entire crystal is converted, yielding an amplification factor that can be as much as 10 million! The amount of development a film receives in a spe- cific application must be adjusted to give opti- mum results. Too much development will pro- duce lines that are too fat and fuzzy. It may also produce high background fog levels in some systems. Too little development will give you skinny lines with low optical density. Usually, PHOTOTOOLS, PART C continues Figure 2: example of a silver halide film structure. Figure 3: Formation of latent image.