Film hangers are available in two designs: channel hangers and clip hangers (Fig. 7-3). Channel hangers tend to retain water and chemicals and need special cleaning and drying to prevent contamination of the dry side. Films must also be removed from the channel hangers to be dried. However, clip hangers are more fragile than the channel type. When the clips are used frequently over a period of time, they become weak and lose the ability to “stretch” the film. The clips also puncture the four corners of the film, which, when filed, can scratch other films in the same envelope. It is important to cut off the corners of films processed with clip hangers before filing to prevent this. When more than one film is processed at the same time in the tanks, the clips on the hangers can scratch neighboring films.

Figure 7-3 A clip film hanger (left) versus a channel film hanger (right).

Wet side.

The wet side of the darkroom is where the actual chemical processing is performed. A darkroom that hand processes films usually consists of three tanks containing developer, water, and fix solutions. Various tank designs are available. The three tanks can be individually freestanding and warmed as required by an immersion heater (placed in the developer). Alternatively, the developer and fix tanks can be placed in one large tank filled with thermostatically controlled water. The latter system is preferred and can be purchased as a complete package constructed with 3- or 5-gallon (9- or 22-L) individual tanks. The water tank is usually four times the size of the smaller developer and fix tanks. The central water tank should have a circulating water system to provide a way to regulate temperature and rinse chemicals off the films during the processing procedure. A thermometer is an essential piece of equipment for the processing tanks because radiographic film is developed for a specified time on the basis of the temperature of the chemicals.

The wet side should also have a film-drying area consisting of either a drying rack or a drying cabinet. The drying rack should be placed in a dust-free area to prevent artifacts from sticking to the wet films. A drying cabinet is a heated forced-air unit that hastens the drying process. A viewing screen is also recommended on the wet side to evaluate radiographs. A “wet” film can be viewed, and, if a second radiograph is required, the radiographer can immediately evaluate the error. This is best achieved in the darkroom before too much time has elapsed.

Darkroom Lightproofing

As mentioned earlier, one criterion of a good darkroom is that it be lightproof. Light leaks in a darkroom can cause significant film fog; therefore taking appropriate measures to lightproof the darkroom is imperative. Lightproofing a room is more difficult than may be expected. The first step is locating the light leaks. Small light leaks may not be perceptible until the eyes have acclimated to the dark, and it may be necessary to spend 5 minutes waiting for the eyes to adapt. To achieve a truly lightproof room, a number of tasks may be necessary.

The entrance to the darkroom is a common site for light leaks. A double-door system or revolving door is preferred but not always practical in a veterinary practice (Fig. 7-4). The first step in lightproofing a standard door is to fit it tightly into its frame against strips of felt or rubber molding. Weather stripping is also useful around doors to prevent the entrance of light. Light entering from underneath the door can be prevented by a vapor seal designed specifically for the bottom of a door. A sliding bolt lock or doorknob lock prevents someone from accidentally entering the darkroom at an inopportune time. A suspended ceiling can be a radiographer’s nightmare. It may be necessary to place a large black sheet of plastic above the ceiling tiles to prevent light in adjacent rooms from entering through the seams.

Figure 7-4 A revolving door for the darkroom. This door is an effective means of entering and exiting the darkroom without risking exposure of the film to light.

It is a common fallacy that the walls of a darkroom should be “dark.” The opposite is true. The walls of the darkroom should be painted white or cream with a good-quality, washable paint. By painting the walls a light color, more reflection of the safelight is produced, providing a more visible work environment. If the quality and intensity of the light are “safe,” the illumination reflected from any surface also is “safe,” regardless of the color of that surface.

Darkroom safelight.

Correct safe lighting in the darkroom is crucial. A “safe” light means that the light produced will not affect the film. Radiographic film is sensitive to ultraviolet light. Safelights use a small-wattage bulb and a special filter to eliminate the light from the blue and green spectrum. The light bulb should be 15 watts or less. The filter varies by manufacturer. The most common types are a brown filter (Wratten 6B, Kodak) for blue-light-sensitive film and a dark-red filter (Wratten 6BR or GS-1) for green-light-sensitive film. The dark-red filter is recommended because of its versatility: both green-light- and blue-light-sensitive film can be used in this lighting.

Safelights should be positioned so that darkroom work can be performed without fumbling. There are two types of safe lighting: direct and indirect (Fig. 7-5). Direct lighting is a diffused light that shines directly over a work area such as the dry or wet side of the darkroom. Indirect lighting is a filtered light directed toward the ceiling and reflected over the entire room. Indirect lighting is often combined with direct lighting. At no time should the safelight be closer than 4 feet from a work area. A safelight that is too close, has a too-high wattage bulb, or has incorrect filtration may cause film fog.

Figure 7-5 Indirect (left) and direct (right) safe lighting for the darkroom.

The efficiency of a safelight can be tested; this is discussed in Chapter 10. Remember, no light is “safe” if the film is exposed to it for a prolonged period. Therefore the film bin should be open only when removing or replacing film. Fogging will result even with a safelight if the bin is left open or if film is left on the counter.