by Leif Kullman
The coin-on the film method, a useful tool to check the
darkroom.
In one of my working places we
recently have had problem with our film quality, all developed films had a low contrast
and were dark. This is a kind of problem that will not exist in the future, when we have
gone digital.
Our darkroom was rebuilt some months ago and among other things, a new safe light was installed. With safelight we usually denote an illumination which is of a low intensity (long, red wavelength) and therefore can be used in the darkroom during handling of the film. To minimize the fogging effect of this light, low effect light bulbs (15 watt) or particular fluorescent tubes are usually used. The light must also be mounted at some minimum distance from the working area and films.
Film emulsions have only reduced sensitivity to the light transmitted through the safelight filter. It is therefore also necessary, besides keeping low safelight illumination levels, to reduce the time for film handling to a safe minimum level. Besides an inappropriate safelight, another source for unwanted light can be light leaks around the doors and around possible wall passages of the equipment of course.
Now to go on to the test, if there is unwanted light that can have influenced the film a simple test can be performed, which is called penny test or coin on the film method. The test is carried out in the following way:
An unexposured (or light exposured) film is opened and placed in the area, where the films are usually handled or unwrapped. After that a coin is placed on the film and is left there for the time it use to take to unwrap and mount the films before developing. The film is then developed as usual. Afterwards no signs of the coin should be visible in the film.
In this manner each type of film, which usually is handled in the darkroom can be tested. Light leaks into the darkroom can be detected by standing in the room and look with eyes accommodated.
In our case our problem depended on the safelight. We had ordered a fluorescent tube of the necessary sort, but had received another one. We trusted in that the correct equipment had been received and installed it.
If we had had radiographic routines prescribed, as for example a regular weekly penny
test, we should have find out the problem much earlier. So an additional advice will be,
nominate somebody to be a director of the darkroom. Let this person in cooperation with
others who are interested draw up rules about things, which should be regularly checked.
Next month I will report on some useful radiographic methods to investigate the
temporomandibular joint and make a case description.
