|
|
|
|
Here
are things to look for: 1.
A light-leak in the darkroom - yes,
even a little light leak matters! Seal
all light leaks with foam weather-stripping or caulking, and buy an inexpensive
rubber "door sweep" from the hardware store to block the light coming
into the darkroom from under the door. A
beanbag draft stopper also works. 2.
A safelight that is not really safe.
You would not believe how common this is !
(See discussion below.)
4.
Processor malfunction - such as the thermostat not working correctly and
allowing the processing chemicals to 5.
Processor not maintained adequately.
Processing chemicals must be changed at intervals no longer than 4 weeks,
and the processor must be thoroughly cleaned each time. 6.
A completely inadequate grid, allowing much scatter radiation to reach
the film. THE SAFELIGHT The
safelight is really only a "relatively safe" light. The light is an ordinary (small wattage) incandescent
lightbulb, covered by a filter that filters out most of the shorter wavelengths
that are more harmful to exposed film, leaving only the longest red wavelengths.
This amount of light should be relatively safe for the short time that
you are handling the films before they are developed.
The maximum wattage of the incandescent bulb
depends on the distance that it is mounted away from your work area. Usually,
a 7½ watt bulb, with its proper filter, should be at least 2 feet away from
your work area. An incandescent
light on the ceiling can usually stand a 15 watt bulb, with its proper filter.
Follow
the directions that come with the safelight.
The filters over the safelight should be installed so that the
identification printing can be read when looking at the lamp. If the filter
orientation is reversed, excessive heat buildup inside the lamp housing may
cause the dye layer to crack and leak "unsafe" light. Fluorescent
"safe" lights are usually not really safe, and they are frequently
responsible for fogging of the films. Remember!
If it is light enough to easily see what you are doing in the
darkroom, you have too much light! SAFELIGHT AND LIGHT-LEAK TEST Everyone should do this ! Expose
an 8x10” film at 50 mA, 50 KV, at one of your smallest time settings, in order
to sensitize the film. Collimate
about one inch from all sides of the film.
(What you want is the amount of exposure that would make a light grey
square on the film, and the exposure factors for this vary widely among
facilities.) This pre-exposure must
be done, or the test will not work. Exposed
film is much more sensitive to light
fogging than film right out of the box, and, of course, that pre-exposed
condition is the condition of all the films you develop in the darkroom.
In the darkroom, after removing the exposed film from its cassette, place
the film under its cassette on the
workbench, leaving 1 inch sticking out. Time
15 seconds. Then pull the film out another inch. Time another 15 seconds.
Repeat to end of film. Develop.
If there are any light leaks in your darkroom - or if the safelight is
not really safe - the developed film will show progressively darker horizontal
stripes. FIXING
THE UN-SAFE SAFELIGHT There
are numerous no-cost ways to fix a safelight problem: A
fluorescent safelight can be modified by winding black electrical around the
tube in a spiral to diminish the amount of light that is emitted.
Be especially careful to wrap the ends of the rube where white light
often leaks out. Incandescent
lights can be moved farther away from the work surface, or turned to face away. Baffles
can easily be constructed from materials readily at hand, such as cardboard or
completely exposed x-ray film. Checking for x-ray fogging To
determine if loaded cassettes are being fogged by scatter radiation, leave one
where they usually are kept and tape a coin or metal marker to the front of the
cassette. After a few days (during
which x-rays have been produced), develop the film and see if the marker can be
seen. |
