DARKROOM FOG

X-ray film used with intensifying screens is very susceptible to fogging from extraneous light. This is because the
film is optimized to respond to the green, blue or ultraviolet light emitted by the interaction of X-rays with the
intensifying screens inside the X-ray film cassette. Improper safelights and light sources inside and outside of the
darkroom can compromise the quality of radiographs by reducing contrast and darkening the image. Unless the
facility performs fog testing as part of a quality control program, there is no certainty that the darkroom is safe for
uncovered film. Facilities that provide mammography are required to perform fog tests semi-annually, but other
facilities should consider testing for fog to ensure the integrity of the darkroom.
Sources of Extraneous Light
Every facility that uses film screen-based radiography can take simple precautions to prevent fogging of film from
extraneous light. Sources of unwanted light include:
 Incorrect or faded safelight filters
 Cracked filters or safelight housing
 Safelights too close to the film handling area
 Excessive light bulb wattage
 Thin-coated red decorative light bulbs (instead of a safelight)
 Indicator or glow-in-the-dark lights such as those on appliances, timers, radios, etc.
 Glow-in-the-dark wristwatches or remote controls
 Light leaks around doors, processors, vents or pass boxes
 Light leaks through perforated ceiling tiles or incorrectly placed tiles in suspended ceilings
 Fluorescent light afterglow
Simple Precautions
Check that the safelight filters are those specified by the film manufacturer. Immediately replace any filters that
appear faded or cracked, and routinely replace filters at regular intervals. At a minimum, use opaque tape to cover
cracks or light leaks in filters or filter housings. Some manufacturers indicate that filters should be replaced
quarterly, but less frequent replacement may be adequate. Light bulbs in safelights should generally not exceed
15 watts so as not to exceed the filter’s capacity or shorten its life. Safelights should be located approximately
four feet from the working surface. A red light bulb may not offer the same protection as a safelight. The film
manufacturer will have specific recommendations. If the darkroom is equipped with fluorescent lights, turn on the
lights for two minutes. With the door closed, turn off the lights and check for an afterglow. If one is visible, either
the lights should be replaced with incandescent lighting or the darkroom procedure should be modified to allow an
elapse of time for the glow to subside. Remember to perform this test again whenever a fluorescent bulb is
replaced. Wait several minutes for your eyes to adjust to the darkness. Remember, the eye is most sensitive
when using peripheral vision to look for weak light sources, so do not stare directly at the source. Look for obvious
light leaks around doors, vents, the processor and in the ceiling. Be sure to look from more than one position, as
leaks may be visible from one angle but not from another. Correct any light leaks that are visible. Reflected light
from tiled, chromed or mirrored surfaces may also cause a problem. Remember, a darkroom need not be totally
dark; only the portion of light in the color range to which the film is sensitive needs to be eliminated. Unfortunately,
common sources of white light contain all colors.
Eliminating Light Leaks
Elimination of light leaks can be a challenge and may require some ingenuity. A solution at one facility may not
work at another facility. Adhesive foam tape, rubber door sweeps, rubber gaskets and wooden molding may be
useful around doors. Heavy tape, cardboard, fabric or caulking may work elsewhere for light leaks. Flat black
paint minimizes reflected light.
Fog Test
The only way to be certain that fog from extraneous light is not a problem in the darkroom is to perform a fog test.
One method of testing for fog is the two-minute test. All fog tests use an exposed film to determine the level of
fog. Unexposed, underexposed or overexposed film is much less sensitive to the effects of darkroom fog. Expose
a film in a cassette so that the background density is between 1.0 and 1.5 Optical Density (OD) units or medium
gray. Experimentation may be necessary to determine the best techniques to achieve the correct density, and it
may be necessary to place a “phantom,” such as a block of acrylic, in the beam. A low peak kilovoltage (e.g.
50-56 kVp) and 5 milliamps per second or less at 40” Source to Image Distance is a technique that may produce
the desired results. Another alternative is to take an image of a step wedge at a higher kVp (about 80) or higher
mAs. This will ensure that a region of optical density between 1.0 and 1.5 will appear somewhere within the image
of the wedge. With the safelights on, place the exposed film on the counter top in the darkroom. Cover half of the
film with the cassette or with an opaque card. Using a stopwatch, timer or wrist watch, allow two minutes to
elapse. Process the film in the usual manner. If more than one type of film is used at the facility, this test should
be performed with each type of film. The results from one type of film can be entirely different than those for
another type of film. Mammography facilities should reference the American College of Radiology Mammography
Quality Control Manual for testing darkroom fog.
Interpreting Results
A film that has been fogged will have a clear delineation between the covered and uncovered portions of the film.
The untrained human eye can discriminate a difference of 0.02 OD. An optical density difference between the
covered and uncovered portions should be no greater than 0.05 OD when measuring with a densitometer. Any
difference greater than 0.05 is cause for concern, and the quality of the radiographs may be less than optimal.
If a densitometer is not available to measure the density differences, interpretation is subjective. A 0.05 OD
difference is quite obvious and would clearly indicate to the viewer that improvements should be made. The
source of the problem should be located and corrected. Of course, if no line or variation is visible, fog from
extraneous light is not a problem and the darkroom is safe enough to leave film uncovered for at least two
minutes, no matter how bright it may seem inside.
If fog is found on the film:
 The test should be repeated with the safelights turned off.
 If the fog disappears, the problem was caused by the safelight.
 If the fogging remains constant, the problem is being caused by a light leak or by other light sources in the
darkroom such as indicator lights.
 If the fogging was reduced, but still significant, the cause may be due to a combination of problems. If this is
the case, several tests may need to be made under different lighting conditions in order to find and eliminate
all of the sources of fog.
References:
 Dupont Diagnostic Imaging. Darkroom Technique: For Better Radiographs Processed Manually or
Automatically
 Perry Sprawls, Jr. 1993. Physical Principles of Medical Imaging. 2nd ed. Gaithersburg: Aspen Publishers, Inc.
 American College of Radiology. 1999. Mammography Quality Control Manual

For more information, contact the Bureau of Radiation Protection at 717-787-3720 or visit www.dep.state.pa.us,
keyword: Radiation Protection.

Commonwealth of Pennsylvania Department of Environmental Protection