Darkroom Fog Test for Medical Facilities and Chiropractic Offices - EH: Minnesota Department of Health

Darkroom Fog Test for Medical Facilities and Chiropractic Offices

Equipment needed:

  • Most facilities have only one speed film. However, if you have more than one speed film, use the "fastest" film in the facility.
  • New box of film.
  • Stopwatch or timer to measure 2 minutes.


  • Select the smallest cassette in the facility (usually 8"x10"). Turn off all lights (red and white) in the darkroom. Load the cassette from a new box of film while in total darkness.
  • In the x-ray room, place the cassette on the x-ray table. Record the distance selected.
  • Establish a technique: (You must pre-expose the film to x-ray to sensitize it.)Techniques will vary depending on speed of film-screen system, processing, etc. Approximately 5 mAs @ 60 kVp will usually work on 400 speed system at 40". Expose the film. Develop the film. Repeat the test (if necessary) until you find a technique which produces a density of approximately 1.0 O.D. (optical density). The exposed film will be a light to medium gray at a density of approximately 1.0 O.D.
  • Once a technique has been established, record it for future use.
  • From the new box of film, reload the cassette in total darkness.
  • Expose the film with the techniques established.
  • In the totally darkened darkroom remove the film from the cassette and place it on the counter. Cover half of the film with the cassette or cardboard. A trick is to run your fingernail (or pencil) down the edge of the cassette or cardboard that is central to the film. This will later help differentiate between the covered and uncovered halves of the film.
  • Turn on safelights.
  • Let film sit for 2 minutes by the clock. Change your position in the darkroom so as not to block any light getting to the film.
  • Process the film. Because your eyes have now partially adapted to the dark, (about 5 minutes), look for white light leaks around the door, around the processor (if it goes thru the wall), around pass boxes (if present), and around ceiling fixtures and vents. White light leaks must be sealed off.
  • Measure the optical density approximately ½" from either side of the line which identifies the covered half from the uncovered half of the film. If the sides differ by more than 0.08 O.D. (0.05 for mammo film), a fog problem exists that needs to be corrected.
  • Date film and record results.

Determining where fog is from:

Run another fog test, this time leaving the safelights off. If the fog is reduced, you have a safelight problem. If the fog is not reduced, there is probably a white light problem.

Some possible sources of safelight fog:

  • The bulb or filter may give off the correct color spectrum for the film being used.
  • The bulb may not be the correct wattage for the distance to the work surface. A 15-watt bulb should be four feet or more form the surface. If the distance is less, switch to a 7 1/2 watt bulb. Use 25 watt or less for ceiling lights.
  • There may be a crack in the safelight filter. If so, replace filter.
  • The filter may be in the receptacle correctly. Follow manufacturer's instructions for installation of the filter. There may be a specific orientation for the filter. (The lettering should be readable form the outside.)
  • Are there other indicator lights that may not be "safe" for your film? If so, run a test with all indicators light covered.

Sources of white light fog:

Stay in closed darkroom (no lights on and indicator lights covered) for approximately 10-15 minutes, and then look for white light leaks that were not seen while waiting for the 2 minute fog test. Seal off any sources of white light.

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Updated Thursday, February 05, 2015 at 02:45PM