How often has this happened to you in the darkroom? You take an extremely difficult x-ray, feed it into the film processor and out comes a completely or almost all black x-ray film. You have just wasted time, money and exposed your patient to dangerous radiation with nothing to show for it. Now you're faced with the task of determining what went wrong.
Since x-ray radiation is the component that exposes film and turns it black, most people jump to the conclusion that the x-ray machine must be putting out too much x-ray. This is a possibility but almost never is the cause of dark films. There are more things that cause black films than just x-ray. Here's a list of the most common things (in order of occurrence) that cause x-ray film to come out black or darker than normal.
1. Light leak in the darkroom. This is a very common cause of dark films. It takes a very small amount of light to expose film. The darkroom should be checked for light leaks anytime you notice dark films. The best way to test for a light leak is to go into the darkroom, turn out all the lights and safe light, place a new sheet of x-ray film on the counter, lay your hand on the film in the middle, and wait 2 minutes. Process the film. When it comes out you should not see anything but a clear film. If you can see a dark outline around where your hand was, you have a light leak. The best way to find where the light is entering the room is to go into the darkroom, turn out all the lights and safe light and wait 1 minute to allow the pupils in your eyes to dilate. Now begin to look all around the room for any light that is entering the darkroom. Pay close attention to the door on all sides, and any ceiling tiles or light fixtures. Any amount of light that is entering the room is exposing the film when it's out of the box or cassette. Completely seal and repair all light leaks.
2. Cracked red filter on the safe light. If the red filter coating on the safe light filter gets cracked it allows white light to escape and expose film. To test for this, go in the darkroom, turn off the white light but leave the safe light on. Wait 1 minute and then inspect the safe light for any white light coming out of the red filter. If any is detected, replace the filter or safe light.
3. Developer in the film processor is too hot. Most automatic processors heat the developer to reduce the time it takes to develop x-ray film. Generally most developer is heated to 92 - 95 degrees. If the heater control circuitry malfunctions, the developer can be heated higher than this and cause overdeveloped dark films. Check the temperature of the developer in the processor by getting a thermometer that will measure in the 90 - 100 degree range and checking the temp of the developer in the processor. Note: Some human thermometers won't measure temperature below 95 degrees so make sure the one you use will measure that lower range. Another cause of hot developer is the water valve got shut off or the water line was hooked to a hot water source preventing cold water from filling the wash tank which also cools the developer in some processors. Check that cold water is coming into the processor wash tank and if not, locate the reason and repair.
4. Expired x-ray film. All x-ray film has a life expectancy. Most film is good for 2-3 years after it was manufactured. The expiration date is printed on almost all boxes of film (some generic and low cost films do not print expiration dates, so buyer beware). Check the film box for an expiration date. It might look something like these, 01/2015, 2015 - 01, Exp Date 2015 - 01. Expired film will develop darker than normal so dispose of any that has passed its life expectancy.
Dark film is a common problem in x-ray. There could be other things causing this to happen but these are the most common causes. Give these a try and if you still have problems it might be time to call a service technician.