Home - Teaching Resources - NDT Tips

-

Lead Foil Screens

Lead foil screens are often used to enhance the radiograph when using X-rays in the range of 150 to 400 kV. The screens increase the exposure of the film largely due to electrons emitted by the screens during an exposure. The following are a few experiments that can be performed to investigate the effects of lead foil screens.

Experiment #1 - Using Lead Foil Screens

  1. Locate a plate of steel approximately ¼ inch thick and 4 inches wide by 5 inches long. Drill a number of holes in the plate.
  2. Load a film cassette with a type 1 film.
  3. Load a second film cassette with a second piece of film and place a 0.005 inch thick lead screen on the source side and a 0.010 inch thick lead screen on the backside.
  4. Place the steel plate of the film cassette and expose the film at 150 kV to create a radiograph with a density of 2.0 to 3.0.
  5. Radiograph the plate a second time using the film pack with screens. Expose the material at similar kV / mA settings but the exposure time will need to be increased since the lead will absorb a portion of the long wavelength radiation.
  6. Process the films and compare on the viewer to determine if the lead produced a sharper, higher contrast radiograph.

More contrast and definition should be seen in the image produce with the screens.

Experiment #2 - The need for intimate contact between the film and the screen.

To produce the desired effect, the screens must remain in contact with the film. The following experiment can be used to demonstrate the effect of an air gap on the performance of lead foil screens

  1. Load a film cassette with a piece of type 1 film and a lead foil screen on the source side.
  2. Cover the half of the film with a thick lead sheet to prevent it from being exposed.
  3. Place a hacksaw blade or other object with fine detail on the source side of the cassette in the area not covered by the thick lead sheet.
  4. Expose this half of the film to produce a density of 2.0 to 3.0.
  5. Return to the dark room and place a couple of pencil in the cassette to separate the screen from the section of unexposed film. Put one of the pencils near the center and the other near one edge of the film so that the hacksaw blade can be imaged between them.
  6. Cover the previously exposed half of the film with a thick lead sheet to prevent it from being exposed a second time.
  7. Place a hacksaw blade or other object with fine detail on the source side of the cassette in the area not covered by the thick lead sheet.
  8. Make a second exposure using the same equipment settings used to produce the first.
  9. Process and compare the two exposures.

More contrast and definition should be seen in the image produce with the screen in contact with the film.

Experiment #3 - Effects of Scratches and Other Artifacts on Lead Foil Screens.

  1. Use a sharp object to scratch a line or pattern into an old lead screen.
  2. In another area of the screen place a dime-sized spot of glue and let dry
  3. Load a piece of film and the screen into a cassette. Place the lead source side to the film and place a small shred of tissue paper or sewing thread between the film and the lead.
  4. Exposure the film to produce a density of between 2.0 and 3.0.
  5. Develop the film and determine the results of the scratch, foreign material, and glue on the exposed film


NDT
Teaching Tips

Ultrasonic
Reflectors
Signal Interpretation
DAC Curve

Eddy Current
Depth of Penetration
Lenz Law

X-ray
Backscatter Radiation
Film Handling Artifacts
Focal Spot Size
Lead Screens
Source/Object Orientation