image industrial radiography

Which errors might occur during film exposure or handling?

Before a particular difference in density in a radiograph is attributed to a defect in the object examined, it must be sure that it is not the result of incorrect handling- or proces- sing of the film. It is, therefore, essential to be able to recognise such faults when exami- ning the film in order to prevent their recurrence. It is often possible to identify faults due to wrong processing by looking obliquely at the surface of the film while facing towards the light, and comparing the two emulsion surfaces. The X-ray image usually is identical on both sides of the film, while a fault in processing will frequently affect only one surface, and can be seen as a change in reflection on the surface.

The most common faults, and their possible causes, are listed below:

Insufficient contrast

a: with normal density:

  1. radiation too hard
  2. over-exposure compensated by reduced developing time
  3. unsuitable or wrongly mixed developer
  4. prolonged development in too cold a developing bath

b: with insufficient density:

  1. insufficient development
  2. exhausted developer
  3. unsuitable or wrongly mixed developer

Excessive contrast (i.e. lack of intermediate tones)

  1. radiation too soft
  2. under-exposure, compensated by prolonged developing
  3. unsuitable or wrongly mixed developer

General lack of density

  1. radiation too soft
  2. under-exposure, compensated by prolonged developing
  3. unsuitable or wrongly mixed developer

General excessive density

  1. over-exposure
  2. prolonged development or developing temperature too high
  3. unsuitable or wrongly mixed developer

Insufficient sharpness

  1. source-to-focus distance too short
  2. source or object moved during exposure
  3. film-to-object distance too great
  4. dimensions of source or focus too big
  5. poor contact between film and screens
  6. wrong type of foil used

Grey fog (local or overall)

  1. unsuitable dark room safelighting
  2. excessive exposure to safelight (i.e. too long or too close)
  3. film accidentally exposed to X-ray or Gamma-ray or to white light
  4. heavy scatter
  5. film out-of-date or stored under unsuitable conditions (ground fog)
  6. extreme under-exposure compensated by excessive developing
  7. exhausted or wrongly mixed developer
  8. film cassette with film exposed to heat (e.g. sunlight, heat from radiators etc.)
  9. cassette not properly closed (edge fog)

Yellow fog

  1. prolonged development in badly oxidised developer
  2. exhausted fixing bath
  3. insufficient rinsing between developing and fixing Note: It may take months before yellow fog becomes apparent.

Dichroic fog (i.e. greenish-yellow by reflected light, pink by transmitted light)

  1. developer contaminated with fixer
  2. film insufficiently rinsed after development and subsequently fixed in exhausted fixer
  3.  ilm stuck to another film when placed in fixer (in which case the development continues in the fixing bath)
  4. prolonged development in exhausted developer
  5. film partly fixed in an exhausted fixing bath, exposed to white light
  6. and then fixed again

Mottled fog

A greyish, mottled fog generally means the film is out-of-date or that it has been stored under unfavourable conditions, e.g. in damp surroundings.

Whitish deposit

  1. water used to make up developer or fixer too hard
  2. wash water too hard
  3. film insufficiently rinsed after development

Clear patches

  1. minute round spots with sharp edges: the film was not kept moving in the first 30 seconds of development
  2. drops of fixer or water fell onto the film before development
  3. marks from mechanical damage to the emulsion before exposure
  4. marks due to rapid and uneven drying of the film (this occurs when there are still droplets of water on the film when placed in the drying cabinet)
  5. clear patches can occur from the film sticking to another film or to the tank wall during development
  6. grease on the film slowing down or preventing the penetration of the developer
  7. screen(s) in poor condition
  8. foreign bodies (for example metal particles ) between film and screen during exposure
  9. small, clear, hollow spots (usually with dark edges) may occur when the emulsion has been subjected to local attack of bacteria. This is generally the result of slow drying in a warm damp climate, particularly if there are impurities in the wash water.

Clear lines or streaks

  1. the film envelope has been scored with a pointed object before exposure.
  2. film insufficiently moved during development
  3. uneven drying (film has been carelessly wiped dry after washing)
  4. drops of fixer or stopbath have fallen on the emulsion before development

Clear shapes

  1. clear crescent shapes may appear when, before exposure, the film has been bent between two fingers
  2. fingerprints may occur when the film has been touched with dirty fingers, contaminated for example with grease, fixer, stopbath or acid

Dark patches

  1. drops of developer have fallen onto the film before development
  2. drops of water have fallen onto the film before development
  3. electrical discharge marks, especially at low relative humidity of the air
  4. marks from mechanical damage to the emulsion after exposure

Dark lines or streaks

  1. the emulsion has been scratched after exposure
  2. the film envelope containing the film has been scored or written on with a pointed object after exposure
  3. insufficient agitation of the film during development
  4. uneven drying
  5. water or developer has trickled down the surface of the emulsion prior to development

Dark shapes

  1. dark crescent shapes (see “clear shapes” above); these are darker than the surrounding area if the bending occurred after exposure
  2. fingerprints: the film has been touched with dirty fingers
  3. electrical discharge (see “dark patches).