How do you view and interpret a radiographic film?
The common term for film interpretation is film viewing. Film viewing in fact means the evaluation of the image quality of a radiograph for compliance with the code requirements and the interpretation of details of any possible defect visible on the film. For this purpose, the film is placed in front of an illuminated screen of appropriate brightness/luminance. The edges of the film and areas of low density need to be masked to avoid glare.
The following conditions are important for good film interpretation:
- brightness of the illuminated screen (luminance)
- density of the radiograph
- diffusion and evenness of the illuminated screen
- ambient light in the viewing room
- film viewer’s eye-sight
Poor viewing conditions may cause important defect information on a radiograph to go
unseen.
EN 25880 provides detailed recommendations for good film viewing conditions. The luminance of the light passing through a radiograph shall not be less than 30 cd/m2 and, whenever possible, not less than 100 cd/m2 (cd = candela). These minimum values require a viewing box luminance of 3000 cd/m2 for a film density of 2.0. The practical difficulties of providing the required luminance for a film density of 4.0 are considerable. The main problem with constructing a film-viewing box for these higher densities is the dissipation of heat from the lamps. However, by limiting the film area requiring such high power lighting, it becomes possible to view radiographs of a film density of 4.
The light of the viewing box must be diffuse and preferably white. Radiographs should be viewed in a darkened room, although total darkness is not necessary. Care must be taken that as little light as possible is reflected off the film surface towards the film viewer. If the film viewer enters a viewing room from full daylight, some time must be allowed for the eyes to adapt to the dark.
A yearly eye-test according to EN473 for general visual acuity is required while especially sight at close range needs to be checked. The film viewer must be able to read a Jaeger number 1 letter at 300 mm distance with one eye, with or without corrective aids. The trained eye is capable of discerning an abrupt density change/step of 1 %.
While interpreting, a magnifying glass of power 3 to 4 can be advantageous.