Image Quality, Sensitivity and Radiograph Interpretation
What makes a good image
Image quality has three components: contrast, definition (sharpness) and freedom from graininess and artifacts. Radiographic contrast comes from subject contrast, film or detector gradient and scatter. Subject contrast depends on thickness difference, density and atomic number differences and photon energy; lower energy raises subject contrast but reduces latitude, and excessive scatter destroys it. Definition is limited by geometric unsharpness, screen unsharpness, motion and detector resolution.
Measuring quality
The image quality indicator is the objective measure. Wire IQIs (ISO 19232-1) and step-hole IQIs (ISO 19232-2) are placed on the source side or film side as the standard requires, and the smallest visible wire or hole defines the achieved sensitivity for that thickness. The required value depends on technique class and nominal thickness. For digital images, normalized SNR and basic spatial resolution replace film-based measures and have minimum values.
Interpretation discipline
View radiographs on a viewer matched to the density, in subdued light, using a mask. Distinguish genuine indications from artifacts: handling marks, screen marks, static, and backscatter images of lead letters behind the film (prevent with a lead backing sheet). Judge each indication by shape, edge sharpness, density and position relative to the weld profile.
Traps
Reading the IQI value off the wrong thickness line; treating a high-density area as a defect when it is a thicker section; blaming low contrast on the film when scatter from poor collimation is the cause; and forgetting that backscatter raises density in supposedly unexposed areas.
Rule of thumb
One half-value layer of extra steel changes transmitted intensity by a factor of about two, so the log-intensity step is roughly 0.3 - the order of change to expect across a weld cap.