Technique Selection, Exposure Geometry and Technique Sheets
Choosing a technique
The technique must deliver the required image quality class for the material, thickness and geometry. The governing standard (ISO 17636-1 for film, ISO 17636-2 for digital, or ASME V Article 2) sets minimum source-to-object distance, maximum geometric unsharpness, number of exposures, IQI type and essential wire, and density range. For welds choose between single-wall single-image, single-wall double-image, double-wall single-image (elliptical) and double-wall double-image panoramic depending on diameter and access. Circumferential butt welds normally need three exposures at 120° with adequate overlap.
Source-to-object distance
Geometric unsharpness Ug = d·b/(F − b), where d is source size, b is object-to-film distance and F is source-to-film distance. Rearranged, minimum F = d·b/Ug(max) + b. Distances in the standards are measured from the source to the source-side surface, not to the film - a frequent source of error.
Technique sheet content
Part identification, material, thickness, standard and class, equipment, source size and activity date, kV/mA or isotope, F and b, exposure time, screens, film class, IQI type and placement, density limits, number of exposures, marking and acceptance criteria.
Traps
Placing the IQI on the wrong side without applying the source-side value; quoting a kilovoltage that gives insufficient penetration; and forgetting that increasing b rapidly increases the required F.
Worked example
d = 3 mm, b = 20 mm, required Ug ≤ 0.2 mm. Minimum F = (3 × 20)/0.2 + 20 = 300 + 20 = 320 mm.