Source-to-Film Distance (SFD)
Source-to-film distance (SFD), also called focus-film distance (FFD), is the geometric distance between the radiation source focal spot and the film, controlling geometric unsharpness and radiation intensity at the film.
Definition
SFD is a fundamental exposure variable. Increasing SFD reduces geometric unsharpness (Ug = F·t/d) but reduces intensity by the inverse-square law, lengthening exposure time. Minimum SFD is calculated to meet a Ug acceptance criterion.
What it governs
The distance from radiation source to film or detector. Together with source size and flaw-to-film distance it sets geometric unsharpness — the blur that limits what detail can exist in the image.
How it is set
Codes set minimum SFD through maximum allowed unsharpness: a larger source or closer geometry needs more distance. Doubling SFD quadruples exposure time, so technique design is a deliberate trade of clarity against productivity.
Governing codes
ASME Section V Article 2 geometric unsharpness limits; ISO 17636 equivalents.
The common shortcut
Shortening SFD to cut exposure time on a night shift. The image gains density but loses definition, and the IQI wire that should prove sensitivity quietly disappears.
Where Source-to-Film Distance fits in an inspection programme
A term is only useful when it connects to a decision. Source-to-Film Distance appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Geometric Unsharpness (Ug) — Geometric unsharpness is the radiographic image blur caused by the finite size of the radiation source, calculated as Ug = F × t / d, where F is focal-spot size, t is object-to-film distance, and d is source-to-object distance.
- Exposure Chart — An exposure chart is a graph or table that gives the required exposure (mA-min for X-ray, Ci-min for gamma) versus material thickness for a specific film, density, kV, and source-to-film distance.
More procedure terms
Contact Coupling · Immersion Testing · Wet Particle Method · Dry Particle Method · Fluorescent Particles · Visible Particles · Demagnetization · Multi-Directional Magnetization · Solvent-Removable Penetrant · Water-Washable Penetrant
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.