RT Geometric Unsharpness Calculator
Geometric unsharpness (Ug) is the blur at the edge of a radiographic image caused by the source having a finite size rather than being a perfect point. It is the single geometry number most likely to fail a film-technique review: ASME Section V sets hard Ug limits by material thickness, and exceeding them means re-shooting. This tool computes Ug from your shot geometry so you can fix it before exposing.
How it works
A finite focal spot of size F casts a penumbra. By similar triangles, the blur projected at the film is Ug = F × (OFD / SOD), where SOD is source-to-object distance and OFD is object-to-film distance. Increasing SOD (moving the source back) shrinks Ug; increasing OFD (object lifted off the film) grows it. The classic fix for a failing Ug is a longer source-to-film distance.
Formula
Ug = F × OFD / SOD
Ug = F × OFD / SODWorked example
A 3 mm focal spot, 600 mm source-to-object, 25 mm object-to-film: Ug = 3 × 25 / 600 = 0.125 mm. For material under 50 mm thick, ASME V Article 2 allows Ug ≤ 0.51 mm — so this geometry passes comfortably. If SOD were halved to 300 mm, Ug doubles to 0.25 mm (still passing, but with less margin).
| Variable | Value |
|---|---|
| input: focalSpot | 3 |
| input: sod | 600 |
| input: ofd | 25 |
| output: ug_mm | 0.125 |
When to use this tool
Use during shot planning to set source-to-film distance, when a technique sheet is challenged in review, or to diagnose why an exposed film looks blurred at defect edges.
Limitations
Where this calculator stops being accurate:
- Accounts only for geometric (penumbra) blur — not film/detector inherent unsharpness or screen/scatter blur. Total unsharpness combines these in quadrature.
- Assumes the object surface of interest is at the stated OFD; thick parts have a range of OFD across the wall.
- F is the effective focal-spot/source size from the certificate, not the housing size.
- ASME V Ug limits are thickness-banded — check the exact band for your material thickness.
Frequently Asked Questions
What are the ASME Section V geometric unsharpness limits?
ASME BPVC Section V, Article 2 bands Ug by material thickness: under 2 in (50 mm) → Ug ≤ 0.020 in (0.51 mm); 2 to 3 in → 0.030 in (0.76 mm); 3 to 4 in → 0.040 in (1.0 mm); over 4 in → 0.070 in (1.78 mm). Many fabrication specs are stricter, so always check the governing code and the client spec together.
How do I reduce geometric unsharpness?
The most effective lever is increasing source-to-object distance (SOD) — Ug is inversely proportional to it. Placing the object as close to the film as possible (minimising OFD) also helps, as does using a smaller focal spot. The trade-off: a longer SOD increases exposure time because intensity falls with the inverse square of distance.
Is geometric unsharpness the same as total unsharpness?
No. Total image unsharpness Ut combines geometric unsharpness Ug with inherent (film/detector) unsharpness Ui, roughly as Ut = (Ug^n + Ui^n)^(1/n) with n≈2. Ug is the part you control through shot geometry; Ui is fixed by the film class or detector. Codes set the Ug limit because it is the operator-controllable term.
References & Standards Cited
- ASME BPVC Section V, Article 2 — Radiographic Examination (geometric unsharpness limits).
- ASTM E1742 / E1742M — Standard Practice for Radiographic Examination.
- ASTM E94 / E94M — Standard Guide for Radiographic Examination Using Industrial Radiographic Film.
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