Radiography Exposure Time Calculator
A 25 mm steel weld with a 60 Ci Ir-192 source and Class II film at 36 inch SFD takes about 4 minutes. Bump SFD to 48 inches and the time goes to 7 minutes because the inverse square hits hard. This tool runs the exposure-factor lookup that every Level II radiographer keeps taped to the inside of their film bag — but with current source activity baked in rather than the certificate value.
How it works
Exposure time scales with the exposure factor E(t) from ASTM E94 lookup tables (E94 Annex A1, Tables A1.1–A1.4 for Ir-192 and Co-60). The working formula is t = E × SFD² / (A × density-factor), where E is read from a kV/thickness or isotope/thickness table, SFD is squared per inverse-square, A is current activity, and the density factor adjusts between film classes. Class I (D2/D3) fine-grain slow film needs ~3× the exposure of Class III (D7/D8) coarse-grain fast film at the same density.
Formula
t = (E × SFD²) / (A × FilmFactor)
t = (E × SFD²) / (A × FilmFactor)Worked example
60 Ci Ir-192 source, 1.0 inch (25.4 mm) steel thickness, Class II film (Kodak AA / Agfa D5), 36 inch SFD. From ASTM E94 the exposure factor at 1 inch steel for Ir-192 is ~0.18 Ci·min/in² for density 2.0 on D5. t = (0.18 × 36²) / 60 = 233 / 60 ≈ 4.0 min = 240 s. Class I film (D3) would double this to 8 min; Class III (D7) would cut to 1.7 min — but Class III sacrifices definition.
| Variable | Value |
|---|---|
| input: activity | 60 |
| input: sfd | 36 |
| input: thickness | 1 |
| input: film | D5 (Class II) |
| output: exposureSeconds | 240 |
| output: exposureMinutes | 4.0 |
When to use this tool
Use to plan every Ir-192 weld shoot, validate vendor exposure times, recalculate when the source has decayed below 60% activity, or train new radiographers on the SFD-squared scaling that drives most density failures.
Limitations
Where this calculator stops being accurate:
- Calibrated for Ir-192 and Co-60 on steel only. Other isotopes and materials require their own exposure-factor tables.
- Density target is 2.0 H&D on film. Higher density (2.5–3.0) requires proportionally more time.
- Does not account for source-to-object-distance vs object-to-film-distance geometric unsharpness — verify Ug per ASTM E94 §17.
- Backscatter from concrete floors can add 10–15% density; not included.
- Film class boundaries are manufacturer-specific. Confirm with Kodak/Agfa/Fuji datasheet for your stock.
Frequently Asked Questions
Why does film class change exposure time so much?
Film speed varies by silver-halide grain size. Class I (D3) is ultra-fine grain — best definition, slowest. Class III (D7/D8) is coarse grain — fastest, lower definition. ASTM E94 Table 1 gives the speed ratios: D3:D5:D7 ≈ 4:2:1 in exposure factor. Pick the slowest class the schedule allows — better grain means smaller indications are visible at lower contrast.
How does SFD affect exposure?
Inverse square — double the SFD, quadruple the time. SFD also drives geometric unsharpness Ug = F·t/D where F is source dimension, t is object-to-film distance, D is source-to-object distance. ASME V Article 2 caps Ug at 0.020 in for thickness ≤ 2 in and 0.030 in for >2 to ≤3 in. Many shoots are SFD-constrained by Ug long before exposure-time becomes the binding factor.
What density target should I aim for?
ASME V T-282.1 and ASTM E94 §11 require base-plus-fog density of 2.0 to 4.0 H&D for routine work, measured under the area of interest. Most procedures target 2.5 ± 0.2. Below 2.0, density curves are too flat to distinguish a 2% sensitivity penetrameter; above 4.0, the high-density region drops off the linear part of the H&D curve.
Can I scale exposure for a thinner or thicker weld using HVL?
Yes — each HVL of additional steel doubles the required time. For Ir-192 the HVL in steel is ~13 mm. A weld that takes 4 min at 25 mm takes 8 min at 38 mm (one extra HVL) and 16 min at 51 mm. This is the practical reason most heavy-wall (>50 mm) work moves to Co-60 or X-ray crawlers — exposure times for Ir-192 become prohibitive past 60 mm steel.
References & Standards Cited
- ASTM E94/E94M-17 Standard Guide for Radiographic Examination
- ASME BPVC Section V (2023), Article 2 Radiographic Examination
- ASNT SNT-TC-1A (2020), Personnel Qualification — Radiographic Testing
- ASTM E1025-18 Standard Practice for Design, Manufacture, and Material Grouping Classification of Hole-Type IQIs
Founder of NDT Connect and Atlantis NDT. 15+ years in industrial inspection across oil & gas, petrochemical, and offshore. ASNT Level III certified across five methods. Drives platform standards for the NDT Connect marketplace.