Radiographic Testing (RT): Physics, Technique, and Image Quality
Core physics you must own
Candidates must be fluent in how radiation is produced, attenuated, and detected. Know the difference between the continuous bremsstrahlung spectrum and characteristic peaks, the practical meaning of half-value layer (HVL) and how it changes with energy and material, and the inverse-square law for a point source. Understand geometric unsharpness Ug = F·t/d (source size F, source-to-object distance d, object-to-film distance t) and why minimizing Ug drives source-to-object distance and film contact.
Technique variables and traps
Expect questions that swap terms: density vs. contrast, sensitivity vs. resolution, kilovoltage vs. exposure (mA·min). Higher kVp raises penetration and lowers subject contrast; more mA·min raises density. Remember film density is logarithmic (D = log(I0/It)), so a density of 2.0 transmits about 1% of incident light. Traps include confusing source-to-film distance with source-to-object distance, and forgetting that geometric unsharpness depends on object-to-detector distance, not total distance.
Image quality indicators
Know that IQI sensitivity is a percentage of thickness, that wire-type and hole-type IQIs measure different things, and that placement is on the source side when practical. Understand that IQI visibility reflects contrast and unsharpness together-not density alone. A common trap: assuming a passed IQI guarantees detection of all planar defects.
Worked rule of thumb
If Ug must be ≤ 0.25 mm, source size is 3 mm, and object-to-film distance is 20 mm, then d ≥ F·t/Ug = 3×20/0.25 = 240 mm minimum source-to-object distance. This single equation answers many technique questions.