NRNDT Radio LU

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X-ray exposure time

Empirical estimate of an X-ray tube exposure time. The method is approximate: it is for pre-setting, not for qualifying a technique.

Caution — indicative result (empirical method). An X-ray tube is set from the generator curve, not from a formula.

mA
kV
mm
m
g/cm³
7.85 for steel.

Exposure time—seconds
Duration
—
mA·min product
—

Method

T = factor × (d × 1000)² × t^1.4 × density / (mA × kV^2.2)

  • T — exposure time in seconds
  • d — focal distance in m (converted to mm in the formula)
  • t — thickness in mm
  • density — density in g/cm³
  • mA, kV — tube settings
  • factor — film factor (dimensionless)

The formula combines a 1/d² falloff, absorption as t^1.4 and emission as kV^2.2. It ignores the real generator spectrum, scattered radiation and detector type. For a qualified technique, use the manufacturer exposure curves and a dose measurement.

Indicative result — check it against your authorisation and your dosimetry.