← X-ray subtopic / Sim: Inverse-Square Law
Inverse-Square Law
Intensity ∝ 1/d². Drag the distance slider and watch the photon fluence at the detector and the required mAs to hold exposure constant.
100
Relative intensity
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Compensating mAs factor
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Photons / cm² (relative)
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Reference: SID = 100 cm, relative intensity = 1.00, mAs factor = 1.0. Change the distance and read the new values.
The physics
X-rays diverge from the focal spot in all directions. The same bundle of photons covers a larger area as distance increases — area grows as d², so fluence falls as 1/d². To hold detector exposure constant when doubling SID: intensity → ¼, so mAs must increase 4×. This governs portable technique choices, SID-adjusted technique factors, and why operators step back during fluoroscopy: even a small increase in distance dramatically reduces occupational dose.