Infographics / X-ray Physics

Inverse-square law & beam intensity

Rays diverge from a point source through three squares at distances d, 2d and 3d. The catchment area grows as the square of distance (relative 1, 4, 9), so the same photon output is spread thinner and the intensity falls as one over distance squared (relative 1, one quarter, one ninth). A plotted curve shows intensity versus distance, and a note states the law holds only for a point source in free space; in tissue or after a filter, exponential attenuation and scatter also apply.

Inverse square law for a point source Rays diverge from a point source through three squares at distances d, 2d and 3d. The catchment area grows as the square of distance (relative 1, 4, 9), so the same photon output is spread thinner and the intensity falls as one over distance squared (relative 1, one quarter, one ninth). A plotted curve shows intensity versus distance, and a note states the law holds only for a point source in free space; in tissue or after a filter, exponential attenuation and scatter also apply. Inverse square law: for a point source, area ∝ distance², so intensity ∝ 1/distance² point source area ×1 I = 1 area ×4 I = 1/4 area ×9 I = 1/9 d 2d 3d the same photons are spread over a larger area intensity vs distance 1 1/4 1/9 d 2d 3d distance relative intensity double the distance → quarter the intensity Holds only for a point source in free space (the primary beam). In tissue or after a filter, intensity is also set by attenuation (I = I₀e⁻µˣ). Scatter and collimation cause further deviation from 1/d².
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