How this calculator works
As radiation spreads over a growing spherical surface, the same flux is shared over area proportional to distance squared. The relation applies to an ideal compact source in a uniform non-absorbing medium. Both intensities must use the same units and both distances must start at the source. Extended sources and focused beams may behave differently.
Formula
I1 r1² = I2 r2²; I2 = I1(r1/r2)²; r2 = r1 sqrt(I1/I2).
Worked example
An intensity of 100 units at 2 m becomes 25 units at 4 m. At 1 m it would be 400 units under the same point-source assumptions.
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Frequently asked questions
Does doubling distance halve intensity?
It reduces intensity to one quarter because the illuminated area grows fourfold.
Does it apply inside an absorbing material?
Absorption adds a separate transmission factor, so the pure inverse-square relation is incomplete.
Can I use any intensity units?
Yes, provided both entries use the same physical quantity and the same units.