Inverse Square Distance of 40” = 5mR New distance = 36”, what is your new dose ____

Inverse Square law: The radiation Intensity is inversely proportional to the square of the distance.   As the distance doubles, the area quadruples and thus, the initial radiation amount is spread over that entire area and is therefore reduced, proportionately.

Imagine we are trying to expose a piece of x-ray film (radiograph) and we move the x-ray source twice as far away on each shot, will the film be more or less exposed?  Therefore, while the inverse square law pertains to radiation safety, it also helps us to determine source to film distances (SFD), time of x-ray exposure, and the intensity (KV) of our x-ray tube.

I₁ = Intensity at  point 1 (R/hr or mR/hr)

D₁ = Distance of first point

I₂ = Intensity of secomd point Distance

D₂ = Distance of second point