200 cm from the source — 2.00× the reference distance of 100 cm.
- Intensity is now 0.25× (a quarter)
- Moving from 100 cm to 200 cm multiplies the distance by 2.00, so the beam covers 4.00× the area. The same total number of x-rays now has to cover all of it, so the intensity at any one point drops to 0.25× what it was. Note what did NOT change: distance affects how many x-rays land, never their energy.
- Holding receptor exposure would take 4.00× the mAs
- This is the square law, and it is the inverse square law turned upside down: mAs₂/mAs₁ = (SID₂/SID₁)². Watch the denominators — in the inverse square law the NEW distance is on top, here the OLD one is. Intensity went down by 4.00×, so the mAs has to go up by the same 4.00× to land the same exposure on the receptor.
- Why this is the radiographer’s best tool
- Distance is the most effective of the three protective measures. Backing from 100 cm to 200 cm cuts what reaches you to 0.25× — no lead required.
- It assumes a point source
- The law is exact for a point source, and only an approximation for anything with real size. That is not a practical worry here: an x-ray tube's focal spot is a millimetre or two across, so at 200 cm — or any clinical distance — the tube behaves as a point source and the law holds.