Which governs the relationship between SID and receptor exposure?

Which governs the relationship between SID and receptor exposure?

• A. Reciprocity law
• B. 15% rule
• C. Inverse square law
• D. Ohm's law

Answer: (C)

The key idea is how the X-ray beam spreads as it travels from the focal spot to the image receptor. Because the beam diverges, its intensity at the receptor drops with distance according to the inverse square law: receptor exposure is proportional to the inverse of the distance squared. In simple terms, if you double the SID, exposure becomes one quarter; if you increase the SID by 15%, exposure falls to about 76% of its original value (roughly a 24% drop). To keep receptor exposure the same, you’d need to raise the exposure factor (mAs) by about 32% in that 15% increase scenario. This is why SID changes are accompanied by adjustments in technique. The reciprocity law relates mA and exposure time for the same mAs, not distance; the 15% rule is just a practical guideline, and Ohm’s law isn’t about radiographic exposure.

The key idea is how the X-ray beam spreads as it travels from the focal spot to the image receptor. Because the beam diverges, its intensity at the receptor drops with distance according to the inverse square law: receptor exposure is proportional to the inverse of the distance squared. In simple terms, if you double the SID, exposure becomes one quarter; if you increase the SID by 15%, exposure falls to about 76% of its original value (roughly a 24% drop). To keep receptor exposure the same, you’d need to raise the exposure factor (mAs) by about 32% in that 15% increase scenario. This is why SID changes are accompanied by adjustments in technique. The reciprocity law relates mA and exposure time for the same mAs, not distance; the 15% rule is just a practical guideline, and Ohm’s law isn’t about radiographic exposure.