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Half-value layer math
Each half-value layer cuts the beam's intensity in half: after n HVLs, intensity is the start × (½)ⁿ.
What an HVL is
The half-value layer, or HVL, is the thickness of absorber that cuts the beam's intensity in half. For diagnostic beams it is measured in millimeters of aluminum. One HVL leaves 50 percent, two leave 25 percent, three leave 12.5 percent and four leave 6.25 percent. Each layer halves whatever reaches it, not the original amount, so the beam never reaches zero.