The most reliable exposure chart a crew owns is the last shot that came out right. Transferring it to the next joint means applying four independent multiplicative corrections to a known-good time.

Distance follows the inverse square law, (d2/d1)^2. Activity is inversely proportional, A1/A2, which handles both decay between shots and swapping projectors. Film class is the ratio of relative exposures, roughly 3.2 between D7 and D4. Thickness is exponential: each extra half value layer of steel in the beam doubles the required exposure, so the correction is 2^((x2-x1)/HVL).

The thickness term is the one that catches people out. It is not linear. Going from 25 mm to 32 mm of steel with iridium is only 7 mm more material, but at an HVL of 12.7 mm that is 0.55 of a half value layer, a 47 percent increase in exposure. Going from 25 mm to 50 mm nearly quadruples it.

Transfer is valid across a modest range. Push it far and the effective beam quality changes – the beam hardens as it passes through more steel, so the effective HVL rises and the calculation over-predicts. Beyond about two half value layers of change, shoot a step wedge and redraw the chart rather than extrapolating.

Worked example

Known good exposure time	10 min
Original source-to-film distance	1 m
New source-to-film distance	1.2 m
Original penetrated thickness	25 mm
New penetrated thickness	32 mm
Original source activity	50 Ci
New source activity	50 Ci
Original film	d7
New film	d4
Isotope	ir192
Override steel HVL (0 = use table)	0 mm
Steel half value layer	12.7 mm
Distance factor	1.44
Film factor	3.2
Thickness factor	1.4653
New exposure time	67.52 min

Distance: (1.2/1.0)^2 = 1.44. Activity unchanged, factor 1. Film D7 to D4: 3.2/1.0 = 3.2. Thickness: 7 mm extra steel at an Ir-192 HVL of 12.7 mm is 0.5512 HVL, factor 2^0.5512 = 1.4653. Total factor 1.44 x 3.2 x 1.4653 = 6.7520, so t2 = 10 x 6.7520 = 67.52 min.

Use at your own risk — verify before you act

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