Emulsion Exposure Calculator
Carry a known-good exposure time to a new lamp distance or wattage using the inverse-square law, instead of starting a new setup from scratch.
Adjusted exposure time
The physics behind the adjustment
Correctly exposing a screen means delivering a specific total amount of UV light energy to the emulsion — enough to fully harden the areas not blocked by your stencil, without overexposing and losing fine detail. That total energy is exposure time multiplied by light intensity, and intensity depends on both how far the lamp sits from the screen and how powerful the bulb is.
Because intensity falls off with the square of distance, small moves add up fast — pulling a lamp back just 50% farther roughly doubles the exposure time needed. This calculator applies that relationship, plus a simple proportional adjustment for wattage, to scale a baseline time to your actual setup.
Frequently Asked Questions
Why does exposure time change with lamp distance?
Light intensity falls off with the square of the distance from the source — an inverse-square relationship. Moving a lamp from 20 to 30 inches away doesn't just cut intensity by a third, it drops to about 4/9 of the original, so the screen needs roughly 2.25× longer to receive the same total light energy.
Why does wattage matter, and how does it combine with distance?
A higher-wattage bulb puts out proportionally more light, so exposure time scales down roughly in proportion to wattage — double the wattage, about half the time. Distance and wattage act independently, so the calculator multiplies both adjustment factors together.
Where do I get the 'known-good' baseline exposure in the first place?
Run an exposure-strip (step-wedge) test on your actual setup: expose one screen with a series of stripes covered at increasing intervals, then wash it out and see which stripe holds detail cleanly without breaking down. That time, at that distance and wattage, is your baseline — this calculator only helps you carry it to a different setup.
Does this account for emulsion type or coating thickness?
No — those change the correct exposure time itself, not how it scales with distance or wattage. Different emulsions, coating counts, and even humidity all shift the baseline you should start from, which is why a fresh step test is worth running whenever any of those change.
Educational estimate based on the inverse-square law — not a substitute for a step-wedge exposure test on your own emulsion, coating thickness, and light source.