How to Expose a Screen Without Guessing
Screen exposure is one of the few parts of screen printing that's genuinely just physics — which is good news, because it means a burnt screen or a washed-out stencil usually has a specific, findable cause rather than being bad luck. The trouble is that most exposure advice you'll find is tied to someone else's specific lamp, distance, and emulsion, and doesn't translate directly to your setup.
What exposure is actually doing
Photo emulsion hardens when it absorbs enough UV light energy. Areas covered by your stencil (the opaque parts of a film positive) block that light and stay soft, washing out in water afterward to leave the open image area. Get the exposure right and you get a stencil with clean, sharp edges. Underexpose it and fine detail washes out along with the image, or the whole stencil stays soft and breaks down during the print run. Overexpose it and fine detail can block in, losing definition the other way.
Why "exposure time" isn't one universal number
The reason exposure times vary so much between shops isn't that everyone's emulsion is wildly different — it's that light intensity at the screen depends heavily on how far away the lamp is and how powerful it is. Move the same lamp twice as far from the screen and the intensity doesn't just halve; light follows the inverse-square law, so intensity falls off with the square of the distance. A lamp moved from 20 to 30 inches away — 1.5× the distance — drops to roughly 4/9 of its original intensity, not 2/3. That's why a "12-minute exposure" quoted online for one shop's setup can be badly wrong for yours if the lamp distance or wattage differs even a little.
Finding your actual baseline
The only way to know the correct exposure for your specific emulsion, coating thickness, and light source is a step-wedge (exposure-strip) test: expose one screen with a series of stripes covered at increasing time intervals, then wash it out and see which stripe holds detail cleanly without the image breaking down or washing away. Whatever time produced the cleanest result, at that exact lamp distance and wattage, is your real baseline — not a number borrowed from a forum post.
Carrying that baseline to a new setup
Once you have a trustworthy baseline, you can scale it mathematically instead of re-running a full step test every time your setup changes slightly. The adjustment combines two effects: the inverse-square law for distance, and a roughly linear relationship between exposure time and lamp wattage (double the wattage, roughly half the time needed). Multiplying your baseline time by (new distance / baseline distance)² and then by (baseline wattage / new wattage) gives a solid estimate for the new setup.
As a concrete example: a screen that exposes correctly in 60 seconds at 20 inches with a 1000W lamp would need roughly 135 seconds at 30 inches with the same lamp — the distance ratio squared, 2.25×, applied to the original 60 seconds. Move the lamp back further, or swap to a lower-wattage bulb, and the required time keeps climbing accordingly.
What this scaling doesn't cover
Distance and wattage are only two of the variables that affect exposure. Emulsion type, coating thickness (how many times you coated the screen and let it dry), and even ambient humidity all shift the baseline exposure itself, not just how it scales. If you switch emulsion brands, change your coating technique, or notice unusually humid conditions in your workspace, it's worth running a fresh step-wedge test rather than trusting the old baseline — the inverse-square scaling only carries a known-good number to a new lamp position, it doesn't fix a baseline that was wrong to begin with.
Once you trust your baseline, an emulsion exposure calculator turns this into a quick lookup instead of mental math every time you move a lamp or switch bulbs — but understanding why the numbers move the way they do means you'll notice immediately if a result looks physically implausible.