How to Find Your Exposure Time: A Step-by-Step Guide to Exposure Calculators

On Reddit I keep seeing people ask: “How long should I expose my screen?” Honestly, that’s asking the question wrong — which is why the veterans keep answering: “It depends.”

That’s not a brush-off. Everyone’s light, mesh, and emulsion are different. Ask for an exact number, follow it, and the result usually disappoints. In my experience, exposure time is the output of these five parameters working together:

• The light source — wavelength, wattage, age. One guy bought a used exposure unit and his times jumped past 20 minutes. Nothing was broken; the tubes were just old.
• Distance from lamp to screen — intensity falls off fast. A 600W UV lamp at 14 cm (5.5 inches) can burn a screen in 20–40 seconds.
• Emulsion type — diazo, dual-cure, SBQ: speeds differ by several times.
• Mesh — mesh count matters, and dyed mesh takes longer.
• The coating — thickness, and the most overlooked one: whether it’s fully dry. A screen that isn’t dry will never expose right.

Same screen, same artwork: a halogen work light might need 16 minutes while a UV LED needs 20 seconds. Copying someone’s “3 minutes” off the internet never works.

The fix: let one test screen tell you

Emulsion is not a light switch — it’s not “cured / not cured.” It hardens gradually with dose (light intensity × time). Too little dose and the stencil is fragile: it washes out fine, prints a few pieces, then the image starts breaking down. Too much dose and the light seals shut the detail you wanted open.

So the real question isn’t “how many minutes.” It’s “how cured is my emulsion right now?” There’s a cheap tool that answers exactly that: an exposure calculator. The classic one is the 21-step Stouffer sensitivity guide.

The principle, roughly:

Lay it on a coated screen and expose. UV gets through each gray step differently:

• Step 1 is nearly clear → the emulsion under it gets ~100% dose → fully cured
• Middle steps block part of the light → partial dose → half-cured
• Step 21 blocks nearly everything → the emulsion gets almost nothing → uncured

One exposure, and the screen holds 21 dose samples from “plenty” to “not enough.” After washout:

• Steps that cured properly: emulsion hard, doesn’t wash away
• Steps that didn’t: emulsion tacky, washes off

Count how many steps stayed solid, and you know whether the dose was right. All 21 solid = over-exposed (you’re sacrificing detail). Too few solid = under-exposed (the stencil won’t last). No more guessing.

Reading the result: count the solid steps

Wash the screen out gently — a shower head or garden hose is enough. One guy posted celebrating his fifth exposure test finally working, then blasted the emulsion off with a pressure washer during washout.

Hold the screen to the light and look at the strip area. Under the light steps, the emulsion got a full dose and survived — hard and intact. Follow the steps toward the dark end, and at some point the emulsion starts washing away. Count: how many steps are fully solid?

That count is your stencil’s health reading. Stouffer’s own instructions set the target: step 7 dense and black, step 8 slightly lighter — that’s a properly exposed screen. Only solid to step 4? Under-exposed. Solid to 12? Over-exposed — the stencil is tough, but your halftones and fine lines are being sealed shut.

Why does this number matter so much? Because “the image washed out” only tells you the stencil is alive. The step count tells you how strong it is. An under-exposed screen won’t fail today — it fails at piece number 50: fraying edges, pinholes showing up.

The math: from step count to new time

Now the useful part. The dose ratio between neighboring steps is fixed: about 1.41× (the “15” in model T2115 means 0.15 optical density per step — run the log math and you get √2). So the gap between your reading and the target step 7 converts straight into time:

Lawson Screen & Digital has a 6-minute demo of exactly this process: Todd exposes one screen three times — 50, 30, and 10 seconds. Result: 50s reads step 13 (too much), 10s reads below 7 (not enough), 30s reads step 9. Two steps over, so: 30 × 1.41⁻² = 30 × 0.50 = 15 seconds. He burns the production screen at 15 seconds and lands right on 7. Watch closely how the strip is placed on the screen, and which side faces which way.

Don’t want to touch a calculator? Use the table (same numbers in Stouffer’s official factor table):

You read	Steps off	New time = old ×
4	−3	×2.82
5	−2	×2.00
6	−1	×1.41
7	0	done ✅
8	+1	×0.71
9	+2	×0.50
10	+3	×0.35

If your reading lands outside steps 4–10, don’t fine-tune — double or halve the time, run the test again, then use the formula.

One last must: verify. Burn one more screen with the new time, wedge on it, confirm it lands on 7. Then you’re set until your setup changes.

Symptom cheat sheet

What you see	What it means
Emulsion slimy, washes off, flakes	Under-exposed
Stencil frays, pinholes, dies mid-run	Under-exposed (it was born half-cured)
Fine lines / halftones won’t wash open	Over-exposed — or your light’s wavelength is wrong
Image area looks dark, won’t clear	Over-exposed, or poor film contact
Sharp on the glass side, fuzzy inside	Contact problem, not a time problem

A few real reference points

From real troubleshooting posts: a 600W UV lamp at 14 cm landed in the 20–40 second range. A small 30W LED wasn’t enough even at 2 minutes. A homemade box with two 180W LEDs took 12 minutes — and that’s fine. The point isn’t the numbers; it’s that they span dozens of times over, depending entirely on the rig. Stop asking whether your time is “normal.” Test it.

Log it, and re-test when anything changes

Your exposure time is a calibration for one setup: this light, at this distance, with this emulsion, on this mesh. Keep a log — light, distance, emulsion, mesh, time, result. Change any one of them (new bulbs, new emulsion batch, different mesh count) and the old number is void; run the wedge again.

One more shop habit from the video: every 20–50 screens, toss the wedge on with the job. Bulbs weaken slowly and your eyes won’t catch it — the step count will tell you before your prints do.

Skip the phone calculator: I built a free Exposure Time Calculator that does this conversion for you — step wedges (Stouffer, SAATI, Ryonet) and the factor-filter calculators (ExpoCheck, Chromaline Dual), with a built-in round-by-round session log that runs until you’re locked in. There’s also a full walkthrough of the tool.

The bottom line

One test screen, fifteen minutes, and you stop gambling on every screen after that. That’s the whole trade.