Setting the exposure time (step test)
Direct answer: the correct exposure time fully hardens the emulsion without closing up fine detail. It depends on the emulsion, the UV source power and the distance. The only reliable method is the step test with an exposure calculator: you expose a strip at increasing times and keep the first one that best survives development, with no loss of detail.
The steps
1. Coat and dry the screen
Emulsion evenly applied, screen perfectly dry and kept away from light.
2. Prepare the calculator
Place your exposure calculator and mask in strips to expose zones at increasing times.
3. Expose in steps
Uncover one more strip at each step, with perfect film/screen contact.
4. Develop
With a low-pressure water jet, until the zones open cleanly.
5. Read the result
Keep the first time that gives a firm emulsion with cleanly open detail.
Practical exposure-time guide by light source
These times are starting values. Always validate them with the calculator: the result varies with the emulsion, coating thickness, mesh, lamp/screen distance, film opacity and the condition of the light source.
| Speed | Light source | Photopolymer / pre-sensitised emulsion | Diazo / two-part emulsion | Workshop note |
|---|---|---|---|---|
| 1 | Metal halide 3000–5000 W | 10 to 40 s | 30 s to 1 min 30 | Very fast, powerful source, suited to busy shops. Excellent UV output, especially effective with good vacuum contact. |
| 2 | UV LED 395–405 nm — compact frame | 45 s to 2 min 30 | 2 to 6 min | Efficient and economical. Times vary widely with the real power, distance and LED quality. A good compromise for a modern shop, at a lower cost than metal halide. |
| 3 | UV mercury vapour | 1 to 3 min | 2 to 5 min | Efficient but hot UV source. Use with care to avoid heating the screen, warping the film or losing contact. |
| 4 | UV "blacklight" fluorescent tubes | 2 to 5 min | 8 to 10 min | Works well with genuine UV blacklight tubes (~350 nm). Tubes age and lose output; diffuse, crossing light can reduce fine-detail resolution. |
| 5 | HPR bulb / compact UV lamp | 8 to 10 min | 12 to 16 min | Efficient source but a narrow beam. Often needs several bulbs or a long distance to cover a large format evenly. Watch the less-exposed edges. |
| 6 | 500 W work-site halogen lamp | 6 to 10 min | 12 to 20 min | Stop-gap solution only. Very bright and very hot, but poor in usable UV. A bad choice for regular use or for good resolution. |
Estimate your starting time
Choose your UV source and emulsion to get a starting time to test:
Starting-time selector — enable JavaScript to use it.
Mind the exposure distance: moving the source further away increases the time sharply (the light received falls off with the square of the distance).
Frequently asked questions
What happens if it's under-exposed?
The emulsion stays soft and soapy during development, washes away, and the stencil won't hold up on press.
And if it's over-exposed?
Fine detail closes up: thin lines and small text no longer open. Also watch the film (positive) quality — it must not let light through.