This is where you’re light bulb and lamp come in. Set up your lamp above a table. You can use a pie tin or tin foil around the light bulb to help the light spread more evenly. Prop your sensitized screen up under the lamp. You want the flat side of the screen facing down, but you want a space between it and the table. Place your acetate image in the middle of the screen. Put the piece of glass or plexi-glass on top of the image. (In my example here, the screen was a bit small for my piece of glass, so I had to just tape the acetate down. But the glass helps make a sharper image).
Arrange the lamp the appropriate distance from the screen, and turn it on. Let it expose for the appropriate amount of time. Speedball has a chart of guidelines regarding exposure times for a 250 watt bulb:
Screen Size . . Lamp Height . . Exposure Time
8" x 10". . . . . . 12 inches . . . . . 10 minutes
10"x14". . . . . . 12 inches . . . . . 10 minutes
12 "x 18" . . . . .15 inches . . . . . 16 minutes
16"x2O" . . . . . 17 inches . . . . . 20 minutes
18"x2O" . . . . . 17 inches . . . . . 20 minutes
When the time is up, take your screen to the nearest hose or sink, and rinse, rinse, rinse. Use cold water. You’ll need a strong blast of water pressure, and it takes a while, but you’ll start to see the emulsion rinsing out, leaving a stencil of your image.
Hold your screen up to the light and be absolutely sure that all the parts of the screen that need to be open, in fact are open. You can use a toothbrush to scrub stubborn areas. If you’ve underexposed the screen, the emulsion will not be hard enough, and you may have chunks of the screen washing out that you don’t want open. On the other hand, if it’s exposed too long, the emulsion will be too hard and you won’t be able to rinse out your image, leaving you with a blocked screen. In the event of either of these cases, (and they’ve both happened to me), you have to wash the emulsion off with emulsion remover, and start again. Which obviously really sucks. If it does happen, don’t wait to clean the screen. Emulsion becomes impossible to remove fairly quickly.