When you see TV cops spraying a wall and using a UV light source to find blood evidence, they're not using luminol. No light source is used. Lights are turned off. The chemical glow lasts only thirty seconds. So the drill is set up the camera first. Use a tripod, shutter release cable, and a rear curtain flash (one that goes off near the end of a long exposure) to provide background or context for the glow. Focus on the area and take a few shots to determine ISO and F-stop. Then spray, hit the shutter release and keep the shutter open for at least 30 seconds. Each subsequent spray of the area decreases the luminescence.

Alternate photography methods include using photoshop to layer the glow shot over the context shot.

Check out "Police Photography" by Larry S. Miller and Norman Marin.