This was mine first attempt to paint using light. I was hunting for small LED to write but rather found that there is small red light indicator on my Nikon SB-600 flash. This was good enough to write. When I started, I was so off in writing as you are writing in dark and and with no upper and lower limits. But anyways after nearly 30 attempts, I got one decent. As I said, this was my first attempt and might try more in future, may be with sparkles.
15 seconds exposure and used red light LED to write