There are lots of ways of doing this. You can print your image in negative, or positive if that mates up with the process, using a laser printer, on a sheet of acetate or mylar. Laser printers "look" good in this application, but their output is not sufficiently dense to occlude the light. Additionally, the toner-side of the mylar must be in contact with the photosensitive surface, else light will "bend" around the edges, and diffuse in the acetate/mylar film.

Perhaps if you print your image multiple times on your medium, you'll get a dense-enough "film" to succeed with this method. You may find that a high-precision color inkjet printer works better, provided you can get precise registration between passes. That's not as difficult as you may think. It's important that you select the right color of ink, as the choice isn't obvious. Black is not necessarily dense enough. Yellow or yellow-green may work out better.

If you hand-draw the "positive" tracks directly on your photosensitive surface, using a very dense, opaque ink, you'll get good results. It's still not clear to me just exactly what you're doing and what you're using.

RE