Hmmm. I just re-did level 18, and the only hints I can give you without posting a screen shot are:


1) Double-mirrors allow the beam to pass through them sideways in addition to reflecting. All three double mirrors will be used this way.

2) The 45-degree angle prism will deflect beams from two opposite directions at the same time. The prism in this level is used in this fashion.

3) The central "box" on the puzzle gets circumnavigated twice. Once diagonally, and once in a straight fashion. This is where the double mirrors come in handy (they reflect the diagonal beams while also allowing the straight beams to pass).

4) The very last thing the beam does is to go down behind the diagonal wall, and split into the last prism hitting those last two bulbs at the ends of the diagonal wall.