I know this isnt very technical, or very specific, but I know that flourecent lights work by exciting the "phosphors" on the inside of the tube. Different phospours release different wavelengths of light. If you just took blue plastic and covered a regular old flourecent, it will act as a filter and only allow blue light to pass through. It wouldnt change the wave length or the amount of light coming out of the bulb. As far as I know, actinics are mostly used to make the corals flourece and make their colors pop. You can see this by the fact that usually actinic PAR ratings are less than a full spectrum bulb(using same brands as comparison) Next time you look at a t5 for a reef tank, look for the spectrum graph on the outside of the box. Actinics have a large peak usually around 420nm and not much else. Where as full spectrum bulbs have a small peak there and then another peak closer to the yellow/red range.