This is an ATS I built as a test bed. It sat across the top of a 55g. It was LED powered and used a heavily corrugated acrylic sheet as the screen.
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I thought I could get better growth if I reddened the light so I did. I think it helped a little.
Summary.
1) The acrylic screen was too smooth and could never be made rough enough.
2) These super bright LEDs were not super enough. You'd need HBLEDs to really make it crank.
3) My water distribution at the top had too many dribble and leak issues that caused water to go places it shouldn't - like down the front window where algae would grow.
4) This was before we realized that you need an ATS big enough or it will never convert to the light green efficient algae. Exacerbating this was the fact that my tank was running a little over 1,100ppm nitrate.
I subsequently converted it to horizontal and learned even more about the issues of horizontals.