Assuming you have the PFO Optimal reflectors mentioned by Joe Reefer and will mount them at 6", I vote for perpendicular. If you have the parallel reflectors, then I would change my vote to parallel. If you have another brand, I would follow what the company recommends (probably parallel).
PFO Website from 2003:
http://web.archive.org/web/200304221...ofit+Items.htm
"OPTIMAL REFLECTOR(Reeflector): This reflector is designed to mount perpendicular to the front of the tank. It is made with 95% reflective aluminum and will put about 40% more light into the tank than a standard flat sheet of reflective metal. "
"PARALLEL REFLECTOR : The parallel reflector is designed to mount parallel with the front of the tank using fluorescents. It is made with 95% reflective aluminum and will put about 25% more light into the tank than a standard flat sheet of reflective metal. "
From Sanjay's Testing I Believe:
Light Distribution for Optimal reflector:
Light Distribution for Parallel reflector:
Using the optimal reflector at 6"-9" from the water surface, the area with the brightest light is greater along the X axis. Notice the shape of the oval near the center is wide.
Using the parallel reflector at 6"-9" from the water surface, the area with the brightest light is greater along the Y axis. Notice the shape of the oval near the center is tall.
Thus, at 6"-9" from the water surface, it is better to mount Optimal reflectors perpendicular to the tank and Parallel reflectors parallel to the tank.
At 12" from the water surface, it looks like both should be mounted parallel.
Info taken from:
http://www.advancedaquarist.com/issu...03/feature.htm
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