I've been playing with an apogee par meter and thought I would post the results of how the mesh tops compare to other popular alternatives.
In these results, I am running a double ended 14k coralife 150 watt bulb. The sensor is approximately 4-inches directly below the bulb. Pumps are turned off to stabilize the reading and give a "best case" scenario. Here are the measurements (units are umol m-2 s-1).
no top 345
mesh top 327 (6.3% reduction)
clean glass top 305 (12.6% reduction)
egg crate 265 (23.2% reduction)
One thing to note is that the glass top is perfectly clean no...no salt creep...no condensate so that's a best case scenario. All measurements were taken directly below the bulb, meaning that with the eggcrate the light drastically is cut down once you move off center. (Maybe I will post some linear results using just eggcrate soon so the offset can be quantified.)
I'm switching out the coralife bulb tonight for an xm 20k. So for the heck of it, I will take some comparison reading with a different bulb.