Hopefully, some light gurus hang out here. I'll cross-post this in the Greenhouse section just in case.

To make a long story short, I'm trying to find a heavy-duty outside plastic cover for the GH I plan to build but am having trouble finding one that allows a significant amount of light through, based on light meter readings. Usually, the specs say the cover allow 88-92 percent light transmissions but I don't get close to that.

Just a thought - I wonder if the people who quote the statistics are using the same measurements I am. All three things I have tried, from some white construction plastic reinforced with fiberglass, to 5.2 oz. covering from FarmTek to plain white (not clear) plastic from the hardware store have Lux reading less than half on the inside of the GH compared to the outside.

But... I live in a very old house and the upstairs does not have good windows, so I line them with plastic during winter time. When I ran out of room for my seedlings this winter, I stuck some of the trays in front of the windows with the plastic covering them. No other supplemental light but the plants did not get leggy and then did not need "hardened off" when planting time came.

Is it possible the plastic is filtering certain spectra and that is what is causing much lower Lux readings? Maybe the scientists or whoever report light transmission rates are using PAR light that is getting through? Or perhaps diffused light does not raise a meter as direct light does?

Mike