I have my onion seedlings under lights, but during the day I put them in the window when the sun is shining. Just this afternoon I noticed that while my seeds were in shaded light, the light hitting the wall on the other side of the room was perfectly clear. The reason? I had left the screen in the lower window slot, and it was the screen that was casting the crosshatched shadow. I slid the screen to the top of the window and voila, clear, bright light.

So don't be a knucklehead like me - give your seedlings the full benefit of what sunlight is available. ;-)