"Polyethylene film is a thick, flexible material that comes in sheets of various thickness (measured in mil) that can be used in single or double layers (for better insulation). A quick and inexpensive option for growers, FarmTek's high quality greenhouse film has ultraviolet (UV) protection and offers excellent tensile strength durability. Polyethylene plastic greenhouse film translucency diffuses light well, bouncing the available sunlight and reducing shadows, both of which aid plant growth. Poly film is also easy to install with homeowner tools and modest building skills."
Stole that from farmtek.com, is straight sunlight best, probably but then you have an outside grow, and all the issues and variables that go with it........