Weedwhacker said:Hmm - I never expected the UV thing to be so controversial 
At any rate - Jim @Moonshadows - am I understanding you correctly, that the "UV protected" means that UV light is actually blocked? (this is just a curiosity thing for me, my plants certainly seem happy enough with whatever sort of light they are getting through the GH film...)
Also - the only way I consider my tomato plants to look different than when they were grown outdoors is that they don't get diseased halfway through the season (I've never nailed down whether from blight or some sort of wilt or whatever, but dead way too early when grown outdoors).
I agree with you about greenhouse tomato plants; mine also tend to do so much better with little, if any, disease as opposed to my outdoor tomato plants. I am not sure why this is, but I know I can control my indoor environment while I can't control my outdoor environment, i.e. light energy, or lack of it, rain, moisture, humidity, soil, etc.
"UV protected" (to my understanding) is not something they apply to the material, but is inherent in the material itself.
Clear stabilized polyethylene film (which is really not clear) is resistant to UV deterioration and also blocks/diffuses a portion of UV rays from reaching the interior of a greenhouse. However, even clear windows in your house block a portion of UV rays, so there is nothing magical about greenhouse film and it's UV blocking abilities. Anything you place as a skin over your greenhouse will block some UV radiation to one degree or another, depending on the material and thickness. When considering UV blocking properties of a greenhouse covering, one only needs to be really concerned with it's ability to extend the material's life, not the effect on the plants within the greenhouse. It should also be noted for anyone who thinks UV radiation is necessary for their plant growth that while UV radiation has benefits for plant growth, it can still cause damage to plants, especially UVB...see next paragraph.
As far as UV goes, there are three different UV's. UVA, UVB, and UVC, and explaining/understanding them can be difficult and not easily understood, demanding much more depth than a few posts in a gardening thread. Actually, UV rays are only a portion of the energy or radiation that the sun produces, and sometimes folks get all caught up on the UV portion without knowing or understanding all the "rays (radiation) of energy" the sun produces.
When talking about plant growth and production, the radiation/energy one should concern themselves with is the PAR energy, or Photosynthetically Active Radiation. (You may or may not remember from science class that photosynthesis is the process that converts carbon dioxide and water into sugars.) By the time the sun's energy reaches the surface of the earth, about 43% of its total energy is within the visible or PAR region, while 52% is in the infrared (IR) or heat region and 5% is in the ultraviolet (UV) region Although plants may be both negatively and positively affected by energy in the IR or UV wavelengths, they do not directly use light in these wavelengths for photosynthesis. They use the PAR energy.
For plants PAR is how they "see" light. For us humans, we "see" light in terms of lumens. Our eyes and plants absorb energy within a relatively narrow range of the electromagnetic spectrum. Humans and plants both "see" light with wavelengths between 400 nm (violet) and 700 nm (red). For us this range of wavelengths is called the visible light spectrum and the unit of measure for the light we see is lumens. For plants, this region of light is called photosynthetically active radiation (PAR). It is the only portion of the suns energy that plants really need to create photosynthesis.
Probably more than you were asking about (or asking for), but if anything, a small taste of how complicated the energy/radiation/"light" coming from the sun is.
Today's modern greenhouse covering materials, both residential and commercial, are the result of many years of experimentation and science, and are the best we have at this point for producing the best greenhouse plants when it comes to the combination of physical protection/material strength and durability/ and energy (light) transmission. Choosing one is a matter of "taste", climatic conditions, budget, and purpose for greenhouse.
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