Evacuated Solar Tube Industry Terminology?

[answer10a]

I am looking to make my own parabolic solar trough but find some terminology used confusing:

What do the phrases "single target", "two target", and "three target" mean in the context of evacuated solar tubes?

Example links of the phrases in use are provided below!

• general search for "solar evacuated tubes":
http://www.alibaba.com/trade/search?fsb=y&IndexArea=product_en&CatId=&SearchText=solar+evacuated+tube

• "single target" phrase example:
http://www.alibaba.com/product-detail/solar-evacuated-tube-solar-water-heater_305692441.html

• "two target" phrase example:
http://www.alibaba.com/product-detail/two-target-solar-evacuated-tube_1008016628.html

• "three target" phrase example:
http://www.alibaba.com/product-detail/all-glass-three-target-heat-pipe_1862391620.html

I thank you in advance for your help; I anticipate you will succeed where Google and DuckDuckGo both failed!

 

[zoobyshoe]

http://www.linkedsun.com/normal-vacuum-tube.html

http://www.linkedsun.com/three-targetvacuum-tube.html

The three target tube just seems to be the deluxe version. I get the sense the one, two, and three designations may refer to the operative coating layers, but they aren't explicit about that.