Sorry for the delay, been busy.
Here is what I found out talking with the company more. Here is a list of all the materials they sell: http://www.pgo.com/link_com_opt/materials.html
. The most cost effective ones are the Starphire and Borofloat.
Just to recap we have UVB transmission at the range of 280-315 nanometers and peak Vitamin D synthesis at 295-297 nanometers.
Looking at the starphire spec sheet http://www.pgo.com/pdf/ppg_starphire.pdf
which was done using 1/4" glass, there is less than 5 percent transmission at 330 nanometers, meaning at around 300 nanometers or 295-297 nanometers there is likely no UVB transmission at all. There may be a bit more transmission at 1/8" glass instead, but we could not find a spec sheet on it and the representative told me it would not be that much different just a little bit more transmission. According to this information, Starphire glass doesn't seem to transmit UVB at all, which brings me back to the question of different grades of Starphire since there has been mention of Starphire passing UVB just fine (in the research article mentioned above and other sources I have found).
Now, let's move on to Borofloat, http://www.pgo.com/pdf/borofloat.pdf
. These numbers were using 1/8" glass, but regardless of that, it seems that Borofloat transmits a great deal more UVB than Starphire. At the 300 nanometer range (which is right near our optimum range of 295-297 nanometers) we have 46 percent transmission. Even in a glass sample of Borofloat that was 1cm thick, I was given the following numbers by the representative: at 280 nanometers transmission is below 5 percent and pretty much begins here, the slope remains straight all the way up to 320 nanometers where it reaches 90 percent transmission, so even in this thicker piece of glass there is around the same transmission at the 300 nanometer range. I am in the process of getting a list of distributors but their price range from the factory was around 17 dollars a square foot with a minimum order of 250 dollars. I got the contact information of a local distributor in my area that I will work with and see if I can buy sheets in bulk from him.
All the information in this post is from the representative and his spec sheets, otherwise I have no other sources to verify this information.