Not sure. It'd have to be pretty thick Tyvek to block all light transmission (although the Orca product is pretty thick at 11mil). My gut tells me that it's not, because as far as I know DuPont still holds exclusivity on Tyvek® and they tend to get pretty extreme when others try manufacturing/selling their products. But I could easily be wrong.
According to Orca's website (linked in post #10), Orca film has a very flat reflectivity profile across the visible spectrum. From a quick read-through of the .PDF linked in post #3 (thanks to HuckFinn for that), it appears that although Tyvek is reflective, it does not have that flat profile (and a flat profile is what we want, along with spectral stability (IOW, wavelengths do not shift) and actual optimum gross reflectivity numbers). Although the study only used a very limited number of samples so there was the possibility of large statistical errors; however, the samples tested did have the same trend within the sample group <SHRUGS>. I'll have to give it a more thorough reread when I've got my maths hat on.
Going strictly off of Orcas's website (admittedly, a bad idea), their product looks like a very good one in comparison to the other readily-available products we routinely consider. And some of their supporting text/graph materials kind of supports what I've always felt about brilliantly white flat paint being an excellent material for the GR walls. That suggests (to me) that there is something to their arguments, perhaps more than what could be expected from a typical marketing department, lol; not because they agree with my thoughts, lol, but because logic suggests that it would be the case. (BtW, they do NOT list bright flat white paint in their Reflectance versus Wavelength chart. Perhaps because that and their product might be close enough in characteristics - if not in absolute gross reflectivity - that many might simply choose to prep/prime/paint their walls instead of purchasing their products, lol. But that is purely speculation on my part and should NOT be taken as anything else.)
I saw on the Orca website that they were at the Indoor Gardening Expo in San Francisco this weekend. Did anyone attend and visit their display? It also shows that they will be at the Long Beach Expo on October 22-23. Anyone who is interested in their product should (if possible) visit, ask for a sample, and inquire in person.