I could be wrong but my understanding wasn’t that it blocked visible light, just 66% of the energy that comes in. So it reflects more UV and IR but still lets in visible light. I think in the visible spectrum they are in the 70-80% range, which is pretty darn clear. I know I don’t see any obvious “tinting” in our low-E windows, but if you open it and look at the light hitting the floor you can see where there is no glass it is a bit brighter.

I stand corrected, just went and looked at the site.  It looks like they do actually tint them more in the visible range.  The jump from Low E to Low E 66 doesn’t look huge, but noticeable.  Maybe you would have to have larger windows to get the same natural light in, which would defeat the purpose of the Low E 66.  I just argued when we built and added as few windows as was allowed by code (and my wife).  The best windows are 10 times worse then the worst wall insulation J

Jelly,

Think of it this way. 

A single pane of 1/8" clear glass has a VT of .9 or 90%.  Add a second pane and the VT drops to 81%.  Add a third pane and the VT drops to 73%.  Add a 4th pane and VT drops to 66%.

The VT published for Cardinal's 366 is based on two lites and the coating.  So in essence, the coating affects the Visible Transmittance to the same degree as adding two more lites to a clear glass dual-pane IG unit.