FWIW, the theoretical maximum transmission of a 4 element multi-coated eyepiece with 4 air-to-glass surfaces is 98.0%, so treat any measurement above that as "approximate".
As to response to a single photon, read:
I agree about the Nagler being interesting. Looks like an outlier if Don's point about light loss is correct.
So, another question . . . How solid is the physics behind the claim for 98% transmission for 4 air-to-glass surfaces?
It occurs to me that if this theory is very solid indeed, it gives us the ability to estimate measurement error in the table.
For example, the Pentax 0-18 is rated at 98.34% max transmission.
Using only that eyepiece we have a minimal error of +/- .34%
If we rinse and repeat comparing the theoretical transmission with the higher element eyepieces and look for any that have above theoretical transmission, we can consider these as highly conservative errors.
If the 98% figure means a .5% loss per lens, then theoretical max for 5 elements is 97.5% (obviously correct me if this is wrong). Nothing in the table exceeds this maximum. So no information there.
However, I believe the Type 6 Nagler has 7 elements, meaning a max 96.5% transmission. And yet the measure for the 9mm claims 96.96% transmission, but a theoretical max of 96.5. A +/- .46% error.
If my assumptions are correct, then we probably want to consider the errors at the very least .5%.