Just to clarify things a bit more.
Most filters let more than 80% of light through. The real issue is reflectance. This is where the filter surface reflects light back in the direction it originated from.
The "standard" Hoya FL-Day reflects approx 4% of light from either side, so it's overall throughput is just under 92%.
The Hoya HMC FL-Day reflects far less light, having a throughput of over 97%.
Reflected light obviously reduces lumen output, but also reduces contrast, affects colour balance & reduces detail.
HOYA Multicoated HMC
& Super HMC Filters
[[[[[[[[ HMC Multicoated Filters
These popular filters are renowned for their ability to mimise reflection at the filter surfaces which reduces flare and ghosting. The result is an average light transmission of over 97% giving sharp contrast and well balanced colour. Hoya HMC filters are recommended for enhancing the performance of today's multicoated lenses. ]]]]]]]]]
Li On isn't an expert on filters as he is using the "standard" FL-Day filter, and asked a very similar question on AVS Forums.
Obviously preferable to get the HMC version, as the cost difference is negligable.
Hope this helps.
p.s any decent camera shop should be able to order this filter, as Hoya are a very common filter in the photography trade.