Recently, I saw two interesting modifications applied to Philips FRC (or similar) fixtures. These normally run two 36W PL-L fluorescent lamps, usually in 4000K CCT.
#1 - Philips Fortimo LED module is used instead of PL-L lamps. Don't know its wattage but it's remarkably brighter than PL-L lamps. While the lantern looks brighter, the shining part is only like 1/3 of the tubes length, so the light output
could be about the same.
There are no other problems with modifications like this, except of that glaring, which is not a problem of PL-L tubes. Almost 100% of the light emitted by the module goes down and partially to sides, which is right. Reflector-less fixtures will always gain in efficiency with a LED module while those with reflectors for SON-T or MH lamps will lose because the LED module light can't be focused so well.
#2 is more problematic.
While the proper HPS/SON-T fixture throws like >90% of the light out of the lantern, here it is very slightly lower because of the white and not silver back. There's 75% direct visibility of the lamp + the reflected light. So it could be again almost 90%.
When compared to original PL-L tubes that partially obscure each other, the ratio of light going out of the lantern is better in favour of HPS/SON-T! Okay, the situation is much worse than with proper SON-T fixtures because a lot of light goes sideways and not downwards as requested. But with PL-L tubes it is the same!
I don't think mods like this are reasonable because of the terrible glaring of the bare SON-T tube and possible overheating of parts near the tube, but there's no problem with efficiency IMO. Well..., maybe there is. The pupil in human eye will stretch because of that bright point and will collect less light from the ground - which is not a problem in case of PL-L lamps.
