You do lose quite a lot of light. Do a lux or PPFD matrix and see how much the average drops.
Malocan did this already and he measured a 20% loss of light by moving the lights from 12" to 24":
31.5cm / 12"
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60cm / 24"
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Although it looks like his fixture was too small for the space. If the COBs would go all the way to the sides, the uniformity would already be fine at 12". Then the wall losses after moving the fixture up would be even greater though.
12" is probably optimal for most builds (without reflectors).
Did you read the quote? Here it is again...
Dimming lowers PPFD(effects photosynthetic rate) as well as total energy(effects metabolism) in the grow area.
When you dim, you provide less over all photons, but maintain the same light/reflection pattern, and capture/incident angles. This shows as lower numbers on the quantum meter. And is ok to compare them 1:1, as all the variables except power were constant.
When you raise the light while maintain full PPF, you don't really lose the energy. You also don't lose the same amount of photons over the crop that is shown on your single spot meter. When the lights are raised, the light pattern is changed as is incident angles. It is impossible to show and compare with out a 3D map showing and recording every bit of light that is received by the canopy from all angles.
I fully understand and said it will change on the spot measurement readings. But without a 3d model of the canopy topography and incident angles of the light source it's not the full story of the entire crop.
If you want PPFD charts at different heights, I got plenty for you. But again, doesn't show the whole picture of full power raised dimmed at the same height.
I have personally grown both ways. And prefer full and raised to hit a lower PPFD. I simple get more growth faster, and is hardier.
I am also a proponent of not dimming or backing off that much anyways.
In the natural environment, a cannabis plants receives the most intense light and most DLI during the peak of veg and beginning of flower. Then decreases as flowering progresses. And even in early veg/spring, get very high ppfd levels, with a slightly lower DLI via the day's durations.