What? Did he have some vanta black walls in that tent or something? Mylar is 98% reflective. You could say 2% is being absorbed by the walls, sure, but 98% is being reflected back inside, how would you lose half the light from 98% reflective walls..
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First of all that 98% is only when it's perfectly flat and clean. You'd be lucky to get 90% after installing and 80% after one grow.
That's also one of the missed issues with reflectors on leds. They work decently when they are clean out of the box, but after one grow they lose ton of reflectiveness. So reflectors clean out of the box you'd get 7 to 8% more light by using bare COBs (comparing bare and "reflectored" at the correct height for same uniformity). Then the reflectors get dirty and you lose another 15% or worse. In total you could have had 24% more light on the plants with bare COBs then.
Even if we take the 98% at face value. This 98% figure is total reflection. Now a portion of that will be diffuse reflection. Diffuse reflection scatters in all directions. Which means half of it will be reflected upwards, back to the light or ceiling. This light does not reach your plants. It's what lights up the ceiling of your tent. That's all lost to the plants. Even though technically it was reflected, but it will be absorbed after enough bounces.
In fact if you look inside your tent and you see the walls looking really bright. Do you think that light that hit your eyes would have been on it's way to your plants? Assuming your eyes are above plant level, the answer would be no. It's a bad trick that light plays on our minds. The lighter the walls look the more light is bouncing in the wrong directions.
So that potentially can already lose you half your light.
Either way. It's measurable effect. Create (or use someone else's) PPFD measurements matrix and sum it up to get an estimate of the total amount of umol/s detected over the measured floor. For instance, take the average PPFD and multiply by total floor surface area. Now compare that to the amount of light which was supposed to be emitted by the fixture.