It's not a factor.
If I take a 1 lumen lamp, and illuminate 1 meter, that meter will be at 1 lux.
If I move that light up, to cover 2 meters, the two meters are illuminated at 0.5 lux.
The circle math people use can look confusing. It says that moving the light upwards, lowered the illumination over my meter. People interpret this to mean that my light somehow wore out as it traveled the greater distance. This is not true. The light has simply spread over a larger area.
Lets say I have a panel of these 14w lamps, making this 40w per foot. But my panel is huge. Say a square mile. And it's 5 meters above my plants. If I raise it to 10 meters. Nothing should happen. At least, not in the middle. At the edge, my lux reading drops as I'm now lighting more area outside the square mile. That is a loss of light by raising the panel. In the middle though, all the photon's are still coming down, and photons don't wear out.
This is upset a little by a dust storm. Fog. Birds and bugs. But a 1mW laser pointer can easily manage 10 miles over a city. It's limitation being the fact it spreads out eventually. It's not perfectly straight.
I know using a light meter can seem to go against this. Because it's a fixed size, and we expect a certain result. So lets try a smaller scale experiment. I'm going to take my torch, that makes a nice defined circle, and point it at the floor. In the middle of the circle, I place a light meter. Just outside the circle, I place another light meter. I take the readings. Then I move the light up, so that both light meters now fall within the circle. Without reading the numbers off the meters, you can see the combined illumination has risen by raising the light. If you had two buds, not sensors, you would raise the light and do better. Yet using a single meter you might be guided not to.
The drawing shows a lamp, giving off 32 of these photons that don't wear out. You can position the sensor to catch any number. Close up looks good, but then you soon surround the lamp. Further looks bad, but look how much space there is. This picture seems simple enough with only one light source. There is an ideal distance the meter can find.
This image is what we are doing.
Once you get between a few lamps, the distance from them becomes less important.
If you can get the 1 lumen at source, to illuminate 1 meter, it's illuminated to 1 lux. The distance is unimportant. Photons don't wear out within the scope of our understanding. They're magnetic waves, not particles.
I must add, this is all in my opinion. I'm not stating it as fact. I think it's fact, but you can all decide for yourselves.
Why are we still at 108 Lumens per watt in our stores.
I was just looking on the philips site, and their dome was just 95. Yet the filament looking led lamp was 138. While it's a few years since they made the 200 lumen per watt dubai lamp, that has to be used in all new builds over there.
I have to wonder if our 108 lamps are measured values with the dome intact, and what sort of performance gain we get taking that dome off. Just a light meter won't help. The dome is a diffuser and by nature scatters the light over a wide area. Removing just that effect will see big gains at a fixed position below. Without any more light leaving the lamp.
