"I've heard of PAR, what is it?"

Photosynthetically active radiation.

Lumens are for humans.

Proximity is not just about heat.
Especially where lensing is used.

For this, I gotta use LUX, (lumens per square meter)

At ~ 91k Lux Cannabis saturates and stops growing.
In the tropics, we would lose about 2 hours a day in growth if we didn't use shade cloth.
If you exceed ~93k Lux you start to get light bleaching which is permanent damage.

Saw a guy buy good led panels, then hang them 2" from his plants.
When the plants quit growing, he didn't waste any time thinking about it, he pronounced the leds to be crap and whined, online, for months.

That, and if an unlensed light is only 4 inches from the crown.
What does the ISL say it will drop to at 8 inches down?!
And then, at 16"?

That's what "penetration" really means.
It's not about shadows.
Sun movement and wind mitigate shading.

It' is, and always has been, about distance, when growing under lights.
The higher above the canopy you can keep your lights, the more, so called, "penetration" you will get.
And that mean bigger trees with green leaf all the way down, indoors.

Lensing, limits the area covered, but allows plenty of headroom at saturation levels.
It's a reasonable workaround to the ISL limitations.

Fingers are tired.
I do run on!

Aloha,
Weeze