How To Use Progressive Web App aka PWA On 420 Magazine Forum
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They use square footage because the leaves on the top layer absorb/block most of the light and the ones below see little of anything. If your light is above the plant, it will hit the top leaves first and stop, so square footage is the most useful way to describe lighting coverage.
Do not get confused tho, they use square footage but they also tell you to hang the light a certain distance above the plant. That means you are actually working in cubic feet.
The reason they dont use cubic feet is because the height needs to be a fixed distance. for example, if the company says an LED light will do an area of 2'x3' at 18 inches, that is 6 sq ft, you ignore the 18 inches because if you used it and did cubic feet, it would be 9 cubic foot. 9 cubic foot could mean anything, that could be 1ft by 1ft area with a light hung 9ft high, which would never work.
Basically, Square footage is the best way to rate lights on the amount of coverage they can do. When you start to talk about cubic feet, you are using a number with worthless value. It may be useful for some applications in growing but for 90% of them, you really dont need it.
Cubic feet is more useful when talking about the volume of air/co2 in the room and less about the light.