This is a great resource for people like me who are about to embark on their first grow and I wanted to say a word of gratitude to all those involved who have made it possible. I have 2 identical LED lights in my 4×4 tent. Using the PPFD footprint map supplied by the mfg. I can calculate average PPFD. Don’t you have to use the entire map, all 25 measuring points, in order to calculate average PPFD, even though one light will be used in half of the tent, and when converting to Usable PPF you use only half of the total square meters to calculate? Can I then double these values to account for the second light? I know within reason there is a more simple way of asking this question but my brain is fried from researching a subject I never imagined. Sorry for any confusion. Please advise.
I think you might be over thinking it. What are you trying to gain by calculating your average ppfd?
In the real world, the “average” ppfd is meaningless because light just doesn’t work that way. The middle area under the light will always be way brighter than the edges, so knowing your “average” isn’t really useful information.
If you are looking for an average number to judge overall light output, look at the PPF, which measures the TOTAL light coming from the fixture.
If you are trying to decide how far to space the lamps, look at the ppfd charts and use the numbers to judge the effective coverage area of the lights you are working with.
More importantly, listen to your plants! You can use coverage chats and light stats to help make purchasing decisions, but when it comes time to actually adjust your lights, the plants themselves are the best indicator.
Watch how they respond to your lights, and adjust from there. 🙂👍
Yeah, I was afraid my state of mind was going to cause some problems with the way I ask my question. That and ignorance. I’m trying to calculate Usable PPF. Part of the equation uses avg. PPFD. I was reading an online study of LED lights in general and they came up with this formula to calculate Usable PPF;
Average PPFD (µmol/m2/s) x square meters = Usable PPF (µmol/s).
They also say that 65 micromoles per sq. ft. is optimum and I wanted to see how my light stacked up.