m0use answered grow question 1 month ago The app is consistent in measuring light on the same device but it is not a true quantum sensor that can give an accurate readout of PPFD, If the app is calibrated using a sensor then it is golden and is almost as good, but in order to calibrate it you need a sensor.
So what you can do is use the app to find the "right number", just know that number is wrong but it will always be the same amount of wrong. so as an example, if at 15" away and its reading 2000PPFD and your plants are doing well under it. make that your number to aim for. 2000PPFD would burn them if it was accurate but its not so it works, numbers are not the same on other devices has always be the same device best to use white paper to diffuse light into the camera lens if going this route.
Also look at your lighting specs, most good lights come with a PPFD map on different hang heights. all Spider farmer and Marz lights have this data.
https://m.media-amazon.com/images/I/71iz91RlXnL._AC_SX679_.jpg
I think this is your light from the information I gathered off your diary.
I'd aim for a hang height of 18-20" to start off. You could also hang it lower 12-15" and dim it down to 50% your call.
Be aware that LED has UV diodes, can be damaging to your eyes and skin wear protection.
Good Luck!
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