While I was home for lunch I downloaded an app called Photone, a popular iphone app for PPFD just to see how accurate it is. I used some standard printer paper over the front camera to act as a diffuser as the app instructed. Picked a spot on the canopy and the app said it at 1960 ppfd, which I was pretty certain was way off. So broke out the Apogee quantum sensor and took a measurement at the exact same spot, the Apogee said 1210, so there was a pretty big disparity. I wouldn’t trust what these phone apps tell you, it’s way off. They may be useful in telling you about coverage and where it drops off (corners and sides) but as far as real PPFD, not even close.