I tested Korona and, more recently, Photone using an iPhone XS Max. I tested Korona against a blurple when I unarchived my grow tent in Jan 2021 and it was not usable so I ended up with an Apogee. When I tested Photone in June of this year against a Growcraft X3 full cycle and Photone was 16% high with all four dimmer settings (25% increments). I traded some programmer to programmer emails with the…programmer of Photone and he said that Android was really tough because, in essence, there are so many different configurations. IIRC, it was something along the lines of 20k different hardware sets. Ugh.I'm guessing that's on an iPhone? The accuracy of phone apps depends on the OS and model of the phone. Photone for android is still in beta and it's crap on many phones. Common problems are hitting a ceiling and maxing out a low number and also locking up. It tops out at something like 400 on my Samsung.
Try PPFD Meter for android. It can pair with this bluetooth lux meter for improved accuracy.
On the iPhone, you need to use a diffuser. That, by itself, was enough for me to lose interest. Add in the fact that the software would have to deal with multiple versions of the iOS + support an increasing number of sensors and I don't want to try to rely on the outcome because there are cheap, reliable options.
I've been using my Apogee for about a year and a half and I see the value in it but, a few months ago, I decided to look into the validity of the conversion factor. I've tested a Uni-T against my MQ-500, which I had calibrated over the summer, and the 0.15 factor is gold when I compare the readings that I get for the Vipar Spectra XS-1500, which I used to germinate some seeds, and, interestingly, when I test the two meters using a Growcraft X2 veg. I can see that the XS-1500 would work out pretty well because it has a broad spectrum but the Growcraft is really heavy blue . Regardless, all of the readings that I've taken, though they range only up to 600µmols since the plants are just under 4 weeks, show that a factor of 0.015 (lux * 0.015 ≈ µmols) gets you really close.
Lotsa words, I realize and…the point? A Uni-T is $35. At that price, I'd be hard pressed to use software on my phone when I can just stick a $35 light meter in the tent and get good readings. Light meters are cheap, phones aren't.
Another point ("But, wait, there's more!") - all meters are wrong. And that's fine.
When I set my light to 140 watts and the Apogee reads 553µmols, in reality, that's somewhere between 525 and 580µmols. Even if it was 553µmols, the exact number does not matter. What matters is that the meter is consistent and, second, that it's close enough that I can set the light level and then watch the plant to ensure that the light level isn't too high. The number is good for reference that "Yeah, I'm in the ballpark." feeling but the only thing that counts, when all is said and done, is if how the plants deal with the light level, regardless of the number.
A meter, of any kind and of any accuracy, will only get you in the ballpark - it's up to the grower to read the plant and fine tune it from there.