You can calibrate it against a light meter, a PAR meter, or by using a known good light source.
I've tested Photone using my iPhone XSMax, I use an Apogee and a Unit-T light meter, and I recommend a Uni-T light meter. The primary reason for not using Photone is that it completely failed when I tested it against a blurple in 2021 (property set up and test) and it was highly inaccurate when I tested it against a Growcraft X3 when I tested is again in 2022. In the latter test, Photone was inaccurate but it was consistent so I could have just calibrated Photone and declared victory. The issue, for me, was that it was reading 11% high, IIRC, when the programmer was indicating that it should be reading "spot on".
One of the major challenges that the Photone programmers face is that they're dealing with so many different hardware platforms. As a programmer for over 30 years, I'm aware of those complexities.
There are fewer sensors for the iPhone and Apple has pretty good standards for quality. In contrast, the Android market is a lot more variable. As the programmer at Photone wrote to me on 4/27/22:
"In general, we’re still heavily working on the Android Beta’s accuracy as we’re supporting over 20’000 Android devices and know that factory calibrated accuracy can be a hit or miss."
In contrast, a general purpose light meter would than likely, I expect, have only one chip over the life of a give model. That means that the software problem is much easier to solve and, that means, that they will tend to me more accurate.
The other issue is cost - I don't want to put my $1,000 phone in a tent when I can put a $25 meter in a tent.
No doubt, as a programmer, I can come up with lots of reasons why it could be so helpful to have the smart of an iPhone but those arguments pertain to a very small number of growers who want to maintain historical records or, as I do, publish their light values in their grow journal (though I probably won't be writing any grow journals for future grows).
All in all, after 2 ½ years and six grows (chop time this weekend!) of using an Apogee, I very firmly believe that there's no reason for the vast majority of growers to do anything but use a light meter.
Below is a copy/paste from an Excel document I put together about using a light meter to set light values.
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I think in terms of PPFD not lux but here's a table of rough values that I created for someone on another site. I'm a "high light" grower, as Bugbee refers to it — get your plants to the light saturation point (800-1000µmols) as quickly as possible and read your plants.
My approach is based on published research that demonstrates a direct, almost linear, relationship between light levels and crop and plant yield and quality. Bugbee says cannabis "loves light". That's one perspective. If you want to justify buying a new light, just think of it in terms of cannabis being not very efficient at converting photons to weed, therefore, a larger light is needed.
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. Light values collapse in the corners. Cannabis will grow at light levels above 64µmols so the plants won't die at 236 but but by day 10, my plants are at 250µmols.