Here is comparison of same light runtime test with different 2 phones at same time. Here you can see how slow are 6a sensors update. 
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What Android app to use to make a relative runtime graph? Also, is there a way of calibrating a phone lux sensor somewhat rather than doing relative measurements (I was thinking of using sun at noon as a constant or spot meter with a good camera and calculating back the illumination from exposure or some other such trick). But this may not be that fruitful upon thinking about it…
Ceilingbounce, more recent Lumenoid and others. Both apps have calibration settings, but Im using it just for tracking runtime and stabilization. But as you can see without phone fast sensor it is nonsense.
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I found this publicly sponsored gem: https://phyphox.org/
It has a graphing lux meter, but I couldn’t find a way to ‘calibrate’ it - on in reality multiply whatever it shows by a constant.