Wow thanks for the info on the app guys. Is it worth it to buy the extras to fit your light type?
Without it, it's a lux meter. The "extras" are the adjustments that allow the phone sensor to give a light reading about the PAR values coming from the light source you've selected.
A white light LED has a different spectrum than a blurple than a CMS than a T5. The sensor in the iPhone is, essentially, a lux meter that reads a very wide spectrum of light. That's because it's designed to take pictures. Only some of that light is "PAR", photosynthetically active radiation = what's used by plants to carry out photosynthesis.
When you purchase a light "type" in Photone, that set of settings is used by the software so that it analyses only those parts of what the sensor is reading that are in the light type that you've selected. If you're using a T5, that's going to be generating a lot of blue and, perhaps, UV. But the sensor is reading the entire spectrum of human visible light. Since the T5 isn't putting out much of anything except blue, a lux meter will read that as "dim". But, wait one! Plants use blue light. By purchasing the fluorescent//T5 set of adjustments, the software will throw out the data it got for the green, red, and IR that the sensor picked up and will give you a more accurate reading about the T5.
One analogy that came to mind is "Separate that wheat from the chaff". That's what the different settings will do. Without them, Photone can't do much. That's not a knock on the app, it's just that's how the software has to work to give an accurate reading.