And
As long as your light is in the 3500K - 3000K arena, your readings are going to be pretty accurate.
People with iPhones can use the Photone app.
For Android phones with actual lux sensors and not newer ones that now use the front camera as a light sensor, the apps PPFD Meter and Tent Buddy can be used with decent accuracy if you go to a lot of effort to get average readings.