Lux Meter

Android phone has a light (illuminace) sensor.
This sensor is visible on the face of the device. So you can easily use it. Just put it close to the light.
This light meter app reports its values in lux value, and has a typical dynamic range between 1 and 30,000 lux level.
But some devices the light sensor may not be linear and not be so accurate. And some earlier devices do not have a light sensor.