This app was mentioned in 2 comments, with an average of 1.00 upvote
I use the old version of Light Meter Tools by WBPhoto
https://play.google.com/store/apps/details?id=lightmeter.hardware.lightsensor
There is a newer app by this publisher. The new one has lower reviews. I'm unsure why that is, because I have simply stuck with the old and never tested the new. There's a LOT more to the app than just a simple meter.
This one:
https://play.google.com/store/apps/details?id=lightmeter.hardware.lightsensor
Try the demo version first to make sure your phone supports a light meter app.
As for the lightmeter: I've bought a Shepherd DM-170 for 15 euros on (our local version of) craigslist. Works like a charm!