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.

// Option A //

To download Lux Meter mod from HappyMod.com.
You need enable the option "Unknown Sources".
1. Click on the above link to download Lux Meter mod APK.
2. Save the file in your device Downloads folder.
3. Now tap on Install and wait for the installation to finish.
4. Once it is done, open the game and start playing it right away.

// Option B //

To download Lux Meter from HappyMod APP, you can follow this:
1. Open your browser and download the HappyMod APK file from HappyMod.com - the only official website of HappyMod.
2. Open Android Settings and go into Privacy or Security.
3. Tap the option to Allow Unknown Sources and enable it.
4. Go to your Android downloads and tap the APK file.
5. Follow the directions on the screen to install it.
6. Search Lux Meter in HappyMod App.