明るさ計測 is an Android app that allows you to measure the brightness using the built-in light sensor on your smartphone. With this app, you can get an accurate measurement of the brightness level, and a list of reference values is displayed below the measurement.

One unique feature of this app is that you can pause the measurement by pressing the volume key, and it will continue displaying the lux value at the time of the pause. This is especially useful when you can't see the screen facing the measurement target.

It's important to note that the accuracy of the brightness value depends on the sensor in your smartphone, as well as the upper and lower limits, which vary from device to device.