here are some notes:
I have used some tissue paper (or other opaque paper would be suitable).
With the camera sensor, turn off Auto Exposure settings and configured white balance to daylight.
Point the camera in the general direction of the sun, and change the exposure time so that there is no red on the display. Then start the test.
Configuring the exposure time in blight sunshine is not an easy task and would need careful instructions.
In the attached images, a gradual reduction in the luminance value can be seen over 45 minutes in the early evening, the big drop in luminence was due to the sun going behind a building and shading the phone.
The other image captures the camera exposure settings.