Darken the phone

Darken the phone
 

John and Sam live in the same room, but John goes to bed earlier than Sam, and John is complaining about Sam's keeping his phone emitting light even when the lights of the room are off.

1. What is the proper way of calling the red-word situation?(keep it on ready mode or?)

2. Is correct to say this:

-John has to tell Sam to darken his (=Sam's) phone before leaving the room.(=to make the phone stop emitting light by pressing the button on the side without powering it off.

keeping his phone lit
to dim his phone

Thank you.