Cipher using flashlights

Hi everyone,

can someone give me a hint what I have to search for when I want my character to use a flashlight to make some sort of cipher visible?
The idea was that the character has to obtain a super awesome high tech flashlight (done) which he may then use to find a cipher text thats written on the wall in a way that it is only visible when the flashlight is shining on it.
How can I detect if my flashlight is actually shining onto the wall?
The Flashlight is a simple spotlight belonging to the character.

Any input would be greatly appreciated!
Christoph

This topic has some ideas.