I have recently purchased an 8-bit gaming console out of nostalgia, of course the light-gun that came with it did not work on modern LCD screens. Now I know that when you squeeze the trigger of the light gun this sends a signal to the console to draw an entirely black screen with just the position of the duck replaced by a white rectangle. Then if the gun is pointing at the duck/rectangle light will hit the photodiode in the gun's barrel and it will register an electrical signal and a subsequent "hit".
I know this does not work with modern TV's because of screen update lag. However I tried shining a flashlight into the gun's barrel and it did not register a hit, either. Then I tried checking if the diode worked, at all, so I used a multimeter to measure the voltage when it was being shined with a flashlight and when not. However, what I got was that the voltage only changed the first second or so when light hits the diode and after that the voltage came down to its usual level, even though the diode was still being shined on.
So far I don't know what I am doing wrong.
EDIT: i appreciate the answers, but it seems my question is unclear:
tl;dr - i know how LCD and CRT works, what i want to know is why the diode does not work when shined constantly with a flashlight