Detect a dark Screen like WinXP-Screensaver

Hello,

i need help with my script here.

I want to detect when my Screen is nearly or completly dark/black.
I'm using PixelChecksum(0,0,1919,1079) and it's giving me random numbers (e.g. 2690216355, 2589900821, 854331631, 481460664)
How can i compare that with a Black like 0xFFFFFF ?