Photo Resistor to Complete Circuit in Light
As others have correctly said, a photo-diode would be the right part.

You can try something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

You need to choose a suitable opto-isolator for your trigger output requirements. That needs a comparator with a suitable output drive for it. As I don't know your trigger output requirements, I've left those.

The pot' adjusts the current at the trip point between 10 uA and 2 mA or thereabouts.