Beam Splitter anyone?

[mrcpickering]

I was reading up on ways to capture bats in flight. The subject of a beam splitter came up and it in interested me. Apparently you can set up a 'beam spliiter' so that when an object passes through the beam it sets the flash and shutter off.

Now I was hoping someone could tell me where i can find a devise like this and how or what i need to set it up...

I use a nikon d7100.