What is the thing atop signal lights?

Factual Questions
1

Atop most signal traffic lights is a little thing.
There is a picture of one here:
http://geocities.com/iheara/traffic.html

But what is it called, and how does it work?
I thought about calling the traffic engineeers but
I shudder at asking, “What is a thing?”

I do know it is NOT affected by headlights. That is a different
thing called a light sensor and is on STREET ILLUMINATION LAMPS and not signal lamps.

2

That would be the sensor to change the light for emergency vehicles. Police cars, ambulances, and fire trucks have strobe lights that flash at a particular frequency that triggers the lights to change green for the emergency vehicle.