Everyone has seen a retroreflector. It could be the eyes of a cat shining in the dark, it could be the multiple images of your face in a corner as you are in a hall of mirrors. A retroreflector simply reflects the light that shines on it back to exactly the same place.
The reflection can bounce off two mirrors as you see in this image and it can bounce off three or more mirrors.
Why is it important how many mirrors the light bounces from? It is only important if you have a special type of radar antenna. A circularly polarized radar antenna makes use of how and how many times a radar wave is reflected. With a single reflection on a flat surface the radar wave gets reflected with the same sense. Think of sense as a clockwise or anticlockwise spiral. Only a screw thread on a bolt with the same “sense” (clockwise or anticlockwise) will go into a nut that has the same sense. It’s just the same with a radar it doesn’t work as well when the reflected wave has a different sense, just like a counter clockwise nut doesn’t fit the clockwise bolt i.e. with the opposite sense thread. When it rains, raindrops reflect the radar wave with the opposite sense. That’s right a circularly polarized radar antenna rejects the reflection from rain. This makes radar work better in the rain.
With certain types of physical retroreflectors the radar bounces three or five or more odd times so that the radar can receive only a little of the signal that is reflected but only that with the same sense as it is sending out. With the odd sense that part of the radar reflection is rejected.
The real trick comes when the retroreflector has two, four or any multiple of two surfaces and the sense is changed an even number of times.
Let’s step back to the retroreflector for a moment. EVERY part of the radar wave that goes to the retroreflector is reflected straight back to the radar. Think of it a a firehose, or shining a light straight into a miror that refocusses the light back into your eyes. Blinding Right? Well that is what happens to the radar – Soaked in its own reflection. It just can’t possibly see anything but the retroreflector. Remember don’t take a bright flash light into a hall of mirrors – it’s just blinding.
When there is rain, the circular polarization reflection is brought down from a firehose to something that the radar receiver can handle. When the retroreflector has 2 bounces, 4 bounces or any even number of bounces the radar gets flooded with its own reflection.
Look at the photograph of the underside of two lane bridge. Just imagine this as a hall of mirrors with the beams holding the bridge, the side walls and the underside of the road all as mirrors. The reflections coming back from all of these to the radar, some with the opposite sense and the other half with the same sense like a fire hose.
No wonder radar operators are taught and the operators manuals caution against operating traffic speed radar near a bridge. We don’t want the radars looking at themselves rather than the speeders right?
