5. Reflector

Any surface capable of reflecting the electro-optical signal will allow distance measurement. However, the more efficient the reflector, the stronger the returned signal and the longer distance which can be measured. Efficiency includes amount of signal reflected along with the direction of its return path. For example, while a flat mirror reflects most of the signal, Figure A-12(a), if it is not perpendicular to the incoming path the signal will be reflected away from the TSI, Figure A-12(b).

 a 12a a 12b 
 (a) Perpendicular to Signal (b) Angled to Signal 
Figure A-12
Using a Mirror

 

To overcome this problem, a corner cube prism is used as a reflector for most TSIs. A corner cube prism is based on a 45° right angle prism. This type of prism has the property that any signal which intersects its long (hypotenuse) side will be reflected parallel to the incoming path even if the prism is not perpendicular to the signal path, Figures A-13(a) and (b). 

a 13a  a 13b 
 (a) Perpendicular to Signal (b) Angled to Signal 
Figure A-13
Right Angle Prism

 

A typical corner cube prism uses a glass cylinder having three 45° facets at one end. This creates three right angle prisms all sharing the glass cylinder's flat front as their hypotenuse. From the front the facets appear as six radial segments, Figure A-14.

 

a 14
Figure A-14
Corner Cube Prism

 

The result is a highly efficient reflector for both signal strength and direction. Efficiency can be increased by using multiple prisms, Figure A-15. This results in more signal being reflected increasing distance range. Using a triple prism can increase range by 50-60% depending on atmospheric conditions.

a 15
Figure A-15
Prism Array

 

Over short distances of a few hundred feet, other objects such as bicycle reflectors and reflective tape will also work. While not as efficient as a prism they have the advantage of being inexpensive.

We will use the terms reflector and prism interchangeably in subsequent discussions.