_:nbc380128ec3748ef9223d912095b438exb0
<http://schema.org/description>
"OR is there a way of  creating very narrow IR light  cone ie. It isn't difficult to calculate how much light you'll receive--the  detected photocurrent is proportional to the optical power received, and  the optical power just spreads out in a cone from the transmitter. To achieve a two degree cone your LED light source will need to be mounted in the center of a tube of radius R, with a length L of greater than or equal to R/0.0175!"
<https://www.polytechforum.com/robotics/loooong-distance-ir-detection-and-ir-collimation-4868-1.htm>