For some reason, everybody thinks LEDs put out nice Gaussian beams. Unless the specs say so, it just ain’t so. Some pix from the Trinity Robotics Contest show the situation.
This is the table used in the RobotWaiter contest. It has a visible red LED on each face to aid in locating and aligning, but you better not depend on the actual beam intensity as a function of angle!
The same picture in pure IR shows the situation with a bit more contrast. Although it’s a visible red LED and I have a visible-cut filter on the lens, there’s enough energy for an image.
These IR LEDs have the same general pattern.