The principle of operation of the infrared LED and the receiver: interruptions or reflections?

Question

Iron

Good afternoon!
The task is to make a counter of the number of people in the room using infrared LEDs and receivers at the entrance to automatically turn on the light.
To determine whether a person has entered or exited, 2 infrared receivers will be used. With the operation of the system, everything is simple and clear. Now you need to choose how the infrared receivers will work: on interruptions (the IR LED is located in front of the receiver, when someone enters the signal is interrupted) or using reflection (the IR LED and the receiver are located nearby, when someone enters the signal is reflected from the obstacle and hits the receiver )
Important operating conditions: minimum distance between receivers, interference, reliability. What is the best way to implement the system?

Answer 1

To interrupt the beam. The range of IR sensors working on reflection does not exceed 20 cm and strongly depends on the color of the object.
The minimum distance between the beams depends on the angle of divergence of the light. Even with the simplest optics (converging lens + lens hood), you can get a spot of 1 cm at a distance of a couple of meters.
To protect against interference, the beam must be modulated. Accordingly, the presence of the desired frequency is checked in the receiver signal, and not the level.