The other unit is a reflector.
Garage door photo eye beam width.
Lower is easy to stand above with a foot on either side.
If your garage door sensors are outside of this range remove them entirely from the garage door frame and install leveled inside the safe zone.
If the photo eye senses an obstruction it stops the door from closing and potentially prevents damage or injury to a vehicle.
Higher than that can miss many low lying objects including pets.
One unit emits an infrared beam invisible to the naked eye.
The photo eye sensor is one of the most important parts of your garage door system.
If something breaks the beam while the door is closing the opener motor automatically reverses direction and opens the door.
The second is photoelectric.
These sensors sit either side of your garage door near the ground.
It consists of two units installed about 4 inches from the floor on each side of the door.
They shoot an invisible beam between them which will sense if something is between them as it breaks the beam.