Optical IR sensors are sensors that use light beams to sense objects. These sensors use a light beam that is invisible to our vision, this light is called infrared, which is commonly abbreviated as IR. These sensors consist of two parts, the light source, and the detector. These sensor units are commonly supplied two different ways.

The most common form is with the light source and the detector in the same package, generally, a plastic enclosure. These IR sensors are called reflective sensors, because they work by detecting a refection from a surface or retro-reflector (a fancy form of mirror, like the reflectors on the back of bicycles). These types of sensors are very popular in stores, you will often see them mounted near the doors, and they ring a bell when anyone enters or leaves the store.

The other form for these sensors is with the light source and the detector being mounted separately. These are then referred to as transmissive sensors, they are mounted facing each other across a gap, which the vehicle to be detected passes through.

The advantage of the reflective sensors are a convenient package and simpler wiring. They usually make for a quick and simple installation. These sensors establish a constant beam that gets broken when a vehicle passes between the sensor and the reflector. The reflective sensors require the reflector to be mounted in a relatively narrow area on the opposite side of the detection gap, and since the light from the sensor is invisible, it can be interesting aligning it correctly, however, most sensors are equipped with a small indicator light that lights when the beam is making connection. Since the reflector is usually much smaller than the light pattern falling on the opposite side of the gap, only a small percentage of the light gets reflected back to the detector, which is also very small. Commercially available reflective sensors can be used for detection ranges of up to 15 feet or so, which would cover a single lane. An occasional problem with these types of sensors is highly reflective trim on some vehicles may cause a false indication. These sensors do consume quite a bit of power, so would not normally be used in a battery powered installation. The advantages of the transmissive sensors are longer distances, lower power, and less prone to false indications. Transmissive sensors can work to very long distances, from a few feet to hundreds of feet. The longer distances would require some sort of optical lenses (like camera or telescope lenses). The alignment requires both the light source and the detector to be pointing at each other, and can be tricky until you finally begin making connection. For vehicle detection across lane(s), these sensors can be operated at low enough power for reasonable battery operation. Since these sensors do not depend on reflections, they will not false trigger for that reason.