Two-minute explainer: Occupancy sensors

We light for people, not for architecture. That has always been the foundation of any light planning.

As energy costs increase, the ability to know when a room is empty has played an important role in managing lighting costs. Generally, sensors provide a way of ensuring that light is only used when a space is occupied. More recently, the introduction of ‘smart technology’ has sensors at the heart of data harvesting, though the knowledge sought by these systems is far more detailed than simply knowing when a room is occupied.

Occupancy detectors can be divided into two basic categories: motion and presence detectors.

Motion detectors do exactly what they say. They are designed to receive a signal, usually the heat signature from a person entering a space, then trigger the lighting. The important thing is that they are detecting movement. The sensor-head is made up of detection zones and it is the movement of a heat source (a person) from one zone to another that causes the device to activate.

Motion detectors work very well in spaces where movement is more or less constant; washrooms, corridors and garages, for example. But there have been many incidences of motion sensors switching the lighting off when it can no longer detect movement – such as when someone is working at a desk, or sitting quietly reading a book. These early models gave occupancy detectors a bad name.

Presence detectors are more appropriate where physical movement is limited. Effectively, these work in the same way as motion detectors, but are tuned to smaller movements across the sensor-head. They are generally more expensive than conventional motion detectors.

There is, however, a third, relatively new addition to the field.

Absence detectors are designed to switch lighting off when the sensors understand the room to be empty, but they do not switch lighting on when someone enters a room. That is left to the human being to decide.

There are four types of sensor: passive infra-red (PIR), microwave, ultrasonic and smart.


PIR detector, from CP Electronics, GI-GEFL

Passive infra-red (PIR) sensors work within the infra-red spectrum, which human beings emit in the form of heat from the body. As the ‘passive’ part of the name suggests, the PIR doesn’t actually do anything. It is a passive device that receives thermal signals from the surrounding environment. The detector is made up of a matrix of sensors which detect the movement of a heat signature from one sensor to the next. It interprets this as being a person moving within the environment and switches on the lights.

The effectiveness of any particular PIR is determined by the sensitivity of the sensors. The PIR provided the original basis of occupancy detection and were motion detectors pure and simple. Recent developments have improved the sensitivity of the PIR units by increasing the matrix of sensors, so that they are capable of working effectively as presence detectors.



Microwave detector, from CP Electronics, MWS6

Microwave detectors are the opposite of passive – they tend to be very busy indeed. They fire regular microwave pulses across the space and then measure the reflected signals. The reflections change when a moving object – most likely a person – enters the field of the detector, triggering the lighting.

Whereas infra-red detectors rely on line-of-sight to be effective, microwave detection can ‘see’ through most building materials (though microwaves do not penetrate metals). This can be a problem, of course, as detection of movement in an adjacent room isn’t usually required. The sensors need to be set to an appropriate level of output when they are installed.

There are concerns about microwave radiation. Manufacturers of microwave sensors point out that the output from these devices is very small. Research is ongoing as to the possible effects of exposure to radiofrequency radiation. As yet, there is no proof that microwave exposure at these levels leads to a higher incidence of cancer (the primary concern), though the International Agency for Research on Cancer (IARC) classifies radiofrequency radiation as ‘possibly’ carcinogenic to humans. The real concern is the proliferation of RF sources and whether we need to be worried about the accumulating effect of microwaves in our interior environments.

In some parts of the world, companies prefer not to use the word ‘microwave’ when describing this type of detector. They are worried that the public will associate a microwave sensor with the far more powerful microwave oven. They are often described as high frequency (HF) detectors.

All microwave detectors should comply with ANSI standard IEEEC95.1 – 1999: standard for safety levels with respect to human exposure to radio frequency electromagnetic fields 3kHz – 300GHz


Ultrasonic detector, from Eaton, OAC-U

Ultrasonic detection works in much the same way as radar and sonar – a sonic signal is radiated out into a space and the reflected sound is then analysed.  The detector recognises the steady state, or background signal, and if that changes it assumes that something has moved into the space. In effect, the ultrasonic occupancy detector is made up of a tiny speaker that transmits the sound waves, and a microphone that picks up the reflected waves.

The sound waves are transmitted at about 40kHz, which is well above the level of human hearing. It’s worth noting that dogs can hear up to 60kHz and cetaceans and bats above 100kHz. While dolphins may not be troubled by ultrasonic occupancy detectors in their everyday lives, dogs and bats may tell a different story.



Smart sensor from Gooee

Smart sensors are similar to PIR in that they are passive devices, but instead of reading infra-red signals they read visible light within the electromagnetic spectrum. They look for movement across the matrix of sensors that make up the sensor-head.

The interesting, and challenging, issue with being able to read visible light is that it makes the smart sensor a crude camera. Smart sensors are making a lot of ground in data harvesting, gathering all manner of data for auditing and analytical purposes. This raises concerns around data protection and personal privacy. Manufacturers insist that their sensors are insufficiently sensitive to identify individual people, but personal privacy is a growing issue. The final gatekeeper for collected data is, of course, the software that interprets it, and it’s important that limits are maintained on character recognition technology.


For applications where more detailed information about the occupants of a space is genuinely useful (such as care homes), plenoptic light field sensing is a helpful technique. Sensors can identify not just the presence of a person but also their size and whether they are standing, sitting or lying.