# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 2 Sensors and Monitoring Technologies
### Occupancy Sensors  
Occupancy sensors are devices used to detect occupancy within a specific space and are designed to send information to building controllers to turn/dim lights and turn equipment ON and OFF based on the presence and motion of people in a particular space. When correctly implemented, this not only helps in saving energy and provides security in buildings [53; 54] but is also useful for climate control [55], making these sensors highly valuable for deployment in buildings.  
Occupancy sensors can be classified into two main types based on technology: (1) Passive Infrared (PIR) and (2) Ultrasonic sensors (see Table 2). PIR sensors; devices that are able to detect movement (as the human body naturally releases infrared radiation) in a specific space using a pyro-electric element [56], are the most commonly adopted occupancy sensors for commercial and household buildings nowadays, which is attributed to their fast response times (as fast as 0.5 s), both wired and wireless capabilities, low cost ($20-$65) and passivity (i.e., these sensors do not emit any energy themselves but transmit a signal based on a change in IR radiation, instead making them very useful during lengthy idle operations when no human activity occurs). Despite these advantages, PIR sensors present some significant drawbacks, such as a requirement for direct line of sight with the object (i.e., these sensors cannot detect movement behind obstacles, e.g., walls, office cubicles, etc.), and are often unsuitable for outdoor locations due to heat interference (i.e., they are only suitable for short distance measurements (3-10 m) in indoor environments) [57]. In comparison, ultrasonic sensors; active devices which are able to transmit and receive ultrasonic sound waves based on the Doppler effect (i.e., an increase or decrease in the frequency of sound or light based on the change in distance between the source and observer), are able to measure occupancy in outdoor environments over long distances (up to 70 m distances), do not require a direct line of sight, and have fast response times (1.3 ms-30 min) [52; 58; 59]. Whilst these functionalities make ultrasonic sensors appealing for use in both large- and small-sized buildings, these devices come at a much higher cost to the end user ($130-$500) and can have higher power consumptions in comparison to PIR sensors due to these added capabilities. In addition, ultrasonic sensors (due to their high sensitivity) can be highly susceptible to false alarms typically triggered by activities such as movement/vibrations from a different room within the building [60; 61].  
In addition to PIR and Ultrasonic sensors, the application of image-based and sound-based detection systems has also been explored in recent years for occupancy measurements in buildings [62; 63; 64]. Image detection systems (based on video cameras being placed at locations with minimal obstructions inside and outside buildings) and sound detection systems (based on the detection of audible sound waves using microphones which convert the measured acoustic signal into an electrical signal) have the capability to provide information on occupant presence, location, count and activity. However, drawbacks such as the requirement for direct line of sight, user privacy and costly hardware in the case of image detection systems and sound waves from non-human sources triggering sensors in the case of sound detection systems can limit their large-scale deployment in buildings [57].