The shape of the detection field of presence sensors is typically 180° (semi-circular) or 360° (circular); the former is characteristic of wall-mounted sensors, the latter of ceiling-mounted sensors.

The detection capability and accuracy of a presence sensor is influenced by several factors:

• the amplitude and speed of movement;
• the temperature difference between the person and the environment;
• the direction of movement (tangential or radial to the detection area);
• the distance of the sensor from the moving person.  

The extent of the detection area is determined by two values in the technical documentation of the devices: the smaller one refers to the central area where even small movements are sufficient for detection, the larger one to the wider area where larger movements are necessary.

The detection area of a presence sensor can however be extended by configuring a presence sensor as a 'master' device and one or more sensors of the same type as 'slave' devices.

The presence sensors can be configured for automatic or semi-automatic operation:

• in automatic operation the lighting is switched on and off according to the presence or absence of movement in the detection field and/or according to the level of daylight measured in the room;

• In semi-automatic operation, the lighting is automatically switched off in the absence of movement and/or in the event of sufficient daylight; it is always switched on manually by means of a bus button, an App on a smartphone or a voice command. With this mode of operation it is possible to achieve greater energy savings than with automatic operation.

In some applications the stand-by function is useful; when no movement is detected for a set period (corresponding to the stand-by time), the lighting can be adjusted to a lower intensity level; if no further movement is detected, the lighting is automatically switched off once the stand-by time has expired.

The switch-off delay is set during sensor configuration and is the time interval between the last detected movement (or reaching a pre-set daylight threshold) and the actual switch-off of the lighting.

Values between 10 seconds and 20 minutes are quite common, but this parameter depends not only on the application and the intended use of the environment, but also on the available light sources. For example, corridors in office buildings are so frequently traversed that occupancy sensors configured with a 15-minute delay time never switch off the lights throughout the day; in the case of fluorescent sources, a delay time of less than 15 minutes is generally not used because of both the long switch-on time and the shortened service life, whereas LED sources can be delayed by a couple of minutes without any problems.

By way of example only:

• passages with low crossing frequency, such as corridors in basements or cloakrooms: 5 to 15 minutes;
• passages with a high crossing frequency, such as main corridors, stairwells or lift landings: 1 to 2 minutes. Here the light source is decisive: delay times of 1 minute only make sense in conjunction with LED technology;
• main rooms such as offices or classrooms: 5 minutes (with LED lamps) to 15 minutes (with fluorescent lamps)

With regard to occupancy, a distinction is made between absence detection and presence detection which correspond to different ways of configuring presence sensors, as shown in the table below.

A distinction is made between two requirements for light level control: daylight harvesting and constant illuminance.

A prerequisite for both is adjustable luminaires.

In the first case, the required illumination is provided totally or partially by daylight during most of the occupancy period and artificial lighting has a simple daylight supplementing role. The estimated energy saving compared to purely manual control is around 60%.

In the second case, the artificial lighting adjusts the luminous flux to maintain the required illuminance in order to compensate for performance degradation over time. This type of control offers energy savings of around 15% compared to purely manual control.

Ekinex combined sensors, capable of detecting both presence and light level, can combine the different control conditions required by the standard and provide savings of up to 75% compared to purely manual control.

The availability of several independent lighting control channels is particularly useful for rooms with multiple luminaires with different functions, e.g. basic/orientation lighting and main/accent lighting.

In building automation, however, it is frequently necessary to control several technical systems - in addition to lighting alone - in the same rooms or areas. A typical example is heating, cooling and ventilation functions: to take into account the particularities of controlling these systems, which differ from lighting, presence sensors usually offer one or more channels specifically dedicated to the automatic control of HVAC functions.

In addition, an alarm channel can activate or deactivate a load or a group of loads according to the number of movements (trigger events) detected in a certain time interval.

In this way, a single presence sensor can be used multifunctionally for lighting, HVAC and alarm signalling with advantages in terms of convenience, ease of connection and aesthetics, thanks to the installation of just one device in the room.