When people think of building automation, the first thought is often automatic lighting control. This is because artificial lighting is intensively used every day and is a major part of energy consumption.
This is why modern building automation requires intelligent and demand-oriented lighting control. This is made possible by occupancy detectors that regulate artificial lighting according to presence and brightness.
But how do occupancy detectors recognise people present? Most modern occupancy detectors can detect movement by measuring infrared radiation. This is emitted by living beings and all warm objects (e.g. cars with warm engines). The detector's system registers the temperature differences in its detection range that occur when a warm object moves. Since the sensor in the occupancy detector itself does not emit any signals, it is referred to as passive (PIR, passive infrared). The detection range depends on the optical system of the detector, and also on the number of PIR sensors installed.
In addition to the PIR sensors, the occupancy detectors also have light sensors (photocells) which can be used to control the lighting depending on brightness. Thanks to the measured brightness value, only as much artificial light is added to the natural light as is needed to achieve the required brightness value for the room. For the dimming function, the presence detector must be connected to the luminaires via an automation system such as DALI, KNX, or 1-10V.
A precondition for optimal functioning of the lighting control system is a brightness value that is as accurate as possible. This requires a suitable installation location on the ceiling, where the measurement is influenced as little as possible such as by reflections, direct sunlight or other light sources. The darkest part of the room should ideally be measured for the brightness value. Nevertheless, the detection zone of the sensor must cover what the user needs.
The light sensor is located in the occupancy detector behind the lens and measures the light in the entire room, from which it averages. An occupancy detector with an additional external light sensor can be used to obtain more accurate results when measuring brightness. The second light sensor in the outer ring of the occupancy detector is aligned by turning the detector towards the wall side, so it is turned away from sources of interference. With some occupancy detectors, the external light sensor can even be swivelled and thus aligned with greater precision. An external light sensor measures the light value in its area at specific points.
To achieve the optimum brightness value, the difference between the brightness at the worktable and the brightness on the ceiling must be compensated in the logic of the occupancy detector. For this reason, the control system does not use the value actually measured by the light sensor to add the appropriate amount of artificial light to natural light, but uses a reflection factor as the ratio of light between the work surface and the ceiling.