Absence Vs Presence Detection

When you see a photograph of London you see blaze of light coming from thousands of offices. It is obvious that the amount of money lighting offices which are unoccupied must be enormous.

However we have all been in rooms where a meeting is being held and the lights go out. Someone then waves their arms madly and magically the lights go on again. If this happens frequently it is very annoying. We have also all been in corridors where the lights come on just after you pass the area which should be illuminated. You’re walking the pool lights flickering on just as you've left. We all know the feeling when the lift doors open on stepping into darkness and hoping the lights come on. We all want to save energy and get irritated when we see the waste of light, yet we don’t want to live with lights flickering on and off at irritating intervals.

We have had cheap movement detection devices available many years now, so what can be so hard? To start with different spaces have different needs. We want lights to be on when we want them on and not when we have left. So a good place to start is an on/off switch. You turn the lights on when you want and off when you leave. The problem is that people forget or assume that someone else will switch off. So let us install some detection which will switch off the lights only when there is no one about. This is called absence detection.

PIR Vs Microwave

Choosing the right detection can be tricky.  You want to detect people only in the space that you want. You have a choice of 2 technologies
1)    PIR Passive infra red – a cheap technology which detects both motion and heat but does not “see” through glass
2)    Microwave sensors – which see through glass and are suitable for large and awkward areas. Because they emit low energy microwaves they utilise more energy than PIR’s

It is possible to get detectors combining both technologies.

By carefully choosing the detectors and planning their locations and spacing it is possible to get good coverage and detection. So now it is possible to add to the simple on off switch with an additional off switch through these presence detectors. Absence detection means that lights never get switched on accidentally by false detection of movement .  This is not a one off choice, you can happily mix and match detectors as appropriate on the same installation.

Absence Vs Presence

Absence detection is often compared to straight presence detection. In simple presence detection when a person is detected as being present the lights go on and when they are no longer detected the lights go off after a delay. This is cheaper to install because there is no on/off switch and associated wiring. However, absence detection will always be cheaper to run because of the lack of unwanted illuminations.

This isn’t the end of the story however. When it is a bright sunny day the illumination from windows can be considerable and that means lights can be burning unnecessarily brightly  It is easy to include a light dimming function which will adjust the light levels to a constant level. This not only saves energy but gives workers a more comfortable environment.  This is often called “daylight harvesting” or “daylight linking”.

Both types of detection and control will give considerable savings not only in energy usage but in increased lifetime of light fittings. 60% savings over a simple switched solution are possible with paybacks way under 2 years.

Generally there are times when absence detection is the solution – these are:-

  • Presentation Rooms
  • Individual offices
  • Classrooms
  • Hotel bedrooms

On the other hand presence detection is better in:

  • Open Plan offices and circulation areas
  • Corridors
  • Toilets
  • Hotel lobbies


Presence and absence detection is not only for lighting. It is great when used in conjunction with HVAC controls.  I wonder just how many offices have been heated over the Christmas period because they only had time controls on them.


The case for simple lighting controls in a commercial environment is overwhelming. You should always consider absence as well as presence detection.