In the modern world these days, we use a number of motion sensing devices on a daily basis to assist us with our day to day activities.
One of the most common examples of these kinds of systems are the beams of light which stores use for their bells/alerts that someone has walked in the door. Two photo sensors are placed at opposite sides of the room near the door. When someone walks through the door and subsequently through the beam of light between them breaking its continuity, there can be a bell or other noise triggered by the system in order to alert the store owner that there’s a customer at the door.
Grocery stores use a similar principle for their automatic doors. Above the doorway, there’s a small box that transmits radio signals down into the doorway, bouncing the waves back in order to form a clear image of what’s below it. If a person walks through the doorway, those radio signals will bounce back to the transmitter differently than if no one was there. This causes the system to trigger the opening of the automatic doors.
These are both examples of what are known as active motion sensors. These kinds of motion sensors transmit a signal or something of that sort out into the environment, where it interacts with the outside world and picks up information based on that which comes back to them. These are based on the same concept that xrays and ultrasound waves function based on.
In the case of the motion sensors you’ll find in security camera systems or security lighting systems, there’s a different system known as “passive infrared”.
Passive infrared motion sensors are those that can detect body heat. The skin surface of most humans hovers around 93 degrees or so, which means that in order to detect the body heat of a human, you’ll need motion sensors that have the capability to detect between 8 – 12 micrometers of infrared energy. The average human body falls somewhere between 9 and 10.
Why Don’t Motion Sensor Lights Stay Activated When Standing Still?
You might have noticed that your motion sensor lights only activate when someone is moving, and will eventually turn off if you’re standing still, even directly in its path. This is due to the fact that the motion sensors that also detect heat are primarily designed to look for sudden or rapid changes in temperature. When a warm body walks by, obviously that is going to be considered a rapid change by the sensors. On the other hand, the general temperature of the ground changing as the sun goes down at night is a very gradual change, and one which the motion sensors on your security lighting is going to ignore.
Another thing that makes motion sensor lights which are triggered by infrared light so great is that tend to have a very wide viewing angle. It seems that they’re practically impossible to sneak around if you’ve ever tried to do it, which as someone who uses them regularly can appreciate.
Since infrared energy is in actuality a form of light, it can be bent or focused pretty easily using simple plastic or glass lenses. That’s how your motion sensor lights are able to cover such a wide area so seamlessly even if it appears that the faces of the lights aren’t facing anywhere near the angle you’re coming from.