Do Motion Sensor Lights Work Through Glass?

When you’re installing a new motion sensor for your outdoor lights, you need to think about where to place it. You might be considering whether it’s easier to install it somewhere indoors that you can access if you ever need to.

But first, you need to check whether that’ll actually work through the window – do motion sensors have any problems detecting motion if they’re behind glass?

Most reliable motion sensors use PIR tech, which uses infrared light to detect heat sources. Glass will absorb this light, so the motion sensor won’t work. The cheapest motion sensors rely on detecting all movement, so the clear glass won’t stop them from working.

The problem is that these motion sensors can be set off by rain, falling leaves, or anything else that moves.

This is why most people prefer a PIR sensor. So you’ll need to be careful when planning your setup if you’re thinking about mounting the sensor behind glass.

In this article I’ll explain:

  • Why PIR sensors don’t work through glass
  • Whether the proximity and thickness of the glass matters
  • Whether dusk to dawn sensors work through glass
  • Where to place the sensor 

Do PIR Motion Sensors Work Through Clear And Frosted Glass?

Hand opening window

PIR motion sensors won’t work through glass, regardless of whether it’s clear or frosted. That’s because it measures infrared signals to detect the changes in temperature in the space in front of it.

So when the outdoor space suddenly rises in temperature because someone’s body heat has entered the visible area, the sensor is triggered, and the light will switch on.

Whether it’s clear or frosted, glass is a heat insulator, which means it absorbs the infrared signal used to measure the temperature.

So the sensor won’t be able to detect the temperature change and will therefore never trigger.

It’s not just a problem with outdoor lights either.

Some people have tried to use motion-triggered cameras to record their cars – for example, if their vehicle is in a high crime area or parked in a tight space where it could be hit.

Dash cams are great when you’re driving, but they’re no good if you need to record constantly when the car is parked – no battery or memory card will cope.

And so while a motion-sensor triggered camera might seem ideal, the windshield of a car is enough to stop the PIR sensor from triggering too.

Does Proximity And Thickness Of Glass Affect The Motion Sensor?

Frosted glass

Technically, not all of an IR signal will be absorbed by the glass if the glass pane is thin enough and the signal is close enough to the glass that it has more chance to shine through.

A windowpane will always be thick enough to stop a PIR sensor from working, but what about if you’re placing the sensor in a glass-protected recess?

The answer isn’t simple because any glass thickness can prevent an IR signal from passing through.

There is a chance that you might get a motion sensor to work if it is placed right up against a thin layer of glass. Still, there is also the chance that the molecules in the glass will prevent the wavelength from passing through.

At best, it will be unreliable.

If you try to set up a PIR motion sensor directly behind a thin glass-protected recess, and it seems to be working, don’t expect it to always trigger whenever there is motion.

Although if the sensor’s signal is strong enough, and the glass is thin enough and close enough that the signal passes through, it may work OK. It is unlikely though.

Do Dusk To Dawn Sensors Work Through Glass?

Outdoor sensor dusk to dawn

Some motion sensors have a dusk to dawn sensor attached to them. In contrast, others completely operate on these sensors, keeping the attached light on at all times when the ambient light is low, rather than checking for motion.

These light sensors can be adjusted to make sure that the sensor only triggers during the dark.

Otherwise, your light will constantly turn on even during the day, when you don’t need it to – wasting electricity.

And while a motion sensor is unlikely to work through glass, it’s worth noting that the dusk to dawn sensor at least will.

That’s because glass doesn’t absorb light in the same way it absorbs IR. So light can pass through – although it may be refracted if the glass is frosted.

Either way, the motion sensor should still have no trouble detecting that the light levels have changed and will still monitor the area when the sun has set.

So, if you’re determined to put your sensor behind a window or other glass panel, and you don’t mind the light being on at all times throughout the night. Then, the photocell dusk to dawn sensor could work.

And if you can find a basic motion detector that doesn’t use PIR tech but does have a dusk to dawn sensor built in then that should work too, although it will be less reliable through frosted glass.

Where Should I Place The Motion Sensor?

installing security light

A PIR motion sensor will remain the best option for most people, which rules out placing it indoors and aiming it through a window.

However, many motion sensors are designed for outdoor use anyway, so you can mount the motion sensor on an external wall, and it’ll work perfectly.

You need to make sure that your chosen sensor is suitable for the outdoors. Look for an IP rating of 44 or better, which will protect it from splashes of water like rain.

Also, you don’t need to worry about wind setting off your PIR motion sensor.

You will need to consider the height you place your sensor at and how you make sure the field of view is adjusted.

Every motion sensor is different, so check the instruction manual before setting it up, and consider the range and when you want the sensor to trigger.

If you’re targeting a pathway to your home, it’ll need to be high enough to have visibility. Still, at the same time, if you want it to also work when you pass underneath the sensor, it needs to be multi-directional.

And if you want to protect your motion sensor behind a surface, use plastic – it won’t absorb the IR signal in the same way glass will.

Thinner plastics are better, but they can be opaque – visible light and IR are different, and so just because you can’t see through the plastic, that doesn’t mean the IR signal won’t pass through.

Related: Can Laser Pointer Set Off Motion Lights?

Final Words

Suppose you were planning on adding a motion sensor to your outdoor lighting setup and were hoping to place it inside behind a window. In that case, you’re likely to be disappointed.

Alternatively, you’ll need to buy a cheaper sensor that triggers more often than you’ll want it to.

The best option is to buy an exterior-rated PIR sensor since you benefit from infrared detection, and it won’t be affected by the glass. At the same time, they’re explicitly designed to be effective when mounted outdoors.

They might be a little more trouble to install, but in the long-term, it’s worth it. What motion sensor have you used at home?