Motion Detectors

My question is simple. If you were invisible would motion detectors work for you?
Michael Easton

Depends what wave lengths of em waves you were invisible to and what em waves the motion detector used (if it used em waves).

If you were invisible, I think motion detectors would be the least of your worries.

IIRC the two common types of motion detectors are passive infrared and ultrasonic. Passive infrared sensors detect infrared radiation generated by body heat. So being invisible will not help; you will need to lower your surface temperature. Ultrasonic sensors emit ultrasonic waves and measure the reflection, so if you want to fool one you need to make yourself transparent to ultrasonic waves.

      • There’s infared(thermal) and microwave motion detectors. Infared is the cheaper/more common ones. You could be invisible to light (like, say, the room could be dark?) and both would still detect you.
  • The infared/thermal ones you can defeat by turning up the thermostat to 98 degrees (they did this in some movie). The infared ones have four “zones” and a pinhole-type metal lens. Any difference between the four zones gets amplified–so if the entire room is 98 degrees, they may not detect humans.
  • I have never heard of any legitimate-sounding way to fool the microwave ones; they can be set up to read through multiple walls. It’s not unusual to set up one detector to “cover” three different rooms in a row, separated by solid walls. OTOH, you could possibly modify a car radar detector to warn you of when you were hear one, but it wouldn’t help you get past it undetected…

Would a black hole wouldn’t trip a motion detector?

A couple of posters have mentioned that infrared motion detectors are triggered by body heat.

I believe this is not the case. The light-control motion detector that I used to have used infrared, but it would turn the lights off after a while if a person stayed very still, and it would turn them on if non-warm things were moving (like curtains in the breeze, for example).

I think this unit (and I’m guessing most of them) used a near-IR emitter and detector - much like the IR remote controls for televisions and other electronics - and turned the lights on when the amount of reflected IR changed. The near-IR light is just outside of the human-visible range, and it has more in common with visible light than it does with radiant heat.

To avoid this type of detector, you would either have to move very slowly or be transparent to near-IR light.

Passive infra red (PIR) devices, which are by far the most common motion detectors, essentially respond to changes in body heat.

Here on my planet, we use Ultrasonic way more than IR.

I have never installed an IR detector but I have installed hundreds of Ultrasonics.

DougC wrote:

FYI, one such movie is The Thomas Crown Affair (remake)

Another is Sneakers.

This page from HowStuffWorks has a good overview of motion detectors.

And handy, a black hole would most likely trigger a motion detector. It would change the distribution pattern of light and reflection pattern of ultrasonic and microwave beams.

My dad owned a company that sold and installed automatic doors (guess what I did when I wasn’t in school), and currently, all of them use a combination of microwave and infrared detectors. The microwave portion of the detector is used to sense someone approaching the door, so that it opens in plenty of time. The infrared sensor is used to detect if anyone is standing in the doorway, and thus in the blind spot of the two detectors. According to my brother (guess what he did when he wasn’t in school), you can fool the microwave detectors by wrapping yourself in aluminum foil. I don’t know the theory behind how all of this works, but I could ask either one of them to see if they can add more to what I’ve already posted.

True, but it detects changes in body heat as a function of displacement, not intensity.


“A many faceted lens surrounds the transducer and focuses heat energy onto the detector. The lens views the area with a multitude of narrow and discrete beams or cones. As such, it does not view the area in a continuous fashion. As an occupant moves a hand, arm, or torso from one cone of vision to another, a positive signal is generated and sent to the controller. The detection pattern of PIR sensors is fan shaped – formed by the cones of vision seen by each segment of the faceted lens.”

In other words, there are many “cones or vision” that sit in front of a dual-element IR detector; this is created by a special “multi faceted lens.” As a hot body moves across its field of vision, the lens will induce “pulses” on IR energy on the detectors. If the pulses “fit the profile” (they must be in succession, above a certain frequency, etc.) the electronics will interpret it as a moving body and send a signal to the main controller.

Agreed; I should not have been quite so brief (they were about to lock up at work).

Why not a physical motion detector? Changes in pressure on a floor, or changes in sideways force as someone walks across a floor?

Admittedly the cost of such a scheme would be goofy, and probably prone to errors, but in a world where people can be invisible, perhaps an alternative… In which case, the motion sensor would be adequate to the task of detecting.

Just interested that a mechanical solution, inelegant as it would be, has thus far been neglected.

You answered your own question: cost.

IR detectors are inexpensive, reliable, small, easy to retrofit, non-contact, and have no moving parts.

While they do have some drawbacks, IR detectors are hard to beat, all-things-considered…

Cost isn’t important to some people (read rich geeks and government), and some things call for physical movement sensors (accelerometers/INS units for example used as part of high accuracy kinematic positioning systems).

Can’t see why the technology couldn’t be applied to help detect those invisible people. :slight_smile:

Pressure sensors, surely, would be lots less cost than this… but of course, orders of magnitude more (and more trouble) than a simple IR sensor… if the person isn’t invisible to infrared…

Ah, but that won’t help the invisible people if they also have no mass or coefficient of friction, will it?

Oh MAN, why didn’t I think of that? :biggrin:

Back to the drawing board…