Can motion detectors be stopped by glass?

[Alpo]

I'm watching this movie. Sneakers. Robert Redford is stealing something from the office, and because of the motion detectors he can only go to inches a second, or he'll set them off.

 

But there is a large tropical fish tank in the office with fish swimming around.

 

Typical Hollywood screw up? Or would the glass and/or the water block the motion detector?

[Sedalia Dave]

Its not the glass but the size of the fish that keeps them from setting off the motion detector. The detector is not sensitive enough for the fish to register.

[Alpo]

Some of them fish are three foot long sharks. BIG fish tank.

[Sgt. C.J. Sabre, SASS #46770]

Motion detectors work with sound, like sonar. The sound waves are bouncing off the fish tank, not showing movement. 

Mythbusters tested various ways to defeat one. The method that worked the best was holding a shaggy blanket in front of you.

[Cliff Hanger #3720LR]

A quick search says motion detectors will not respond through glass.

[Duffield, SASS #23454]

There are two types of motion detectors:  Infrared and microwave.  Glass will stop both types.

Also, the microwave detectors' sensitivity setting can be adjusted to ignore smaller objects, such as a dog or cat.

The detector should ignore two"baby steps" of motion but alarm with any more motion than that.

 

Duffield

[Ozark Huckleberry]

[Charlie Harley, #14153]

Some modern “motion detectors”, such as Amazon’s Blink, are based on a camera image, not sonar/microwave/etc. 

 

When a person moves through the field of view,  they create a change in the pixels going across the image. The camera’s software detects this change and registers it as “motion”.

 

Thus, it would not be defeated by glass as long as the fish or person were visible. 

[Alpo]

Movie was 1992. That camera image was probably not available.

[Ace_of_Hearts]

The term "motion detector" is not technically correct. 

 

Microwave detectors work exactly like police radar detectors but are limited by density of an object in the path. When they are installed properly they are adjusted so that the detector will not detect the movement of objects outside of the desired detection zone as the microwave will easily detect motion through walls and windows. They are usually pointed at dense wall materials to assist in reducing the energy passing through.

 

Passive infrared detectors are looking for a heat signature. There are inexpensive and not so inexpensive. The cheap ones do not have any discrimination as to size or shape of an object. These are usually found in the detectors used on lights. Detectors used in the security field can discriminate by size of heat signature as well as orientation. A vertical signature over, say, 40 pounds will be determined to be a valid detection signature where a horizontal signature would be ignored. 

There are also combination detectors which utilize both technologies to reduce false trips. However the combination of two cheap detectors results in a more complicated cheap detector. Physical object motion as well as an infrared signature changing position are required to trigger these detectors.

 

There are also sonic detectors which are not in common use today. They work similar to a radar detector but just above the frequency of human hearing. They are only used in very stable, quiet areas and are turned off during the occupied hours as they can cause headache in humans and animals.

 

Camera type detectors use the change in individual pixels to detect motion. Better units are "adjustable" to detect pixel changes only in areas desired so that objects that move outside the desired areas are ignored.

 

There are also "so called" motion detectors that are actually sound detectors. They listen for sound and most typically found in vaults.

 

Capacitive detectors are looking for a change in static voltage that all objects that are ungrounded have. They are typically used on safes that are insulated from the floor and walls. Touching or almost touching specific protected objects changes the charge and sets off the detector.

 

To address your original post. The type of detector that is being avoided is a passive infrared detector. Any object, including glass, can and does block infrared energy that the detector is looking for. Moving slowly is another method (although not very effective) of avoiding the detector sensing a change in the energy sources position in the area. Fish are cold blooded and do not generate infrared energy that can be detected by these detectors. 

 

[Ace_of_Hearts]

7 minutes ago, Alpo said:

Movie was 1992. That camera image was probably not available.

That technology has been in used since the early 1960s. It was typically used in high security areas in which people were not allowed to enter or leave without scrutiny. A money count room in a casino would be an example. It is NOT used in burglar alarm systems as things like large dust particles can trigger false alarms. Especially if illuminated with infrared sources located on or near the camera.

 

 

 

[Alpo]

5 minutes ago, Ace_of_Hearts said:

To address your original post. The type of detector that is being avoided is a passive infrared detector. Any object, including glass, can and does block infrared energy that the detector is looking for. Moving slowly is another method (although not very effective) of avoiding the detector sensing a change in the energy sources position in the area. Fish are cold blooded and do not generate infrared energy that can be detected by these detectors. 

They said that the detector (the best on the market) also detected thermal energy. It would see him because his 99° was hotter than the 68° room.

 

Would that be the same as the infrared you're talking about?

 

They defeated that in the movie by getting into the air conditioning system and increasing the temperature in the office to 98.6. But he still could only move two inches a second.

[Ace_of_Hearts]

17 minutes ago, Alpo said:

They said that the detector (the best on the market) also detected thermal energy. It would see him because his 99° was hotter than the 68° room.

 

Would that be the same as the infrared you're talking about?

 

They defeated that in the movie by getting into the air conditioning system and increasing the temperature in the office to 98.6. But he still could only move two inches a second.

Passive infrared detectors look for a change in background temperature. In a normal situation the detector is "looking" at a wall or large object. The lens of the the detector is divided in multiple zones. Any object that is of different temperature than the background is suspect. If that object then moves from one zone to another it is considered a trigger. Human bodies are not a constant temperature everywhere. The head may be hot whereas the feet will be cold. Most detectors used by security companies are temperature compensated. That is to say as the temperature approached the range of 95 to 105 degrees the sensor adjusts its sensitivity to compensate for its lack of ability to discriminate the change in background temperature. 

Good companies install different detectors in higher risk environments.  

And alarm in a pawn shop/jewelry store has better components (or should) than a corner candy store.

 

Although mostly used outdoors, photoelectric beams are still in use in some situations. (And NO, you cannot see the beams by suspending some sort of powder in the air.)

 

 

[Cold Lake Kid, SASS # 51474]

My system is set to ignore the movement of a small dog on the floor.

It picks me up walking, crawling or crouched, but ignores a small dog.

I'll have to try a belly crawl.

Or maybe not. My gut may not let my get down that low.

[Ace_of_Hearts]

1 hour ago, Cold Lake Kid, SASS # 51474 said:

My system is set to ignore the movement of a small dog on the floor.

It picks me up walking, crawling or crouched, but ignores a small dog.

I'll have to try a belly crawl.

Or maybe not. My gut may not let my get down that low.

It is a limitation built into the Passive Infrared by the manufacturer of the device.

PIRs on the market today fall into several areas of coverage.

1 - No "pet" rating.

2 - Pet friendly up to 1 - 40 pound or 2 - 10 pound animals

3 - Pet friendly up to 1 - 80 pound animal.

 

These detectors do a good job of ignoring small animals that have a horizontal signature and still detect vertical heat signatures. They are designed for use in residential systems. Most Alarm Companies use them in Commercial systems also to cut down on false signals generated by rats and other vermin.

 

Trying to avoid being detected by a sensor as in the original post is NOT how to defeat an alarm system of any kind. Cheap DIY system or Professionally installed.

 

 

[Alpo]

So if I had an alarm system with a motion detector, and it was set up to ignore Worthless, who is about 20 in at the shoulder and weighs about 80 pounds, would it ignore a 15/16 year old boy kid crawling on his hands and knees?

 

Your number three only goes up to 80 pounds. So if I had a hundred and fifty pound Saint Barnyard, I'd be SOL?

[Ace_of_Hearts]

5 minutes ago, Alpo said:

So if I had an alarm system with a motion detector, and it was set up to ignore Worthless, who is about 20 in at the shoulder and weighs about 80 pounds, would it ignore a 15/16 year old boy kid crawling on his hands and knees?

 

Your number three only goes up to 80 pounds. So if I had a hundred and fifty pound Saint Barnyard, I'd be SOL?

You are correct. Which is why these detectors are used sparingly and almost never in a commercial installation.

 

There are other ways to "trap" burglars and still allow you St. Bernard to patrol the home.