# Why don't security camera systems emply a simple motion detection algorithm

**URL:** <https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517>\
**Category:** Factual Questions\
**Created:** [July 21, 2006, 6:55pm UTC](https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517 "2006-07-21T18:55:51Z")\
**Posts on this page:** 4\
**Page:** 2

<div class="post-metadata">

**Author:** ![Seven](https://avatars.discourse-cdn.com/v4/letter/s/3d9bf3/32.png) [@Seven](https://boards.straightdope.com/u/Seven)\
**Post date:** [July 23, 2006, 6:53am UTC](https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517/21 "2006-07-23T06:53:49Z")

</div>

> [@Q.E.D.](#):
>
> I do have to address this, however. No, PIR does not see through glass. Glass is largely opaque to IR at the long wavelengths PIR looks for.

I had to think about this for a bit (it’s been a while).

The PIR’s we used were dual sensor. So they had both IR and MW. The fact they had MW means they were, in fact, “radar”. I just never thought of it in those terms. In any case, that’s why **we** never pointed them at windows.

I suppose a cheap standard PIR would be fine pointed right at a window but I would happen a guess this wouldn’t be the best choice. I figure the glass would reflect the IR.

---

<div class="post-metadata">

**Author:** ![Boyo\_Jim](https://avatars.discourse-cdn.com/v4/letter/b/87869e/32.png) [@Boyo\_Jim](https://boards.straightdope.com/u/Boyo_Jim)\
**Post date:** [July 23, 2006, 6:55am UTC](https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517/22 "2006-07-23T06:55:22Z")

</div>

Sprry about this partial hijack, but the motion sensors I witnessed most often were things attached to floodlights, usually at a home entrance. Walk up to or past the door and the lights go on. I grew up in a poor neighborhood, buy lots of my neighbors had them.

I had the impression these were sound based, sonar rather than radar. Is this correcr?

---

<div class="post-metadata">

**Author:** ![Seven](https://avatars.discourse-cdn.com/v4/letter/s/3d9bf3/32.png) [@Seven](https://boards.straightdope.com/u/Seven)\
**Post date:** [July 23, 2006, 8:12am UTC](https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517/23 "2006-07-23T08:12:32Z")

</div>

I think those are often passive infrared.

---

<div class="post-metadata">

**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\
**Post date:** [July 23, 2006, 4:29pm UTC](https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517/24 "2006-07-23T16:29:22Z")

</div>

> [@Seven](#):
>
> I suppose a cheap standard PIR would be fine pointed right at a window but I would happen a guess this wouldn’t be the best choice. I figure the glass would reflect the IR.

Well, the quality of the PIR doesn’t matter; even the best ones available don’t see through glass–it’s a physical limitation of the IR band our bodies emit in and the stuff we like to make windows from. But you’re correct, pointing at glass can cause false triggering from reflections, unless care is taken with placement and alignment. Microwave radar has no such trouble with glass, as you note.

[Previous page](https://boards.straightdope.com/t/why-dont-security-camera-systems-emply-a-simple-motion-detection-algorithm/365517.md?page=1)