Fresnel Lens History Lesson
Motion detectors, [? PIRs, ?] in particular, use Fresnel lenses. They've been around for a long time, since the early 1800s. They were first used to be a lighter, more economical way to spread light from lighthouses. So a glass lens is heavy, it's expensive, and this guy named Fresnel, or Fres-nel, as some people say it that way, invented this way to diffuse light and spread it out. So now, Fresnels are used in everything from photography, which I'm sure you know, to stage lighting, to motion detectors in the security industry. These motions use Fresnel lenses in two different ways. In some cases the lens is marked with a pattern, and that pattern on the lens determines what the zones are. So you can kind of see on the inside of this one-- so on the motions that use the Fresnel lens and the little swirly things on the inside, to detect what the patterns are, those are usually convex or kind of flat. And if you hold it up to the light you can see the little swirly things on the inside. That is that kind. The motion, or the mirror optic motions, they still use a Fresnel lens, but it's concave, kind of flat, there's a little bit of a dip in the side. And what they do is the lens focuses the infrared energy in, it hits this mirror, which has all the little segments on it, so that's where your zones come from, and it bounces off this onto this little-- this is the pyro-electric sensor that senses the infrared energy. So these-- this is the-- they tend to be a little bit smaller, a little bit neater looking. This one has a look down, so you push the mirror thing back in there. This is your look down. If you don't want it, you pull this out, if you do want it, you push it in. And this one's got all kinds of other adjustments. And we've got another PowerPoint to go through that compares these two. I'm not sure we'll get to it, but we'll try.