Infra red

Hi, Roger. Window glass passes infrared well -- it's UV that's attenuated.

Find out how far away the 100 watt light bulb has to be from the sensor to reliably turn it on without the glass. My WAG is, the distance with two perpendicular panes of glass between should be well more than half that distance.

Cheers Chris

If it's sensitive enough, and/or if it also responds to visible light.

--

Bye.
   Jasen

If the IR sensor is a phototransistor or a photodiode of any common type especially silicon, then glass will not make much difference. Glass is largely transparent to infrared wavelengths out to about 2 micrometers (2,000 nm).

Most transparent materials are transparent to infrared out to at least about 1.5 micrometers.

But if you want to detect thermal infrared (wavelength several micrometers), then the ballgame becomes very different and expect the usual and more ordinary transparent materials to be opaque. A non-contact thermometer will read the temperature of a sheet of glass or acrylic rather than see through it. More specialized materials are used to make lenses to handle those wavelengths.

- Don Klipstein ( snipped-for-privacy@misty.com)

Not all UV - wavelengths down to about 350 nm go through pretty well! Ever see how much of a shadow is cast against ouput of a blacklight or a

350 nm blacklight?

But the tanning portion of the UVA spectrum (roughly 315-340 or 315-330 nm) is fairly well blocked and UVB and shorter wavelength UV are well-blocked.

- Don Klipstein ( snipped-for-privacy@misty.com)

Hi, Don. You're right -- in standard window pane glass, some UV does pass.

Interestingly, many glass panes these days are made with coatings that will reflect IR. I'd guess that's why it's important for the OP to just do an experiment and see.

Cheers Chris

Hi Chris. I'd be careful with making statements like this. "IR" means a pretty broad range. By the time you're out at 5 microns, glass is an absorber of radiation.

That being said, what you say is about glass transmitting IR is true for most of the IR that is emitted by a typical lightbulb, and detected by, say, a silicon sensor. I guess my criticism is because a lot of people on these usenet groups talk as if whatever happens in one small region of the IR (or UV for that matter) applies to the entire range. 'tain't so.

Regards,

Mark

two

What I am thinking of doing is using an ordinary el cheapo off-the-shelf IR sensor with a pair of light bulbs (you know the sort of the the DIY fellow screws on the side of the house) inside the garage and pointing one of the bulbs through a garage window and through a house window to shine on an IR sensor. The garage is on the sun free side of the house (south here) and the sun will not shine directly on the IR sensor inside the house. I do not want to run wires between the garage and the house.

R

--- A few years ago I designed a system for taking infrared videos of wildlife at night, and part of the system was a passive infrared (PIR) sensor which detected motion by sensing the temperature change across a pyroelectric transducer working at about 5000nm, as I recall, when the animal walked across the field of view of the sensor. One of the system requirements was that all the electronics had to be mounted in a waterproof enclosure, so one of the things we tried was to use was a window in front of the PIR sensor lens made of plain old window glass. It didn't work at all, and neither did many other window materials we tried, and it turned out that things were going to get pricey if we used a glass IR filter, so what we did was redesign the optics and used the plastic Fresnel lens itself, mounted in a sealed lens holder exposed to the weather, as the seal. It worked perfectly.

You haven't explained what your system is supposed to do, but I suspect that if you're using a conventional PIR tuned to around

5000nm those two sheets of glass are going severely attenuate anything around that wavelength coming out of the lamps, and as I recall, there's not that much coming out in the first place.

But why not just try it? It's certainly cheap enough to do, no?

-- John Fields Professional Circuit Designer

Hi, Roger. You may have still problems with sunlight and background heating here. IR *is* used for sensing and transmitting of information (TV remorte control and industrial sensors), but usually the IR is modulated at a specific frequency (38KHz is most common for remote controls).

Because of the thermal lag of incandescent lights, modulation at higher frequencies is out. For lower frequencies, the change in bulb temperature causes work hardening of the filament leading to early bulb failure.

The easiest hobbyist way to do this would be using a radio remote pendant. If you've got an outlet available where you want something to turn on, you could do a lot worse than Radio Shack's Wireless RF Remote-Control On/Off Switch Catalog #: 61-2667 for only $9.97 USD.

This is such an easy and inexpensive solution, you can even get a

120VAC relay and just wire the coil up to the outlet to switch just about anything.

But if you really want to use IR, I'd think using a laser diode for longer distances on the sending end, and a photodiode with a lens in front and demodulation on the receiving end might be your best bet. You'd have to modulate the IR to filter out extraneous sunlight and heating. This would be a little complicated.

But please post again if this is of interest.

Cheers Chris