Infra red

[Roger Dewhurst]

Will the infra red radiation from an ordinary 100 watt bulb pass through two
sheets of window glass about 5 metres apart to trigger an IR sensor?

R

 

[Chris]

Will the infra red radiation from an ordinary 100 watt bulb pass through two
sheets of window glass about 5 metres apart to trigger an IR sensor?

R

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

 

[jasen]

Will the infra red radiation from an ordinary 100 watt bulb pass through two
sheets of window glass about 5 metres apart to trigger an IR sensor?

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

 

[Don Klipstein]

Will the infra red radiation from an ordinary 100 watt bulb pass through two
sheets of window glass about 5 metres apart to trigger an IR sensor?

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 ([email protected])

 

[Don Klipstein]

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

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.

<SNIP>

- Don Klipstein ([email protected])

 

[Chris]

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.

<SNIP>

- Don Klipstein ([email protected])

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

 

[redbelly]

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

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

 

[Roger Dewhurst]

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 ([email protected])

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

 

[John Fields]

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.

---
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?

 

[Chris]

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

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.

http://www.radioshack.com/product/index.jsp?productId=2103886&cp

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