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Infra red

Discussion in 'Electronic Basics' started by Roger Dewhurst, Jul 28, 2006.

  1. 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
     
  2. Chris

    Chris Guest

    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
     
  3. jasen

    jasen Guest

    If it's sensitive enough, and/or if it also responds to visible light.
     
  4. 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 ()
     
  5. 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 ()
     
  6. Chris

    Chris Guest

    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
     
  7. redbelly

    redbelly Guest

    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
     
  8. 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
     
  9. John Fields

    John Fields Guest

    ---
    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?
     
  10. Chris

    Chris Guest

    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
     
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