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How do IR goggles work?

  1. May 14, 2010 #1
    Greetings all!

    I am doing this experiment with my son:
    http://www.ehow.com/how_2092756_make-infrared-goggles.html" [Broken]

    Since IR is a segment of the EM spectrum that human eyes cannot see, how is it that this experiment works? To clarify my question further, since visible light and IR are both EM waves (just different wavelengths/frequencies), is this experiment decreasing the wavelength of IR to, in effect, turn it into visible light?

    I hope I'm making sense. I look forward to your input! :smile:

    Thanks
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. May 14, 2010 #2

    mgb_phys

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    No they don't really work.
    They block most visible light except some red and if you make the image dark enough (and the goggles block all other light) you don't see colors because your night vision is mostly black and white.

    You used to be able to buy filters like this for photographers to estimate what a scene would look like in B+W
     
  4. May 14, 2010 #3

    ZapperZ

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    Loosely, the sequence of events for IR detection in such a device is as follows:

    1. The IR is focused onto a suitable photocathode
    2. Electrons are emitted by the photocathode
    3. The photoelectrons are passed a microchannel plate. This plate multiplies the number of electrons via secondary electron emission
    4. The amplified electron signal then hit a phosphor screen (in the case of an analog viewer) and emit visible light that you see with your eye.

    Zz.
     
    Last edited by a moderator: May 4, 2017
  5. May 14, 2010 #4
    There are a few comments to make here.

    1. The human eye can in fact see some IR light (700 nm to 1000 nm), but it does so with very low sensititity. The eye's response decreases rapidly as wavelength is increased beyond red light (650 nm). In full daylight, the amount of near IR light is so intense that it can be detected by the eye. You just don't notice it because the visible light is even more intense and your eye is much more sensitive to the visible spectrum.

    2. This experiment is not decreasing the wavelength. Simple color filters can't do that. It turns out that converting IR to visible light is difficult (with a simple transmission pannel) because the photon energy of visible light is higher than that of IR light. It's much easier to convert ultraviolet to visible light.

    3. I think the above answer by mgb_phys is basically correct. The color filters block the visible light and pass the infrared spectrum. This allow your night vision to kick allowing the rods (> 100 times more sensitive than the cones) in your eyes to be able to detect the IR light. Keep in mind that the IR light you are seeing has a wavelength relatively close to the visible light spectrum. I'd guess you are not seeing anything more than a wavelength of 1000 nm. This is a very small portion of the IR spectrum. So, you're not really seeing "heat waves" like a thermal imaging camera, but basically long wavelength light rays. A typical movie camera and many camera films pick up this portion of the spectrum too, but they do so much better than the human eye.

    4. I think ZapperZ is talking about a different device than your experiment.
     
  6. May 14, 2010 #5
    I'm so grateful for the wonderful responses! Thanks! :smile: I do have some further comments/questions below if anyone is interested in providing more input.

    I appreciate the laymans explanations, they were exceptionally helpful. Regarding your #2 comment above: in general, is it safe to say that it's easier to increase the wavelength of higher-frequency EM waves (to make them visible to human eyes) than it is to decrease the wavelength of lower-frequency EM waves?
     
  7. May 14, 2010 #6

    mgb_phys

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    Yes, high frequency (short wavelength) have more energy, so you can easily convert them into lower frequency (longer wavelength) lower energy photon.
    But you can't convert low energy infrared photons into higher energy visible ones without an external source of power *

    There are ways you can make infrared visible, you can have a material that you 'charge up' with high energy UV light and then a low energy infrared photon can trigger it into discharging and giving off visible light - since you get out much less visible energy than you put in UV energy this is OK, you don't violate energy conservation.
    You use this to make infrared detecting cards for lasers and you used to use it in early infrared vision systems. You have to leave them in sunlight to re-charge them



    (* because this is PF - yes you can under certain circumstance frequency double lasers with non-linear optical materials)
     
  8. May 15, 2010 #7
    There is a difference between night vision and infrared. Night vision goggles used in the military are simply amplifiers of light. Evan at midnight the goggles amplify light emitted from stars moon and so on. This technology is expensive. But if you are talking about IR, they emit a light that can be seen with a filter. The goggles would need big IR LED lamps on the front to see what you are illuminating. the reason they don’t use this in the military is because at night you don’t want the enemy to see you. So putting big lamps on your head is not a good idea. Although you can’t realy see the light emitted from the LED lamps any camera with an infrared filter will see it like a Christmas tree. So don’t get your sons hopes up for night vision if you don’t have a heavy wallet. But not all is lost, look online for display glasses and buy a portable camera system. attach the mini camera to the glasses and the av cords that would go into your tv into the glasses and see the world through IR. Looking for night vision is costly. If you want the result seen in movies it will cost you about 10 to 20 grand. But no ware near the power of the military ones.
     
  9. May 15, 2010 #8
  10. May 15, 2010 #9
    Is the OP asking about "night vision" or "thermal imaging"? The latter is IR, very expensive, and a different technology than night vision.
     
  11. May 15, 2010 #10

    mgb_phys

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    The link they posted is to a x-ray specs type gadget, it's just a dark purple filter to make the scene look other-worldly / infrared.
     
  12. Sep 1, 2010 #11
    if you were to shine a few IR LED's at an object in the dark and use the goggles to pick up some of the reflected IR?
     
  13. Sep 3, 2010 #12
    There is a link to a related article where he uses IR LEDs to illuminate a scene to be viewed through those two gels. If it was just a simulation visual effect, I would think the invisible LEDs would have no effect.
     
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