Built on Facts

All right ladies and gentlemen, today we’re performing an experiment! More of a demonstration, really, but one that’s very easy and will impress your friends. You will need:

1) One remote control.
2) One camera phone.

The vast majority of remote controls operate via an infrared light-emitting diode situated at the front of the device. You press a button, and the diode lights up in a particular pattern of pulses corresponding to the button you pressed. A little sensor at the front of the TV detects these pulses and converts them into the electric signals that tell the TV to switch from Monday Night RAW to Mythbusters. (Or whatever your preference may be.) Your eyes can’t see this light, because your eyes contain cellular chemistry that only reacts with a particular slice of the electromagnetic spectrum. Infrared light happens to be too low in frequency to set off the chemicals in your eyes, and so it’s simply not seen.

The people who manufacture digital cameras don’t use those same chemicals, because what works in a biological environment is an engineering nightmare in an electronic device. Digital cameras use one of a few different technologies that generally speaking don’t involve chemical changes – diodes and photosensitive capacitors tend to be the major players. As such, they don’t generally do quite the same things as your eyes. One can approximate the other pretty well, but there’s some exceptions. The one we’re exploiting is this: the sensors in digital cameras are usually sensitive to infrared light. But the internal electronics in your camera have no way of knowing this, and so it displays the infrared light as the visible light it wrongly thinks it’s detecting.

So if I take a picture of the front of a remote control with no button pressed, I see something like this:

i-2905e5b40abd07b3ba3ab498834acc08-remote1.png

And if I take a picture with me holding down one of the buttons:

i-6760ac59be66b56a998af8b5a6b1afad-remote2.png

The heretofore invisible infrared light shows up as a blaze of visible light that may range from blue to pink depending on the specifics of your camera. Interestingly the prominence of this effect is usually inversely proportional to the quality of your camera. Higher-quality cameras are interested in the most faithful reproduction of the visible light scene and so they include special filters to remove the infrared light before it reaches the sensor. If you try this with a quality dedicated digital camera rather than one in a cell phone you may not see the effect at all. (Hence doing the experiment with a camera phone)

While there’s not a whole lot of other things that emit a lot of IR at this wavelength, there are a few. One of them is the thermal radiation of a very hot object. Here’s a camera phone picture of a stove just when a very dim red light starts to be visible with the naked eye:

i-64b906173087c74f661e4796e5c6f951-hot.png

You may be able to find other examples. Please feel free to share them, but be careful not to burn your house down!

Comments

  1. #1 Chris Hertlein
    June 2, 2009

    I think you’re lying. You just have a purple stove!

  2. #2 Tom
    June 2, 2009

    Cool! I had not thought to look at a burner that was on the verge of being incandescent when I was tooling around with my IR webcam. (I have a gas stove)

    IR photos are one area where cheap often works better than expensive, because the IR filter is an added cost.

  3. #3 yud
    June 2, 2009

    I noticed this effect on my digital camera when I tried taking a photo of campfire embers. The embers were a nice dark red to my eyeballs, but looked bright pink to my camera.

  4. #4 rpsms
    June 2, 2009

    Some video cameras have a night-shot mode which, AFAIK, removes the IR filter and enables an IR spotlight which is typically just under the lens.

    YOu can use this feature when searching for bugs and cameras in hotel rooms ;)

  5. #5 Rhett
    June 2, 2009

    You actually CAN see IR with your eyes. At least the near-IR. Our eyes aren’t very sensitive in that area. Here is a good page on how to see this with your own eyes. I have not tried this, but I trust Bill Beaty.

    http://www.amasci.com/amateur/irgoggl.html

  6. #6 jbrydle
    June 2, 2009

    That’s cool! I found out about the cell phone camera trick a little while ago. I helped a friend build an interactive gaming table that uses IR pens on a table surface as a mouse cursor:
    http://www.wired.com/geekdad/2009/04/diy-gamer-build/

    I built the IR pens out of highlighters and IR-LED’s, but it was very frustrating to test it because I had to use my multimeter rather than just look to see if the light was on. A friend told me about the cell phone camera and it made the project much easier.

  7. #7 Uncle Al
    June 2, 2009

    Foliage chlorophyll violently flurosesces in the NIR as a leaf cooling mechanism. The middle “star” in Orion’s sword is one of the most intense IR emitters in the night sky. It’s much bigger in the NIR.

  8. #8 John
    June 2, 2009

    My senior design project actually involved using infrared LEDs and a reflective glove to make an interactive sign language tutor. The camera trick was great help since I’d never soldered before.

    Not quite as cool as that gaming table though.

  9. #9 Eamon
    June 3, 2009

    This featured on a Japanese Programme recently – as a way to spot cheap bugging devices using IR Comms.

  10. #10 Brian X
    June 7, 2009

    I have a cooking show and occasionally shoot on a one-chip camcorder. I’m very familiar with this — it was incredibly eerie the first time I saw it.

  11. #11 John S. Wilkins
    June 8, 2009

    Way cool. I just found out where the IR source under the piece of black plastic on my TV remote is…

  12. #12 Tony P
    February 3, 2010

    I’ve got a Hasbro Kiddie Cam that I modified. I took out the IR filter. I’ve got to try it out, it’s low resolution but adequate for a little bit of fun.

  13. #13 kmul
    February 3, 2011

    Tony P, thats’s creepy.

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