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How do i measure light frequency?

  1. Mar 2, 2008 #1
    Hi There!

    I have a bit of a problem....

    I work in Audio Visual and install video matrix systems.

    We are findind that the newer LCD screens are throwing out so much IR interference that they are blocking the Remote Controls from operating the Sky Boxes...

    The manufacturer of the matrix panel have introduced new "shielded" IR receivers but this has not helped the problems.

    So, my question is...

    Firstly, how do i go into a room and measure the frequency of the IR coming out of the TV and how would i do the same to the Remote Control?

    The reason I ask this question is because I am wondering if it would be possible for the manufacturer to "remove" the unwanted IR from the system and just leave the SKY IR signals.

    My thoughts are this would be possible AS LONG AS the IR coming out of the screen is not on the same frequency as the Remote Control is transmitting...

    Sorry if there are flaws in my theory but i am not a scientist. just need some help on IR and light frequency - i think - lol


  2. jcsd
  3. Mar 2, 2008 #2
    I think you will have a problem with the appropriate devices. Sorry.


    Just one thing: I don't think its the actual frequency of the IR causing this problem but its rather the IR impulses emmitted from the screen causing the problem. An IR remote works by conveying certain patterns of impulses to the target. A constant overlay over the beam of a IR-source with the same frequency would not effect that as long as its not to strong. Its rather that the IR-impulses from the Screen mix with the pattern from the remote.

    Last edited: Mar 2, 2008
  4. Mar 2, 2008 #3
    thanks for the reply...

    so, i need to get a prism and put it in front of the tv and then get a night camera and look at the light coming out of the prism through the camera?

    what would i expect to see? how do i then measure it?


  5. Mar 2, 2008 #4
    ah, i see, i think....

    that would make it quite problematic/difficult to "scrub" then and remove the unwanted pulses would you think?

    any thoughts on a solution?

  6. Mar 2, 2008 #5
    I've found an interesting article on "How Remote Controls Work".

    On page 2 of the article it mentions that "To avoid interference caused by other sources of infrared light, the infrared receiver on a TV only responds to a particular wavelength of infrared light, usually 980 nanometers."

    Article is at http://electronics.howstuffworks.com/remote-control1.htm

    Does this mean that if everything except 980 nanometers was filtered out it would solve the problem or is it still liable to have "overlay" pulses from the screen?

  7. Mar 2, 2008 #6
    In reply to #4

    This heavily depends. But from what you ask I don't think you are a professional at tweaking circuit boards and electronic components?

    Logically there will only be a few possible solutions:

    • Arrage the screen and the sensor the way the sesor doesnt receive any (major) impulses from the screen (shield it correctly)
    • Tweak the circuit so it will not react to the length of impulses emitted by the screen. Imagine conveying a morsecode standing next to a flashing beacon. Now make the morse signals longer so they can be told apart from the beacons
    • Increase the amplitude and decrease the sensitivity for the remote and the receiver respectively so it will not react to the screen IR any longer

    All this you better double check. I'm not an engineer either. I'm just giving my best to assist you with what I know.
    Last edited: Mar 2, 2008
  8. Mar 2, 2008 #7
    In reply to #5

    If the the article is correct (which I presume) you were right with what you assumed before. That the screen actually matches the frequency of the SAT exactly. This discloses another option for fixing it.
    If you manage to change the receivers and the remote frequency to one which is not present elsewise (this means change it from 980nm to something else), you will not sustain any interferences any longer.

    Actually I think thats the most convenient way to do it. I think doing so is the very easiest to do. Just know how. Maybe your TV/screen ships with a circuit scheme explained in the manual. I know some TV's which do come with a detailled explaination of their interior.

    With respects to "filtering 980nm" out: Of course it would also fix it if you could filter out the range of frequencies (like from 970 to 990 or something like that) at the screen itsself. But I doubt that this is possible without covering the monitor with something which one can hardly see thru. I don't even know what it would be called. I know IR-Filters but those do just the opposite.

    Well, maybe "inverse-IR-filters" are fully transparent (which would make sense assuming that they really only filter out IR and no visible light) and you find one for 980nm. Good luck.
    Last edited: Mar 2, 2008
  9. Mar 2, 2008 #8
    what about using an infrared camera to see the IR coming from the TV and the remote when it is being used?
  10. Mar 2, 2008 #9
    hi Tshock

    thanks for the reply.

    i need to know if the light sources are overlapping and interfering with the ir. with the suggestion you have made im not sure i'd be able to do this...

    do i need a spectrum analyser i wonder?

  11. Mar 2, 2008 #10
    Look, I don't understand where this is ment ot be going. You know the screen interferes with the remote, don't you? So you can certainly assume that they are on the same wavelength. Now why do you want to analze it further? Is'nt it enough to know whats wrong and how to fix it, but do really need to put your hands on the fault and touch it?
  12. Mar 2, 2008 #11


    User Avatar

    Glass is opaque to IR, but I'm not sure about this case. You could try placing the screens behind glass and testing to see if there is still interference.

    Remember that just because something is transparent to visible light does not mean it is transparent to IR, and similarly, if it is opaque to visible light, it is not necessarily opaque to IR.
  13. Mar 2, 2008 #12
    Though this is hypotetically possible I'd like you to name a material which meets this, because I can't think of anything usual. Or I just appear not to know one which is common.

    Putting glass in between is actually a good idea. I didn't think of glass as an IR absorbing material.
  14. Mar 2, 2008 #13
    Hi ManDay,

    I think you are right. by assumption. that must be the answer.

    This thread doesn't make sense out of context really...

    maybe this thread throws more light on the subject... exscuse the pun...

    this is the background of my newly born interest in IR...


  15. Mar 2, 2008 #14

    Claude Bile

    User Avatar
    Science Advisor


    Your only real option in terms of measuring the wavelength of the IR radiation is using an optical spectrum analyser, which unfortunately is not cheap.

    The manufacturers should have extensive data on the emission characteristics of their screens, see if you can find details of the emission spectrum, specifically the portion that overlaps with the spectral bandwidth of the remote control.

    Silicon is probably the most common material I can think of that is opaque in the visible and transmits IR. Chalcogenide glass also has these properties.

    Also, silica glasses do not absorb near IR radiation, only the mid-far IR from wavelengths of about 2-3 microns and above. There are probably other glasses whose wavelength cut-offs are closer to the visible, but are less common, and consequently, I can't recall their names :redface:.

    Last edited: Mar 2, 2008
  16. Mar 3, 2008 #15
    Semiconductor infrared detectors are not very frequency sensitive. The photons of radiation of room temperature objects do not have enough energy to give the electrons in the detector enough energy to get across the band gap to the conduction band, but higher frequencies such as visible light will still produce a signal in the detector.
    All Infrared communications use an infrared source wich is switched on and off with an intermediate frequency, roughly 100 Khz for remote controls, higher for faster communication. The resulting IR, amplitude modulated by an intermediate frequency is then switched on and off much slower to get the signals of the remote across.
    The resulting signal can be picked up by a receiver with a phase locked loop, that isn't very sensitive to wich intermediate frequency it picks up, because 100 Khz modulated IR doesn't occur in nature.
    I think the problem occurs because the backlight of the LCD panel is run of a switched power supply that produces flicker at a frequency that the infrared receiver picks up. The exact frequency of visible and near-infrared light that is sent out isn't relevant.
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