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IR-light focusing and collimation

  1. Jul 31, 2007 #1
    HI, im thinking of how can i focus and make a home made collimation to an IR beam.

    I have a lens and an IR-led mounted to a center of a focal point. But its hard to collimate the beam properly without visual sight to IR.

    B/w-video cams are usually lit with a series of IR leds to have enough light to have a clear view, so that could be one solution, but i have only color-cam :( . Are these color cams also sensitive to IR ? What other possibilities there might be to do the job?

    TechSpec
     
    Last edited: Jul 31, 2007
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  3. Jul 31, 2007 #2

    chemisttree

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    I used to collimate IR in my FTIR instrument by placing a temperature-sensitive LCD film in the path at the focal point. The temperature-sensitive LCD film is of the type used in 'mood rings'. It was black at room temperature and became blue then green and ultimately yellow at its extreme range. This was fairly intense IR from a globar. Your IR LED may not be intense enough to accomplish this.

    You can use an IR-sensitive photodiode to do this as well. Place the photodiode exactly in the path you wish to focus upon and then adjust the focus to maximize the signal obtained from the photodiode. A voltmeter should do the trick. You may have to mask the diode to allow only the response from a small spot to be measured.
     
    Last edited: Jul 31, 2007
  4. Jul 31, 2007 #3
    Please excuse me, but what are we talking about? Are we discussing ways of turning a telescope into an infrared camera? Or are we talking about producing infrared photons as a sort of laser?

    - Bryan
     
  5. Jul 31, 2007 #4

    chemisttree

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    I was referring to focusing an IR beam generated by an IR diode. I don't believe that would apply to a laser or a telescope. My reference to an FTIR is to a fourier transform infrared spectrometer used to measure the infrared spectrum of a laboratory sample using a very intense source or the type of IR beam used to measure the position of something like the IR beam in an intrusion alarm system.

    My idea is to place an IR photodiode in the path of the IR beam and then focus the beam to a minimum spot size. This can be done with one or more IR photodiodiodes (different sizes/areas) and an appropriate mask(s). If the surface of the IR photodiode is large relative to the collimated spot, the response voltage will not change appreciably if the slightly defocussed IR spot is approximately the same size as the IR photodiode. Thus the mask. This reduces the IR sensitive area of the photodiode and allows for a more dynamic response near optimum focus.

    If you are trying to collimate a telescope in the IR range, I think you will have to mount an IR-sensitive CCD at the focal point of the telescope and observe the image.
     
  6. Jul 31, 2007 #5
    Hi, first of all, i made a simple illustrating picture what i was thinking. I put a file into here:Link

    The purpose is not just focus the beam into a spot, but focus it to an area of a size of that lens, about 30mm. Thatway the beam would be similar size on all of its lenght.

    That photodiode came to my mind, but then i only know the strenght of a one spot, i dont know is the area of illumination circular or not, or is its strenght equal on the whole area. Offcourse i could move the sensor to check the area on a certain points. The focus is important, but also the shape is important, it should be round, not oval or stretched. Offcourse certain tolerances must be accepted.

    Hmmmm.. Thats why i thought i should use cameras so i could see live picture and instantly see how my adjustments takes effect.

    Now i remember, that my friend has a infrared viewing device, it has a capability to view infrared images as it has its own diodes to do the extra light. It is similar like this Meade unit:
    http://www.meade.com/nightview/index.html
    What do you think of using that to do collimation and focusing?

    TechSpec
     
    Last edited: Jul 31, 2007
  7. Jul 31, 2007 #6

    LURCH

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    The simplest solution would probabbly be to put a visible LED in place, locate it to the best spot for focus, mark that spot, and replace with IR LED.
    If your Cam has night vision, it will see IR (my brother found this out accidentally by pointing the remote at the TV while the Camera sat on top of it, filming himself).
     
  8. Jul 31, 2007 #7

    chemisttree

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    What you need to do is to paint a target with an IR reflective pigment (http://news.thomasnet.com/fullstory/474510/1303) and visualize it with a camera set on night vision or low light illumination. Many digital cameras have this feature (my Sony digital camera does). It is probably better to buy an IR photodiode that has the lens integral to its construction and arrange a series of these so that the 'hot' spot is near the center of vision at the distance you are going for.
     
  9. Jul 31, 2007 #8
    Visible led should have exactly the same radiation pattern and the housing to get the equal results between these two diodes. And because the IR has a different focal point than a visual light, the result would not be the same.

    TechSpec
     
    Last edited: Aug 1, 2007
  10. Jul 31, 2007 #9

    mgb_phys

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    If this is a very near infrared Silicon diode 905-920nm then the easiest way is to use a simple B+W tv camera.
     
  11. Aug 1, 2007 #10
    I just red that a digital camera is a very sensitive to IR, but this is blocked away with internal filtering. If its not filtered, IR can cause unsharp images as it has a different focal point on lenses than visual light :
    http://www.naturfotograf.com/UV_IR_rev07.html

    On that link above, they removed IR filtering to make a powerful IR photographing device from a camera.

    Has anyone seen a spectral response curves for digital cam's ?? Im using 850-880nm leds.


    TechSpec
     
    Last edited: Aug 1, 2007
  12. Aug 1, 2007 #11

    mgb_phys

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    For a black and white TV camera without an IR filter the response curve is basically that of silicon so should be better than 10% at 850-880.
    Even with the IR filter in place, in a dark room a camera with autogain would be enough to focus an 850nm IR-LED. Security cameras generally don't have IR filters.
     
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