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IR LED direction accuracy

Discussion in 'Electronic Design' started by Pimpom, Jan 10, 2011.

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  1. Pimpom

    Pimpom Guest

    A couple of weeks ago, I posted a question in s.e.b. about
    sending a trigger pulse over some hundreds of metres. I apologise
    for not responding to each reply. The reasons were social
    obligations of the season and the death of my mother-in-law.

    I'd previously rejected the idea of sending the pulses by IR, but
    suggestions in seb made me consider it again. I've thought of
    some techniques to extend the range and preliminary calculations
    indicate that it's doable.

    I've already been using IR links for other purposes over
    distances of ~10m, but while experimenting to see how much
    distance I can get without any special technique, I ran across an
    unexpected snag: I found that the no-name IR LEDs from the local
    market focus their beams in directions way off the LED axis.

    So I've been looking for sources where I can get specific models
    of IR LEDs with datasheets. I've tentatively selected the Vishay
    TSAL6100 with a half-power beam width of 10 degrees, but it will
    take at least 2 or 3 weeks for me to get the devices. Does anyone
    here know if such branded LEDs can be relied on to project their
    beams accurately along their axes?
  2. Pimpom

    Pimpom Guest

    I was not really thinking of focussing the LED output. My
    approach (so far theoretical) is to use an array of LEDs for
    higher intensity. Their combined IR emission would be impossible
    to focus into a parallel beam with a single lens. The lens will
    rather be at the receiving end.

    My concern with the direction of emission has to do with ease of
    aiming the emitter at the receiver. According to the datasheet,
    the Vishay TSAL6100 I mentioned has high intensity due to the
    narrow angle: 130 mW/sr typ compared to 20-40 mW/sr for wider
    angle types, and still has 90% intensity at +/- 4 degrees. 4
    degrees is 7' at 100', or 28m at 400m. It shouldn't be difficult
    to aim the beam with that degree of accuracy, provided that the
    direction of peak emission is approximately along the axis of the

    However, if the direction of peak radiation for individual diodes
    is unpredictable, it will also partly defeat the aim of combining
    the IR outputs from several devices.
    I've also been considering that and have not completely rejected
    it. The only laser device I can get hold of without too much
    hassle is a pointer.
  3. Pimpom

    Pimpom Guest

    Interesting. But I might have given the wrong impression by
    mentioning the beam width of the TSAL6100 LED. There's no reason
    to confine the beam to a narrow angle except insofar as to
    concentrate the intensity at the receiver and thus extend the
    usable range. The TSAL6100 has a stated radiant intensity of 130
    mW/sr at 100mA compared to 20-40 mW/sr for wider angle types.

    I was thinking of using several LEDs, something like 25 of them
    in a 5x5 grid, and a lens at the receiver. But the effect will be
    partly cancelled if they don't all radiate in the same direction,
    at least within a very few degrees.
  4. Hammy

    Hammy Guest

    Look at the Data-sheet those LED's are meant to be pulsed and are
    rated as such.Radiant intensity at 1A pulses goes up to 650 to

    What you could try is a light pipe this could be as simple as a bic
    pen with the ink part removed.This would help to increase the focus
    of the beam and improve the range, other then that you need a lense.
  5. Pimpom

    Pimpom Guest

    Ah yes, a burner laser. I'd forgotten about them. Thanks for
    bringing it up. My son and I started a project using those last
    year, but never finished it because other things took priority
    over it. I wonder how practicable is the idea of projecting a
    laser beam onto a receiver from 400m, especially since the system
    has to be set up quickly, used for a day and then dismantled. An
    advantage of using IR is that the receiver can be a standard
    remote control receiver module.
  6. Pointing the laser will be difficult.
    But sending IR to a remote control receiver over 400 m will be next to impossible,
    at least I had big problems with much shorter distances.
    At least with a laser in the visible range you can see where it hits (points).
    I have tried no more then 50 meters with a helium neon laser though, YMMV.

    Are you using some kind of telescope at the receiving end, does it pass IR?
  7. PS
    there is also a safety issue with lasers, the risk of
    shining somebody in th eyes, somebody may topple your transmitter,
    anything, aircraft..
    I would use a radio link if I were you.
    Cheap about 430 MHz modules in a free band are available from many places,
    with digital IO too.
    Most optical link announcements I have seen over time always stayed just that: announcements.
  8. Pimpom

    Pimpom Guest

    The datasheet does not directly give a duty cycle vs. pulse
    duration vs. current rating. The radiant intensity figure for 1A
    is only for a single 100us pulse. The current vs. pulse duration
    curves are drawn up to 1A for duty cycles up to 0.05, so I assume
    that's the maximum permissible level. I'd like to use a duty
    cycle of at least 0.25 with 25usec pulses. Interpolating the
    curves implies that this would be safe only at around 0.25A max.
    The problem with trying to focus the beam with a lens is that I
    don't think a single LED will be powerful enough to cover the
    distance, and a single lens will not be able to focus radiation
    from multiple LEDs into a parallel beam. OTOH, a lens at the
    receiving end will be able to focus the radiation from several
    LEDs onto a single receiver.
  9. Pimpom

    Pimpom Guest

    When I first started thinking seriously about this project, I
    also quickly discarded the idea of using IR at such distances. I
    felt that RF was the only realistic option. But then I got to
    thinking about IR again and came up with this:

    I've been using cheap no-name, no-datasheet receiver modules and
    LEDs (see my opening post) successfully at distances of around
    10m outdoors under bright sunlight. With high-intensity LEDs and
    sensitive receivers, both with known characteristics, I hope to
    get at least twice that range, i.e. 20m. The target distance is
    20 times that or 400m.

    An array of, say, 25 LEDs, will give 25/20^2 = 1/16th of the
    radiant intensity at 400m.

    The receiver's sensitive area is about 0.2" in diameter. A 2"
    lens will catch x100 radiation. This gives a theoretical received
    radiation of 100/16 = 6.25 times that with a single LED at 20m.

    Factors that will inevitably reduce that figure in practice are -
    1. Atmospheric absorbtion: No idea except a wild guess. At least
    there's only minimal pollution at the site. There may be slight
    seasonal haze.
    2. Partial opacity of the lens glass to IR: Most sources found
    with a quick google search for ordinary glass gave only vague
    statements like "fairly transparent to near infrared", "passes
    near IR very well", and so on. The one curve I found for window
    glass gives a transmissivity of about 0.86 at 940nm. (I wonder if
    a cheap magnifying glass will be better than a coated lens).
    3. Imperfections in alignment and focus: If the Rx is somewhat
    offset from the focal plane of the lens, it will reduce
    efficiency, but will be more tolerant of alignment errors.

    If all these factors reduced the received radiation to 0.2 of the
    theoretical figure, it will still be in the same range as that
    for a single LED at 20m. Now please feel free to point out any
    flaws in my reasoning.
  10. TTman

    TTman Guest

    Outdoor 'laser' Paintball by any chance ????
  11. I have read of some amateur project that used a parabolic reflector
    and visible light (light bulb) on the transmission side,
    telescope on the other end.
    It is possible to slowly modulate a light bulb, or perhaps faster
    with some LCD shutters in front (from some LCD 3D glasses).
    Then you could use a powerful car headlight :)
    LCD glasses can do 50 on / off flips per second at least mine can.
    Kerr cell ? IIRC.
    Hey I just invented that LCD thing.
    Signal lights like that are as old as the world,
    and with a normal telescope can bee seen far away,
    and are not dangerous.
    IR has more attenuation than radio, but it could work.

    Sounds OK, but then again, the proof of the pudding is in the eating.
    My experiences with IR LEDs are not that hope giving, but it is
    many years (30 or so) ago.
    I was interested in this for transmitting simultaneous translation
    to a group of people with receivers from a big IR LED panel.
    Only indoors.
    These systems exist.
    But outdoors you will also get a lot of noise from daylight and other sources.
    Give it a try, I am curious.
    Anyways, for outdoors we went to radio.
  12. TTman

    TTman Guest


    Because IR detecters are the 'preferred' reception device....
    You can use 632nm laser, (class1e.g. ) and a fibre optic array to collect
    the beam.... as used by LaserRunner systems.
  13. There's no spec for accuracy of beam alignment. Getting the body
    aligned would require snug holes in a plate or something like that.
    Or just throw a 10W or 20W white LED at the problem (modulated and
    with higher peak power), or perhaps a 3W with a lens.
  14. Pimpom

    Pimpom Guest

    Don't know if they're brighter or not. But IR has at least one
    advantage: I can choose from a range of receiver modules that
    already incorporate amplification, AGC, interference suppression,

    If necessary, I guess I could use a digital camera to monitor the
    IR from the transmitter at the receiver's location and direct an
    assistant to adjust the transmitter's direction. However, I don't
    think that will be necessary as even the narrow-angle TSAL6100
    LED outputs 90% of the axial intensity at 4 degrees off center. 4
    deg is 7' at 100'. Yesterday, I marked out these distances in my
    compound and decided that it shouldn't be difficult to aim the
    transmitter with that level of accuracy.

    All of this, of course, is *IF* the LED's peak radiation is along
    its axis, at least approximately. Which was really my original
  15. Charlie E.

    Charlie E. Guest

    You have to be careful with massed IR LEDs, as they can cause eye
    damage pretty easily! Be very careful with them...

  16. Yes, at least you can see how (literally) blinding a 20W white LED is,
    and avoid it, as when looking at the sun.
  17. nospam

    nospam Guest

    And you know that because?
  18. Jasen Betts

    Jasen Betts Guest

    spend a few bucks for a "keyring" laser pointer.
    visible's light's much easier to aim than IR.
  19. Pimpom

    Pimpom Guest

    My intention, if I do decide to use IR, is to use a standard
    remote control receiver module. I have some experience with this
    and it already has all the features you mentioned -
    amplification, AGC, frequency tuning, noise suppression. What
    needs to be tackled is the problem of making the system work at
    long distance.
    There's that to consider, of course.
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