Loooong distance IR detection, and IR collimation

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Hi,

Every body know about the IR remote controls that we use everyday for our
TV, music set etc. They are short range, almost omni direction (due to
reflections from almost any surface) controllers.

Imagine that we have a tube/pipe of some sort with radius R, and lenth L. We
have positioned the IR transmiter at the end of this tube and alligned it in
such a way that its illimunation axis parallel to the tube so that the IR
source can only be visible if the observer/receiver aligned with the
tube/pipe. BUT the inner surface of the pipe/tube must be coating with an IR
absorbing material so that the tube will not act as a wave guide. Or the
tube/pipe must be made out of an IR absorbing material.

Do you know any IR absorbing material?

Is it possible to collimate the IR light source with the above mentioned
method to 2degrees?  OR is there a way of  creating very narrow IR light
cone ie. collimation around 2degree?

If we use an ordinary IR remote control receiver (or some other low-cost IR
sensor) to detected the IR light about 100 meters away from the collimated
IR source how much IR power (how many IR LEDs or what else)  we need?

Thanks

Rico Maxle



Re: Loooong distance IR detection, and IR collimation


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If i didn't get the impression that you were trying to figure out a new
way to set of an IED I might offer a few suggestions.  I must be getting
paranoid in my old age.

Shawn

Re: Loooong distance IR detection, and IR collimation


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Sure.  Lots.  Black paint, for example.  It won't help you.
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No problem, your collimation scheme will work fine.  But it won't help
you.  You will wind up with a very dim, approximately collimated beam.
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All that will do is to absorb some of the light before it leaves the
instrument.  This won't do anything to increase the amount of light you
will receive at the far end, because the light you absorb would have
missed the receiver anyway.  If you put it at the receiver end instead
of the transmitter, it will help quite a bit in reducing background
light, which will blow you out of the water otherwise.  (Sunlight is
*very* bright.)  If you use lenses, you'll do much better, but success
won't happen by accident--do it by the seat of your pants and you're
liable to be off by 6 orders of magnitude from practicality.

It isn't difficult to calculate how much light you'll receive--the
detected photocurrent is proportional to the optical power received, and
the optical power just spreads out in a cone from the transmitter.  The
transfer efficiency is just the ratio of the detection area (lens or
photodiode) divided by the cone's cross-section at the distance of the
receiver.

Assuming you're using kilohertz modulation like a TV remote, your noise
will be dominated by the shot noise of the background light, so that's
pretty easy to calculate--just measure the photocurrent I_BG caused by
the background in a realistic situation and compute the 1-Hz noise as
i_N = sqrt(2*e*I_BG), where e is the electron charge (1.6e-19 coulombs).

Then you can figure out what detection bandwidth will give you the
signal-to-noise ratio you need.  On your first try, it will be a very
small fraction of a hertz.

When you find out how frightfully bad your SNR will be, you can improve
it in several ways:

1.  Lenses
2.  Very small detectors
3.  Lots of transmitter power
4.  Baffles to exclude stray light
5.  Laser sources

Unlike hobby laser radar, this is not at all impossible--it just takes
work and thought.

Cheers,

Phil Hobbs



Re: Loooong distance IR detection, and IR collimation


Phil Hobbs wrote...
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 [ snip ]

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 There's Rico Maxle's answer, use an IR laser.


--
 Thanks,
    - Win

Re: Loooong distance IR detection, and IR collimation


You probably won't find a paint which does the job. Most paints are
reflective at grazing angles.  For a surface coating some kind of
flocking (a fuzzy surface) is better.  Better still is to put a series
of thin black painted baffles (like washers) inside the tube.  The last
one should be slightly larger diameter so no energy can directly
illuminate the inner edge.

You really need to define the system better.  Whether you can pass the
needed signal depends on:
1. transmit power
2. power density at the receiver (determined by distance, atmospheric
absorbtion, and level of collimation (beam angle).
4. detector acceptance angle is as important as beam angle
5. detector area
6. ambient illumination (usually solar energy in the receiver optical
bandpass)
7. detector noise -sets ultimate detection signal-bandwidth limit.
8. required signal bandwidth. Matched modulation and detection schemes
can extract very weak low bandwidth signals out of broader bandwidth noise.

To get better signal to noise it's common practice  to use optics to
narrow the beamwidth, but that may not be practical for hand held or
vehicle mounted systems.  Other methods use more power and/or narrower
bandwidth signals. Modulating the transmitter can allow detection of
your signal in the presence of optical or thermal noise.  Reducing the
optical bandwidth  using a partially collimated laser plus interference
filters on the receiver can minimize optical interference (usually from
sunlight).  Cooling can reduce detector thermal noise.   You're
requirements and budget will determine what is possible and what is
practical.


Rico Maxle wrote:
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Re: Loooong distance IR detection, and IR collimation



RicoMaxle wrote:
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Hi,

Is this a theoretical exercise, or do you have some intended use?  If
you tell us just what you are doing with this, we may be able to
provide more directly useful answers.

It is somewhat of a mystery to me why so many posts here have no
reference to the actual inteded application.

Luhan


Re: Loooong distance IR detection, and IR collimation



"Luhan"
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** It is characteristic of usenet that posters secret the real purpose
behind asking a question.

There are three major reasons for this;

1.  The purpose involves doing something illegal.

2.  The purpose is highly dangerous.

3.  The purpose is completely stupid.


Sometimes the purpose is illegal, dangerous and stupid all at the same time.

Since folk cannot get free, expert advice on illegal dangerous and stupid
ideas in the real world  -  they naturally head straight onto usenet.

Where they find dozens of obliging fools ready and willing to help them on
their way.





.......   Phil






Re: Loooong distance IR detection, and IR collimation




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NOTA (None of the above.)

The majority of such posts (on sci.optics at least) appear to emanate from
EEs
or MEs and the like who appear to have a very limited understanding /
knowledge
of optics in general.




Re: Loooong distance IR detection, and IR collimation



Phil Allison wrote:
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I'm only slightly less cynical.  Perhaps they cannot say because they
are doing it for a company that doesn't want its secrets revealed.
Even in that case, it would be nice to preface the question with 'I
cannot reveal exactly what I am doing here.'

Luhan


Re: Loooong distance IR detection, and IR collimation


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Thanks Phil...I feel like I'm not alone.  I try to use common sense when
judging the validity of a post before responding.  Unfortunately some
people can't overcome their egocentric desire to spew onto the world
their knowledge. In this unfortunately sometimes ugly world, a little
common sense and proper judgment can save lives and limbs.  If I knew
how to make a bomb, or even a small component of one (which I do), I
would not tell some guy I have never met how to make either of them.
I'd rather be paranoid than the reason for another human(s) losing their
life.  Thanks for getting it.

This is an open forum, and if your question has nothing to do with
robotics or an air of illegal or questionable moral standing, or it you
can't post your project in detail, you'll get no help from me.  Sorry.

Shawn

Re: Loooong distance IR detection, and IR collimation


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<rest snipped>

The first thing I thought of was the OP is wanting to create YAO (Yet
Another Opticon) device for the *illegal* control of traffic light
signals. You see these types of requests all the time on the hacker
forums. Of course, it could be a legitimate question, but since
collimators are common finds in any optical catalog (or use a spotting
scope in reverse) you have to wonder. Why go to the obviously lossy
approach of a baffle tube, other than the fact that it's smaller and
more stealthy.

-- Gordon

Re: Loooong distance IR detection, and IR collimation



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I saw some guy who was allegedly using one of them, but got caught when he
kept showing up on the red-light camera. ;-)

Cheers!
Rich



Re: Loooong distance IR detection, and IR collimation


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Is an interesting topic I've never heard of or thought of before.

First, what is an Opticon?

Then, how does a (self-actuated) traffic-light work?

  It contains a *camera* of some kind?

   (I had imagined that it used some kind of sonar to tell when
     a car appeared.)

   It "photographs" in red?   Or infra-red (heat)?  Or what?

Interesting stuff -- please do elaborate a bit.

Thanks!

David  (usually just "lurking" here)



Re: Loooong distance IR detection, and IR collimation


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Well, an "Opticon", in this context, to the best of my knowledge, is a
photocell (electric eye) mounted up on the stoplight bracket, and
ambulances and fire trucks (and, presumably, the cops) have some secret
code that they flash through their windshield with a strobe - the Opticon
picks up that signal, and changes the cross-lights to red, and changes
the light for the emergency vehicle green. Some guy had decoded that code,
and was breezing through town, but presumably got caught because the
red-light cameras were timed to the moment when the light was
_supposed_ to have turned red - there was this car in the intersection,
and the "switch to red" signal had been overridden!

They caught the guy. :-)

Cheers!
Rich


Re: Loooong distance IR detection, and IR collimation



David Combs wrote:

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The answer to this is "it depends". Some use coils embedded under the
road (often used at highway on-ramps in Long Island, for instance).

The more modern scheme is a camera in the traffic light that exploits
the highly reflective license plate on a car to detect if the
intersection is occupied.

I read something - here maybe - about a motorcyclist who was tired of
not triggering this type of detector. He put a strip of orange
reflector tape on his helmet and all was well.


Re: Loooong distance IR detection, and IR collimation



<Rico Maxle> wrote in message
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I see no reason why this can't be readily achieved.  As someone has
already mentioned glancing blows tend to reflect better, so a tube with
an inner surface with texture (ie: black cloth or velvet lined for
example) will probably work somewhat better than a shiny interior
surface.  To achieve a two degree cone your LED light source will need
to be mounted in the center of a tube of radius R, with a length L of
greater than or equal to R/0.0175.  In other words, select R, then
solve for L>= R/tan(1degree) = R/0.0175.  For example, if you select a
tube with a 3 cm diameter (R= 1.5cm), then the length needs to be at
least 86cm long.  The LED will need to be located at one end mounted at
the very center of the tube and pointed straight down the tube.  Using
this method will not however increase total range, it will only make
the light source more directional.  In order to get any gain in range,
you would need to use at least one lens.

However...  Given that 100m is fairly modest and not too hard to
achieve with the right semiconductors, I would probably avoid pesky
optics and lenses and whatnot.

The SFH 4503 infrared LED by OSRAM (available from Digikey) claims to
ouput typical infrared power of 40mW (at 100mA), and probably around
six times that at 1A pulses.  The LED claims a half angle of intensity
of +/- 4 degrees.  If we assume half of the power is contained in the 8
degree main cone, then at 1A it should provide something like 120mW
(typical, min. is much less) into the cone.  At a distance of 100m, you
can find the output in mW/square meter by using some basic
trigonometry.  It should be something roughly like 100meters * tan(4
degrees) = r.  (assuming a fixed circle projected at a distance 100m
away).  The area of the circle is approximately 3.14*r^2 = 154 m^2.  So
the energy delivered at that range will be around 120mW/154m^2 =
0.78mW/m^2.

The TSOP1256 infrared receiver by Vishay (available from Mouser) might
be able to receive a signal of that strength under ambient irradiance
levels of up to perhaps around 10W/m^2 (which equates to around 1.4klux
of incandescant illumination or 8.2klux of sunlight like light).  Those
levels of illumination are much weaker than direct unshaded sunlight,
however, they are probably quite a bit brighter than your house is lit,
or even your office or grocery store is probably lit.  You will
definitely want to put a hood over your receiver module to make sure it
is shaded from direct sunshine if this is an outdoor application.

Datasheet for the TSOP1256 can be found here:
http://www.vishay.com/docs/82013/82013.pdf

Datasheet for the SFH4503 LED can be found here:
http://catalog.osram-os.com/media/_en/Graphics/00029741_0.pdf

Your milage may vary in practice.  These are all very rough numbers.
Multiple LEDs will require a much larger diameter tube (and
consequently much longer as well), so if one LED isn't quite enough for
your task, a lens might not be totally out of the question.


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Re: Loooong distance IR detection, and IR collimation



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Dear Fritz,

THANK YOU for your very informative posting.
I'm sure many people who read your reply feel the same as me and appreciate
the info and your help.

Regards,

Rico Maxle
 



Re: Loooong distance IR detection, and IR collimation


Actually in practice, if you are using the half angle as the measure of
collimation, the tube you describe for a B1% 4B0% LED even assuming a
reflectivity of only 50% for glancing blows over 85B0% angle of
incidence will only reduce the 1/2 angle cone to approximately 3B0%.  If
you used a highly reflective material you would get practically no
change. That's a long way from the 1/2 angle cone of 1B0%.  My own
estimate is the tube would have to grow to at least 500 meters to
actually reduce the signal to a 1/2 angle of 1B0%.  Not too practical
and not much signal left.


Re: Loooong distance IR detection, and IR collimation


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a convex lens would be a good start.  the focus fo IR will be behind the
focus for visible light.

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Edmundson Electronics in the 80s made a 100m infrared security beam using a
pulsed signal between a single LED and a single photodiode they had plastic
lenses at both ends.

--

Bye.
   Jasen

Re: Loooong distance IR detection, and IR collimation


Take a look at

http://ronja.twibright.com /

I had my senior design students build this and the LED based link works
at about Ethernet for about 1 km. We had difficulty in making it
ethernet compatiable, and pointing the system.  We also didn't test for
a full KM, just indoors since the semester ended.

However the short answer is that about 1 km with 1 LED at 10 Mbps.

john muth


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