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Posted by on May 10, 2006, 10:18 am
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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
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Posted by Shawn B. on May 10, 2006, 10:28 am
Please log in for more thread options
Rico Maxle wrote:
> 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
>
>
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
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Posted by Phil Hobbs on May 10, 2006, 10:45 am
Please log in for more thread options Rico Maxle wrote:
> 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?
Sure. Lots. Black paint, for example. It won't help you.
>
> 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?
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.
>
> 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?
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
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Posted by Winfield Hill on May 11, 2006, 7:02 am
Please log in for more thread options Phil Hobbs wrote...
>
> Rico Maxle wrote:
>>
>> Is it possible to collimate the IR light source ... to 2degrees?
>> OR is there a way of creating very narrow IR light cone ...
>
> 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.
[ snip ]
> When you find out how frightfully bad your SNR will be, you can
> improve it in several ways:
> 5. Laser sources
There's Rico Maxle's answer, use an IR laser.
--
Thanks,
- Win
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Posted by Louis Boyd on May 10, 2006, 1:31 pm
Please log in for more thread options 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:
> 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
>
>
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>
> 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
>
>