detecting laser beam in sunlight

P

pave

Guest
I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
laser beams. It is working somehow but I have one Major problem.
Sunlight is disturbing my detector because the diode I'm using is more
sensitive at 850nm. I have used red perspex to filter those infrared
wave-lengths but it is not enough when there is a very sunny day. The
detector receives hits from sunlight.

Have you guys solved this kind of problem and how? Should I use
different kind of photodiode or perspex? I haven't found very good
alternatives.
Or could it be solved by electronics?

Thanks!
 
gratis@jippii.fi (pave) writes:

I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
laser beams. It is working somehow but I have one Major problem.
Sunlight is disturbing my detector because the diode I'm using is more
sensitive at 850nm. I have used red perspex to filter those infrared
wave-lengths but it is not enough when there is a very sunny day. The
detector receives hits from sunlight.

Have you guys solved this kind of problem and how? Should I use
different kind of photodiode or perspex? I haven't found very good
alternatives.
Click to expand...
You can buy narrow-bandwidth optical filters to remove the sunlight
but let the laser through. But I think these are expensive.

Or could it be solved by electronics?
Click to expand...
The "electronic" solution would be to modulate the lasers. You then
put the photodiode signal through an electronic filter that only
accepts the modulation frequency.


--

John Devereux
 
On 25 May 2004 05:22:53 -0700, gratis@jippii.fi (pave) wrote:

I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
laser beams. It is working somehow but I have one Major problem.
Sunlight is disturbing my detector because the diode I'm using is more
sensitive at 850nm. I have used red perspex to filter those infrared
wave-lengths but it is not enough when there is a very sunny day. The
detector receives hits from sunlight.

Have you guys solved this kind of problem and how? Should I use
different kind of photodiode or perspex? I haven't found very good
alternatives.
Or could it be solved by electronics?
Click to expand...
The usual way is to modulate the laser beam in some way, and build a
receiver that is (mostly) only sensitive to that modulation. The
sunlight isn't modulated, and will be (partially) rejected.

A suitable modulation would be 100% AM at several kHz. (Or perhaps
much faster, depending on the speed of your rotating laser beams.)

Please read this (long) sci.electronics.design thread about an IR link
with a double conversion superhet receiver:
http://groups.google.com/groups?threadm=JyhW7.91717%24pa1.29931771%40news3.rdc1.on.home.com

Regards,
Allan.
 
On Tue, 25 May 2004 23:01:49 +1000, Allan Herriman
<allan.herriman.hates.spam@ctam.com.au.invalid> wrote:

Please read this (long) sci.electronics.design thread about an IR link
with a double conversion superhet receiver:
http://groups.google.com/groups?threadm=JyhW7.91717%24pa1.29931771%40news3.rdc1.on.home.com
Click to expand...
On closer inspection, Win's circuit (post #9 in the thread) actually
modulated the light with a 2.5kHz carrier with 250Hz BPSK.

Regards,
Allan.
 
In article <74457dfa.0405250422.33c7b8e@posting.google.com>,
gratis@jippii.fi (pave) wrote:

I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
laser beams. It is working somehow but I have one Major problem.
Sunlight is disturbing my detector because the diode I'm using is more
sensitive at 850nm. I have used red perspex to filter those infrared
wave-lengths but it is not enough when there is a very sunny day. The
detector receives hits from sunlight.

Have you guys solved this kind of problem and how? Should I use
different kind of photodiode or perspex? I haven't found very good
alternatives.
Or could it be solved by electronics?

Thanks!
Click to expand...
Try a differential receiver. One photodiode is exposed to the laser and
the sunlight, the other is exposed to just the sunlight. Detect the
diffference between the two. This won't work if the sunlight saturates
the photodiodes.

Al

--
There's never enough time to do it right the first time.......
 
John Devereux wrote:
gratis@jippii.fi (pave) writes:


I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
laser beams. It is working somehow but I have one Major problem.
Sunlight is disturbing my detector because the diode I'm using is more
sensitive at 850nm. I have used red perspex to filter those infrared
wave-lengths but it is not enough when there is a very sunny day. The
detector receives hits from sunlight.

Have you guys solved this kind of problem and how? Should I use
different kind of photodiode or perspex? I haven't found very good
alternatives.


You can buy narrow-bandwidth optical filters to remove the sunlight
but let the laser through. But I think these are expensive.
Click to expand...
Interference filters are tricky in that they are angle sensitive, which
will let more sunlight in than expected from their transmission graph.
And since diode lasers don't have very tight wavelength tolerances, the
filter may need some angle tuning to maximize transmission of the
lasers' wavelength.

If the two wavelengths are going to the same detector, than an
interference filter can't be used.

In either case some good long wave pass and short wave pass filters can
be stacked together to make as narrow a bandpass as possible. Such
Colored glass filters don't have angle tuning issues. But they will
attenuate the signal quite a bit along with the sunlight.

A filter stack combined with the differential method suggested below by
Al might do a good job if fast response is needed without needing to
resort to modulation.



--
_____________________
Christopher R. Carlen
crobc@earthlink.net
Suse 8.1 Linux 2.4.19
 
gratis@jippii.fi (pave) wrote in message news:<74457dfa.0405250422.33c7b8e@posting.google.com>...
I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
laser beams. It is working somehow but I have one Major problem.
Sunlight is disturbing my detector because the diode I'm using is more
sensitive at 850nm. I have used red perspex to filter those infrared
wave-lengths but it is not enough when there is a very sunny day. The
detector receives hits from sunlight.

Have you guys solved this kind of problem and how? Should I use
different kind of photodiode or perspex? I haven't found very good
alternatives.
Or could it be solved by electronics?

Thanks!
Click to expand...
Red acrylic 'perspex' is absolutely transparent to near IR. It blocks
only blue and green wavelengths, so helps only a little with sunlight.
However, it can be used in conjunction with a 'hot mirror', which is
a kind of short wavelength pass dielectric filter. There is a piece
of hot mirror filter glass over most video camera chips, or you can
buy some from scientific suppliers. Alternatively, use a red filter
with 'heat absorbing glass'. This is a slightly greenish appearing
material, usually about 5 mm thick. You can salvage a piece of the
latter out of any old slide projector.

Paul Mathews
 
Make a densitometer.

Take the appropriate IRleds, and a phototransistor, and set up the circuit
so that an op-amp drives the LED to get a specific current out of the
phototransistor.
Set up the mechanicals to allow samples of the material to be placed between
the emitter and detector.
Measure the LED drive current relative to the current with just air, and you
have the density of the material.

Plastic dyes that have a lot of carbon, absorb IR.
Exposed photographic negatives make a pretty passable IR pass filter.

I used this to investigate a plastics substitution problem, where the stuff
they used turned out to be "frosted glass" for IR. The stuff I specified
was IR Opaque, and of course the substitution caused a problem.
 
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