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'[ee]: photodetectors ( tag changed from OT)'
2002\01\16@100427 by Jon Baker

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I am working on a similar but slightly more complex project at the moment
which needs to detect 4 different wavelengths of light emitted from a sample
luminescing. The amplitude ( brightness??) of the light at each frequency
must be measured independently so I can draw 4 graphs- each frequency
corresponds to a particular molecule and the graph is to show the
concentrations ( brightness ) with time of each. I am positive this can't be
done with filters over photo detecetors becuase the number of photons I need
to detect is so small it would be absorbed into the filter. What would be
ideal is a several photodetectors each of which is sensitive to a
(different) narrow band of wavelengths. Does anyone have any pointers where
I could look for such specific devices?

I had thought of using a CCD, but with all 4 wavelengths being measured by
the same device I think it would be almost impossible to extract the 4 sets
of data I need from the RGB value the CCD would give me.

Any help much appreciated.

--
Jon Baker


{Original Message removed}

2002\01\16@110414 by Alan B. Pearce

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>I am working on a similar but slightly more complex project at the moment
>which needs to detect 4 different wavelengths of light emitted from a
sample
>luminescing. The amplitude ( brightness??) of the light at each frequency
>must be measured independently so I can draw 4 graphs- each frequency
>corresponds to a particular molecule and the graph is to show the
>concentrations ( brightness ) with time of each. I am positive this can't
be
>done with filters over photo detecetors becuase the number of photons I
need
>to detect is so small it would be absorbed into the filter. What would be
>ideal is a several photodetectors each of which is sensitive to a
>(different) narrow band of wavelengths. Does anyone have any pointers where
>I could look for such specific devices?

This is exactly the way it is usually done on satellites which are used for
earth resources research. I had dealings with an instrument being used to
check the upper atmosphere gasses which had 21 CCD's in it, each with its
own filter for the specific wavelength emitted by a specific gas. The
filters were done by a university group who were part of the instrument
team. Unfortunately I cannot give you any specific help on the subject
except to say it is probably worth trying to contact a university which has
involvement in this sort of field to see what expertise you may be able to
gain from them.

I would expect this sort of approach to work for you unless the spectral
lines are extremely close together.

As an after thought, have you considered trying to pass the emitted light
through a prism, and then using a linear CCD to look at the resulting
spectral line image? The only problem I see with this approach is that you
seem to think the light output will be that low, that filter losses will be
too great. If that is the case you are probably going to have to cool what
ever detector you use to minimise the internal detector noise. The
"standard" method on spacecraft is to use a helium pump to get close to
absolute 0.
;)

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2002\01\16@112229 by Douglas Butler

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If you are looking at a diffuse glow you will probably want four filters
over four detectors.  If your sample is small so you have a point source
of light you could use a prism into a linear CCD or onto four discreet
detectors.  You can't use a prism with a diffuse source because the
output will also be diffuse.

If you need extreem sensitivity and can't afford cryogenics, look at
photomultiplier tubes.  They can be pretty cheap on the surplus market
(there are several under US$10 at the moment on Ebay).

Sherpa Doug

{Quote hidden}

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2002\01\16@112410 by Jon Baker

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> I would expect this sort of approach to work for you unless the spectral
> lines are extremely close together.

I haven't got the wavelengths handy at the moment but I dont think they are
_that_ close
together so maybe CCD would be an option after all.

Probably have to use something more powerful than a (current) PIC to do
spectral analysis though :)

> As an after thought, have you considered trying to pass the emitted light
> through a prism, and then using a linear CCD to look at the resulting
> spectral line image?

Clever ******* :-) Thats definately worth looking into.

> The only problem I see with this approach is that you
> seem to think the light output will be that low, that filter losses will
be
> too great. If that is the case you are probably going to have to cool what
> ever detector you use to minimise the internal detector noise. The
> "standard" method on spacecraft is to use a helium pump to get close to
> absolute 0.
>  ;)

I may have to cool the detector and I am 'familiar' with helium cryogenic
pumps as featured on
shall we say .. satellites used for watching other peoples activity :)

Maybe I'll give the M.O.D research department a call and ask them if they
have a spare lying around :-) They probably
wouldn't be too obliging though. Last time I 'heard' anything they were
trying to use ultrasonic not-quite-so standing waves
to pump the helium around. Be interesting to know if they got anywhere with
that.

Thanks for the insights.

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2002\01\16@113904 by Alan B. Pearce

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>If you need extreem sensitivity and can't afford cryogenics, look at
>photomultiplier tubes.  They can be pretty cheap on the surplus market
>(there are several under US$10 at the moment on Ebay).

This may not be much use either, as the "snow" that you see in the image is
thermal noise, which will drop as the temperature is lowered.

In terms of dealing with cryogenics it may be enough to enclose the detector
in a thermal housing and then pour liquid nitrogen through some pipes by
gravity feed from a thermos. LN is very cheap and gets you a significant
amount below 0C.

If you do try this do be very careful in handling it because it is all too
easy to end up with skin burns.

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2002\01\16@114122 by David VanHorn

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>
>I may have to cool the detector and I am 'familiar' with helium cryogenic
>pumps as featured on shall we say .. satellites used for watching other
>peoples activity :)

I always wanted to paint a test target on my roof.
Squares, crosses at various sizes.  Maybe a cute message.


In your position, I would be very tempted to use a prism, combined with the
PMTs.
The physical size isn't small though, unless the wavelengths are pretty
much spread out.  You can put a slotted mask over the PMT, so that it only
looks at the right wavelength.

My father-in-law had a spectrometer that worked that way, though he had
more than a few photons lying around.  Process control in a steel mill.
"Yeah, toss in 10' of half-inch copper, and a couple of car batteries".

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2002\01\16@114339 by Alan B. Pearce

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>Last time I 'heard' anything they were
>trying to use ultrasonic not-quite-so standing waves
>to pump the helium around.

Hmm, my current project is to simulate a J-T cooler which uses piston pumps,
so they can develop software for a DSP in the drive electronics without
having to run an expensive-to-run cooler and risk damaging it. ;)

Did hear something about an ultrasonic method of pumping as well, but it was
going to be another project.

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2002\01\16@123254 by Kirk Lovewell

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A middle of the road solution for moderate sensitivity is to use a PMT in a
Peltier cooled housing, easy to get down around 0 degress without the hassle
of LN2 (Also easy to do the temp control with a PIC!)  This is also much
more feasible for a field installed monitoring application.

The solutions proposed cover a huge range of sensitivities and wavelengths,
you really need to do some rough calcs on the sensitivities before you can
choose a path.

I would recommend getting hold of a set of catalogs from Hamamatsu, they
make a huge range of photonic devices, CCD's, photodiodes, PMT's and lots of
useful info in their catalogs.

Kirk

> {Original Message removed}

2002\01\16@141905 by Josh Koffman

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What if you used a prism instead of a filter? Then place the
photodetectors at the correct output angle. I'm not that well versed in
optics, so please excuse me if this doesn't really work :)

Josh
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Jon Baker wrote:
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2002\01\16@192349 by ksutton

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Jon Baker wrote:

>I am working on a similar but slightly more complex project at the moment
>which needs to detect 4 different wavelengths of light emitted from a sample
>luminescing. The amplitude ( brightness??) of the light at each frequency
>must be measured independently so I can draw 4 graphs- each frequency
>corresponds to a particular molecule and the graph is to show the
>concentrations ( brightness ) with time of each. I am positive this can't be
>done with filters over photo detecetors becuase the number of photons I need
>to detect is so small it would be absorbed into the filter. What would be
>ideal is a several photodetectors each of which is sensitive to a
>(different) narrow band of wavelengths. Does anyone have any pointers where
>I could look for such specific devices?
>

Alternately, why not use a prism to separate the light frequencies to
focus on four separate spots and position four photo multipliers, one at
each spot to measure the separate colors?  It's an "old tech" solution
but a good one.

- Karen

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2002\01\17@042151 by Russell McMahon

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> >Last time I 'heard' anything they were
> >trying to use ultrasonic not-quite-so standing waves
> >to pump the helium around.
>
> Hmm, my current project is to simulate a J-T cooler which uses piston
pumps,
> so they can develop software for a DSP in the drive electronics without
> having to run an expensive-to-run cooler and risk damaging it. ;)
>
> Did hear something about an ultrasonic method of pumping as well, but it
was
> going to be another project.


I would be interested in hearing about the JT cooler - perhaps under a
different [EE] topic tag to avoid confusing this thread. What sort of size,
what piston pumps are being used? JT is usually a largish scale industrial
process and requires rather high pressures to be useful (2000 psi+ typical)
so I would be interested in hearing if this is small or lower pressure.
Gifford-McMahon (no relation :-) ) (poor man's Stirling) lends itself more
to smaller systems.


       Russell McMahon

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2002\01\17@045252 by Vern Jones

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A method used for photoastronomy using CCDs, is to use a Peltier cooling
bank taking the CCD array down to between -40 - -50 C, then integrating
the exposure time over 10 to 60 seconds, in the process there is a dark
frame to zero out the detector, and a flat field process is used to
balance out the detectors photo response. This way with even a very few
photons, filters or prisms will work, unless you need high speed
detection.

Vern Jones

"Alan B. Pearce" wrote:
{Quote hidden}

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2002\01\17@090907 by Michael Vinson

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Josh Koffman wrote, in part:
>What if you used a prism instead of a filter? Then place the
>photodetectors at the correct output angle.

I'm puzzled by all this talk of prisms. A diffraction grating works
much better, at least in all the (physics) applications I've
worked with. Regarding another comment someone made, you *can*
use a diffraction grating with a diffuse source, you just need to
send the light through a slit and maybe some additional optics.

Michael V

Thank you for reading my little posting.


_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp.

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2002\01\17@092533 by Douglas Butler

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You can turn a diffuse source into a point source with a slit, but you
lose most of your light doing it.

Also, I built the electronics portion of a CCD spectrometer a few years
ago, a Harvard physics PHD did the optics.  He chose a compound three
element prism over a grating but I never got a good answer why.
Sometimes he claimed it made the calibration math easier.  Other times
it seemed to just be a visceral hatred of gratings!  What are the
relative merits of prisms vs. gratings?

Sherpa Doug

> {Original Message removed}

2002\01\17@100828 by Roman Black

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Jon Baker wrote:
>
> I am working on a similar but slightly more complex project at the moment
> which needs to detect 4 different wavelengths of light emitted from a sample
> luminescing. The amplitude ( brightness??) of the light at each frequency
> must be measured independently so I can draw 4 graphs- each frequency
> corresponds to a particular molecule and the graph is to show the
> concentrations ( brightness ) with time of each. I am positive this can't be
> done with filters over photo detecetors becuase the number of photons I need
> to detect is so small it would be absorbed into the filter. What would be
> ideal is a several photodetectors each of which is sensitive to a
> (different) narrow band of wavelengths. Does anyone have any pointers where
> I could look for such specific devices?
>
> I had thought of using a CCD, but with all 4 wavelengths being measured by
> the same device I think it would be almost impossible to extract the 4 sets
> of data I need from the RGB value the CCD would give me.
>
> Any help much appreciated.


Maybe some type of specialised prism, to separate
the light frequencies, with very little gain
lost. If you need a string of light sensors old faxes
have a linear element light sensor, basically one
sensor for each pixel, a prism in front of one of
these might be a start, something like a simple
mass spectrograph.
-Roman

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2002\01\18@121614 by Jon Baker

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The wavelengths I am looking for are:

545nm 570nm 600nm and 620nm if this might make any difference to the best
way to proceed..

> Jon Baker wrote:
> >
> > I am working on a similar but slightly more complex project at the
moment
> > which needs to detect 4 different wavelengths of light emitted from a
sample
> > luminescing. The amplitude ( brightness??) of the light at each
frequency
> > must be measured independently so I can draw 4 graphs- each frequency
> > corresponds to a particular molecule and the graph is to show the
> > concentrations ( brightness ) with time of each. I am positive this
can't be
> > done with filters over photo detecetors becuase the number of photons I
need
> > to detect is so small it would be absorbed into the filter. What would
be
> > ideal is a several photodetectors each of which is sensitive to a
> > (different) narrow band of wavelengths. Does anyone have any pointers
where
> > I could look for such specific devices?
> >
> > I had thought of using a CCD, but with all 4 wavelengths being measured
by
> > the same device I think it would be almost impossible to extract the 4
sets
{Quote hidden}

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2002\01\18@124328 by Martin Peach

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----- Original Message -----
From: "Jon Baker" <TakeThisOuTjonEraseMEspamspam_OUTHAYSEED.NET>
To: <RemoveMEPICLISTspamTakeThisOuTMITVMA.MIT.EDU>
Sent: Friday, January 18, 2002 12:12 PM
Subject: Re: [EE]: photodetectors ( tag changed from OT)


> The wavelengths I am looking for are:
>
> 545nm 570nm 600nm and 620nm if this might make any difference to the best
> way to proceed..

You could probably find LEDs with peak output very close to those
wavelengths (many flavours exist in the yellow through red range).
Connect a LED with anode to ground and measure current at the cathode caused
by photons hitting the LED. You will need to amplify the signal a lot with
op-amps. I have not tried this...You may end up just measuring thermal noise
and ionizing radiation events.
/\/\/\/*=Martin

{Quote hidden}

measured
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2002\01\18@140509 by downes

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> > 545nm 570nm 600nm and 620nm if this might make any difference to the best
> > way to proceed..
>
> You could probably find LEDs with peak output very close to those
> wavelengths (many flavours exist in the yellow through red range).
> Connect a LED with anode to ground and measure current at the cathode caused
> by photons hitting the LED. You will need to amplify the signal a lot with
> op-amps. I have not tried this...You may end up just measuring thermal noise
> and ionizing radiation events.
> /\/\/\/*=Martin

Yes, in theory LEDs will work as (bad) photodiodes but the problem here is that,
assuming they are in a colorless package, they will respond to any wavelength
shorter (higher energy) that the wavelength they are designed to emit.  So a 620nm
LED (if they exist) will respond to all the (above) required wavelengths....  You could
work-out some subtraction scheme but it might get a bit messy...... :-(

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2002\01\19@073307 by Vispanathan Naicker

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I have , in my efforts to increase the reception range of my IR remote ,
bunged the ordinary LED in the breadboard by mistake - and sat puzzling what
happened to the range - the circuit then only received signal from within1/2
metre. Also , another buggy circuit where I was just trying for maximum gain
out of an electret microphone/amp ( for listening to the neighbours ) , and
turned the radio on low  .... but the blip bip was heard in my headphones -
so I narrowed down the remotes ir beam to the electret - it was receiving
the signal . I never investigated properly so I don't know if it was
sensitive to the IR or was receiving some other? sort of Electromagnetic
radiation .

Vis Naicker

======= in reply to ========
= Martin Peach  :
= I have heard and theoretically it should be true that LEDs work
'backwards'
= as well...as photodiodes. You 'just' need to amplify and measure the
output
= current of different coloured LEDs. But it may not be anywhere near the
= sensitivity needed, I've never tried it.
========================

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