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not_important
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PbSe is good from ~1 um out to 4 um at room temperature, out to 5.5 to 7 um with cooling to -50 C down to LN2. Cooling also boosts the sensitivity by
an order of magnitude.
Some characteristic curves
http://www.lasercomponents.de/uk/fileadmin/user_upload/home/...
It is easier to move a mirror or grating, usually done as a rotation, than to move the detector, which will have to follow an arc that traces the
focal plane.
And trust me on this, it's not just measuring while moving, but changes in resistance and noise pickup caused by moving even after stopping, that
make waving a hi-Z detector about less than desirable. Remember, you're working with a detector with a resistance of 100K to several meg, a bias
voltage of several 10s to several hundred volts, and an amplifier front end with an input Z of 10s to a couple hunder megs.
For PbSe, a chopping frequency around 1 KHz works well. Use a PLL to extract the actual frequency from the amplified signal, and drive a synchronous
demodulator with the PLL followed by a low pass filter to recover the 'DC' signal.
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LSD25
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OK, I'm currently looking for details on the feasibility of moving the mirror/grating, the trouble is the horrid lack of comprehension on my part.
The reason for trying to avoid cooling is simply that to do so will really make this a lot harder for me to do and probably for a whole lot of others
to even contemplate trying.
http://www.cappels.org/dproj/syncdet/syncdet.html
To separate the wheat from the chaff so to speak, or to isolate the signal from the noise. You suggest this with a PL Loop? Here is a google-book on
the theory, so at least we are on the same page:
http://tinyurl.com/35kamv
Now for mine, I suspect that this one is getting closer to where we want to go (they even provide sample code):
http://www.edn.com/archives/1998/061898/13di.htm
And here is an article pointing us to where to find this circuitry essentially ready-made:
http://www.radio-electronics.com/info/receivers/synchdet/syn...
So, as my motto is 'why buy when I can make and why make when I can modify...', can I yank the ready made circuitry out of something else in order to
make this project as simple as is humanly possible?
Whhhoooppps, that sure didn't work
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not_important
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PLL - like this:
http://www.uoguelph.ca/~antoon/gadgets/pll/pll.html
http://www.fairchildsemi.com/pf/MM/MM74HC4046.html
http://yyg.stanford.edu/projects/rangefinder/LM565.pdf
sync demod
http://www.analog.com/en/prod/0,,773_862_AD633%2C00.html
http://www.analog.com/en/prod/0,,773_862_ADL5391%2C00.html
For gain blocks, ordinary op amp should do nicely. You could also use switched capacitor filter chips for a a signal bandpass and the output lowpass,
although ordinary R-C filtering will likely be enough for the LP filter.
http://para.maxim-ic.com/cache/en/results/4431.html
This is all audio frequency stuff, there are also chips use for RF that will not work at low enough frequencies for your application. There may be
something out there that already has the combination of functions on it, but the IC building blocks make it pretty easy to do.
Wow, I wasn't aware what bad shape the US dollar was in. When I was working with this stuff a decade ago, these were all jelly bean parts, the
budgetary pricing at the hundreds level was a little over a dollar on down.
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LSD25
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Do you perhaps know of some way to source PbSe photodiodes that would be more effective than what I have found so far? I mean, and this extends way
beyond the continuing argument over pro or contra drug chemistry, this could be of serious moment to virtually everyone on this site if only it can be
made to work.
It is just that what appeared to be attainable the other day, is really not that easy on further research. It is potentially not so much that the
photodiodes aren't available in the suggested equipment, it is more that there is very little data on how to know which equipment contains the same.
PS I am not looking for specific suppliers, it is more that I need a suggestion as to where they can be found more generally.
Whhhoooppps, that sure didn't work
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microcosmicus
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While it is directed towards infrared thermometry, this page contains lot of useful
data about detector sensitivities, absorption of materials, instrument design and
the like which should be just as useful in building a spectrometer:
http://www.omega.com/literature/transactions/volume1/thermom...
As for where to find PbSe devices, maybe have a look at flame detectors and
thermometers because I have seen them mentioned in that context. I
even came across mention of one in an alcohol detector.
[Edited on 21-2-2008 by microcosmicus]
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not_important
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For NIR purposes flame detectors are indeed the most likely source.
Contactless/remote temperature sensors usually use micro thermocouple or bolometer elements, and include a filter that cuts off wavelengths short than
some value, generally 4.5 to 5.5 um. This would almost work in traditional scanning IR instruments, although the short wavelength cutoff is a
problem.
Contactless sensors intended for metal working or ceramics applications might be another source for NIR sensors, above 400 C emissivity at wavelengths
shorter than 3 um becomes significant so the detectors used will be sensitive to shorter wavelengths than the more general purpose ones.
Low cost sensors for flame detection and temperature measurement often are based on lithium tantalate. These are robust but lower sensitivity, an
order of magnitude or two less sensitive that the sensors typically used in IR-type analytical instruments. Because they are directly sensing the
emitter, flames or hot material, the lower sensitivity in not much of a problem. In dispersive spectrometers only a small fraction of the IR
emitter's light is hitting the sensor, just a narrow band of wavelengths, and that gets reduced further by the absorption of the sample being
analysed. This can drop the signal right down into the noise level,
This book looks like it might be a useful reference on this general topic
http://www.blackwellpublishing.com/book.asp?ref=1405121033
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LSD25
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So you mean something like these:
http://www.accutherm.com.au/html/test_products_international...
This is not to say that I have used or would use this supplier, I just want to know about the range of products offered and whether any of the same
are useful.
The range of the infrared dealt with by these instruments is from 7µm-14µm on average. I assume the cheaper instruments of this type which are
capable of measuring the same range of temperatures would also use this range of wavelengths?
The ones that utilise a laser to emit IR radiation and then measure the return, are they of interest or not?
Finally, is it possible to remove the filter (I assume there is a filter which cuts off the emmissions below 7µm? or is this likely to be the minimum
wavelength of the sensor?).
PS This would be the far-IR would it?
Is there any absorbence data in this range that could be utilised?
What is the approximate range of the wavelengths used in IRDA? What about the wavelengths used by those IR tracking devices &/or the IR testing
devices?
Lastly, what about the IR thermometers with the massive range - the ones that can measure from 30C-1700C, wouldn't they utilise the lower wavelengths?
Would it be possible to use NIR & FIR to generate a reasonable range of absorbence, the difficulties involved in finding a low-cost MIR sensor are
beginning to look prohibitive...
Whhhoooppps, that sure didn't work
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not_important
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| Quote: | Originally posted by LSD25
So you mean something like these:
http://www.accutherm.com.au/html/test_products_international...
This is not to say that I have used or would use this supplier, I just want to know about the range of products offered and whether any of the same
are useful.
The range of the infrared dealt with by these instruments is from 7µm-14µm on average. I assume the cheaper instruments of this type which are
capable of measuring the same range of temperatures would also use this range of wavelengths? |
Yes, the range is used in part because the blackbody radiation curves for the temperature range is always a slope increasing as the wavelength
decreases, and higher temperatures are brighter than lower ones so a simple 'brightness' reading can be used as a measure of temperature.
Another reason for that range is that it is basically the transmission range of silicon, which as a hard, fairly chemically inert, and reasonably
temperature tolerant material makes good windows for the detectors.
| Quote: | | The ones that utilise a laser to emit IR radiation and then measure the return, are they of interest or not? |
The laser is just for targeting, as with gunsights.
| Quote: | | Finally, is it possible to remove the filter (I assume there is a filter which cuts off the emmissions below 7µm? or is this likely to be the minimum
wavelength of the sensor?). |
Some of the sensors look like the filter is part of the casing, on the inside surface of the cap. Those might be difficult as the sensors are small
and you'd be working close to the actual sensor. Also some sensor materials are sensitive to water and/or air, if you didn't know what the sensor was
you'd be taking a gamble.
Some sensor materials have response curves not much wider than the filter, others are much wider. The thermopile type have their range mainly
determined by the 'blackness' of the hot junctions for a given wavelength. Some of the others become transparent at various IR wavelengths. Again,
you'd need to know the sensor type to say.
I noticed that some suppliers of these sorts of sensors offered a range of window materials, with overlapping ranges but overall covering noticeably
more than 7-15 um.
| Quote: |
PS This would be the far-IR would it?
Is there any absorbence data in this range that could be utilised? |
Long wavelength IR or IR-C might be better, because far IR often means beyond 15 um. In astronomy it falls within the midrange IR
This is roughly wave numbers of 1400 to 700. In that range are many of the carbon-carbon bond bands, C-F, Cl-C-H, C-SO3 salts, sulfamides, sulfates
organic and inorganic, some C-H, and -O- . This range is sometimes called the "fingerprint region", because the bands are affected by other parts of
the molecule, giving patterns that at somewhat distinctive of a certain compound.
But you're still missing important parts, you want to go to at least 4 um if not even shorter. This range contains the generic bands for classes of
bonds; 2 to 4 um is O-H, N-H, C-H, and similar, 4 to 5 um is CC and CN triple bonds plus H-C where the carbon is also double bonded, 5 to 7 is C=C,
C=O, C=N, and certain C-C single bonds
charts of bands
http://www.kayelaby.npl.co.uk/chemistry/3_8/3_8_5a.html
http://www.kayelaby.npl.co.uk/chemistry/3_8/3_8_5b.html
| Quote: |
What is the approximate range of the wavelengths used in IRDA? What about the wavelengths used by those IR tracking devices &/or the IR testing
devices? |
IRDA : 850 to 900 nm
trackers - depends on which generation. 2 to 5 um for hot objects, 300 to 1600 C, including the 4,2 um CO2 emission band in jet exhaust. Newer ones
also use 8-13 um, cooler targets and in an atmospheric window. Remember the sky is rather dark in the IR; Rayleigh scattering goes down as the
wavelength increase, and space is cold looking like -50 to -150 C.
IR testers? Likely the 7-15 um thermal range, never really looked into them.
| Quote: | | Lastly, what about the IR thermometers with the massive range - the ones that can measure from 30C-1700C, wouldn't they utilise the lower wavelengths?
|
They might be "two colour" devices, looking at two different bands to handle the shifting of the blackbody curves.
| Quote: | | Would it be possible to use NIR & FIR to generate a reasonable range of absorbence, the difficulties involved in finding a low-cost MIR sensor are
beginning to look prohibitive... |
The problem is that cheap sensors means high volume production, which also means many devices sold and eventually junked.
Optics can be expensive as well, especially for the wider bandwidth used by conventional investigative (lab) IR. While visible light spans less than
an octave, lower cost UV-Vis a range of 3:1 and better UV-Vis a half decade, NIR a 3:1 range, full analytic IR goes from 3 to 15 um - 5:1 range - or
better. And many materials exhibit absorption bands somewhere in that range. Mirrors can show appreciable changes in reflectivity, air itself gets
in the way to a degree.
--------------------------------------------------------------------------
An alternative that you may want to do some serious reading on is Raman spectroscopy. It yields information quite similar to conventional IR
spectroscopy, but rather than bands at fixed wavelengths it uses re-emitted photons that have had their frequencies shifted by small amounts related
to the particular local molecular structure. And it can work with aqueous solutions.
Postage-stamp overview - shine an intense beam of monochromatic light at or through a substance, capture some of the light coming off at 80 degrees to
the beam. Block the wavelength of the original beam, run the rest through a spectroscope and look at the spectrum.
The shifts are such that with yellow to Really Near IR light the shifted frequencies stay within the range silicon diodes sense, meaning scanner and
camera CCDs would function as detectors. As the scattered light is very weak, the smaller the area of the detector the less background ionizing
radiation adds to the noise, so scanner CCDs might be better.
The holographic blocking filter might be expensive, the gratings for dispersing the light are also not real cheap. But the rest of the optics should
be conventional, glass lenses and so on. CD (640 nm) or DVD (780 nm) lasers might do as the light source, as might the lasers in laser printers
(diodes @ 655 or 790 nm). The scattered light is longer wavelength than the laser.
The volume phase holographic gratings favoured for these applications are expensive, but also are possibly within the reach of amateur production.
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LSD25
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Like this one:
http://www.umich.edu/~morgroup/virtual/labeled/virtual.html
Here is another basic introduction:
http://carbon.cudenver.edu/public/chemistry/classes/chem4538...
The first one is probably the most interesting, it describes in detail how it works and describes the use of a, what appears to be, fairly normal IR
camera. My question with this is, how and why would we need a holographic grating? Couldn't we use a normal grating, like the one in DVD/CD systems?
This coupled with a laser from a DVD/CD and then using a Si/CCD from a scanner would appear to be the easiest, lowest tech, option for home use.
The only question is, would it work? From what I understand the spectra would not be anywhere near as well organised as the one in the link, although
if we used a laser as the excitor, then the monochromatic beam should exclude any need for a filter?
Yes, I agree this might well be the best option for the home chemist.
Scratch that about the holographic grating, they are hardly expensive:
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm...
Also, I assume that by carefully selecting the incident laser light, the Raliegh scattering could surely be excluded - given that surely some lasers
are used more than others and surely there is limited wavelength filters available to exclude them?
I don't know for sure, but that is what I am now going to look for.
[Edited on 23-2-2008 by LSD25]
I just found something which makes me wonder if the makers of the infrared thermometers with the attached IR laser device which measures the reflected
light haven't already dealt with this problem, simply because this book:
http://tinyurl.com/3c79y7
Suggests that they must be filtering out the wavelength coinciding with the IR radiation they illuminate the article with.
Also, would it be correct to suggest that if we used a narrow wavelength of light - like a true red LED and then a true red filter, we would
substantially remove anything in the red range? If for instance we did this again with a blue LED and a blue filter we would have something to compare
the red LED spectra with, that which was ommitted by the red & blue filters could then be worked out by reference to the other spectra?
I don't know, but by filtering out the wider wavelengths, the entirety of the Raliegh scatter from each colour ELD should then be excluded? That which
is not Raliegh scatter from each should then be present in the spectra from the other colour of LED should it not? Just a wild theory anyway...
PS Have a good look at all the links on that first one with the CCD camera, it goes right into the necessary design features and is fucking brilliant.
[Edited on 23-2-2008 by LSD25]
Whhhoooppps, that sure didn't work
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not_important
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The gratings in CD/DVD systems are specialised for beam generation needed for that application, they are not GP diffraction gratings.
Those hologratings are replica gratings and likely don't work well enough for this application, although they are good for conventional spectroscopes.
See
http://www.edmundoptics.com/onlinecatalog/DisplayProduct.cfm...
http://www.edmundoptics.com/onlinecatalog/displayproduct.cfm...
for the types used for NIR Raman work.
LEDs are not that narrow band, nor are the intense enough. The scattered light is very weak, you need a intense light source to get useful amounts of
scattered light. Before lasers this would be done with an array of mercury vapour discharge lamps enveloping the sample.
Because the shifted lines are so weak, you need very good grating to avoid having the excitation wavelength light scattered by the grating from
contaminating the spectrum, swamping it out. That's also why a narrow band blocking filter is used, 10 to 20 nm wide with less than 5 to 10 nm going
from OD 0,3 to 4,0 (50% to 0,01% transmission), to block unshifted laser light from the analyser.
A similar narrow bandpass filter is used to clean up the laser light before it illuminates the sample. Many lasers have small side bands, which would
confuse the resulting spectrum.
The umich people are using a packaged laser, possibly with a cleaning filter as they label a filter holder in the path from laser to sample. That's
also a fairly powerful laser, | Quote: | Laser:
The laser is the source of the light used to induce the Raman effect. In these experiments, the laser is a Neodynium Yag laser (often abbreviated as
"Nd:YAG") which emits light at a wavelength of 532 nm. The intensity of the light can often be around 0.5 Watts.
TECHNICALLY SPEAKING: This is a flash-lamp pumped, mode-locked, doubled Nd:YAG which weighs more than most of the graduate students in the lab and
works sometimes only when it wants to. Plans are to upgrade to a diode-pumped 2 Watt laser sometime in the future. |
or class 3B to 4 - eye protection, no shiny objects in the lab including watches and jewelry. The LSO would write you up if you
forgot those rules on class III and up lasers, and tell you stories of people hearing a "crack" as the beam fried their eye.
Note that they use a holographic notch filter and holographic grating, which may be a VPH one similar to the second set in the Edmonds links I gave.
The filters used in contactless temperature measurement are very broad band. For devices with IR illumination, they are using the illumination to
determine the emissivity of the surface in the IR, I suspect the equipment just turns the laser on to calibrate, then off to measure. Alternating
those states would give a signal whose high/low values are laser+thermal and just thermal, allowing continuous calibration and monitoring.
Ah, LN2 cooling on their CCD, and they are working in the visible range so it is just for noise reduction meaning really low light levels.
notch filter example attached
Attachment: 1050.pdf (275kB)
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LSD25
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Hang on a minute,
Don't most CCD camera's come straight out of the box with IR filters already installed?
I am unsure what this would give in the scattered light side (the stuff we want), but you were saying that this would be a shorter wavelength than the
incident light?
So, thereotically at least, if we could match a simple laser type device to the bottom end of the IR filter on CCD cameras to start with, this should
remove the Raleigh scatter and the reflected incident light as well, leaving only the shorter wavelength ('Stokes shifted'?) light to be recorded
(what wavelengths would we be talking about?).
Would this work sufficiently to allow us to escape having to purchase a notch filter? It is just that that would probably be the single most expensive
part of the entire system. This may require, given the crude nature of the available laser devices, the building of an adjustable monochromator
(tuneable mirrors, a grating, etc. same as would be used in an IR diffraction device), but that looks to be feasible (just). This would allow for
greater choice in light sources, which could then be narrowed down to the appropriate wavelength while the matching of this incident light to the
bottom end of the existing IR filter would thereotically mean that the built-in filter would remove the problems?
Whhhoooppps, that sure didn't work
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not_important
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| Quote: | Originally posted by LSD25
Hang on a minute,
Don't most CCD camera's come straight out of the box with IR filters already installed?
I am unsure what this would give in the scattered light side (the stuff we want), but you were saying that this would be a shorter wavelength than the
incident light?
So, thereotically at least, if we could match a simple laser type device to the bottom end of the IR filter on CCD cameras to start with, this should
remove the Raleigh scatter and the reflected incident light as well, leaving only the shorter wavelength ('Stokes shifted'?) light to be recorded
(what wavelengths would we be talking about?).
|
No, normally the Stokes scattered light, longer wavelength than the source, is used as it is much more intense than the anti-Stokes at ordinary
temperatures.
Yes, digital cameras do generally have IR filters on them, but those are not sharp cutoff filters and useless for this application, ignoring them
"facing the wrong way".
| Quote: | | Would this work sufficiently to allow us to escape having to purchase a notch filter? It is just that that would probably be the single most expensive
part of the entire system. This may require, given the crude nature of the available laser devices, the building of an adjustable monochromator
(tuneable mirrors, a grating, etc. same as would be used in an IR diffraction device), but that looks to be feasible (just). This would allow for
greater choice in light sources, which could then be narrowed down to the appropriate wavelength while the matching of this incident light to the
bottom end of the existing IR filter would thereotically mean that the built-in filter would remove the problems? |
You can use a monochromator to clean up the laser before illuminating the sample. If you use real good gratings you can use multiple monochromators
to extract the spectrum information. But a notch filter makes it easier; the Stokes scatter is 10^6 to 10^8 times less in intensity that the Raleigh
scatter.
Note that most instruments that use multiple monochromaters seem to be scanning ones, with a single detector and moving parts.
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LSD25
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I am trying to cope with the start of uni at present and I am having time-issues as it is (I still have over 10 projects ongoing). I would just like
to know have you had any success finding a cheaper, useable variant of the notch filter or alternatively, the PbSe photodiode?
I am going to have to order one of these in and I have to think very carefully which is going to be the easiest and most likely outcome. At a pinch, a
few overtimes and I could probably afford both in time.
I think for the moment I will try and make a working monochromator, or a couple of the same - so that if I manage to get either (especially the PbSe
diode) I have a good headstart on the scanning part of the proposed equipment.
If anyone reading this has any ideas to contribute, especially on where to find a PbSe photodiode or a useful notch filter in Oz, please feel free...
{EDIT}
I was just reading up on the theory of the bolometers(?), the idea is that the sensor includes an IR absorbent material which, when it absorbs IR
radiation emits a miniscule amount of heat which is read by a secondary part of the sensor and which can then determine what and how much radiation
was absorbed depending upon the nature of the absorbent material and the wavelength of light is was irradiated with.
I was thinking to myself, hmmm, how to use this to build a sensor for IR spectroscopy... Perhaps if I used an aldehydic material as the sensor, it
would pick up and absorb the light... Nah, wouldn't work, BUT
What if the heat sensor was mounted immediately adjacent to the sample to be irradiated and a tuneable monochromator was used to vary the wavelength
of light the sample was irradiated with? Heat sensors may be somewhat easier to design and build, the sample will absorb IR radiation and will emit
heat. The nature of the monochromator could then allow us to determine (probably by extensive calibration with known compounds) what wavelength of
light is emitted at what mirror rotation position(s)?
Of course this will be slower than any other approach, although I suspect that the heat increase would be miniscule and could probably be factored in
if a heatsink and fan were used on the casing.
The thing is, that by measuring the heat emission caused by absorbance of IR radiation, we are actually measuring absorbence, not (as in most IR
spectroscopy) the level of the remaining IR radiation passing through the sample AFTER absorbence by the sample.
[Edited on 25-2-2008 by LSD25]
The design of microbolometer type spectroscopy should, if it is feasible, probably use low-cost thermistors with a measurable change in resistance
with a 0.1C increase in temperature (I think this is feasible). This would be mounted series with another thermistor backing IR transparent material
(dunno, how about NaCl/KBr?). The difference in the resistance would be the heat increase of the first less noise. If another was mounted as well,
with IR opaque (ie. full absorbence) material, the percentage difference between the 1st & second will be able to be worked out by expressing it
as a percentage of the difference in resistance between the second (no increase) and third (maximum increase).
The DOD type of microbolometers use Si as the heat sensor. This is possibly as cheap as it is going to get.
From what I understand, microbolometers of this type are not normally cooled (that is why they are so bloody big at present). The change in resistance
would allow us to get a clear, electrical signal, which when plotted with the known position of the tuning mirrors (and the wavelength(s) attributed
thereto) should allow for a relatively simple plotting program.
Please tell me whether or not this will work?
[Edited on 25-2-2008 by LSD25]
Whhhoooppps, that sure didn't work
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LSD25
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Double-posting, well sorta unavoidable at present...
I have been looking into this further - apparently the new Si gap thermosensors (digital output apparently - they are also quite cheap) can pickup a
0.05C change in temperature - they are also virtually linear in gain so it is quite possible that by reference to a small heat increase / time, it
should be possible to operate this fairly well without a major heat increase being necessary. In order to allow the overall speed of the unit to be
improved, a peltier type junction-board with thermostat could probably be used to rapidly cool and maintain the sensor at a set temperature before and
after irradiation of the sample.
The only question I can find nothing on is whether or not the concept underlying microbolometers, that when an IR absorber is irradiated with IR it
gives off heat (conversion/conservation of energy - I have heard of that somewhere?) can be applied succesfully to organic/non-organic chemicals which
absorb in discrete wavelengths and not in others. I cannot see why it would not work, I would just like to know for sure.
Whhhoooppps, that sure didn't work
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not_important
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In my day microbolometers used extremely find gold powder, chemically produced, as thei absorbing surface. There's a very black form of nickel that
might do.
These ICs - MAX6633, MAX6634, MAX6635 : http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3074
will resolve 0,0625 degree C
The problem with many of these sensors is that they are too large, their thermal time constant is fairly long. This means the scan takes a long time
to complete, or that you will have very low resolution. If you stretch the time to complete a scan out too far, drift gets to be a real problem; even
the IR source drifts some.
I think you'd have problems trying to use a feedback loop with a TEC. For maximum sensitivity you want the sensor to heat up quickly, meaning it has
low thermal mass and is thermally isolated. To cool it down you want it to have a good thermal path to the TEC, which has fairly large thermal mass.
I think you run into problems using the temperature rise of the sample. First off you'll need a very small sample to make it responsive; a test tube
full isn't going to heat up much in the monochromator beam. Remember that you're tossing out most of the radiation at any given time, and broadband
IR emitters aren't terribly intense.
Second is the thermal capacity of the sample. Different materials will heat up a different delta for a given amount of absorption. This can make it
difficult to determine structure based just on the spectrum, and even matching against known spectrum could be harmed by a relatively minor amount of
a second substance that affect the heat capacity of the sample.
I think you'll find it's easier to detect small changes in intensity than it is to detect small changes in temperature.
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LSD25
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I was actually just thinking of using a silicon thermistor - these really are as small as one is likely to see. They are fairly efficient at picking
up changes in temperature and the responses are fairly quick. The cost of these is absolutely negligable, especially when viewed alongside trying to
detect changes in intensity.
As to the ambient heating of the sample, I was considering using a blank sample of either KBr or NaCl as a zero, presumably the heat increase in this
would be a good reference to that of the sample? The rise in temperature appears to cause a straight-line increase/decrease in resistance, as such
this should be able to be worked out.
As to the problem you foresee with the whole concept, that is the main thing I am worried about. It goes to the very heart of the idea - is the heat
increase of an organic/other substance when exposed to a narrow wavelength of IR radiation a measurable and useable indicator of that compounds
absorbence at that wavelength?
I 'hope' that it will prove to be so, but I am far from convinced... However, I suspect that it might well be, given that the emission of heat caused
by the absorption of IR radiation must be in some way connected to the amount of absorption, I just hope that this is measurable. I have looked and
cannot find any data, one way or the other, which would enable me to answer this question properly in relation to organics, not withstanding the fact
that this is what appears to be the conceptual background to the use of microbolometers themselves (albeit, from what I have found so far, all
non-organic).
However, if the half-baked theory holds good (and worse have), then the mere adulteration of the sample with other compounds will obviously lead to
the corruption of the absorbence data, as it does with any other type of spectroscopy. The spectrum, based upon the heat emission / absorbence should
really not be all that different, within the boundaries of measurement, from that obtained by detecting the intensity of the remaining radiation.
BUT, this has yet to be tried and I cannot even find a patent on the idea... Nonetheless, I cannot see why it would not work (which worries me, as
people tend to patent the damndest things), if absorption of IR radiation = emission of heat (and both NASA & the US DOD appear to be of that
opinion), and leaving aside the difficulties in actually collecting the data, then this would seem to be a fucking good bet for a purely speculative
patent application.
Whhhoooppps, that sure didn't work
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LSD25
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Double-posting, well nobody to blame but myself
Anyhow, I was thinking - I can buy an entire (admittedly fairly clunky) Michelson interferometer online for under $200, so this set me to thinking (Oh
god NO, I hear you say?), there are also good topics on making the same:
http://www.colorado.edu/physics/phys5430/phys5430_sp01/PDF%2...
If a microbolometer works by (1) absorbing IR with the IR absorbent coating; and (2) measuring the change in resistance of the internal resistor as a
result of the increase in heat transferred from the IR absorbent part, is it possible to use one (or a number of these - say 20-30 silicon resistors
coated with an IR absorbent - at the right ranges of course) of these as the sensor for actual FT-IR?
It seems to me that the benefit of using it this way would be that the increase in heat with each absorption of IR radiation would be limited by the
fact that the whole thing would be over and done with more rapidly.
Whhhoooppps, that sure didn't work
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not_important
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| Quote: | Originally posted by LSD25...
As to the problem you foresee with the whole concept, that is the main thing I am worried about. It goes to the very heart of the idea - is the heat
increase of an organic/other substance when exposed to a narrow wavelength of IR radiation a measurable and useable indicator of that compounds
absorbence at that wavelength?
... |
The absorbed IR will show up as heat, however:
1) the change in temperature is proportional to the intensity of the radiation and the heat capacity of the substance. The heat capacity of the
substance depends on a number of things, including the degrees of freedom of the molecule - directly related to the complexity of it. So differing
compounds will have different heat capacities, and thus differing temperature increases for a given amount of some wavelength absorbed. So unlike a
transmission IR where the intensity of say the C=O band is roughly the same for most compounds, the temperature based one will give radically
different amounts of change for differing compounds. In effect the spectrum is flattened out for the more complex molecules.
2) there's not a lot of energy at a given wavelength for most IR sources, making for a very small temperature rise.
3) for solutions, mulls, or other forms used when working with solids, the heat capacity gets hit over the head by the other substance, even if it is
transparent for the wavelengths being measured.
Putting those together I think it would be difficult to determine what an unknown is from looking at its structure.
I'm not sure I understand what you're trying to do with the multiple pickups - care to amplify on that?
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LSD25
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I was looking at a site earlier tonight where it showed a java-applet of FT-IR and what is the actual end-result.
I was thinking to myself, that if it were possible to place several radiation-heat receivers in a small area, then the changes in the fringes would be
more apparent more quickly - thus less radiation & less heat.
I must admit though, I am still hopelessly befuddled about how one converts that signal to anything, even the one that would be received with a proper
sensor.
Whhhoooppps, that sure didn't work
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not_important
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Not sure why several receptors would do that, focusing all the IR onto a single detector should do the task.
The FT part of FT-IR is close to FM if you've not the maths background. But there are plenty of code libraries around. Get to understand the basic
concepts needed, the several types of transforms, windowing, and so on, and you could cobble together a functional FT system.
As FTs are used in NMR and some newer MS, it's a concept you likely want to become a little more familiar with.
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LSD25
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Righto,
I'll make that my main effort for the next week or so...
That is the joy of this, I ain't seeking to build a better mousetrap - just a cheaper one - the information exists, it is just a matter of
assimilating enough of it to work out what is necessary and how shit works.
The main problem I have is working out how to visualise 'seeing' something when the sensor is only sensing the presence of an IR signal, whether by
direct absorbence or via heat absorbence. This is somewhat easier to get my head around when dealing with a single wavelength obviously the difference
in the signal intensity when it goes through a sample is the absorbence level of the signal and the wavelength is able to be determined by reference
ot the position of the mirrors in a monochromator...
OK, now from the little I have read so far, the fringes resulting from running light through the interferometer are the radiation from the output
separated into discrete fringes, from which one can derive the entirety of the spectra of both the source of the light, but also that of the source
passed through the sample - which effectively filters out part of the spectra.
That is obvious, the problem I have is not so much the FT, but more how does a single-point type sensor collect suffifcient data from which to collect
enough of the fringes so as to allow the FT to be run?
I mean, once I comprehend this - the FT part could be worked out, without even being required to understand the equation - simply by virtue of the
fact that it is mathematics and that is what computers do best. The same (or a virtually similar) program could then be used in whatever spectroscopy
was utilised to collect the signal.
Whhhoooppps, that sure didn't work
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not_important
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If you took the simple Michelson interferometer design, and have the IR source be an idealised one at infinity so the photons can be thought of as
flat wavefronts - parallel with or perpendicular to the end mirrors, the the fringes exist in time, or in space along the axis of radiation traveled,
but not as the conventional bullseye pattern. Think of it with a single wavelength, nice straight wavefronts marching along, reflecting off the beam
splitter and mirrors. When they arrive at the detector plane, it's rows of peaks and troughs parallel to that plane; the entire detector surface
brightens and dims simultaneously.
The real world isn't that pretty, but close enough. So far as the detector goes, it's not seeing a bullseye fringe pattern but just a modulation of
the intensity of the light, modulation tied to the rate of movement of the moving mirror.
The difficult part of an IR interferometer is the beamsplitter, it has to work across a decade of wavelengths. There have been IR systems that use
purely reflective optics, interdigitating the fix and movable mirrors, as a means of getting good performance across a wide range of wavelengths.
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LSD25
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Thanks,
Here is a good introduction to the principles:
http://mmrc.caltech.edu/FTIR/FTIRintro.pdf
I also found this which is probably the most simple version of the theory I have yet found:
http://www.laserfocusworld.com/display_article/25338/12/none...
So, looking at the problem from that viewpoint and taking part of what you said to its logical conclusion - the sensor only really takes in the
intensity of the light at the point at which the sensor operates and that this when viewed in conjunction with the position of the moveable mirror
gives the basic information which is deduced by the computer?
Ok, so when the system is first built, one just takes a spectrum of the light source, probably one of a sort which has detailed spectral information
available - using this to work out what is what & where it is, then various samples of known compounds, etc.?
Whhhoooppps, that sure didn't work
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LSD25
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Ok,
Back to this for a specific question - if one had a Class 2, Laser Pointer, 0.95mW output, wavelength 630-680nm, would it be feasible to use a grating
(as linked to above), a filter (is it possible to just cutoff from 630nm on up?), a lens and a CCD device?
If the Class II device is too light-duty, is there a way to modify the same in order to increase the output?
Alternatively, could the weak raman spectra be intensified by using a photo-editing package as used by High School students in fluorescence microscopy
(I think I cited that idea here), IIRC they used the photo-editing package to increase contrast and thereby intensify the weak image to something
useable.
Whhhoooppps, that sure didn't work
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not_important
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If the shifted (Raman) light is too weak, it is too weak - down in the noise and not recoverable. You're never looking at these with your eyes, it's
always electronics and data massaging before it's put on a display for you to see. The problem is the huge difference in intensity of the laser and
simple scattered light from it vs the Raman light; defects in the grating and lens and dust in the air can scatter so much of the laser frequency that
the portion that lands elsewhere is still many orders of magnitude greater than the Raman light, more than the resolution range of the sensor.
Post processing would be useful to remove 'hard' background data errors such as adjusting cells with higher or lower sensitivity than average, or
mapping out stuck cells. Some linear (scanner) CCDs have support for some oft hat processing built into them, scan a uniform surface and then load
the resulting intensities into a small RAM on the chip; the values are used by the chip to tweak the apparent sensitivity of each pixel.
You can't boost the output from a laser diode very much, at least for any useful amount of time. Better to get a higher powered diode; that's why
CD/DVD drives were talked about because their diodes are higher power.
You can get sharp cutoff filters, but they will be expensive. If you have good quality gratings you can do the task using them, but there is strong
emphasis on 'good quality'
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