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LSD25
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Can Digital Photography be used as the receptor array for FT-IR?
I have been putting some thought into this,
Now there are very small Near Infra-Red, Mid-Infra-Red & Far-Infrared filtered Digital camera lenses as well as the usual visible wavelength
lenses.
I have also been looking up the use of diffraction gratings and how they split various light up into it's wavelengths, and more especially the overlap
between the various sub-divisions of light when passed through a grating.
http://scitoys.com/scitoys/scitoys/light/cd_spectroscope/spe...
http://www.jobinyvon.com/SiteResources/Data/Templates/1divis...
http://www.rsc.org/ej/JA/2000/b001066i/b001066i-f2.gif
Now, I may well be wrong, but it seems that if one mounted 4 (or more) lenses on a rotary servo (Vis, NIR, MIR & FIR), it should be possible to
record the amount of each wavelength passing through the grating. It might even be possible to use a high shutter speed and adapt a cheap(ish) digital
camera to take onboard the images viewed through all 4 of the rotating lenses. Now if this was passed through to a PC it might even be possible to
write some basic code (or to modify the code on an existing shareware package) to analyse the images, extract the peaks and identify the wavelengths.
However, it might well be a lot easier to manually plot the various points on the images with the computer, identify the wavelengths and then allow
the computer to extrapolate from this to provide an overall graph of the data taken from the images.
Just a thought, given the extraordinary prices charged for this machinery it might well be that a useable variant may be able to be made at home (it
may lack a lot of things, but that is life). Anyone with ideas as to whether or not this will work are more than welcome to have their say, as I said,
this is only an idea - I am far from convinced that it will work.
Whhhoooppps, that sure didn't work
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niggaknow
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Everything you always wanted to know about IR, but were afraid to ask.
http://www.4shared.com/dir/2077108/ab6c3d06/sharing.html
http://www.geocities.com/antiquesci/PE337/PE337-1.htm
[Edited on 7-2-2008 by niggaknow]
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niggaknow
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See this article.
Attachment: JCE1992p0077-monochromators.pdf (1.5MB)
This file has been downloaded 606 times
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niggaknow
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FT-IR specs on the market (with Michelson interferometers) are different from the dispersive type (gratings and prisms).
Attachment: JCE1979p0681-FTIR.pdf (2.5MB)
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not_important
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OK, first off the CCD in digital camera only responds to about 1100 nm http://www.andor.com/learn/digital_cameras/?docID=315 so you're not going to even get all the NIR. Here's a short list of detectors used for IR
work, you can use it as a starting point for further research http://www.brukeroptics.com/accessories/Notes/FTIR/detectors...
Second FT-IR is Fourier transform IR, which does not use dispersive methods but rather is a multiplex spectrometer. You can build dispersive IR gear,
that's what I learned on, but calling it FT-IR is wrong.
[Edited on 8-2-2008 by not_important]
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LSD25
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OK,
Finally some feedback (Excuse my not replying earlier, I had to track down the password, etc.).
I would like to now more, I simply (obviously erroneously) believed that by using a single light source/pulse and a diffraction grating that all such
variants were described as variants of FT-IR spectroscopy.
As to the limits on the digital cameras, that is one of the things I feared - but did not know. Thank you.
Do you happen to know if there is any way one could use the IR photodiode-array from a CD/DVD player and the various parts therein (although I believe
a different light source would be needed) in order to come closer?
Would it be possible to come up with a way to build something approximating those extremely expensive array's?
The major difficulty I have with those units is the price - no question - but it would also be nice to come up with something...
For instance, is it possible to use Near IR spectroscopy to identify various functional groups? Or is it possible to use some of these ideas with N/IR
microscopy (I have a wierd little paper where they use photoshop to increase the contrast between different areas of digital photography - could this
be useful?).
I have done a few searches on NIR spectroscopy & here are some of the more interesting results:
http://www.thermo.com/com/cda/technology/detail/0,2165,13191...
http://www.spectroscopynow.com/coi/cda/detail.cda?id=1881&am...
http://tinyurl.com/2d4zh5
In any event, it may be possible for members here to build UV-Vis/NIR spectorophotoscopes and build up a database of various compounds. Although it
appears that these are nowhere near as well defined as IR spectroscopy, this would have to be set-off against the cost and availability of the
necessary equipment. In any event, if such a thing were possible it would represent a large leap forward when compared to the extremely basic
analytical options available to most members now.
From the link you posted, a digital camera may well be an option as a replacement for expensive arrays in this respect, while a light source will
still be needed, this is a step forward.
I'd seriously like this to be worked through to a conclusion, it would be nice to know with some precision what is in the tubes after working up a
reaction. It may not be unequivocal, but it would provide a sound starting point for working out what is happening.
Whhhoooppps, that sure didn't work
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not_important
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Near IR is mostly used for (C,O,N)-H and C=O bond analysis, meaning you're not looking at carbon-carbon and maybe not carbon-oxygen single bond as I
recall.
The cutoff of CCD arrays is such that your are looking at 3rd and 4th harmonics, which are rather weak and broad; I don't know how useful they are
without the lower overtones and fundamental.
The photodiodes for CD/DVD application are in a array of eight diodes that really would effectively be only six in dispersive applications. They are
also optimized for wavelengths right on the edge of visible down into the red proper http://www.datasheetcatalog.com/datasheets_pdf/T/Z/A/1/TZA10...
UV/Vis gives different sorts of information, useful when you know what is in there and want to know how much, but less so when you're trying to find
out exactly got brewed up.
For near-UV/Vis/very-near IR people have used the imaging arrays of scanners as detectors in dispersive arrays, the one dimensional stream of data is
just fine for looking at a spectrum. You get 1500 to 4000 points of data, which is a high enough resolution to be real useful; a 2D sensor would hand
you much more data which you would the need to fold down into a 1D stream.
Using a laser, some narrow band pass and block optical filters, and a CCD array you might be able to do Raman spectroscopy, which can give similar
information to that from standard IR spectroscopy.
FT spectroscopy does not use dispersive elements, prisms or gratings, but basically collects a blob of frequencies with a single detector and throws
computers at the data to get a spectrum. The detectors can be expensive because of the difficulty of getting rapid, sensitive detection across that
range of frequencies; some include integral thermoelectric coolers.
http://www.chemistry.adelaide.edu.au/external/soc-rel/conten...
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vulture
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Most CCDs are in fact sensitive to IR light, try pointing a remote at a digital camera. However, most cameras are also equipped with a hotmirror to
block as much IR light as possible (it leads to moire and blooming IIRC). So you might want to check astrophotography sites as they usually do IR
conversions to get that end of the night sky spectrum.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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not_important
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| Quote: |
Most CCDs are in fact sensitive to IR light, try pointing a remote at a digital camera. However, most cameras are also equipped with a hotmirror to
block as much IR light as possible (it leads to moire and blooming IIRC). .... |
This is true, but as shown in the link I gave, this is the very near IR. Most CCDs themselves poop out around 1000 nm, silicon CCDs can't go above
1,1 micron. See http://www.wrotniak.net/photo/infrared/ for IR and digital cameras.
The Fuji S3 Pro UVIR cameras were designed to support UV and IR photography. Spectral response is 350 to 1000 nm.
Even NIR is considered as 0,8 to 2,5 microns, the 1,1 limit only takes to higher overtones of the bands in that region.
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LSD25
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Righto,
Here are a couple of links to amateur websites (actually pages) where they have modified cameras for astrophotography by removing the IR filters and
they give details of how to acheive this.
http://www.vub.ac.be/STER/www.astro/AstroCCD/AstroQC.htm
http://www.ayton.id.au/gary/Science/Astronomy/Ast_PhotoDigit...
Here is an abstract of an article on the limits and details of astrophotography:
http://adsabs.harvard.edu/abs/1976S&W....15....4B
Thanks Vulture...
Not Important,
given the discussion in the google book I linked to above (read as much of that as possible - it is truly interesting), is there any chance of using
these modified cameras in the manner suggested? (I had a look at the Fuji & also the Nikon, but the prices are horrendous & if amateur
photogaphers are modifying cameras to get similar shots, I'd like to imagine the members here were able to do the same).
Here for instance is a page (including spectra) examining the perfomance of several cameras and modified variants of them in the relevant range:
http://www.pbase.com/terrylovejoy/image/29755088
NB This site also gives details on how to modify the cameras in question as well as great links to other sites.
[Edited on 12-2-2008 by LSD25]
Whhhoooppps, that sure didn't work
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not_important
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I'm not sure what you mean by " using these modified cameras in the manner suggested".
They will work with a dispersive element for visible light spectroscopy, more properly for the near UV down to aboy 350 nm on up to 900 to 1000 nm.
(note that human vision goes up to the high 600s to mid 700s, depending on the person)
This is useful for atomic emission spectra, and absorption spectra. With a detector with a large enough number of elements the UV end can be extended
downwards using a phosphor to convert the short UV to visible or near UV that the detector will sense.
For absorption use, traditional uv-vis model, you don't even need a lot of elements as the bands generally are rather wide. Even the cheap "webcam"
units will give enough resolution for that. However:
| Quote: | | The uv-vis spectra have broad features that are of limited use for sample identification but are very useful for quantitative measurements. The
concentration of an analyte in solution can be determined by measuring the absorbance at some wavelength and applying the Beer-Lambert Law.
|
For emission spectra - atomic spectroscopy, atomic-absorption, and atomic fluorescence, you'll want more resolution, which means more elements in the
detector and better dispersive optics. That's why I suggested a flatbed scanner detector as these generally have 2000 to 4000 elements, which very
roughly corresponds to a 4 to 16 megapixel camera. However because they are linear sensors you only have to deal with a few 10s of kb of data per
scan, instead of the megabytes of a digital camera.
Note the 'atomic' in these types of spectoscopy. You are dealing with electrons around isolated atoms, not molecular structures (except for some high
temperature stable simple compounds). For organic molecular structures you need near-to-mid Iror Raman equipment. The first can't use the CCDs
sensitive range, the second can but also needs narrow line lasers and optical filters. The sensor is not the most expensive element. These types of
spectroscopy usually benefit from FT methods, see the Fellgett advantage, and use a single sensor element.
One other place digital cameras might be useful is with chromatography, where the extended pickup range and contrast enhancement might help visualise
spots. Only simple software qould be needed, IrfanView would likely do the job.
It is important to note that the astronomers often refer to the the very near IR range as simply 'IR'. The atmospheric absorption bands start
punching holes in the spectrum at about 800 nm with strong, wide bands around 4 and 5-8 microns. This limits astronomical viewing to windows such as
those at ~870, 1100, and 1500 nm. The astronomers IR includes some of the emission lines of hydrogen, iron, and helium. Not all of these can be
detected by silicon CCDs, as they lie above 1 micron in the NIR proper.
http://www.globalwarmingart.com/wiki/Image:Atmospheric_Absor...
http://www.atnf.csiro.au/people/mdahlem/pop/multi/ir.html
in a fashion related
http://pagesperso-orange.fr/redlum.xohp/laser/gratingOSA.htm...
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LSD25
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Ok, Long, long way above my head at present - I will be doing some more reading and then I'll modify this post.
In case I forget, thanks for your help.
[EDIT No.1]
I decided to go right to the start of what you had written above and look up this:
http://en.wikipedia.org/wiki/Charge-coupled_device
I'll now start from there in an attempt to get up to speed.
[Edited on 12-2-2008 by LSD25]
[EDIT No.2]
I looked this up:
http://en.wikipedia.org/wiki/Image_scanner#Applications_Prog...
I must point out that the use of infrared cleaning as is mentioned at the bottom of the page - or the use of infrared light to find those parts of a
scan which absorb/filter IR - suggests that not only do scanners have the capacity to decode IR signals (or probably NIR), but they can be used to
pick up absorbance. Is this correct? or am I on the wrong track?
[Edited on 12-2-2008 by LSD25]
Whhhoooppps, that sure didn't work
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not_important
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IR cleaning -
check the link to the full article; they use a NIR source to get a NIR image. This technique is being applied to colour film, which is somewhat
opaque to NIR no matter what the visible colour is. Scratches and dust have a different brightness than the relatively flat toned IR background, so
the IR imge can be enhanced to create and adjustment mask.
The scanner CCD isn't giving a special "I'm NIR" signal out, except in the context of "there is NIR illumination being supplied"
The scanner CCD has similar properties to the camera CCDs, the same 350 to 1000 nm range.
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LSD25
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Yeah, I thought as much - but as NIR offers probably the best chance of this being developed for home use and also offers the use of glass containers
for samples, this is probably the best we could hope for.
NIR spectroscopy is gaining some serious attention according to the literature:
Handbook of NIR Spectroscopy (Googlebook): http://tinyurl.com/ywtbg2
A brochure:
http://las.perkinelmer.com/Content/RelatedMaterials/Brochure...
A review article dealing with NIR spectroscopy:
http://www.odin.life.ku.dk/News_letters/Q2_Q3_2005/news001.p...
This is good as it gives a basic overview of the strengths & weaknesses of NIR, as well as giving some detail of the procedures used.
Whhhoooppps, that sure didn't work
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not_important
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I fear that it is of limit use in the way that you seek, as it really only gives information on hydrogen to (C, N, O, S) bonds, and very little on the
carbon skeleton. Besides that:
| Quote: | In conclusion, NIR absorption bands are typically broad, overlapping and 10–100 times weaker than their corresponding fundamental mid-IR absorption
bands. These characteristics severely restrict sensitivity in the classical spectroscopic sense and call for chemometric data processing to relate
spectral information to sample properties
...
pharmaceutical industry and regulatory bodies have been slow to adopt the NIR technique, most probably since it lacks the ability of mid-IR to
identify samples by mere inspection of spectra and involves calibration by sophisticated mathematical techniques |
For identification you need to have the sample libraries built up, which sort of defeats the purpose of "know with some precision what is in the tubes
after working up a reaction" unless you aleady can make the various products in pure form and are optimising the reaction or trying alternative
methods.
The main advantages, being able to measure samples in containers or tablets, is of little use to the amateur lab, where it is easy to obtain a drop of
the materials for analysis and generally it is not intentionally being mixed with other materials besides what's in the reaction mix.
The other problem with what you propose is that the Si CCDs only sense up to 1000 nm, maybe 1100 nm in some types with cryocooling. The real
information in NIR is in the 1500 to 3000 nm range, and you data from most of the NIR range to untangle the signals into something useful.
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LSD25
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Yeah, I get that it is nowhere near as useful as the dedicated MIR range... Nonetheless it may provide a useful tool for a home lab and that is all
that I was considering it as. I am willing to do the testing and develop the process with solvents and pure reagents to start with in order to develop
the procedure, as well as being willing to separate the component fractions of the mixture in order to try and determine which one is the one I wish
to keep.
Even if this works out to be of little utility, it is an interesting exercise and given the current price of electronics - it is hardly likely to
break the bank. If it also provides a useful starting point for further development, that would be a plus - but the entire topic came from wild
speculation, so I am suprised that it has any utility at all.
Whhhoooppps, that sure didn't work
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not_important
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It's just not the NIR is less useful for identification of unknowns, but that the range that silicon CCDs respond to has only very limited usefulness.
I just don't think you'll find it useful for product identification, especially as there is little to no data on the carbon skeleton - n-propyl,
iso-propyl, allyl, and tert-butyl alcohol would all look rather similar, you might not even see carbonyl oxygens, ethers, and substituted amines and
amides.
First, note that there can be confusion as to what is meant by the term NIR
Astronomers typically use:
* near: (0.7-1) to 5 µm
* mid: 5 to (25-40) µm
* long: (25-40) to (200-350) µm
Based on the types of detectors that are useful:
* Near infrared (NIR): from 0.7 to 1.0 micrometers, from the approximate end of the response of the human eye to that of silicon photodiodes
* Short-wave infrared (SWIR): 1.0 to 3 micrometers (from the cut off of silicon to that of the MWIR atmospheric window. InGaAs covers to about 1.8
micrometers; the less sensitive lead salts cover this region
* Mid-wave infrared (MWIR): 3 to 5 micrometers (defined by the atmospheric window and covered by InSb and HgCdTe and partially by PbSe
* Long-wave infrared (LWIR): 8 to 12, or 7 to 14 micrometers: the atmospheric window, covered by HgCdTe and microbolometers
* Very-long wave infrared (VLWIR): 12 to about 30 micrometers, covered by doped silicon, microbolometers, and pyroelectric detectors.
Yet another scheme:
* Near-infrared : 0.75-1.4 µm
* Short-wavelength infrared 1.4-3 µm
* Mid-wavelength infrared also called intermediate infrared (IIR): 3-8 µm. Used to track jet aircraft by their engine plumes.
* Long-wavelength infrared : 8–15 µm, thermal imaging
* Far infrared (FIR): 15-1,000 µm
Second, the information in this range is pretty piss poor. Look at the chart on this web page - the frequency range you are trying to use has bands
that are 3rd and 4th overtone. The intensity is 1/1000 to one ten-thousandth as strong as the fundamental in the NIR, generally used for IR
spectroscopy. Path lengths are on the order of 10 cm, besides being weak the bands are broad, fuzzy, and over-lapping.
http://www.spectroscopynow.com/coi/cda/detail.cda?id=1881&am...
The main use of the range covered by Si CCDs seems to be measuring the ration of reflected light at 500 and 800-900 nm in order to determine the
amount of vegetative coverage, and combined with 350-450 nm estimating soil moisture. All of that is based on the chlorophyll absorption spectrum.
Aromatic Thai Rice Identification by Near-infrared Reflectance - starting at 1300 mn and longer
http://www.tropentag.de/2005/abstracts/full/158.pdf
Mini-NIR spectrophotometer for measuring water content
http://newjournal.kcsnet.or.kr/main/j_search/j_download.htm?...
Attached is a chart of NIR bands. Remember that 3rd overtone is a bit like identifying distance shapes in heavy fog - that blob can be a person,
shrub, or mailbox.
Attachment: nir-chart_grid_rev-3.pdf (668kB)
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LSD25
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Thanks for that, my view is that as this is THE current research topic in Spectroscopy (as demonstrated by the sheer number of articles - because
industry wants to be able to spend less money for better data in process management), I suspect that this area of spectroscopy will become more and
more useful in the near future.
In fact, there are a large number of scholarly articles being written about the more accessible ranges of NIR and how to make better use of the data
provided thereby. While this is not at a breakthrough stage, I strongly suspect (given the apparent volume of time and money thrown at the problem)
that somebody will make a fundamental discovery to make better use of this data (I'll attach some of the more interesting ones soon).
I'd dearly like to be in a position to take advantage of such a breakthrough - I know I ain't smart enough or knowledgable enough to solve these
problems, although I enjoy trying to take advantage of the hard work of others (standing on the shoulders of giants, so to speak). The best part of
this idea is that unlike most labs, I have access to a variety of experienced computer programmers, so if the breakthrough is software based (as I
strongly suspect it will be), the playing field will be evened out far more than it is with the oh-so-dear MIR arrays, etc.
Whhhoooppps, that sure didn't work
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not_important
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It is a current hot topic, but mostly for QC and similar monitoring applications, not for general research. Best when you know what you should have,
or shouldn't have, and you want to know how much of it is there, how pure it is. In these applications you already know what you have and what might
show up that you need to control for, you can dispense with much of the information that is needed to determine structure and focus on a few
wavelengths that distinguish A for B.
Because the wavelengths used can be transmitted through fiber optics, it's particularly useful for industrial process control and monitoring
applications. The NIR band has the advantage of being less sensitive to stray thermal radiation. There is also a much wider range of optics materials
to chose from as well; sapphire with its transmission out to 5 um works well and its hardness and temperature and solvent resistance are quite useful
in industrial settings. But I doubt your personal applications include taking the IR spectrum of abrasive slurries being pumped over the viewing
port.
Part of the reason it is hot topic is there's a lot more demand for industrial gear than for research lab instruments. More sales == bigger profits.
I don't understand why you keep referring to detector arrays, as in your last sentence above. Simple dispersive scanning and FT spectrometers can use
a single element detector, arrays are used for dispersive instruments that give multichannel output and for imaging devices.
As I'm not in the business of chemical product manufacturing, I'd much rather have a FT-IR that covers 2 to 20 um than a half dozen of the 1 to 4 um
process control ones that simply don't see much of the useful information.
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LSD25
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Thanks mate, yeah I know the limits of the proposed equipment and trust me, I'd much prefer a proper instrument (in fact, I'd also like to have the
$'s to buy the same without qualms). Unfortunately this is not likely to be the case in the foreseeable future. I'll try and get 'something' going to
start with and then we'll see what works, what doesn't and whether further experimentation is worthwhile.
I'd like to try and set something up later which utilises the components of the DVD/CD type setups, this seems a lot more involved and I would prefer
to get some runs on the board (actual hands on experience) before I even attempt such a thing.
Now, I do not understand (guess you are probably not all that suprised?) how the units could work with only one detector? My problem is in
understanding how a single detector differentiates between the various wavelengths?
But yes, I too would honestly and sincerely prefer a true FT-IR setup that could acheive everything my little heart desires, I just don't see how to
get there.
Whhhoooppps, that sure didn't work
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not_important
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Scanning dispersive spectrometers use a prism or diffraction grating to get the spectrum, and allow a narrow range of frequencies to react the
detector at any one time. The 'scanning' is the sweeping across the frequency/wavelength range to measure all the spectrum.
FT models split the 'white' light into two paths, slightly wiggle the length of one path, and recombine the paths to get a complex interference
pattern which the computer then uses FFT processing to convert into the frequency spectrum.
In both cases you want the detector to have as flat a response to all wavelengths as it can, although differential models reduce the need. Most
dispersive IR spectrometers were/are scanning, specialised ones use multiple detectors to avoid scanning by measuring a few wavelengths of interest;
these generally are process control or specific monitoring applications.
Spectrometers for UV/visible light, usually called spectrophotometers, are mostly dispersive in nature, earlier ones and simple models are similar to
scanning IR spectrometers in that they measured one wavelength at any given instant. More recently CCDs and other linear arrays have been used as
detectors to allow sampling of the entire spectrum at once. However for many applications the single wavelength measurement wasn't a problem, because
the task was measuring the absorption at some important wavelength to track the concentration of a particular chemical.
Spectroscopes also use film to record the full spectrum at once, and can give very high resolution; the newest ones use linear detectors. These are
used for light emitted from excited atoms, while spectrophotometers are used for measuring light absorbed by molecules.
For a generally useful IR spectrometer, even for NIR, you'll be using a single detector. There's some applications for IR below 1 um, but they are
fairly limited, QC related to water content and similar, or some biological research stuff.
http://en.wikipedia.org/wiki/Infrared_spectroscopy
http://elchem.kaist.ac.kr/vt/chem-ed/spec/vib/ir-instr.htm
http://en.wikipedia.org/wiki/Ultraviolet-visible_spectroscop...
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LSD25
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OK, sorry but the diagrams I have seen to date didn't make that sufficiently clear, thus the failure to comprehend.
So, if I am correct - either the difraction grating (and therefore also the difracted spectrum) moves or the detector moves? This would, I assume
require a fair degree of calibration insofar as the location of the various wavelengths within the spectra, as the only way to differentiate between
the various wavelengths (or most, I would assume that by using the three photoreceptor diodes in a CD unit would differentiate between some of them)
would be the location of the same within the spectrum?
So, working on the assumption (which well may be misplaced) that this is indeed possible, what you are in effect saying is that a functional and
functioning IR spectroscope is within the reach of the home hobbyist?
I mean, it is technically interesting - although not exactly difficult - to imagine a road culminating in a 'shuttle' which moves from one side of the
spectra to the other, measuring, collecting and sending to a PC data on the intensity of IR radiation. Similarly, it is hardly difficult to imagine
that such an apparatus could be calibrated via the comparison of the spectra obtained thereby with a range of pure, known compounds against the known
spectra of those compounds. Lastly, if this were indeed feasible, the sending of radiation-intensity data to a PC would enable the automatic plotting
of the IR spectra obtained by this method (once a value for the IR-wavelength is assigned to the position of each component of the spectra where it
intersects the shuttle track).
The only real problem with this is whether or not you were serious when you stated that the receptor array used in DVD/CD players descends into the
actual IR range... I suppose one could also look about and try and find similar instruments which contain diodes/transistors which are capable of
working in that range. The existence of these receptors at a decent price, appears to be the decisive factor in the design of a home-model. If the
hypothesis that one could assign a wavelength value to the positions of the shuttle holds true, then this would obviate the need for the unit to be
able to identify as well as quantify the wavelengths within the spectra.
PS I apologise for the late reply, I was working on converting a temperature controlled ceramic hotplate to a speed controlled, stirred,
heat-controlled ceramic hotplate. You may have guessed, I like gadgets.
Whhhoooppps, that sure didn't work
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microcosmicus
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| Quote: |
I mean, it is technically interesting - although not exactly difficult - to imagine a road culminating in a 'shuttle' which moves from one side of the
spectra to the other, measuring, collecting and sending to a PC data on the intensity of IR radiation.
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Another possibility is to use circular as opposed to linear motion My thought would be to make a
carefully balanced mirror wheel mounted between centres (like a gyroscope) and turn off the power
to the wheel while measuring. This way, even if you didn't know the exact angular speed, you could
be reasonable confident that it was constant during the scan. Using your spectral lines of known
substances as reference (likely several spectral lines from one substance would do) you could then
calibrate the spectrum. To scan both at once, perhaps an arrangement whereby the reference spectrum
goes through the top of the slit, the spectrum to be measured through the bottom --- measuring the
calibration simultaneously with the real McCoy rules out issues of conditions changing between
the calibration and the measurement.
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not_important
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for dispersive instruments the grating can move, the slit can move, or a movable mirror sweeps the spectrum across a fixed mirror. These are rotary
movements.
Dual beam models do the same thing, but they also send a beam through the sample and another through a similar path without a sample. The beams share
the detector, a rotating mirror sends one than the other beam to the detector/monochromator. This allows absorption from water vapour and CO2 to be
compensated for, as well as other wavelength related variations in response. If the beam selector also gives some dark time, those periods can be
used to determine the background signal level.
Even single beam instruments chop the beam, this gives an AC signal to process, getting away from problems related to DC drift in the detector and
electronics; it's easier to build a low noise AC amp that a DC one.
Moving the detector is rare, some types have a physical structure that would generate false signal from vibration; as the signal levels are generally
low moving the detector gives the possibility of electrical noise pickup.
As the detectors in the early machines were fairly slow to respond to changes in intensity, the sweep rates weren't high; it could take a minute or
more to get a spectrum. Newer detectors are faster, but given that you are looking at a very narrow band of wavelengths, and IR sources are not very
intense, scanning instruments are generally not real fast.
FT-IR moves a mirror along the light path, parallel with the beam. Often the mirror is mounted on a transducer which can be similar to a speaker
cone, or a slab piezoelectric material. FT-IRs are usually self-calibrating, they include a gas laser (laser diodes tend to be too broadband and
noisy) that functions as a reference frequency.
A simple array like that in CD players really isn't of any help here, you need to look at hundreds to thousands of wavelengths. The most use you'd
would get of it is in monitoring light at right angles to the spread of the spectrum; this usually represents misaligned or poor quality optics.
Any silicon photodiode, be it a single diode or transistor or a CCD, likely is dropping down to zero response at about 1 um, and just about all do so
by 1100 nm. Anything you build using Si diodes will be limited to about 1000 nm and shorter wavelengths, in that range you'll mostly find the very
weak 3rd overtones for H-O and H-N bonds.
I think that you would get more useful information building a Raman spectroscope working in the visible and/or very near IR; it could give you
information close to what conventional MIR gives.
By the way, this equipment used a 2D CDD, kept at -50 to -100 C
http://www.avaloninst.com/content/raman_information/Echelle....
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LSD25
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Sorry, I was somewhat overoccupied for the last few days,
Anyhow, I was looking at the dedicated mid-IR photodiodes, correct me if I am wrong but it appears that the PbSe variety are useful for detecting
these wavelengths and do not require anything below room temperature in order to operate properly?
http://sales.hamamatsu.com/assets/applications/SSD/irdetecto...
Here is where they are used in Oz:
http://www.capgo.com/Resources/Temperature/NonContact/NonCon...
This is where my thought process is leading me at the present time anyhow...
As to the detector moving, I was visualising moving the detector a 10th of a mm, then stopping for a period and taking in the spectra at that point.
Quite frankly, if it takes 10 hours for this to completely log the spectra - so fucking be it.
So, what do you think?
Whhhoooppps, that sure didn't work
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