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Author: Subject: Laser-Diode based Raman Spectroscopy
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[*] posted on 28-1-2010 at 19:18
Laser-Diode based Raman Spectroscopy


Ok, we've all seen the number of articles popping up on the internet on the use of either green or violet laser diodes for Raman Spectroscopy, some of which are extremely interesting in that they use laser pointers (OTC & cheap as piss) and one interesting one where they have methodically replaced the dearest components of a working Raman Spectrometer with cheap, OTC components...

They used a laser pointer with a notch filter, coupled to optical fibre cable, which is then used to take the laser light to the sample, a second optical fibre cable, with a notch filter included, is then used to take the Stokes/Anti-Stokes radiation to the spectrometer, where it is analyzed and converted for graphing purposes.

Now, the spectrometer is the expensive bit in the equation - whereas it really shouldn't have to be, it should be possible to use a CCD, which converts images, by reference to their specific wavelength & intensity, into electronic signals which can then be decoded by a number of devices...

What I am suggesting is that if we could find out how to harness the ACTUAL raw data from a CCD chip, we wouldn't need much more than that (well, apart from a lens & grating), to get ALL the information we need for a Raman Spectrum. We then utilize a CCD chip directly at the vision port of the spectrometer.

The programming side of it should interest someone, after all, it would use the same numbers/letters as are assigned in jpeg/png type files, only this time instead of reconstructing the entire picture, we'd only want to count the precise number of pixels of each specific color, and assign a wavelength to that color. That way we can map wavelength v intensity, giving a Raman spectra

PS Here are two J.Chem.Ed articles (uploaded by Polverone in another thread):

Wakabayashi & Hamada, A DVD Spectroscope: A Simple, High-Resolution Classroom Spectroscope J.Chem.Ed 83(1) 2006 pp.56-58

Wakabayashi, Resolving Spectral Lines with a Periscope-Type DVD Spectroscope, J.Chem.Ed 85(6) 2008 pp.849-853

And here is the site the author's of the articles just cited, mentioned. Of particular note is their description of the spectral emission of laser diodes as being effectively monochromatic, that is another area where the big savings lie, both in time & money.

Although, looking at the second article again, they aren't just using it as a spectroscope, but as a veritable UV-Vis Absorption Spectrometer... Interesting.

So we could have UV & Raman capabilities with fuck all outlay, can anyone access the supporting information for both articles? I'd dearly love to see the pattern for the latter version of the spectroscope

Because what I am thinking of, is if the software to digitize the image from a digital camera is truly opensource, then it would digitize it regardless of whether it was from a UV-Vis adsorption spectrum or a Raman spectrum - so 1 camera, one DVD and one cuvette, with two light sources, could effectively be used to generate both the UV-Vis adsorption spectra and the Raman spectra of whatever is in the cuvette. That should eliminate a LOT of guesswork I'm betting:cool:

PS Anyone here with programming experience? There are actually open-source image manipulation tools online and one or more of these could be modified to fit with this concept, which would be an absolute boon to amateur scientists and underfunded educational institutions to boot.

PPS Here is a good step by step - they use a grating, but the idea is the same - extremely simple - just imagine a Y junction with the grating in the middle - with two light sources at the other ends (etc. one a laser, one a mercury vapor lamp), turn one light source on, record the spectrum, turn it off, turn the other on, record the spectrum and we're done...

Edit 30-odd IIRC:P

Now, I have sussed it out a little - what would probably be the easiest solution would be to convert the image to a greyscale image (it's a spectra still, so provided it is properly aligned, the greyscale will accurately represent the original spectra), then make a histogram - X=Shade & Y=Intensity (or the number of units of that shade).

Now Adobe Photoshop has this ability, but it shouldn't be the case that the image conversion software costs more than the fucking hardware. I will now go and scout around for some freeware/opensource applications that can translate a greyscale image to a histogram, once that is to hand, then building a spectrometer would be the next step.

[Edited on 29-1-2010 by unome]

The best option I can see from a quick perusal of the internet information, is the RAW Format - ie. allows for the receipt of unmodified RAW data from the CCD (nb open source) and collating that data so as to build a histogram directly therefrom. The RAW data format group have already got a shitload of information & more importantly code which should be able to be made to work with this concept, after all, we aren't doing any image modification, simply polling the data to build the histogram.

The only bit that really worries me is the calibration of the spectra - which wavelengths = what part of the histogram... Of course in a perfect world, the spectra would be in order - based upon the concept of planning for the worst & hoping for the best, it may be necessary to program in the various swaps/changes to positions in order that the histogram correlate to the required spectral analysis.

[Edited on 29-1-2010 by unome]
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[*] posted on 29-1-2010 at 13:04


The software is the easy part, IMO. There are already many free programs and toolkits that can be used for data interpretation. Building the hardware is the part that requires money and (at least metaphorically) getting your hands dirty. If you want to build the hardware I will be glad to figure out the software side.

I was a little late getting in my JCE renewal this year, but I will be able to get the supporting information from those articles once I've been renewed. Or you may be able to ask in References.




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[*] posted on 29-1-2010 at 13:44


From what I can see, the hardware, particularly for the UV-adsorption version, is pretty simple.... The hardware for the Raman variant is going to be a little harder, optical fibre isn't exactly given away and the bandwidth & notch filters are going to be sticking points. The diodes, blue diodes particularly, have improved out of sight since the blueray thing took off, the prices have also dropped too.

I'd really like to try and build something that could be attached to a USB port, take its power from the USB port, and send its information back through the same port.

Now, what would be nice is if anyone has the necessary know-how to design or explain the design of the bandpass filter and the notch filter - seems to me they are nothing but colored glass/plastic and not worth the $3-400 price tags, especially given we'd be using laser diodes, which by definition should be near enough to monochromatic anyway.

[EDIT]

This article (attached) is very enlightening, it suggests (to me at least) that the only filters one would need would be a laser-line filter (lower cost than bandpass) and a cut-on filter that corresponds closely to the laser line filters wavelength. That would make for a narrow bandwidth laser, which lies just under the steep cut line for the cut-on filter, which would allow us to avoid using notch filters (which cost a fucking fortune) and just use relatively cheap cut-on filters.

Of course, with the sort of outlays that would be required, it would make sense to invest in a cheap holographic grating to match, but yeah, should be able to keep the prices reasonable

But, insofar as the UV-Vis absorption spectrometer, I cannot imagine that many people on this forum cannot access a DVD, some cardboard, some aluminium sheeting, a halogen light and a digital camera...

The graphics manipulation might be somewhat daunting (although there are freeware programs that should be able to handle it, not just photoshop), but if it were made easier, then it would be an EXCEPTIONAL development for amateur chemistry. Everyone who wanted one, could build one and have it to hand in their lab/garage/etc. Sure would beat the hell out of TLC for measuring the progress of a reaction.

Attachment: Erdogan.Mizrahi.Thin.Film.Filters.Raman.Spectroscopy.pdf (159kB)
This file has been downloaded 823 times

I just found this image (screencapture) at this site

Looking at the screencapture, it would appear that they have simply taken a cross section of the ccd generated picture, aligned it with the known spectra and then worked out the brightness via the histogram/graph... Sounds simple when said quickly, anyone here any good at programming?

I think if it were possible to generate such graphs from the simple adsorption (UV-Vis) spectrometer (using the DVD as the grating), then and only then, would it be worthwhile spending the couple of hundred bucks needed to try for the Raman


QCAPSCR.JPG - 43kB

I just found another article with some bearing on the problem, I haven't read it all yet, but maybe some here could make use of it

[Edited on 30-1-2010 by unome]

Attachment: Petrov.FilterBased.Program.Freeware.pdf (328kB)
This file has been downloaded 804 times

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[*] posted on 1-2-2010 at 10:58


Relatively expensive digital cameras are more likely to have the CCD imagers and RAW mode data capture preferred for this application. However, they also deliver far more spatial resolution than is useful here and are expensive.

The good news is that amateur astrophotographers have been putting webcams on their telescopes for years. They also have done some good research on e.g. combining multiple exposures to reduce noise levels, finding the cameras with best low-light sensitivity, and modifying cameras to capture raw data. If you can use a web cam, it already has host-computer control and should be cheaper than any digital still camera that supports RAW mode.




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[*] posted on 1-2-2010 at 16:37


Yeah, I've been reading through a lot of the information, some take 100 odd images to remove noise/distortion by averaging it out. It is resolving the image to the histogram without extensive, manual manipulation that is the issue for me. I seem to recall seeing a PHP type backend program to which you send the image and it comes back with the spectra/histogram (wavelength v intensity).

Building something lightproof out of wood and actually investing in a diffraction grating, that is all possible (they aren't expensive), using a digicam to take the image is simple as too (fuck, I can even make it build-in to the structure of the spectroscope), and I am quite happy to do so and provide plans.* But I think it would be a lot easier if we could process our images without extensive "learning", ie. using software to do the step-by-step conversion of an image to a histogram/spectrum.

Once that is sorted, we can start looking at the various possibilities opened up by Surface-Enhanced Raman Spectroscopy and laser diodes - the picture will be different, but in greyscale it will still convert to a histogram/spectrum (I think). Have to have a look at the use of either Fe or Cu for surface-enhancement, fucked if I want to use colloidal silver.

* For starters, given the research using DVD's, I'd prefer to use them first to establish whether or not the idea has merit.

[Edited on 2-2-2010 by unome]
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[*] posted on 1-2-2010 at 17:08


I assume you mean "learning" as in training/calibrating the software on a set of images.
I see that there are already at least some spectral images on flickr.com and other places. For at least experimentation I can see how hard it is to work with them.

One possible complication with the visible spectra that should be absent for laser-based Raman spectroscopy is different response of the imager to different wavelengths of light. I don't know if this is a notable problem with getting quantitative data from digicam images; it's probably putting the cart before the horse to worry of it now.




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[*] posted on 1-2-2010 at 20:24


Yeah, I just made a wooden version of the one from the first article - be aware you have to trim the bloody DVD to make it fit (there is NO way a full DVD will fit in the box specified - half a DVD fits well), but there is visible spetra - which are obviously & visibly different under normal lightbulb/halogen/sunlight.*

I think it might be time to look around for some cheap holographic gratings to try and get to grips with this...

PS I was thinking instead of using a mirror/periscope arrangement in the second article, what about carefully removing the shiny side from a dvd, the lines are still there...?

PPS No, I meant as in learning to do a repetitive set of manual tasks, using various different image processing tools (that various members may or may not have), each one requiring the steps to be (a) undertaken in a different order; or (b) done altogether differently.

* Pics to come - what is cool is that on top of the visible spectra through the viewing window - is the reflected spectra which is thrown onto the counter/benchtop in front of the unit.:D I suspect that that would be intercepted by the CCD and provide a nice picture - just have to play around with the angles.

[Edited on 2-2-2010 by unome]
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[*] posted on 2-2-2010 at 21:03


Here are a couple of sites, dealing with "Littrow Spectrographs" (originally they were done for astronomers), but they have been adapted to Raman Spectroscopy.

Now, 532nm, 10mW laser lights (like the one used in the attached article) aren't exactly expensive or even terribly hard to come across... The mirrors, lenses & the dichroic filter, that may be the bugger. The most expensive part of their unit is the spectrometer itself - the lightsource and CCD were the new-ish cheap bit.

I'm wondering if it wouldn't be better to narrow the laser wavelength (532nM (+/-)2nM laser line filters are $44 online), then use a longpass/shortpass filter to collect everything above (>550+)/below(<500) 532nM? It would certainly save fucking around with the dichroic mirror. They afe both available and not too expensive. here is a useful site on the nature of diode lasers and what they ARE being used for NOW.

PS If you check out the pdf's from that maryspectra site - she actually ended up getting a useful spectra from toluene with a 5mW 532nM laser pointer and a CCD camera (She also documented all the fuckups and half-ass fixes). This is starting to look like it might be a very interesting journey.

PPS In terms of getting a Raman Spectra, it is just me but wouldn't she have been better off placing mirrors so as to reflect as much as possible of the scattered light back into her collection lens?

Attachment: DeGraff.etal.Inexpensive.Laser.Raman.using.CCD.Detection.pdf (178kB)
This file has been downloaded 1101 times

Damn, I really, really hope these become useful to the optic fiber people (RT MID-IR photodiodes).

[Edited on 3-2-2010 by unome]
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[*] posted on 5-2-2010 at 03:17


As noted, the woman (I assume her name is Mary) from maryspectrogra.org, had serious problems with the initial setup of the Raman Spectograph because of the off-wavelength excitation of the sample, due to the fact that she used a laser-pointer, which although monochromatic enough for most purposes, was a LONG way from monochromatic enough for this one (where the Rayleigh scattered light is the main thing that has to be excluded from the spectra). Thus, above where I suggested she would probably have been better off getting a laser-line filter - thereby narrowing the possible wavelength (with a (+/-)2nm filter, she would have had 530-534nM laser light) to an acceptable level, especially given that the off-the-shelf edge-filters are either going to cut-off/cut-off steeply @ 500/550nM respectively. Thus the Rayleigh scattered light from the narrowed laser would NOT get through to the spectroscope (which is probably why so many of the companies advertising 'cheap' (used relatively speaking) Raman Filter sets suggest using a laser-line filter and a longpass edge filter, such as here, here and here - although to be honest, the later only supports the use of the narrow laser-line bandpass filter, it suggests using the much more expensive notch filter).

Incidentally, using only "normal" mirrors, the set-up should be a hell of a lot easier... We aren't trying to direct light in two different directions at once using the one mirror.:P That makes alignment a shitload easier and the use of camera parts I'm yet to understand (thankfully for me). Personally I'd probably use lenses for a microscope to get the laser light into the sample and the scattered light out. I'd be inclined to try and bounce the laser light around the sample-area for as long as possible, the more it bounces around the more it will elicit responses and focusing those responses back to the collection point only makes sense.*

* Is it possible/feasible to put the cut-on/cut-off filter below the collection lens? Theoretically that would bounce the laser light back into the sample would it not?

PS I honestly think the amount of machining is overrepresented in the PDF's off maryspectra.org - honestly, with a little less haste and a lot more forethought (including the ability to build on her mistakes), it should be possible to build this a whole lot more easily than she did. The first and foremost tip - don't go machining metal until you can get a plastic/wood one to work. Get prototyping & testing out of the way before trying to get things to "LOOK" or "FEEL" right. I cringed when I read that she had only aligned the laser in one axis (horizontal) and that it was misaligned in the vertical axis. Working out a way to hold the sample should have occurred to her WAY before it did.

She does get the kudos for doing the job and actually making the thing, I have GREAT respect for her in that regard. I just think it could possibly have been a little better thought out (this from the person typing with fingers covered with skin from his thighs due to a poorly thought out way of heating glassware:D). I honestly think some of the techies from this forum could contribute GREATLY to the idea and on the way, assist the astronomers to build BETTER spectroscopes for their purposes as well (that thought might well encourage them to be helpful with software design).

[Edited on 5-2-2010 by unome]

[Edited on 5-2-2010 by unome]
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[*] posted on 6-2-2010 at 15:30


It's mentioned on Mary's site that there are problems with photobleaching and fluorescence using the shorter-wavelength lasers, even though they also give stronger Raman signals. Using longer-wavelength lasers with more power than a laser pointer may be a better way to go. It looks like red diode lasers in the 10-100 mw power range aren't all that expensive.

The Raman signal may also be cleaner with a single-mode laser instead of the multimode found in cheap laser pointers and the like. But single-mode lasers can hop between frequencies depending on operating conditions (temperature, other conditions too maybe?). Single-mode diode lasers in the appropriate power/frequency range again aren't all that expensive, but when I see "frequency stabilized" laser modules the price goes WAY up, which makes me wonder how hard it is to achieve. I probably also need to read more about Raman spectroscopy to really understand where the limitations lie with frequency/power/stability choices in the light source.

I think what Mary accomplished is really impressive, no doubt. But the bulk of the work appears to be mechanical and optical. It seems wise to optimize the laser, as far as possible on a restricted budget, since picking a better laser should not make the instrument harder to assemble and may substantially improve the speed, ease, and accuracy of data collection.




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[*] posted on 7-2-2010 at 00:14


Yeah, but the apparent issue is that the signal:noise ratio is excessive at the higher wavelengths, that is why the el-cheapo systems are coming out now using violet/green (violet/Blue ~ 405-450 & green is mid @ 532nm). Prior to that there was too much power needed thus causing excessive heat/etc in the solutions.

Yeah, but minimal bucks in the pocket makes this a low-budget project for me. I have the camera and the pc/laptop, they are the big budget items. I am REALLY interested in using Fiber-Optics for this, it would save untold fucking around with mirrors, collimators, lenses, etc as well as determining intersecting lightpaths, it would also allow for hermetic seals around the CCD end of the project, no stray light can get into that part. Plus with fiber-optics, we could also have a halogen lamp for the UV-Vis region adsorption spectrometry, the output of which would also go to the same hermetically sealed spectrometer (or separate one, the line period/blazing would make trying to use only one hard).
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[*] posted on 7-2-2010 at 15:51


Here is what I think is necessary, a laser-line filter (as narrow as the budget allows) and a steep-edge filter (allows all wavelengths above the cut-on to pass through) as close as possible to the excitation wavelength. There is a couple of interesting filters, namely a 550nM (10CGA-550) (if using 532nM laser) or the 420nM (10CGA-420) (if using the 405nM laser) 1"/25mm colored glass filter from Newport for $25.00. That is hell cheap and would make exclusion of Rayleigh Scattering and stray laser light from being taken into the spectrometer. The best online explanation I could find of the various [url==https://www.omegafilters.com/index.php?page=prod_raman_filter_needs]filters[/url] was good insofar as it explained notch filters - which simply block a portion of the wavelength, ie. that around the laser wavelength, but which transmit both Stokes & Anti-Stokes radiation, whereas Cut-off (aka Shortpass) filters, only transmit Anti-Stokes and Cut-on (aka Longpass) filters only transmit Stokes Radiation.

An interesting idea just came to me, if we used Fiber-Optics, with a miniature shortpass filter on one probe and a miniature longpass filter on the other, the light could be combined to effectively give the same performance as a notch filter, at a fraction of the cost (notch filters are high-tech, whereas edge filters are essentially colored glass).

In terms of narrowing the laser excitation, the laser-line filters from Thorlabs are probably going to be the best option, the 532nM is available in two variations, the [url=http://www.thorlabs.com/thorProduct.cfm?partNumber=FL05532-10(+/-)2nM[/url] (for $44.00) and the (+/-)0.2nM (for $85.00). The latter would probably be the better option, although I am wondering, with the edgepass filter being 17nM above the laser output, do we need a laser line filter at all? I mean, surely, despite the known difficulties with laser diodes, they won't output energy at more than 1/20th of the determined wavelength?

Getting that idea sorted, whether or not the laser-line filter is needed, is going to be the source of significant savings if we can work it (especially as there is NO off-the-shelf filter I can find for the 405nM laser diode).

[Edited on 8-2-2010 by unome]
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[*] posted on 8-2-2010 at 07:51


The green laser pointers are not simple laser diodes, but a VNIR pump diode generally at 808 nm, an NIR laser of Nd:YVO4 at 1046 nm, and a KTP doubler crystal to get 523 nm. There's a lot of the IR wavelengths coming through with the green so you need serious IR filtering to remove those as the CCD will sense them and can be overloaded if the IR is scattered in the equipment. Ordinary green glass or plastic may work for that purpose, if the IR density is 5 or so.

Sometimes there's other wavelengths at low intensities, including Raman light from the optics in the laser device. The intensity is low, and it's not detrimental for use as a pointer put will mess up use in Raman spectroscopy if not filtered out. Neodymium doped glass will clean the beam up a bit, but true line filter may be needed.

Remember that the Raman lines are roughly 7 orders of magnitude less intense than the pumping energy, anything you do to easy the removal of scatter laser light will improve the observation of the Raman spectrum.

With a 532 nm laser the Raman light will be in the range of 550 to 700 nm or so. For this range a neon light works well for generating calibration lines in the green through red. Small neon lights are cheap:


http://www.goldmine-elec-products.com/prodinfo.asp?number=G1...

and can be switched on and off using low cost opti-triac ICs:


http://www.allelectronics.com/make-a-store/item/MOC3022/OPTO...

http://parts.digikey.com/1/parts-kws/pic-opto-isolated-triac


This gives you a reference line source controlled by a few volts at 2 or 3 ma, for less than a Euro. You may be able to design the system so that all the needs to be done to switch between calibrate and to a spectrum is switch either the lamp or laser on/off.

Because of the low intensity of the Raman light noise in the CCD becomes important. For this application there are three important types of noise
1) bad cells (pixels in 2-D detectors such as cameras)
2) impulse noise from electrical circuits or cosmic rays
3) flicker or thermal noise

The first is handled by taking a few scans of the spectrophotometer in the dark, and a few of a 'white' background; bad cells show up as wavelengths much brighter or darker than the neighboring wavelengths. The software picks these out and remembers them, either at power-up or before each scan; replacing them with averaging of neighboring cells during spectrum reading.

Impulse noise is handled by taking a number of consecutive scans and for each cell/pixel/wavelength looking for significantly outlying readings in the set of readings. Any outliers are replaced with values calculated from the remaining readings for that cell/pixels/wavelength.

Flicker noise is reduced by averaging the consecutive reading together, the more readings the better the noise suppression. It is also reduced by cooling the CCD; this is often done with multistage TEC assemblies which obviously add to the cost and complexity.

Note that using such a dispersive system + CCD as a UV-Vis spectroscope - the imaging devices generally don't go very far into the UV and in some cases run out of steam around 380-400 nm making the system a Vis-VNIR spectrometer for 400 to 900 nm.



http://astrophys-assist.com/educate/noise/noise.htm

http://www.fen-net.de/walter.preiss/e/slomoinf.html




http://pagesperso-orange.fr/redlum.xohp/laser/gratingOSA.htm...
http://pagesperso-orange.fr/redlum.xohp/laser/spectra.html


http://www.physics.umd.edu/courses/Phys401/bedaque06/discret...


http://www.astrosurf.com/~buil/us/spe2/hresol4.htm

http://www.amateurspectroscopy.com/color-spectra-of-chemical...






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[*] posted on 8-2-2010 at 12:12


Just a thought, would a simple prism and slit do a good job of removing leftover IR from the 523nM beam ?
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[*] posted on 9-2-2010 at 04:34


Thanks not_important, was kinda wondering when I'd hear some sage advice from you...

So if the Raman light is going to be 550-up, then the edge filters would be the way to go (steep edge, cuts on @ 550nM)?

I'm hoarding funds now to purchase the line-filter and the edge filter, what about with the 405nM blue/violet lasers? How bad are they - I mean, how far off-wavelength can we expect the light to be? I ask because I cannot find a reasonably priced line-filter for that wavelength.

What size/line spacing grating would you recommend? There are shitloads of options and a wealth of contrary opinions available...:(
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[*] posted on 10-2-2010 at 00:14



Quote:
Just a thought, would a simple prism and slit do a good job of removing leftover IR from the 523nM beam ?


In theory, yes. However I've read reports that some cheap green DPSS laser pointers have about the same amount of light at 0,8 and 1 micron as green coming out the front. Typical simple homemade light absorbers don't do much better than absorbing 99% of the incident light, may get only 95-97 percent. This leaves unwanted light that is 4 to 5 orders of magnitude more intense than the Raman signal, so careful design is needed to keep that IR under control. Using a green glass or plastic filter, just conventional filters rather than specialised sharp edge ones, to reduce the IR right out of the laser is likely a good starting point so long as the cheap filter does block the IR; this means it might be better to build the simple spectroscope first and check the filters for doing what you want them to; a CCD will see 808 nm and might respond to 1 micron.

If you can get the laser collimated into a very marrow beam you can use a prism or grating plus slit to clean the beam up o a degree. Combined with a Nd glass filter this might do well in place of a line filter, at the cost of extra messing around with optic.


As for violet laser systems, I suspect that they need several more years for prices to drop on components; first for the diodes themselves and then time for those to be used in other applications and drive price drops in supporting optics such as filters. I've not seen much on the diode performance, what information I have is on the early ones which likely are different than those going into high density optical drives.

I'd go with green lasers for a number of reasons, including

A) they and the filters needed are easier to find and cheaper

B) fluorescence is a bit less of a problem at 532 than down around 400 nm

C) the CCD is several times more sensitive to the Raman frequences for 532 nm excitation than for 406 nm; this is in part because of the way the CCDs work and in part because the human eye is most sensitive to frequencies in the 532 system and much less so to light below 450 nm so there's little incentive to push the CCD response down there.

Quote:
Damn, I really, really hope these become useful to the optic fiber people (RT MID-IR photodiodes).

Actually that range of 1-3 microns is near IR. Why do hope this is so? That range isn't too useful for general structure puzzling out, it's mostly O-H N-H S-H and some C-H lines, used for monitoring production quality when you know what compounds to expects or for looking at local environments in biological situations.
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[*] posted on 10-2-2010 at 10:23


The company I was thinking of that frequently advertises in JCE is DeltaNu. The Advantage Series of desktop instruments is what they direct academics to. I could have sworn that the company was at least once so indiscreet as to quote prices in the JCE, something like $9995 for their entry level model, but I can't seem to find that ad while flipping through my big unorganized stack of JCE back issues.

You'll see that they make instruments operating at 532, 633, 785, and 1064 nanometers. For reasons unknown to me the 1064 nm instrument is export-controlled under ITAR regulations. I am mentioning this company because I think their products might serve to set expectations about what a "low cost" commercial instrument can do, which may serve as a basis for comparison with homebrew efforts.

I have attached the instrument brochures and lab activities found on their web site, since normally you have to register to read them.


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[*] posted on 10-2-2010 at 14:00


Yes, I've just found a 30mW laser module that is within my price range (532nM), now I just have to work out the circuit design/preparation. Now, what grating am I looking for? Some ideas on blaze & lines/mm would be nice. Also, what mirrors, etc. am I going to need?

PS I intend to purchase the laser-line filter, saves a LOT of fucking around, then use the el-cheapo >550nM longpass colored Schott glass filter to remove Rayleigh scatter/stray laser bands. I'm actually looking for a shortpass filter <550nM cutoff, that could be a potential low-cost way around it - the only light going into the sample would be <550nM and the only light accepted from it would be >550nM, so only the Stokes-Shifted radiation would make it to the CCD

PPS Could these be used for FT-NIR? I have seen some papers on the subject. From what I understand, changing the RF frequency and amplitude changes the wavelength that is passed through the crystal? I realise NIR is not "exactly" what we need, but if it is possible, it would be a great advance on what we have now.

[Edited on 10-2-2010 by un0me2]

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[*] posted on 11-2-2010 at 15:17


Here's a good article about monochromators.

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[*] posted on 11-2-2010 at 23:00


Nice stuff coming in:D But let's take a step back, look at the difference in performance this person got when using a CD-RW with the reflective coating removed, as a transmission grating, which is also used by this to make an even better spectrometer (this design is actually the design that is being sold on the internet as being complete with a scale, I'm unsure if any benefit accrues to the designer).

Now, given that with a transmission grating we can avoid having multiple lenses and mirrors, also given that we can determine the actual spectral alignment with mathematics, I propose starting first with a Vis-type spectroscope using a CD-RW, then once I get that working, I'll try with a denuded DVD-R (although that will entail working out the formulae for where to put the various components, especially the scale.

I also have a sneaky suspicion, given that spectrometers can be used as a modular component, that I could use the same transmission grating with the Raman signal...

[Edited on 12-2-2010 by un0me2]
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[*] posted on 12-2-2010 at 00:11


A DVD-R will give much better performance than a CD.

Transmission grating do not eliminate all lens and mirrors, especially for the higher performance required for Raman work. The higher losses in a CD/DVD derived transmission grating may needed to be considered when designing a Raman spectroscope; with a 25 mw excitation laser the total Raman energy is less than 10 nanowatts.

It's hard to beat using a reference source's lines for establishing a scale, or calibrating a CCD. I mentioned neon lamps, several of the documents regarding Raman spectroscopy used neon lamps for calibration. Neon has lines throughout most of the visible spectrum, easing calibration. For Raman work a secondary calibration using Raman references is needed; polystyrene, naphthalene, and sulphur are easy to obtain primary standards.

Quote:
PPS Could these be used for FT-NIR?

I don't see how. Those are an electronically controlled version of an adjustable defraction grating. Fournier Transform spectroscopy does not use dispersive elements, instead collecting all the light without dispersion onto a single sensor and using DSP (AKA Kenneth) to resolve the differing frequencies in the light stricking the sensor.

===============================

Note that a 10 mw or higher laser is Class IIIb/Class 3B, and requires protective eyewear. If you do not know the level of diode pump light (typically 808 nm) and 1064 nm light in the output beam, you really should get eyewear rated for protection against all three frequencies. You can avoid that if you mount a IR filter on the front of the laser so that no light can avoid passing through the filter and the filter rating is at least O.D. 2 for the IR bands; with that just protection against 532 nm should be OK.


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[*] posted on 12-2-2010 at 14:23


(Just quickly, is anyone else having a bitch of a time with acrobat & firefox? It's taken me a dozen attempts to try and get this reply together and posted).

not_important, I was actually considering purchasing a 'real' transmission grating (or a reflective grating) from either Thorlabs or Edmund Optics for the Raman instrument. I have not been able to glean enough information from the materials I have read to date to determine what exactly I should purchase.

As to the other, I have attached a number of papers, one - Instrumentation for FIR Spectroscopy, is a wonderful read... It appears to be suggesting that the decisions made with regard to instrumentation were based upon limited computing power and that we are now stuck with the resulting compromise.

I have been reading a LOT of articles (all that I can access in fact, be that by fair means or foul), on these AOTF's, and they are being used extensively in NIR/MWIR scanning spectroscopy. The following quote is lifted directly from F. Kowol et al., Mid infrared acousto-optical tunable filter-spectrometer for rapid identification of black plastics from automobile construction, J. Near Infrared Spectrosc. 6, A149–A151 (1998) (attached):

Quote:
...With black plastics the problem exists that NIR light (0.7–2.5 mm) is strongly absorbed by electronic resonances of the colorant (carbon black). The application of mid infrared spectroscopy (MIR) in the spectral fingerprint region (2.5–20 mm) suffers from noise generated by the thermal background. However, it can be demonstrated that the restriction to the wavelength region between 2.5 and 4 mm, where the fundamentals of the CH– and NH–stretching vibrations are observed, is sufficient for a reliable distinction of the major types of (blackened) technical plastics. To provide a rugged and cost-effective alternative to the FT-technique often applied to this task, an MIR spectrometer with an Acousto-Optical Tunable Filter (AOTF) as a wavelength selecting device and a new type of peltier cooled Mercurium Cadmium Telluride (MCT) detector has been developed. In Figure 1 spectra of (blackened) plastics used in automobile construction are shown, which were obtained with the new spectrometer.

Of course, the spectrometer is not restricted to black plastics only; it is also applicable for the identification
of different types of polymers used for electronic devices. Since spectra can be taken within less than one second, the system can manage high throughputs of, for example, electronic waste. The basic set-up of the system is shown in Figure 2.


Yes, I was wrong insofar as I was suggesting this might be of utility in FT-IR spectroscopy, but I am seeing an awful lot of papers where this technology is being used in 'Scanning NIR/MWIR Spectrscopy', which with the peltier cooled detector and no moving parts (and limited difficulty in building by the looks of it), it might be a better option than FT-IR for home use. The caveat being, that the components, unless they become cheap because of Fiber Optics or somesuch technological innovation, they are likely to remain fucking hard to find and too expensive to contemplate purchasing.

EDIT

For some reason this board is not uploading the first paper I upload, but only the ones after that (ie. like here, numbers 2,3 & 4).

Here is the link to Griffiths & Holmes, Instrumentation for Far-Infrared Spectroscopy.

And Gata, Imaging Spectroscopy Using Tunable Filters: A Review is an invited paper to some techexpo on the subject.

Here too, is a mini-review on the subject, Tran, Principles, Instrumentation, and Applications of Infrared Multispectral Imaging, An Overview, Analytical Letters, 38: 735–752, 2005

Attachment: Kotidis.etal.Optical.Tunable.Filter.Based.Micro.Instrumentation.for.Industrial.Applications.pdf (605kB)
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Attachment: Kowol.etal.MIR.AOTF.Spectrometer.pdf (53kB)
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Attachment: Varasi.Verona.Integrated.Acousto.Optical.Tunable.Filters.AOTF.Development.Technology.and.Applications.pdf (229kB)
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[Edited on 12-2-2010 by unome]

[Edited on 12-2-2010 by unome]
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[*] posted on 12-2-2010 at 16:57


I'll have to double post this, as I have to mention that I have been trying my fucking damndest to remove the aluminium backing off some CD-R's and have had no real success, I also checked the protector disc in the spindle (the blank one) and couldn't see any discernable spectra, thus leading me to conclude that it did not in fact, have the requisite lines/mm. That left me with trying out the idea in this (thanks to Jokull) paper, which does in fact work. I will next try it out from the word go, first I have to clean up a shitload of mess from trying to scrape the fucking aluminium off (with the attendant scratches, etc. that that would leave on the non-grating side of the transmission grating, which presumably would have serious consequences for any spectra done with the artifacts in place).

I will try to carefully, with a scalpel-type blade, work my way around the extreme edges of the coating, then try and lift it in one go with reinforced tape. I'll then attempt to use a toilet roll tube/paper towel tube to set up one of these as proof of concept. That done, we'll proceed:P

PS Although, thinking about it, if one were to cut the circles into an intact CD



Then attach the tape to the shiny side, presumably the edges of the circle that is cut would be sufficient to remove the need to carefully dislodge the hold of that layer from the inside and outside extremities of the CD-R (presumptively, if the cut on the inner diameter and outer diameter, allow the entire thing to come off, then it is only really adhering on the inner and outer diameters, not through the middle). Kinda hard to explain, does anyone follow my logic?

PPS I was thinking, based upon the article above where the proper camera angle, relative to the grating is 19.2', that if the grating were inset in the middle part of the tube, such as to allow the camera to be placed directly over/inside the lower end of the tube (upper end is the slit), then that should give the best results for our purposes, no?



I am also seriously wondering if I could cut round 'transmission gratings' from a DVD-R, would it be easier to separate the two layers (actually there are apparently 3 sandwiched into it), allowing me to utilize the much higher line/mm resolution for this project:D

[Edited on 13-2-2010 by unome]

Cutting the circle out, THEN removing the aluminium layer works like a fucking dream. Now, I am in the process (sans protractor, there is NOT one in the fucking house:mad:) of determining the notch I have to take out of a second tube in order to put the camera at the most advantageous angle of 19.2'.

I also have to work out the most advantageous angle for DVD-R's as the same technique works a charm with them too*

*Personally, I find it easier to draw the circle, then cut the piece out of the DVD-R/CD-R then trim it, it avoids a lot of stray scratches and possible damage to the flatness of the transmission grating.

PS Given I have no protractor, I have to work it out - I'm guessing here (well, not quite, but it has been a LONG time), but tan 19.2 = O/4, so tan 19.2 being 0.365531827, then that x 4 = O, ie. O=1.46cm, as I am using a decent ruler, 14.5mm should be easily doable.

PPS I am currently reinstalling the drivers for a Microsoft LifeCam VX-1000. It is about the cheapest digicam I can think of, with absolutely crap resolution, etc. But it should suffice for this part of the project. We'll cross the other bridges when we come to them;)

FUCKING HELL

Anyone got a fix for those fucking cameras? I have two sitting here, neither of which I can get to work:mad:

BTW I have just attached the threaded upper neck section of cordial bottles (that are the right diameter) to both ends of a toilet roll tube with tape, then spray painted the whole lot, inside & out, black. I cut out the center of on of the lids and mounted a circular 'transmission' grating in it with araldite. In the morning, both the Araldite and the spraypaint should be dry and I can start putting the whole thing together (I'm going to cut a small rectangle out of the other lid then use two pieces of metal (straight edges) to form the slit over the same.

I just thought that threading everything would be a worthwhile idea, given the purpose of the plan, to make a modular spectroscope.:cool:

[Edited on 13-2-2010 by unome]
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[*] posted on 13-2-2010 at 23:04


Finally managed to get the bloody camera(s) (found another one, a VX3000 - which has much better resolution and gives better results) working and the pics are attached (from my wooden version of the JChemEd article (the first one). Next, I'll grab that online image tool (that is mentioned in one of the articles) and try and convert them to recognisable spectra.

Spectra3.jpg - 54kB Spectra4.jpg - 67kB Spectra1.jpg - 62kB Spectra2.jpg - 53kB

[Edited on 14-2-2010 by unome]

SPECTRA7.jpg - 100kBSPECTRA8.jpg - 81kBSPECTRA6.jpg - 66kBSPECTRA5.jpg - 95kB
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[*] posted on 14-2-2010 at 19:30


Has anyone else got anything to add to this:

Gigavision - Cheap naked chips snap a perfect picture
"Cheap naked chips snap a perfect picture"
* 07 October 2009 by Paul Marks
* New Scientist Magazine issue 2729.

HOW can image sensors - the most complicated and expensive part of a digital camera - be made cheaper and less complex? Easy: take the lid off a memory chip and use that instead.

As simple as it sounds, that pretty much sums up a device being developed by a team led by Edoardo Charbon, of the Technical University of Delft, in the Netherlands. In a paper presented at an imaging conference in Kyoto, Japan, this week, the team say that their so-called "gigavision" sensor will pave the way for cellphones and other inexpensive gadgets that take richer, more pleasing pictures than today's devices. Crucially, Charbon says the device performs better in both very bright light and dim light - conditions which regular digital cameras struggle to cope with.

While Charbon's idea is new and has a patent pending, the principle behind it is not. It has long been known that memory chips are extremely sensitive to light: remove their black plastic packages to let in light, and the onrush of photons energises electrons, creating a current in each memory cell that overwhelms the tiny stored charge that might have represented digital information. "Light simply destroys the information," says Martin Vetterli, a member of the EPFL team.

A similar effect occurs aboard spacecraft: when energetic cosmic rays hit a cell in an unprotected memory chip they can "flip" the state of the cell, corrupting the data stored in the chip.

What Charbon and his team have found is that when they carefully focus light arriving on an exposed memory chip, the charge stored in every cell corresponds to whether that cell is in a light or dark area. The chip is in effect storing a digital image.

All very clever, you might say, but why would anyone want to do that? The answer is that the two types of sensor chips used in today's digital cameras store the brightness of each pixel as an analogue signal. To translate this into a form that can be stored digitally, they need complex, bulky, noise-inducing circuitry.

The charge-coupled device (CCD) sensors used on early cameras and camcorders, and the cheaper and more modern complementary metal oxide semiconductor (CMOS) type both operate on a similar principle. On each, the area that forms an individual pixel can be thought of as a small charge-containing "bucket". The size of the charge contained in one of these buckets depends only on the amount of light falling on it.

In a CCD, the contents of each bucket of charge are "poured" into the bucket next door, and then the next until the signal reaches the edge of the chip. There, an analogue-to-digital converter (ADC) typically assigns it an 8-bit greyscale value, ranging from 0 to 255. In a CMOS sensor, the charge is converted to a voltage local to each pixel before being shunted off to an ADC at the edge of the chip - where it too is assigned a greyscale value between 0 and 255 (see diagram).

A memory chip needs none of this conversion circuitry, as it creates digital data directly. As a result, says Vetterli, the memory cell will always be 100 times smaller than CMOS sensor cells; it is bound to be that way because of the sheer number of signal-conditioning transistors the CMOS sensor needs around each pixel. "Our technology will always be two orders of magnitude smaller," he says.

So for every pixel on one of today's sensors, the memory-based sensor could have 100 pixels. A chip the size of a 10-megapixel camera sensor will have 100 times as many sensing cells if implemented in memory technology - hence the choice of the gigavision name.

But don't expect a gigapixel camera any time soon. Unlike the pixels in a conventional sensor, which record a greyscale, the cells in Charbon's memory-chip sensor are simple on-off devices: they can only store a digital 0 or 1, for which read either light or dark. To build a sensor that can record shades of grey, EPFL engineer Feng Yang, who presented the Kyoto paper, is developing a software algorithm that looks across an array of 100 pixels to estimate their overall greyscale value.

It's a technique called spatial oversampling - and while it's early days, he's getting somewhere. "It's turning out to be a lot more accurate than the greyscale values you get from regular CMOS sensors," says Vetterli. "Analogue to digital conversion gives only poor estimates of the actual analogue light value."

They'll have their work cut out, observers say. A major problem they will have to overcome is that of the poor sensitivity of their pint-sized pixels. Their size means the number of photons that can be scooped up by each of them will be small - and that can make for a very noisy signal.

The prospect of producing image sensors as cheaply and easily as memory chips is bound to attract attention, says Alexis Gerard, an analyst and chief executive of the consultancy Future Image in San Mateo, California. "It will be pretty interesting if they can make these sensors using regular memory-chip-making technology."

From: http://www.newscientist.com/article/mg20427295.100-cheap-nak...

That may be the answer to our prayers, work out what they mean and how to access that digital image, given that there would be no UV filter, the image may be able to go from true UV-NIR

The alternative would be working out how to coat the CCD detector array with "Lumogen" (a BASF product), a phosphor that enables the standard CCD to be utilized well into the UV region from what I'm reading.

[Edited on 15-2-2010 by un0me2]
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