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All Over But the Funding: Open Hardware Spectrometer Kit
New submitter mybluevan writes "The Public Laboratory for Open Technology and Science is putting together an open hardware spectrometer kit
on Kickstarter. The kits are built using an HD webcam, discarded DVD, and a couple other odd bits. They've also put together a kit for your smart phone and open-source software for desktop, Android, and iOS. Need to analyze the contents of your coffee, the output of your new grow lights, or a distant star on a budget? Just build your own spectrometer, or pick up the limited edition steampunk version."
Besides making cool hardware, they'd like to "build a Wikipedia-style library of open source spectra, and to refine and improve sample collection and analysis techniques. We imagine a kind of 'SHAZAM for materials' which can help to investigate chemical spills, diagnose crop diseases, identify contaminants in household products, and even analyze olive oil, coffee, and homebrew beer."
One of the steps of building the kit is removing the IR filter from the webcam.
I did my PhD on a topic that involved building some gear related to spectroscopy. A good deal of my work involved cludging together optical parts from seemingly unrelated sources. As such I just love this sort of thing. Unfortunately I have to agree with the above commenter that the precision is just not going to be good enough from a webcam for real chemometric measurement. However with a little more investment the precision could be improved. The real difficulty will come from the need to calibrate the thing. You need to know exactly where each wavelength of interest is centred ** exactly** on the sensor at the time of measurement and this is not a trivial task. Unfortunately proper calibration is so often just not achieved... and that's not even beginning to consider the issues of background/ stay light, Johnson noise etc. I suppose this design could be used for crude detection like the article says rofl.
Lots of people have been working in this field. The most impressive results are achieved by the astronomy community. link It is possible to produce a home made spectrometer that gets useful results. Some of these are capable of resolution sufficient to identify chemicals. These are sophisticated and often use a peltier cell to cool the CCD in order to reduce noise. link
I did a project whose aim was to produce a cheap spectrometer to match paint colors. link The problems I found were:
My own engineering trade-off was sensitivity vs. resolution. To get spectra for dark colored paints, I widened the slit which reduced resolution. That, as far as I could tell, was reasonable because I wasn't trying to identify chemicals and the spectra from paints weren't particularly sharp.
The folks in TFA have a site where people can upload spectra. That's fine but a huge database of spectra is not too useful. The spectra have to be organized somehow. Here's an example. In fact the problem can be quite daunting
I have converted consumer junk to remove IR sensors on many devices from simple webcams to high end DSLRs. It is simple.
The hardest part about the process on cameras is working with tiny ribbon cables and sourcing replacement optics if you wish the resulting camera to operate again (since removing the filter completely will result in your camera becoming nearsighted.
Neither of these are a problem with webcams, and the only issue you're likely to have is that some webcams have their filter glued onto the CCD rather than just placed on and retained with a small amount of pressure from the case.
I don't want to sound all negative here, but... I don't have a choice, do I? A visible light spectrophotometer will not "detect toxins", no matter how much you try to make it open-source or crowd-sourced. The very concept of identifying compounds by visible light absorbance is very much flawed. Thing is, *most* molecules will not absorb visible or near UV light in a way that is specific enough. Real Chemists (TM) traditionally use the so-called IR fingerprint region for this purpose. This region is from approx. 700 to 1500 cm-1 (about 6 to 20 uM - that is 6000 to 20000 nanometers). A special detector is needed for these wavelengths. The one we have in our lab is cooled with LN2 and costs south of $15K. We also have a UV-Vis spectrophotometer, which has its own purpose. That purpose is not "identifying toxins", or analyzing any unknowns. Now, on to my next point. Identifying molecules is challenging, because they are very, very, very mindbogglingly small. Chemists have been grappling with this challenge for a long time. There are many spectrometric methods out there, including IR and UV-Vis (briefly discussed above), near-IR (900 to ~1800 nm, useful for *some* fingerprinting), and NMR (60-1000 MHz, very informative, bit needs a BIG magnet). Spectral data for many molecules of interest has been compiled into readily accessible databases, and is easily accessible. Some of the databases are proprietary/pay-per-view: https://ftirsearch.com/features/libraries/sea407.htm Some are semi-public: http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi And some are government/public: http://webbook.nist.gov/chemistry/ The people who started this project do not seem to grasp of the very basic concepts of chemistry, nor did they do any research on the subject. Reading a Wikipedia article on UV-Vis would have been a good start. What is even more disconcerting is that the fundraising effort behind this cardboard spectroscope has been a success. One just has to hope that nobody buys this to screen their food for "toxins", or to teach their kids chemistry.
Non-invasive glucose measurement is one of the holy grails of portable spectrometer design. As it happens, it is most practical using NIR and IR. There are products that will be on the market soon. The kit from TFA won't do it, of course. Spectral resolution is too low, and it looks at all the wrong wavelengths.
The bit depth of the sensor is more or less irrelevant. Spectral accuracy is a function of their diffraction grating and the size of the sensor: it's a spatial effect, not a detection/encoding one.
The bit depth does matter for sensitivity and dynamic range. For sensitivity, the dark noise of the sensor is also important, since you can effectively average for a very long time.