
RamanPi – The 3D Printable Raspberry Pi Raman Spectrometer - MichaelAO
http://publiclab.org/notes/flatCat/08-29-2014/ramanpi-the-3d-printable-raspberry-pi-raman-spectrometer
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jlev
Love the PublicLab's enthusiastic embrace of open source hardware and software
licenses. If you don't want to 3d-print your own, you can get an awesome
bluray laser spectrometer + webcam at their Kickstarter.
[https://www.kickstarter.com/projects/publiclab/the-
homebrew-...](https://www.kickstarter.com/projects/publiclab/the-homebrew-oil-
testing-kit)

Participatory citizen-science FTW!

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mcmancini
This is neat, but I wonder what the actual resolution of this device is (and
how stable are the measurements)? Is it useful for anything other than just
novelty?

Not sure that I really see the point of 3d printing the parts (although
scratching many geek itches, yes) when your optics are going to be a much
larger cost than the mechanical mounts, and the quality of the mechanical
mount is so very important.

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VLM
"when your optics are going to be a much larger cost than the mechanical
mounts"

It would be nice if that were always the case. I suspect this is very much
like my electronics stuff, a $2 MMIC amp mounted in a $30 aluminum diecast
box. You can blow a lot of money on chassis and component mounting.

Also, speed. Could print something where an exact model just drops in place
with superglue and a perfect fit. I could probably replicate their chassis in
aluminum by hand in a days work, but I'd much rather go lazy and print one.
Also each engineering revision drops from "days work" to "hit print and come
back later"

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mcmancini
A custom housing could certainly be on the order or more expensive than the
optics, but if you were using catalog parts the mechanics should not be the
largest cost.

For example, a "cheap" 1" notch filter from Thor would run about $500, whereas
a kinematic mount would be somewhere closer to $40.

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ycui1986
For Raman spectroscopy, a "cheap" notch filter won't even work. a 0.5" long
pass filter made for Raman can easily cost $2000. When you have a $2000
filter, you have to use temperature stabilized laser, otherwise the laser
wavelength shifts, thus, spectrum shifts. Grating and concave mirror can also
cost a fortune.

Also, Raman typically is very weak, roughly 10^-6 of the laser intensity. The
CCD/CMOS has to be low dark count.

The things is, if you have spent $5000 on components, why should you save
couple hundreds of dollar on good optical mounts?

This 3D printed plastic base/structure will never have good stability and
precision for any serious scientific application.

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mcmancini
Nonsense, you can run a perfectly good student lab with a cheap notch filter
and pull a usable spectra off something like polystyrene or Tylenol. What I'm
wondering is if this thing could even do that. Looking at the abuse the
diffraction grating has taken suggests the answer is no.

As far as a decent scientific bench instrument goes, we're mostly in
agreement. I'd love to know if you could make a decent optomechanic system
like this on a SLS with a metal substrate.

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flatCat1597
Hey, I just want to thank everyone for the kind words I've been getting..! I'd
love to hear any suggestions or comments anyone has on my project..!

Take a look if you're interested at the project page on hackaday.io...
hackaday.io/project/1279

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CamperBob2
From the article:

 _Ordinarily, an expensive notch filter would be used which is cost
prohibitive for most average people. My system avoids this cost by using two
less expensive edge filters which when combined in the correct manner provide
the same benefit as the notch filter...at the minimal cost of a little extra
computing time._

If the goal is to cancel the incident light "carrier" frequency, couldn't this
also be done interferometrically, just using mirrors and beam splitters?

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nmz787
That's exactly what expensive notch filters do.

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CamperBob2
Cool. I guess it was naive of me to visualize a piece of colored glass. :)

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discardorama
I'm not a physics grad, but I'm curious: can one _predict_ the Raman response
of a molecule? For example: suppose I'm interested in detecting molecule X. I
have a laser of frequency f. Given these two, can I predict that the reflected
light (after Raman effect) will definitely be of frequency f' ?

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gipp
In principle, yes. The relevant equations can't be solved exactly for any non-
trivial molecules. But there are a large number of different approximation
methods, which boil down to a numerical optimization problem that converges on
a bounding value.

These methods generally don't scale well, however, so as the size of the
system grows, the approximations that can be successfully solved quickly grow
too coarse to be useful.

There are also a lot of simple heuristic methods that a chemist can employ
that don't require collaboration with a theorist, e.g. particlar chemical
"motifs"/"functional groups" generally have a Raman resonance at a consistent
frequency that is affected only slightly by the surrounding parts of the
molecule (and the general direction and order of magnitude of that shift can
also be approximated heuristically).

Finally, there are enormous catalogs of recorded spectra for a huge range of
molecules at e.g. the NIST webbook
([http://webbook.nist.gov/chemistry/](http://webbook.nist.gov/chemistry/)).

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discardorama
Thanks. Followup question: does the light source _have_ to be a laser? I read
on the Wiki that CV Raman demonstrated the phenomenon with sunlight. So would
it be possible to just observe the reflected light from an object and
determine the molecules on it that are lighting up? If so, can (someday)
someone come with a Star Trek-style tricorder that you look through and figure
out the molecular composition of an object? Maybe at a distance of 100 meters?

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0xdeadbeefbabe
How does this compare to vessyl[0]? Is building one of these harder than
building a radio kit?

[0] [https://www.myvessyl.com/](https://www.myvessyl.com/)

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VLM
They are opposites, in that the vessyl site strictly discusses apps and
salesmanship without mentioning how it works whereas this project is strictly
how it works and what it does and you figure out what to do with it.

I enjoyed reading the vessyl site because it explained nothing at all about
how it works, so I immediately applied my engineering gut sense to trying to
figure out how I'd make one if I wanted to. Small scale calorimetry heating
and cooling with a peltier device a couple degrees at a time? That works for
metals and phase transitions but probably not enough to tell diet sodas apart.
Some kind of high res ultrasound to analyze waves and thus viscosity and thus
density/composition? Personally I'd go all EE on it and shove a modest AC
signal thru a big capacitor and get the dielectric constant of the "stuff" in
the cup. A really accurate capacitive sensor to tell exact volume with a
really accurate force sensor on the base to read weight/mass and a decent
thermistor to calibrate for temp and you've got density at a specific temp.
Once the raw data is in there, I'm guessing its all just lookups and best fits
and the like.

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fhnjh
I'd do all of it, and then pull it apart with machine learning.

