
Make a Public Lab Spectrometer (2017) - Tomte
https://publiclab.org/wiki/economist
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joshvm
One thing to bear in mind with these kind of DIY spectrometers is that
typically you only perform a wavelength calibration (e.g. you can transform
from pixel number on the detector to wavelength). That's reasonably easy to do
with a CFL bulb and some known spectral peaks.

What's much more challenging is accurately measuring colours, or the relative
intensity of certain spectral features. The problem is that your image sensor
has its own spectral sensitivity which you usually don't know. This also means
that you can't really quantitatively compare spectra between spectrometers
because the sensors will be different.

Most of this time this isn't much of an issue, but suppose you want to measure
the spectrum of some tungsten halogen lights around your house (or the sun).
You won't see the characteristic blackbody curve. You'll see the spectral
response of your detector with some absorption lines on it. It shouldn't
matter much for e.g. differentiating between two species because you'll have
the same instrumental reponse for each.

A fairly simple way to do this is to use a tungsten bulb with a known colour
temperature. You can use the blackbody equation to generate the expected
spectrum and work out the transform between your spectrometer and the
calibration lamp.

If you want to do spectroradiometry (measure the absolute intensity per
wavelength), you need to know the power spectral distribution of the lamp too.
In the real world you'd need to buy a NIST-traceable (or equivalent)
calibrated lamp.

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burfog
It hurts me to know that I can't cheaply buy a spectrometer, colorimeter,
spectrophotometer, densitometer, spectroradiometer, turbidimeter, or similar.
There are toys sold with essentially the same level of complexity that go for
roughly 1% of the price. It seems that silly games are far more popular than
chemistry labs.

Consider a 3-channel colorimeter. You need a pair of tri-color LEDs, a
squareish glass test tube, a power source (a USB cable or battery holder), a
chunk of black plastic, and a multimeter. In place of the multimeter, a USB
interface can be had via a very cheap chip. Let the user move the tri-color
LED, and you get a turbidimeter too.

The whole OLPC XO laptop, with that custom screen and support ASIC, cost only
about $188 to make.

The problem is popularity. Almost nobody wants to study science. Meanwhile, we
can get Pokemon Power Action Pikachu for $19.95, or Nerf Laser Ops Pro
(2-Pack) for $41.25, Boxer (Interactive A.I. Robot) for $64.99, and so many
other useless things.

~~~
dejv
I wonder what kind of experiments/projects you will do with given instrument.

My company tried to sell "cheap" (as in ~1000/1500 USD) VIS/NIR spectrometers,
but it was very hard to actually find some viable market.

You can actually find modules[0] that will give you quite a good performance
for given price. Electronics needed to run those devices are quite complex,
but I even seen some open hardware boards available that might be half decent.
Still, you are going to be in territory of 500 USD even in fully DIY build.

[0] [https://www.hamamatsu.com/eu/en/product/photometry-
systems/m...](https://www.hamamatsu.com/eu/en/product/photometry-systems/mini-
spectrometer/micro-spectrometer/index.html)

~~~
burfog
I'd teach AP Chemistry. It's useful for teaching AP Biology too. The
equipment, with some application-specific packaging and instructions, might
also work for: aquarium keepers, gardeners, pool owners, beer brewers, people
on well water.

Full DIY builds are cheap for a colorimeter, photometer, and turbidimeter. One
can make all three for about $30, not counting labor and a computer.

The trouble is manufacturing volume. Sadly, not every student takes AP
Chemistry. I realize that the manufacturing processes with low per-unit cost
(casting, forging, blow molding, extrusion...) all have high set-up costs.
This really is my complaint: if people cared about learning science as much as
they cared about pointless toys, the production volume would let us have these
things at prices ranging from a few dollars (simple colorimeter) to a hundred
dollars (stuff with decent lenses and moving parts). The affordability of low-
end cell phones makes it clear how low prices could go. I know you're having
trouble due to the costs associated with low volume, but to an ordinary
purchaser your price looks like a rip-off.

FYI, there are about 320000 people learning AP Chemistry, of which about 9600
are homeschooled and thus unlikely to reuse equipment from prior years.
Assuming the equipment in schools needs replacement every 20 years, perhaps
25000 could be sold per year if the price were low enough to not be a budget
problem. I'm thinking you'd have to knock about 90% off the price, and I
suspect that 25000 per year is nowhere near the volume you'd need to justify
the set-up costs of high-volume manufacturing processes. I may be wrong
though... maybe you can get a good deal on plastic molds or custom aluminum
extrusion dies or molds for some sort of casting process.

~~~
joshvm
>I know you're having trouble due to the costs associated with low volume, but
to an ordinary purchaser your price looks like a rip-off.

Yup, being a hardware engineer makes you appreciate just how cheap some things
are. Bear in mind that our motivation to develop our instrument was shock at
how much the incumbents were charging! I think getting to O($500) for a
quality instrument without high volume would be fairly doable with some clever
design work. Getting to O($100) yes, if you could make the entire optical
bench as a MEMS product and you have scale on your side (this has been done
partially by SCiO).

A big limiting factor is that there aren't many perceived use-cases beyond
"science". The big consumer one that everyone touts is food inspection, but
outside a few very specific uses, it's a gimmick to most people. Maybe for
avocado ripeness... It's also not ready for primetime yet. The real markets
are places like astronomy where people don't hesitate spending thousands on
glass and silicon. Brewing is another. You want hobbies which are taken up by
data nerds.

The other problem is that spectrometers are overkill for a lot of stuff.
Colour measurement for instance - (calibrated) colorimiter ICs are incredibly
cheap. For applications where you need the resolution you either need an
experienced user or software to automatically interpret the results.

There are some other off the shelf solutions: Hamamatsu make a low resolution
spectrometer that's good enough for general demonstration - costs a few
hundred[1]. AMS make an 18-channel "spectral" sensor which costs $10 (ish?).
There have been a few Kickstarters which are trying to address this e.g. the
SCiO [2]. That's about the closest we've got so far to a mass market
spectrometer and it costs $299.

The problem as you say is: how many schools can afford even $100 on a device
which may only get used once or twice a year? Holding a transmission grating
up to your eye will solve maybe 90% of the needs of most teachers. It's enough
to show different line emission from a bunch of lamps. The public labs
spectrometer would do for the other %10.

[1][https://www.consumerphysics.com/scio-for-
consumers/](https://www.consumerphysics.com/scio-for-consumers/)

[2][https://groupgets.com/manufacturers/hamamatsu-
photonics/prod...](https://groupgets.com/manufacturers/hamamatsu-
photonics/products/c12880ma-micro-spectrometer)

~~~
burfog
I don't think MEMS is required. For $60 you can get a 2 TB hard drive. It
contains platters that are several inches in diameter, and there is a swinging
arm that is a couple inches long. All these parts move with incredibly tight
tolerances. DVD writers are $26, and Blu-Ray writers are $120. Those are an
even better comparison, with moving optics.

Food inspection is silly unless that is your job. People who keep aquariums,
particularly saltwater ones, are a more likely market.

18 channels sounds great for most school labs. It's enough for a crude graph
of chlorophyll's absorption and/or florescence. It might even be enough to
distinctly identify the four main sources of color in spinach leaves. Repeated
sampling at about 10 Hz would be plenty for all the labs involving reaction
rates; most could be done with 0.1 Hz. For example, a popular lab involves
measuring the changing rate at which Crystal Violet is destroyed by lye. Some
less-demanding popular labs involve measuring the amount of copper in brass
(as copper nitrate via nitric acid) or the molarity of blue food coloring in
drinks.

~~~
joshvm
Do you fancy chatting further about this? I've been toying with writing a
hacker's guide to spectroscopy, and it would be interesting to know what sort
of experiments schools would find useful. Feel free to get in touch at josh at
iayc dot org.

> Food inspection is silly unless that is your job.

Nevertheless, that's what the majority of the consumer devices are going for.
Ripeness detection, "quality" measurements (eg measuring brix) and so on. SCiO
also mention things like pill identification and body fat measurements. This
is the problem: these are all gimmicky things that get VC funding but aren't
all that useful in the real world.

This is the AMS sensor btw [https://www.digikey.co.uk/product-
detail/en/ams/AS72651-BLGT...](https://www.digikey.co.uk/product-
detail/en/ams/AS72651-BLGT/AS72651-BLGTCT-ND/9486614)

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knolan
I bought an older version of this for a student of mine to get her started on
her dive into optical measurement. It’s a nice simple kit and helps demystify
some of the technology.

She ended up building a nice polariscope for measuring microfluidic flows of
viscoelastic fluids.

