
Public Lab DIY Spectrometry Kit - stevewilhelm
http://www.kickstarter.com/projects/jywarren/public-lab-diy-spectrometry-kit
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tomkinstinch
I attempted this before, even on Kickstarter:

<http://www.openspectrometer.com/>

[http://www.kickstarter.com/projects/makerhaus/open-
spectrome...](http://www.kickstarter.com/projects/makerhaus/open-
spectrometer?ref=live)

Unfortunately we did not hit our fundraising target, and it's now a dead
project. I'd love to see this new Kickstart project succeed, especially the
online spectral database. Commercial databases of spectral signatures are
expensive (thousands of dollars), and they shouldn't be.

Part of our problem, I think, was that we were attempting the project while
taking classes full time, and working.

We were planning to release two different designs at different price points.
One would have used inexpensive ruled transmission grating, while a more
expensive unit would have used a concave holographic aberration-corrected
reflective grating. The latter would have had better system efficiency, less
"smear" across pixels, and better linearity. Each would have made use of a
linear CCD array with good sensitivity (the same chip used in the Ocean Optics
units). Concave gratings add a couple hundred bucks to the price, but are well
worth it if you need to do UV.

A challenge we didn't get to solve was how to make something DIY that could be
used to detect UV. It's difficult to sputter a uniform phosphor coating or
remove a sensor window without specialized equipment.

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forgottenpaswrd
"Part of our problem, I think, was that we were attempting the project while
taking classes full time, and working."

I think your project was fantastic.

I see the main problem is the price. Most of the normal people can't justify
expending $800 on an spectrometer. They don't even know what they could use it
for.

People that could justify it because they need it will buy tested equipment
anyway, they have money but not time.

So you need to fill the gap. The new project is making it super cheap so they
could get traction like Reprap machines. At first they are terrible, but with
enough people and huge market improvement is so fast like with the PC that at
some point in the future it could get better than commercial closed systems.

Start small, if you can't detect UV it does not matter, remember it is 100x
better than Newton ever had.

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nmz787
Thanks for the kind words!

The $800 price tag was just a kickstarter only price, since we were still
largely in the development phase we weren't sure of final pricing. Now it
looks like a UV capable scientific instrument with a chinese grating would
cost around $400, if you want the same optics but U.S. made it would be around
$900.

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zdw
I'm no optical scientist, but seeing as the diffraction grating being used is
recycled from a DVD and the sensor is a consumer webcam, I wonder how
accurate/reproducible the results would be.

A conventional diffraction grating is pretty inexpensive ($2.50):
<http://www.rainbowsymphonystore.com/difgratfilsh.html>

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tomkinstinch
Gratings like this (or a CD) will work, but they introduce plenty of
artifacts. Scratches and material nonuniformities can scatter light in a way
you don't want, and reduce the (already limited) amount of light hitting the
right spot on the sensor.

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daniel_reetz
I applaud the approach, but it seems like a challenge to achieve useful
accuracy across many devices. I am a little skeptical about the 3nm claims.

First, the (bandpass) Bayer color filter arrays on cameras limit their color
resolution pretty sharply. It might be a better (if less elegant) approach to
use the ambient light sensor and scan it across the spectrum generated by the
diffraction grating.

Also, the resolution of a spectrometer is in part determined by entry slit
width. Old timers and DIY'ers use abutted razorblades to make a clean,
straight slit. Problem here is that there is a trade-off. You need a wide slit
to get light in for these insensitive webcam sensors. But then you lose
spectral resolution because your slit is wide. Alternately, you narrow the
slit to get more spectral resolution, but then your image is dominated by
noise. I'm sure these guys have found the sweet spot.

Second, the spectral response of the color filter arrays varies both from
camera to camera and even within same-model sensors (I personally own over 40
of one model of Canon Powershot, and the differences in color response are
fairly extreme). This could in principle be at least partly calibrated out, if
you had each device take a picture of a calibration standard.

Third, the image signal processors are closer and closer to the sensor these
days. It may be impossible to get data which hasn't at least been somewhat
mucked with, meaning some people will have a useful spectrometer and some
won't.

This goes doubly for phones with bad imaging pipelines, where the image
processing is out of your control. Lots of spatial and spectral processing
going on. It will be interesting to see, for example, how the auto white
balance and sharpening algos deal with having zero natural image content in
the captured data.

Aside from the within-sensor variation/image processing problems it would
definitely be usable for relative data - I see that they used a relative
approach on their UV detection of a bluing dye in detergent. Nice.

All that said, I run a somewhat-similar project using cheap cameras to do work
previously only reserved for hard-core SLRs and scanning line sensors, and
I've found that over and over again, something is vastly better than nothing
and often "good enough" is better than expensive, complicated, and pain in the
ass. I've also dealt with a lot of people laying out a bunch of complicated
technical "Here's why it won't work" talk and have learned to counter with my
own "But look, it does work" answer.

Blah blah: I think it's important for the capture software captured as much
metadata about the image sensor as possible. Their software looks great and a
database of spectra would also be great. It would be AWESOME if the Shazam-
style auto identification worked at all. Good luck, I'll probably be
supporting the project.

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sp332
>First, the (bandpass) Bayer color filter arrays on cameras limit their color
resolution pretty sharply. It might be a better (if less elegant) approach to
use the ambient light sensor and scan it across the spectrum generated by the
diffraction grating.

The diffraction grating spreads the different frequencies physically across
the sensor. So the camera sensor doesn't need _any_ color information, you
only need position and brightness information.

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daniel_reetz
Right, but the different physical locations on the sensor have different
spectral sensitivities due to the bayer CFA. Ideally, you'd have panchromatic
pixels with no bayer pattern.

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sp332
Oh I hadn't thought of that. I assume some calibration would be required for
each device anyway, I wonder how hard it would be to detect and correct for
this effect?

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daniel_reetz
People have done it for a variety of purposes (natural image statistics,
astronomy, etc). I can post references later. Problem is that it almost
universally requires you to photograph a standard light source or a standard
reflectance target under a standard illuminator. Or a monochromator. So you
can calibrate your cheapy device but you need an expensive/scarce standard.

It's an interesting problem and I look forward to seeing how they solve it.

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tomkinstinch
Back in the day, standard illuminants were candles made from spermaceti--the
wax substance from the cavity of a sperm whale (it was also used as space-
grade lubricant until very recently). Nowadays, CIE Illuminant D represents a
sort of model of natural daylight. I bet a blue sky sunny day could be used
for calibration, if the instrument is pointed straight up and a function
corrects for lon/lat and time of the year.

For reflectance measurements, the highly-lambertian Spectralon material is
basically just well-controlled Teflon sheet. Some PTFE sheet from McMaster
might do the job for DIY purposes.

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sp332
Sunlight is tricky because colors can be distorted according to the angle of
the light through the atmosphere, hazing, dust, etc.

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tomkinstinch
Certainly, but you could account for much of it--and average away the rest.

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defdac
When I do this with a Project STAR spectrometer and my cameras and point it to
a candle I get two distinct dips in the spectral distribution. It should be
even like a blackbody. I have made a parser for the RGB-picture to turn it to
a B&W power distribution and I thought I'd might do a simple software filter
to make it completely even, like a calibration of sorts. Like so:
[http://www.flickr.com/photos/defdac/7874867702/in/photostrea...](http://www.flickr.com/photos/defdac/7874867702/in/photostream/lightbox/)
I use the power distributions coupled with lumen and wattage-specification as
input for my home made spectrally based global illumination renderer to
calculate PAR in aquariums. Do you think this will work?

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defdac
The IR-filter most cameras have is a real problem too. I have a webcam with
the IR-filter removed.

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chm
Best of luck with the project. I think the really good idea is the open-source
spectral library.

You need to educate your readers a bit more, though. They need to be aware of
the kind of spectroscopy the machine does, i.e UV-Vis, and what information it
provides.

I might try DIYing it, but what would be really interesting is an IR
spectrometer, under 250$. Tough.

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Jun8
What are some of the cool experiments one can do with a spectrometer, other
than the ones mentioned here (I especially liked the wine analysis
application)?

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tomkinstinch
If you can transmit light through a sample curvette to the spectrometer, you
could use it to measure optical density (i.e. the amount of stuff in the way
that blocks or scatters light). You could use that to measure the
concentration of particulate in engine oil, or the strength of coffee.

If you can detect at 405nm, you could measure blood coagulation time/extent
(separate blood cells from plasma, measure attenuation at 405nm though
plasma).

You could cludge together a pulse oximeter if you can detect at 660nm and NIR
(905, 910, 940nm) and have light sources to emit at these wavelengths.
Absorbtion at these wavelengths changes as hemoglobin picks up and loses
oxygen. The ratio at 660nm and one of the other wavelengths could be used to
show blood oxygenation.

If you were into photography or cinema, you could measure the emission
spectrum of a light source and fit a blackbody curve to it to find its color
temperature--useful for white balancing.

If you wanted to match paint colors, you could use a spectrometer to do (along
with some additional calculation and knowledge about your pigments).

If you perform a flame test, you could use a spectrometer to learn about
elemental composition.

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nsp
rze

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nsp
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