
Inexpensive chip-based device may transform spectrometry - charlysl
http://news.mit.edu/2018/inexpensive-chip-device-spectrometry-1023
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analog31
I help design spectrometers. Every once in a while, a new idea for a mini
spectrometer floats across my desk. Many of the ideas are based on silicon IC
fabrication technologies or micromachining.

The drawback is the same in every case: The sheer amount of light that can
pass through the optics of a spectrometer depends on its size. This in turn
affects what signal-to-noise ratio can be realized.

These devices are certainly interesting, and performance is relative to
requirements, so a novel application could make use of whatever sensitivity is
made available by a particular device. But a tiny spectrometer isn't likely to
be a drop-in replacement for a big one unless the big one is overkill for its
application.

Still, I read these articles with interest, because my "size matters" rule is
somewhat ad hoc, and there might be a factor that I'm overlooking.

~~~
DenisM
Maybe you can help me with this... I want to measure the quality of light in
my home and office. CRI seems to be the right metric, and it seems that the
cheapest option is a $2000 (Sekonic) hand-held device. Are there cheaper
options? I don't need much precision, just enough info to understand that
these lightbulbs over here are much better than those other lightbulbs over
there.

I am puzzled as to why it has to be so expensive. I would think one can put a
prism, project the light over the surface of a black-and-white CCD sensor and
measure the light intensity along the length, giving a reasonably good idea of
how much energy is deposited into each wavelength range. I feel like one of
these can be made for $20 in parts?

~~~
danielvf
You can get a plastic DSLR camera body cap for less than 50 cents each (ebay
from China). This cap goes where the lens would attach the camera when you
don't have a lens on. Sometimes I cut a hole in the center of a cap and make
really hacky camera "lenses" for fun. You could put a prism on the outside of
a cap.

You would then need to calibrate your new spectra-cam on something with a
known radiation (usually just find a good blackbody radiator.) You can't just
convert to black and white without calibration because the RGB sensors on the
camera are going to have their own sensitivity to different wavelengths. This
would be an awesome project!

\--

I write firmware for high end professional LED lights, and have a $3K
spectrometer on my desk. Here are things I've learned:

1\. CRI is simply a bad formula to judge light quality overall and gives
almost meaningless results at lower color temperature levels.

2\. There are lots of fantastic python modules for working with spectrograph
and light data.

3\. One awful thing cheap LED lights do is blink slowly to control their
voltage. This is at least as bad a poor CRI for aggravating your eyes.
(Sometimes you can see this just by using slow motion video on your phone)

~~~
tyingq
Interesting detail. On this, though:

>One awful thing cheap LED lights do is blink slowly to control their voltage

I assume you're talking about PWM. That's not just to be cheap. It's the only
real way to control brightness.

~~~
danielvf
(Sorry, I meant to say that I hate cheap LED lights that blink slowly to
control their _brightness_.)

Blinking the LED on/off to control its brightness with a PWM is definitely not
the only way to control output. And if you are controlling by blinking, PWM is
still the bottom of the pile in ways to blink, from a quality of light point
of view.

~~~
CamperBob2
What other ways are there? Nobody is going to want to dissipate heat in a
linear element somewhere, are they?

I guess that because LEDs are actually so efficient, that isn't such a bad
idea as it sounds at first.

~~~
jononor
A constant current source does not have to be linear, one can use a switchmode
powersupply with current regulation. Then the current though the LED flows in
a continuous manner.

~~~
solarkraft
Switch-mode regulators don't pulse?

How wrong am I to think that a capacitor after the PWM-regulator would give a
reasonably good approximation of a constant voltage?

~~~
jononor
Sure they pulse/switch, but that is into/from the inductor/capacitor used as
intermediate energy storage, not through the load (LED). At the output a
constant voltage is seen (plus some ripple).

A capacitor on PWM output will give a decent approximation, _if_ it is big
enough. For large LEDs this can be very costly (relative to other parts in
system). Though the inductor in a switching converter is also a big cost
driver.

Both for the capacitor, inductor and wrt to human perception increasing
switching frequency helps. So that is what modern designs focus on normally.

A challenging aspect of LEDs is their nonlinear voltage versus current
characteristic. A small change in voltage gives a very large change in
current, and thus in output power. The characteristic is temperature dependent
and has per device variation. Hence LED drivers are usually constant-current
sources, ie they measure and attempt to regulate the current.

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keithnz
This is cool, a while back when I used to do fruit sorting machines, we used a
spectrometer for working out the sugar content of fruit.

Nothing quite like setting up the machine to siphon off the biggest sweetest
peach you can find at one of the largest peach farms in the world. best.
peach. ever. :)

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softgrow
If you are wanting an inexpensive spectrometer, then Public Labs has
instructions on how to build one using an old DVD as a diffraction grating
[https://publiclab.org/wiki/spectrometry](https://publiclab.org/wiki/spectrometry)

I got a kit as a present and really enjoyed it.

~~~
dejv
It depends on what you want this spectrometer to do. Public Lab spectrometer
is interesting and cheap, but the performance is very very limited.

If you need something better you can search eBay for used (scanning)
monochromators. Next step is to check Ocean Optics or Hamamatsu catalogs: they
have good line of products, but those are in 1000 USD+ range.

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rfinney
Open access Nature Communications paper :
[https://www.nature.com/articles/s41467-018-06773-2](https://www.nature.com/articles/s41467-018-06773-2)
High-performance and scalable on-chip digital Fourier transform spectroscopy

 _Here we demonstrate a transformative on-chip digital Fourier transform
spectrometer that acquires high-resolution spectra via time-domain modulation
of a reconfigurable Mach-Zehnder interferometer. The device, fabricated and
packaged using industry-standard silicon photonics technology, claims the
multiplex advantage to dramatically boost the signal-to-noise ratio and
unprecedented scalability capable of addressing exponentially increasing
numbers of spectral channels._

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starshadowx2
FTIR devices seem to cost around 50000 right now, would this be something that
would lower that cost?

FTIRs are definitely in demand with harm reduction groups both in cities and
for drug testing at music festivals. If we could get these at lower costs it
would honestly save lives.

~~~
mediaman
How is FTIR used for drug testing? Is it used to test for contamination based
on the spectrum profile?

~~~
dejv
Peaks of each chemical is unique so you can scan database of known chemicals
and compare.

You can even measure the amounts of various chemicals in solution, but that
would require complex calibration. In this case you are basically writing ML
system that is trying to fit various peak heights/areas to amount of compound.

