poor mans spectrometer.

[George Herold]

So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.

 

[Martin Brown]

On 06/02/2020 21:53, George Herold wrote:

So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

Depending on what it is for and how much dispersion you might be better
off with a camera and a CD at glancing incidence. That configuration is
easily good enough to see the Fraunhofer lines in the solar spectrum
without the complication of needing any kind of slit. You need a
aluminium coated one or you get junk from the dye absorbtion bands.

If you can build something and don't mind infringing PE patents then one
of the cutest high resolution 2D spectrograph imaging designs uses a low
dispersion prism on one axis and a high dispersion grating on the other
with a little bit of overlap on each line. No moving parts at all.

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.

What sort of resolution are you looking for?

--
Regards,
Martin Brown

 

[Phil Hobbs]

On 2020-02-06 16:53, George Herold wrote:

So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.

I've had good luck mounting gratings on HS-7945TH RC airplane servos,
using a 525130 servo hub and 585440 right-angle aluminum channel (all
from servocity.com). Your normal Richardson / Jobin Yvon one winds up
rotating with the axis almost exactly in the plane of the grating.

The servos are super easy to drive from a LabJack--I'll send you the
code if you like, but all it needs is a PWM with 50-60 Hz rep rate.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[George Herold]

On Thursday, February 6, 2020 at 5:23:30 PM UTC-5, Martin Brown wrote:

On 06/02/2020 21:53, George Herold wrote:
So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

Depending on what it is for and how much dispersion you might be better
off with a camera and a CD at glancing incidence. That configuration is
easily good enough to see the Fraunhofer lines in the solar spectrum
without the complication of needing any kind of slit. You need a
aluminium coated one or you get junk from the dye absorbtion bands.

If you can build something and don't mind infringing PE patents then one
of the cutest high resolution 2D spectrograph imaging designs uses a low
dispersion prism on one axis and a high dispersion grating on the other
with a little bit of overlap on each line. No moving parts at all.

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.

What sort of resolution are you looking for?
1nm at 780/650 would be enough.. I assume width is at least 10 nm.

--
Regards,
Martin Brown

 

[George Herold]

On Thursday, February 6, 2020 at 5:06:07 PM UTC-5, Phil Hobbs wrote:

On 2020-02-06 16:53, George Herold wrote:
So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.


I've had good luck mounting gratings on HS-7945TH RC airplane servos,
using a 525130 servo hub and 585440 right-angle aluminum channel (all
from servocity.com). Your normal Richardson / Jobin Yvon one winds up
rotating with the axis almost exactly in the plane of the grating.

The servos are super easy to drive from a LabJack--I'll send you the
code if you like, but all it needs is a PWM with 50-60 Hz rep rate.

Rotate the grating... that is the obvious choice.
Can the servo wiggle it back and forth where you want it?

George H.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-02-06 18:23, George Herold wrote:

On Thursday, February 6, 2020 at 5:06:07 PM UTC-5, Phil Hobbs wrote:
On 2020-02-06 16:53, George Herold wrote:
So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.


I've had good luck mounting gratings on HS-7945TH RC airplane servos,
using a 525130 servo hub and 585440 right-angle aluminum channel (all
from servocity.com). Your normal Richardson / Jobin Yvon one winds up
rotating with the axis almost exactly in the plane of the grating.

The servos are super easy to drive from a LabJack--I'll send you the
code if you like, but all it needs is a PWM with 50-60 Hz rep rate.
Rotate the grating... that is the obvious choice.
Can the servo wiggle it back and forth where you want it?

Sure thing. It uses a pot for an encoder, so the resolution is very
good. Lifetime is more of an issue, but then we're building protos here.

Cheers

Phil Hobbs


--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[whit3rd]

On Thursday, February 6, 2020 at 1:53:42 PM UTC-8, George Herold wrote:

So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.

Rotate the grating, and if you get the RIGHT grating, that also focuses the output light on a
slit. Change the slits for narrower bandpass.

I coupled a geared-down stepper to this item
<https://www.ebay.com/itm/Jobin-Yvon-H-20-VIS-Monochromator-w-PMT-Detector-300-800nm-Horiba-ISA/113926146367>
back in the day, and just clocked the readings every few steps of the motor.

That monochromator uses a hologram as grating, which does focusing as well as dispersion,
so the light path is nicely devoid of transparent objects (like lenses).

 

[Rick C]

On Thursday, February 6, 2020 at 6:23:22 PM UTC-5, George Herold wrote:

On Thursday, February 6, 2020 at 5:06:07 PM UTC-5, Phil Hobbs wrote:
On 2020-02-06 16:53, George Herold wrote:
So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.


I've had good luck mounting gratings on HS-7945TH RC airplane servos,
using a 525130 servo hub and 585440 right-angle aluminum channel (all
from servocity.com). Your normal Richardson / Jobin Yvon one winds up
rotating with the axis almost exactly in the plane of the grating.

The servos are super easy to drive from a LabJack--I'll send you the
code if you like, but all it needs is a PWM with 50-60 Hz rep rate.
Rotate the grating... that is the obvious choice.
Can the servo wiggle it back and forth where you want it?

I was picturing moving the sensor/slit with obsolete electronics like printers or using the sensor of a flat bed scanner (lots of resolution per inch), but then I realized, a simple camera will get you 4,000 pixels or more. The setup can't get much simpler.

You can arrange an IR and UV LED with the device under test to get calibration peaks.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Jasen Betts]

On 2020-02-06, George Herold <ggherold@gmail.com> wrote:

So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

spin the grating with a motor


--
Jasen.

 

[Phil Hobbs]

On 2020-02-06 21:27, Rick C wrote:

On Thursday, February 6, 2020 at 6:23:22 PM UTC-5, George Herold
wrote:
On Thursday, February 6, 2020 at 5:06:07 PM UTC-5, Phil Hobbs
wrote:
On 2020-02-06 16:53, George Herold wrote:
So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some diffraction
grating. (real spendy one or just consumer CD.) And say I have
just one detector rather than an array. If I could have the
detector translate across the spectrum, I could watch it in the
time domain on my 'scope. (assuming some trigger in whatever
causes translation.)

So how best to translate/ vibrate... my detector? (det. is sm
photodiode bpw34.. maybe with tape as slits?)

George H.


I've had good luck mounting gratings on HS-7945TH RC airplane
servos, using a 525130 servo hub and 585440 right-angle aluminum
channel (all from servocity.com). Your normal Richardson / Jobin
Yvon one winds up rotating with the axis almost exactly in the
plane of the grating.

The servos are super easy to drive from a LabJack--I'll send you
the code if you like, but all it needs is a PWM with 50-60 Hz rep
rate.
Rotate the grating... that is the obvious choice. Can the servo
wiggle it back and forth where you want it?

I was picturing moving the sensor/slit with obsolete electronics like
printers or using the sensor of a flat bed scanner (lots of
resolution per inch), but then I realized, a simple camera will get
you 4,000 pixels or more. The setup can't get much simpler.

That's basically an optical multichannel analyzer (OMA). They're pretty
good for many things--we have two ourselves--but they don't have
anything like the dynamic range or spectral selectivity of a regular old
Czerny-Turner monochromator. (Double and triple monochromators are much
better still, but they're a bear to get right unless you have serious
optomechanics chops.)

You can arrange an IR and UV LED with the device under test to get
calibration peaks.

LEDs aren't good calibration sources, because their peaks are broad,
poorly controlled, and drift with temperature. A He-Ne laser is
dramatically better. Also, one-point calibration is good enough for
most spectrometers, because there's a first-principles expression for
the centre wavelength vs. grating angle.

I have an old Burleigh Wavemeter that uses a He-Ne and a Mach-Zehnder
interferometer--it counts fringes of the unknown laser and normalizes by
the number of He-Ne fringes. The power supply arcs a bit at startup,
but then works fine. Not bad for $300 on eBay.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Rick C]

On Friday, February 7, 2020 at 11:56:07 AM UTC-5, Phil Hobbs wrote:

On 2020-02-06 21:27, Rick C wrote:
On Thursday, February 6, 2020 at 6:23:22 PM UTC-5, George Herold
wrote:
On Thursday, February 6, 2020 at 5:06:07 PM UTC-5, Phil Hobbs
wrote:
On 2020-02-06 16:53, George Herold wrote:
So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some diffraction
grating. (real spendy one or just consumer CD.) And say I have
just one detector rather than an array. If I could have the
detector translate across the spectrum, I could watch it in the
time domain on my 'scope. (assuming some trigger in whatever
causes translation.)

So how best to translate/ vibrate... my detector? (det. is sm
photodiode bpw34.. maybe with tape as slits?)

George H.


I've had good luck mounting gratings on HS-7945TH RC airplane
servos, using a 525130 servo hub and 585440 right-angle aluminum
channel (all from servocity.com). Your normal Richardson / Jobin
Yvon one winds up rotating with the axis almost exactly in the
plane of the grating.

The servos are super easy to drive from a LabJack--I'll send you
the code if you like, but all it needs is a PWM with 50-60 Hz rep
rate.
Rotate the grating... that is the obvious choice. Can the servo
wiggle it back and forth where you want it?

I was picturing moving the sensor/slit with obsolete electronics like
printers or using the sensor of a flat bed scanner (lots of
resolution per inch), but then I realized, a simple camera will get
you 4,000 pixels or more. The setup can't get much simpler.

That's basically an optical multichannel analyzer (OMA). They're pretty
good for many things--we have two ourselves--but they don't have
anything like the dynamic range or spectral selectivity of a regular old
Czerny-Turner monochromator. (Double and triple monochromators are much
better still, but they're a bear to get right unless you have serious
optomechanics chops.)

Do you really think the OP is asking for anything like that? He's talking about using a CD as the diffraction grating.... I think the camera will be quite good enough easily satisfying his 1 nm resolution requirement if aligned correctly.

You can arrange an IR and UV LED with the device under test to get
calibration peaks.

LEDs aren't good calibration sources, because their peaks are broad,
poorly controlled, and drift with temperature.

Just like the devices he is measuring.

A He-Ne laser is
dramatically better. Also, one-point calibration is good enough for
most spectrometers, because there's a first-principles expression for
the centre wavelength vs. grating angle.

Perhaps you could lend him a laser or two to get his setup calibrated.

To use a one point calibration you need to know all the details of the geometry and optics. With a two point setup you need to know only the wavelengths of the two references... that's it.

I have an old Burleigh Wavemeter that uses a He-Ne and a Mach-Zehnder
interferometer--it counts fringes of the unknown laser and normalizes by
the number of He-Ne fringes. The power supply arcs a bit at startup,
but then works fine. Not bad for $300 on eBay.

Why not lend him your entire setup?

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

[Piglet]

On 06/02/2020 21:53, George Herold wrote:

So I want to look at the spectrum of led's (LD's below
threshold) Lotsa light mostly. And say I have some
diffraction grating. (real spendy one or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the
spectrum, I could watch it in the time domain on my
'scope. (assuming some trigger in whatever causes translation.)

So how best to translate/ vibrate... my detector?
(det. is sm photodiode bpw34.. maybe with tape as slits?)

George H.

Does $9 qualify as poor man affordable?

<https://publiclab.org/wiki/papercraft-spectrometer>

piglet

 

[Jon Elson]

On Thu, 06 Feb 2020 13:53:38 -0800, George Herold wrote:

So I want to look at the spectrum of led's (LD's below threshold) Lotsa
light mostly. And say I have some diffraction grating. (real spendy one
or just consumer CD.)
And say I have just one detector rather than an array.
If I could have the detector translate across the spectrum, I could
watch it in the time domain on my 'scope. (assuming some trigger in
whatever causes translation.)

For one measurement, it doesn't make sense. But, if you want to do this
often, a fast-scan spectrophotometer can be built. You need one of the
linear image sensors from an old page scanner, they typically have 2048
photodiodes in a line and a CCD array to scan them out. I recently
hacked up some minimal circuitry to fire up one salvaged from a dead
scanner. It puts out an analog "video" signal similar to an old NTSC
scan line. You then need a slit and a grating, and tweak the optical
arrangement until you get the right range of wavelengths spread across
the sensor. If you can find an old scanner that has about a 3" square
board with 5-8 chips on it, and the sensor in the middle, you have almost
the complete electronic part of the instrument in one piece.

Jon

 

[Rick C]

On Friday, February 7, 2020 at 2:41:17 PM UTC-5, Jon Elson wrote:

If you can find an old scanner that has about a 3" square
board with 5-8 chips on it, and the sensor in the middle, you have almost
the complete electronic part of the instrument in one piece.

Then you just need an interface spec. A few hundred hours of analysis later you might have one. Why not just use a camera you can buy for $50 or use your phone?

BTW, how does the scanner scan across the width of the page? Does it use optics? That sounds a lot like a camera.

--

Rick C.

-- Get 1,000 miles of free Supercharging
-- Tesla referral code - https://ts.la/richard11209