poor mans spectrometer.

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

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.

What sort of resolution are you looking for?

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

George H.

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

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 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).

I was picturing moving the sensor/slit with obsolete electronics like print ers 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 calibrat ion peaks.

spin the grating with a motor

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.)

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

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.

Just like the devices he is measuring.

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.

Why not lend him your entire setup?

Does $9 qualify as poor man affordable?

piglet

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

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.