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Public Lab Research note


  • 3

a paper spectrometer design

by mathew |

mathew was awarded the Photo Documentation Barnstar by amysoyka for their work in this research note.


What I want to do

build a spectrometer that doesn't use a conduit box, out of mostly paper and cardboard. I want it to be easy to open and close, and ideally all the pieces could be printed on 8.5' x 11" paper

suggested by @cfastie in his research note on

In light of various ideas in the history

My attempt and results

Eschewing the more flexible concepts (more 1, 2 ) that Warren and I were prototyping before the breakthroughs in the oil testing kit assay ( 1 2, 3, 4, 5, 6, 7) that lead to a realization that a fixed geometry was fine-- we don't seem to really need to play with the geometry much.

That doesn't mean we couldn't do better than the existing conduit box, as @stoft, @cfastie, and others have pointed out. Getting the relationship of spectrometer's grating, slit, and camera just right is hard in the short conduit box spectrometer.

Following @cfastie's empirical data (from the note linked above), I wanted a spectrometer with a 20cm distance between the camera and slit, and therefore a 45 degree camera angle and 60 degree grating angle.

I started by making a very rough prototype from an existing desktop spectrometer insert and parts, and some black aluminum foil.

IMG_20141021_201721.jpg

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I made a little cap for the 45 degree wood block that holds the grating at 60 degrees:

IMG_20141021_201854.jpg

And used another small block to step the .18mm optically printed slit 5mm back from a 2mm wide baffle. this "double slit" a wider baffle and second slit, came out of an experiment to limit stray light. That was very effective, but needed a longer distance to the camera.

IMG_20141021_201904.jpg

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Then I set about making an open-and-closable shroud out of the prototype parts. To make sure everything was in easy metric relationships, I prototyped on some Open Structures grid paper I printed out. i then rapidly violated their guidelines regarding tape :-P open_grid_8.5x11.pdf

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after getting a prototype of the tab closures working, I made some vector plans:

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And then taped them to black paper and cut them out with an exacto.

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here they are in their assembly relationship, and partially assembled:

IMG_20141106_151328.jpg

Here are the printable PDFS. I should really differentiate the cut and fold lines, this is a first draft so they aren't super clear yet: endcaps.pdf

wrapper.pdf

angle.pdf

Assembly

I glued two blocks together so they were the full width (4cm) of the spectrometer chamber-- it makes easier alignment.

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I then glued down the bottom panel of the rear end cap onto the cardboard panel that forms the bottom of the spectrometer:

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I mounted the camera assembly flush to the rear cap, and also taped a dvd grating in place:

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unfortunately I didn't account for where the webcam cord was going to go. so I cut a slit in the rear panel and slid it out, and then taped everything closed. Big oversight:

IMG_20141106_160602.jpg

I then lined up the optical slit, little block, and front endcap with baffle slit:

IMG_20141106_162920.jpg

and then glued its base down too:

IMG_20141106_163221.jpg

then it was time to close the wrapper around the spectrometer and close all the tabs:

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Then It was time for light-tightness testing with a halogen. Some light can slip underneath the gap between the cardboard panel and the front cap. that was the only spot I could find.

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spectra look nice and clear. The two green peaks are visible (though blown out in this image)

Photo_on_11-6-14_at_4.50_PM.jpg

Questions and next steps

I'm really happy with the tabs, they let the whole thing be opened and closed for adjustments to the camera or slit, etc.

I'm going to replace the cardboard with a piece of plywood or other more rigid material, its not rigid enough.

the connecting tabs need to be 1mm shorter to make them easier to insert into the slits.

I need a solution to the USB cable that is straightforward.

I want to make a laser-pointer/cuvette holder that attaches to the front.



spectrometer desktop-spectrometer fold-up dsk oil-testing-kit paper

barnstar:photo-documentation


7 Comments

Bravo!

usb cable

There are the little plastic crimp-on things made to protect electrical wire going through holes in metal boxes. Those come in black and might fill a hole in paper without tearing it up.

Or, perhaps easier, tho' potentially messier and more permanent, there's opaque black silicone caulk/glue stuff used to hold door gaskets in place for automobiles


This looks great!

I am really impressed with the resolution of your spectra

On the USB cable.

My spectrometer broke when the pins came out of the modular plug on the USB cable.

Can I recommend trying to house the join between the cable & the camera chip board itself?

On the light from the USB hole.

In keeping with what @hank said.

Most offices have blue tack lying around sparsely. Would that work?

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Hi Mathew,

Nice work and than you for posting detailed steps. I have two questions:
  1. Why is DVD diffraction grating pasted at 60° angle?
  2. How do you determine the required distance between slit and camera?

    Thank you in advance!

SJ

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@spectraljugad The distances and angles come from @cfastie's empirical tests linked above in "relationship between grating, slit, and camera" for some reason on mobile I can't copy and paste the link back in here.


Mat, That's a pretty nice spectrum. Are you going to try adjusting the grating angle by 5° or so in either direction? I'm not sure it would make any difference or if the web cam could capture the difference, but it might be worth a try before you lock down the angles. When I tested a DVD+R the best angle for a 20 cm slit to grating distance was about 65°. The angle was closer to 60° when grating film was used. I never tried DVD-R Chris

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Thats a good idea. I was thinking of making a version of the paper grating support that had a protractor-like series of angles so someone could fold it for a different angle too.



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