Especially with the new stability upgrades being explored, I wanted to look at different ways to reduce reflected and ambient light leaking and into and reflecting around the inside of the spectrometer box, because that makes high baseline noise when we capture spectra.
(Note that this is not a question about noise over time, like where a pixel can vary in brightness randomly -- a separate issue.)
I've seen a bunch of different solutions for basically darkening and better sealing the box, and wanted to explore which work best (easiest/cheapest/simplest/darkest), especially for the above upgrade, which is currently shown in bare wood (not black).
- Lining with black paper/cloth: (see below image by @stoft) be aware that not all black colored material is absorbent in the infrared range too! Some are reflective.
- Ink: sumi ink, india ink, matte paint -- could suffer same problem as above)
- Baffles: paper "gates" which narrow the field of view to reduce "incident" light (see block with hole in below image)
Other solutions?
Is baseline light sometimes related to camera temperature? If it helpful to use the spectrometer in a cold room?
Another approach is to subtract out the baseline light in software, using a "blank" spectrum (covering the front slit) to read in just the baseline light. This is a little more complex, but explored here (read the comments especially).
I know other folks have tried lots of techniques, so please post them here and we can compare/contrast. Thanks!
I have been considering this too, and am looking seriously at carbon covered sponge that we have at work. We sell it in 2 products. One is for soldering iron smoke extraction. The other is a carbon sponge matting for storing electrostatic sensitive components.Its basically activated carbon bonded to some sort of sponge material. I think the one for storing components appears to be better and more robust in regards to it not falling apart and dropping charcoal dust falling off inside the spectro'.
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After having a look at the 2 different products I decided to go with the carbon filter for the soldering fume extraction. It is more black than the other product which had some greyish reflective specks all over it. I will cut and install it on my box. It should be better than the black cardboard.
Hi, I'm really interested but I'm not familiar with that material... Could you share some links to where it could be bought, and some photos?
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I tried using similar ink to paint the inside, and it works pretty darn well; I'm thinking of posting it as an easy upgrade!
Update: I posted this as an upgrade: https://publiclab.org/notes/warren/09-22-2016/reduce-interior-reflections-in-your-spectrometer-with-black-ink?_=1474547664 -- try it out and post a response!
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Regarding a baffle, I wonder if we could just repurpose the paper structure that was holding the DVD, after upgrading with wood blocks:
@stoft -- your wooden baffle shown in the main body of the question is abotu 1" away from the webcam -- any comments on placement? And, does it matter if the baffle is not as rigid as the rest of the optics -- I suppose it could cause slight variance in background light levels if it's not completely rigid, so I could try making another "stand" from three more wooden blocks, then re-use the black card paper? (interested in re-use because those pieces are cut on the same die as the spec kit, so every kit owner will have them already). Thanks!
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@warren, the black ink looks quite dark in the photo and seems easy to apply. Hope it cures well.
I believe the primary factors are 1) the black color being "flat" as opposed to "reflective" and 2) the surface texture. So, the same black color value but of a "soft" surface will absorb light better than the same black color used to make a "hard" surface paper. The simplest very non-reflective balck surface is black felt. It has the advantage of having a very high surface area which, like with acoustics at a much larger scale, dramatically increases the scattering -- and the light (like sound) looses a little energy (turns to heat) on each "micro-reflection". Also quite good is soft black fabric. However, for simplicity, simple black paper, construction paper and (some) cardstock are often much easier to cut and install.
As for the baffle, the idea came from having some bright reflections from light leakage from the source (lamp) being observed -- as opposed to exclusively light from the slit itself. So, the baffle is of more or less importance dependent on the source of "leakage" light. Since the baffle is just a "helper" it's design is less critical. As for re-purposing the old grating holder, a factor to watch for is to avoid surface angles which might tend to cause any reflected light on the camera side to be directed back at the DVD surface. It also appears very simple to just add another "slit mount" as a baffle support since the pieces are all the same and the baffle is just another piece of black paper; all very easy and cheap.
As for the slit mount position, a half-inch variance isn't going to matter. I mounted it back from the end just to avoid having that part interfere with the paper housing when the assembly is inserted. Ideally, the entire device should be longer to improve the optical performance.
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it would be interested to use Vantablack.. maybe the manufacturer would be willing to donate some considering the purpose
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